ISSN:0003-2654    

DOI:10.1039/AN97398FX025

出版商:RSC    

年代:1973

数据来源: RSC

ISSN:0003-2654    

DOI:10.1039/AN97398BX027

出版商:RSC    

年代:1973

数据来源: RSC

摘要:

iv SUMMARIES OF PAPERS IN THIS ISSUE [July, 1973of Papers in this IssueMass and Charge Transfer Kinetics and CoulometricCurrent Efficienciesand Lewartowicz Methods, and Apparatus and Procedures forRamping VoltammetryA comparison is made of the provenance and application of patterntheory, Tafel, Allen and Hickling and Lewartowicz methods for the deter-mination of charge-transfer kinetic parameters. A general-purpose electro-chemical cell is described together with a ramping potentiostat - galvanostatfor slow single-sweep voltammetry. The mass-transfer characteristics, whichare governed by the geometrical placing of the various appurtenances thatdip into the solution and by the stirring speed, were examined by determiningthe apparent diffusion layer thickness at various stirring speeds for asystem of known mass-transfer properties, copper(I1) - copper(1).It wasfurther established that the limiting current of reduction of hexacyano-ferrate(II1) is reproducible and linearly related to concentration.E. BISHOP and P. H. HITCHCOCKChemistry Department, University of Exeter, Stocker Road, Exeter, EX4 4QD.Analyst, 1973, 98, 465-474.Part V. Comparison of Pattern Theory, Tafel, Allen and HicklingMass and Charge Transfer Kinetics and CoulometricCurrent Efficienciesthe Effects of Oxidation of PlatinumPart VI. The Pre-treatment of Solid Electrodes, and a Review ofThe nature and condition of working electrode surfaces are set in thecontext of reaction speeds and current efficiencies. The formation of filmsand their effects are examined for platinum and other metals and alloys.Adsorption and specific adsorption on electrode surfaces are briefly reviewed.An attempt is then made critically to appraise the current state of the artin respect of the activation and deactivation of electrodes.Methods ofcleaning electrodes are canvassed. The more significant theories of activationand deactivation are reviewed with specific reference to platinum electrodes.These theories include the impurity theory, the platinisation theory andthe various oxygen-containing surface theories. For the last, the formationof oxides, the measurement of film thickness, the film thickness and thenature of the oxide reducible a t 0-6 V are discussed, followed by a selectivereview of the oxygen-bridge theory, the half-reduced oxide theory and theplatinum - oxygen alloy theory.Tentative conclusions are reached. Finally,gold electrodes and their behaviour are briefly examined.E. BISHOP and P. H. HITCHCOCKChemistry Department, University of Exeter, Stocker Road, Exeter, EX4 4QD.Analyst, 1973, 98, 475-484July, 19731 THE ANALYSTTHE ANALYSTVEDITORIAL ADVISORY BOARDChairman: H. J. Cluley (Wembley)*L. S. Bark (Salford) W. Kemula (Poland)*G. F. Kirkbright (London)G. W. C. Milner (Harwell)G. H. Morrison (U.S.A.)*J. M. Ottaway (Glasgow)*G. E. Penketh (Billingham)S. A. Price (Tadworth)D. 1. Rees (London)E. B. Sandell (U.S.A.)*R. Sawyer (London)A. A. Smales, O.B.E. (Harwell)H. E. Stagg (Manchester)E.Stahl (Germany)A. Walsh (Australia)T. S. West (London)P. Zuman (U.S.A.)R. Belcher (Birmingham)L. J. Bellamy, C.B.E. (Waltham Abbey)L. S. Birks (U.S.A.)E. Bishop (Exeter)E. A. M. F. Dahmen (The Netherlandsj*J. B. Dawson (Leeds)A. C. Docherty (Billingham)D. Dyrssen (Sweden)*W. T. Elwell (Birmingham)*D. C. Garratt (London)*R. Goulden (Sittingbourne)J. Hoste (Belgium)D. N. Hume (U.S.A.)H. M. N. H. Irving (Leeds)A. G. Jones (Welwyn Garden City)M . T. Kelley (U.S.A.)*A. Townshend (Birmingham)*J. A. Hunter (Edinburgh)* Members of the Board serving on the Executive Committee.NOTICE TO SUBSCRIBERSSubscriptions for The Analyst, Analytical Abstracts and Proceedings should beThe Chemical Society, Publications Sales Office,Blackhorse Road, Letchworth, Herts.Rates for 1973(other than Members of the Society)sent to:(a) The Analyst, Analytical Abstracts, and Proceedings, with indexes . .. . €37.00(b) The Analyst, Analytical Abstracts printed on one side of the paper (withoutindex), and Proceedings . . . . . . . . . . . . . . €38.00(c) The Analyst, Analytical Abstracts printed on one side of the paper (withindex), and Proceedings . . .. . . . . . . . . . . L45.00The Analyst and Analytical Abstracts without Proceedings-(d) The Analyst and Analytical Abstracts, with indexes . . . . . . . . €34.00(e) The Analyst, and Analytical Abstracts printed on one side of the paper (withoutindex) . . . . . . . . . . . . . . . . . . €35.00(f) The Analyst, and Analytical Abstracts printed on one side of the paper (withindex) .. . . . . . . . . . . . . . . . . €42.00(Subscriptions are NOT accepted for The Analyst and/or for Proceedings alone)v1 SUMMARIES OF PAPERS I N THIS ISSUE [July, 1973Ionic Polymerisation as a Means of End-point Indication inNon-aqueous Thermometric TitrimetryPart IV. The Determination of Catecholamines(-)-Adrenaline, adrenaline hydrogen tartrate, L-noradrenaline, dopaminehydrochloride, L-dopa, DL-dopa, L-ct-methyldopa, D-a-methyldopa and ( + ) -Corbasil have been determined in amounts down to 0.0001 mequiv by catalyticthermometric titration of their basic and acidic functions. Basic functionswere determined by titration with 0.1, 0.01 and 0.001 M perchloric acid byusing the ionic polymerisation of a-methylstyrene to indicate the end-point,while acidic functions were determined in a similar manner with tetra-n-butyl-ammonium hydroxide as the titrant and acrylonitrile as the end-pointindicator.The L-dopa contents of tablets and capsules have been determined byusing these techniques and the assay results have been compared with thoseobtained by alternative methods, namely, the recently described B.P.pro-cedure involving non-aqueous titration, and ultraviolet spectrophotometry.Magnesium stearate, which is used as a lubricant and flow promoterin tablet manufacture, is titrated as a base in the solvents used, but in titra-tions of the acidic function of catecholamines its effect is negligible.E. J. GREENHOW and L.E. SPENCERDepartment of Chemistry, Chelsea College, University of London, Manresa Road,London, S.W.3.Analyst, 1973, 98, 485-492.The Determination of Di-n-alkyl Phthalates in CosmeticPreparations by Gas - Liquid ChromatographyAn improved gas-chromatographic method for the direct determination ofC,-C, di-n-alkyl phthalates in toiletry samples that contain ethanol is des-cribed and a range of perfume essential oils and perfume synthetic chemicalsis examined for possible interference.E. W. GODLY and A. E. MORTLOCKDepartment of Trade and Industry, Laboratory of the Government Chemist, CornwallHouse, Stamford Street, London, SE1 9NQ.Analyst, 1973, 98, 493-501.Residues of Prophylactics in Animal ProductsPart 111. The Determination of Carbarsone in Poultry MeatA method for the determination of carbarsone in poultry meat is de-scribed.The carbarsone is extracted from the sample with methanol and,after clean-up on an ion-exchange column, hydrolysed to arsanilic acid withsodium hydroxide. The arsanilic acid is diazotised, coupled with 2-amino-ethyl-1-naphthylamine and determined spectrophotometrically.R. A. HOODLESS and K. R. TARRANTDepartment of Trade and Industry, Laboratory of the Government Chemist,Cornwall House, Stamford Street, London, SE1 9NQ.Analyst, 1973, 98, 502-505July, 19731 SUMMARIES OF PAPERS I N THIS ISSUEThe Determination of Microgram Amounts of Sulphate byEmission Spectroscopy of Barium with a NitrousOxide - Acetylene FlameThe sulphate content of aqueous solutions has been determined in-directly, in the ranges 0-5 to 5.0 and 1.0 to 10.0 p.p.m.from barium emissionmeasurements. By using a slight excess of barium, the sulphate is precipi-tated in a 50 per cent. solution of propan-2-01 and its concentration is calcu-lated from the decrease in the barium content of the solution. The amountof barium in solution is determined from its emission a t 553-55 nm in a nitrousoxide - acetylene flame. The only major flame interference detected is thatfrom the band emission of calcium; 410 p.p.m. of calcium gave an emissionintensity equal to that of 1 p.p.m. of barium. Sulphate has been determinedin both pure solutions and in synthetic sample solutions containing otherelectrolytes. Major interferences were noted for potassium and ammoniumoxalate, sodium orthovanadate, nickel chloride and to a lesser extent forsodium fluoride and perchloric acid.The sulphur content of biologicalmaterial, digested by oxygen-flask combustion, has been determined satis-factorily by using this method.E. A. FORBESRuakura Soil Research Station, Private Bag, Hamilton, New Zealand.Analyst, 1973, 98, 506-51 1.Pyridylazonaphthols (PANs) and Pyridylazophenols (PAPs) asAnalytical ReagentsTheir Determination in AlloysThe reactions between copper(I1) and 2-(2-pyridylazo)-l-naphthol(0-a-PAN) and 4- (2-pyridy1azo)phenol ($-PAP) have been studied by spectro-photometry and graphical analysis, and their equilibrium constants havebeen determined. A rapid and simple method for the spectrophotometricdetermination of copper in aluminium alloys has been devised.A solutionof the alloy with pH 2 is extracted with an equal volume of a 5 x Msolution of o-a-PAN in carbon tetrachloride and the absorbance of the extractis measured at 590nm. The method has been checked by the analysis ofstandard alloys. The effect of interferences is discussed. The extractionbehaviour of several cations with o-%-PAN is discussed and the sequentialseparation of copper(II), zinc(II), manganese(I1) and lead(I1) is outlined.D. BETTERIDGE, D. JOHN and F. SNAPEChemistry Department, University College of Swansea, Swansea, Glarnorgan,SA2 8PP.Analyst, 1973, 98, 512-519.Part 111. Formation of Copper(I1) Complexes andPyridylazonaphthols (PANs) and Pyridylazophenols (PAPs) asAnalytical ReagentsPart IV. Formation of Complexes with Titanium(1V)The formation of complexes of titanium(1V) with PANs and PAPs hasbeen investigated in order to ascertain the types of complexes that areformed and to elucidate the optical and stability constants of such complexes.The competing effect of hydrolysis on these reactions has also been con-sidered. The reaction with 2-(2-pyridylazo)-l-naphthol (0-a-PAN) has beenexamined in order to determine the analytical utility of this complex formation.D. BETTERIDGE, D. JOHN and F. SNAPEviiChemistry Department, University College of Swansea, Swansea, Glamorgan,SA2 8PP.Analyst, 1973, 98, 520-524

ISSN:0003-2654    

DOI:10.1039/AN97398FP073

出版商:RSC    

年代:1973

数据来源: RSC

摘要:

July, 19731 THE ANALYST ixPlease mentionTHE ANALYSTwhen replying to advertisementsAPPOINTMENTS VACANT~~~ ~LANCASHIRE COUNTY COUNCILCounty Analyst's DepartmentASSISTANT ANALYSTSalary AP. 4/5 f;Z,lOO - L2,GGlApplications are invited for the above post from candzdates prefer-ably with G.R.I.C. or A.R.I.C. or with a degree and analyticalexperience in food and drugs analysis. Commencing salary willdepend upon qualifications and experience. N. J.C. for Local Author-ities A.P.T. and C Conditions of Service will apply.The post will be superannuable and subject to medical fitness.Applications should be made to the County Medical Officer ofHealth, Serial No. 9545, East Cliff County Offices, Preston, andreturned within 14 days of the appearance of this advertisement.Preference will be given to applications received from L. G.Officersserving in the North-West.DECEN Nl AL INDEXESTO THE ANALYST1906-1915t .. fZ*lO1916-1925* . . f2.101936-1945* . . €2.10l9461955* .. €51956-1965* . . €6.75Bound in cloth boardst Paper boundObtainable fromThe Society for Analytical ChemistryBook Deportment9/10 Savile Row,London, WIX IAFChief AnalystWith knowledge of X-RayFluorescence techniquesRockware Glass Limited i s one of the leading UK manufacturers of glass containers.We are now seeking a Chief Analyst to run our six-strong Central Analytical Depart:ment a t the Company's head office in Greenford, Middlesex. As well as assisting withdevelopment and technical work in the Technical Division, this department providesan analytical service for our five factories located in various parts of the country.Duties for the Chief Analyst will include the immediate supervision of the analysis ofsilicate materials by X-Ray Fluorescence and conventional solution methods and theadministration of the department's work load under the general supervision of theHead of the Materials Development and Control Department.After an initial periodof 1 2 months in Greenford (requiring a certain amount of travel), he will then bebased in Yorkshire.Ideally, applicants will be aged between 25-35 with a minimum LRlC qualification,though ARlC or equivalent is preferred. He is also required to have had at least fiveyears' experience in an analytical laboratory. A first-hand knowledge of X-RayFluorescence analysis or similar techniques would be a distinct advantage.Analysts who feel they are able to meet the requirements of this interesting andprogressive appointment, are invited to write, giving full career details, to the HeadOffice Personnel Officer.ROCKWARE GLASS LIMITEDRockware Avenue, Greenford, MiddleseX THE ANALYST LTulv, 1973IMPERIAL COLLEGEASSISTANT RESEARCH OFFICER/RESEARCH OFFICERApplications are invited for work in the analyticalinstrumentation laboratory of the Department ofChemical Engineering and Chemical Technology.Applicants should have a university degree orits equivalent, together with practical experienceof mass spectrometric and spectroscopicmethods of analysis in research. Some back-ground knowledge of computing is desirable.The laboratory develops techniques for specificresearch projects and investigates new 'on-line'computer applications of analytical instruments , for process control.A 'real-time' computersystem links with a range of techniques which~ include mass spectrometry, spectrophotometry,gas and liquid chromatography, and theircorn binations.Salary will be in the ranges %I155 to S2238: or€2397 to S2874 plus London allowance, depend-ing upon qualifications and experience.Applications in writing giving full details withtwo referees t o Professor A. R. Ubbelohde,C.B.E., F.R.S., Imperial College, London SW72AZ.BOOKSM 0 N OG RAPH SREPRINTSorders for all publications ofthe Society (except journals)should be sent direct or througha bookseller to-THE SOCIETY FORANALYTICAL CHEMISTRYBook Department9/10 Savile Row,London, WIX IAFMONOGRAPHSFOR TEACHERSModernAnalyticalMethodsby D.BETTERIDGEand H. E. HALLAMModern Analyfical Methods is one of TheChemical Society's series of paperbackmonographs which present concise andauthoritative accounts of selected welldefined topics in chemistry for those whoteach the subject at 'A' level and above andfor students of further and higher education.It discusses the principles underlying themost important methods of quantitative andqualitative analysis used today. Samplesfor analysis may arise from diverse sourcesand contain a variety of molecules orelements at various levels of concentration.Thus separation methods, organic reagents,nuclear, electrochemical, spectroscopic andtitrimetric methods are amongst those dealtwith in some detail.Within the bounds ofelementary algebra, equations are developedwhich show how the optimum conditions forthe application of a method may be deducedand conditional constants are usedthroughout. The numerous illustrationssupport the text by clarifying principles or byexemplifying important methods which aredealt with briefly because they do not involvenew principles.234pp 75 diagrams $2.00(CS Members 21.50)ISBN 0 85186 759 6Orders enclosing the appropriateremittance, should be sent to:The Publication Sales Officer, TheChemical Society, Blackhorse Road,Letchworth, Herts SG6 1 HN.For information on other titles in the serieswrite to: The Marketing Officer,The Chemical Society, Burlington House,London W1V OBN.THE CHEMICALSOCIETJuly, 19731 THE ANALYST xiSELECTED ANNUALREVIEWSof theANALYTICAL SCIENCESVolume 2 - 1972lust publishedCONTENTSThe Techniques and Theory of ThermalAnalysis Applied t o Studies on InorganicMaterials with Particular Reference toDehydration and Single Oxide Systems- D.DollimoreDevelopments in Ion Exchange -F. VernonThermometric and Enthalpimetric Titri-metry - L. S. Bark, P. Bate and J. K.GrimePp. vi + 149 f5.00; U.S. $13.00Obtainable from-The Society for Analytical Chemistry,Book Department,9/ I0 Savile Row, London, W I X I AFMembers of The Chemical Society may buy personalcopies at the special price of €3.00; U.S.$8.00S PECl ALlST ABSTRACTJOURNALSpublished bySCIENCE AND TECHNOLOGY AGENCYAtomic Absorption and FlameEmission Spectroscopy AbstractsVol. 5, 1973, bimonthly €30X-Ray Fluorescence SpectrometryAbstractsVol. 4, 1973, quarterly $28Thin-Layer Chromatography AbstractsVol. 3, 1973, bimonthly €28Gas Chromatography-MassSpectrometry AbstractsVol. 4, 1973, quarterly f37Nuclear Magnetic ResonanceSpectrometry AbstractsVol. 3, 1973, bimonthly f30Laser-Raman Spectroscopy AbstractsVol. 2, 1973, quarterly €30X-Ray Diffraction AbstractsVol. 1-2, 1973, quarterly f30Neutron Activation Analysis AbstractsVol. 2-3, 1973, quarterly €30Electron Microscopy AbstractsVol.1, 1973, quarterly €30Liquid Chromatography AbstractsVol. 1, 1973, quarterly €30Electron Spin Resonance SpectroscopyAbstractsVol. 1, 1973, quarterly €30Sample copies on request from:SCIENCE AND TECHNOLOGY AGENCY,3 HARRINGTON ROAD,SOUTH KENSINGTON,LONDON, SW7 3ES01-584 808xii [July, 1973 SUMMARIES OF PAPERS IN THIS ISSUEAn Ultramicro- scale Method for the Determination of theUranyl CationA method that requires the use of very simple equipment has beendeveloped whereby the uranyl cation can be determined a t the micro-scaleand ultramicro-scale levels with high precision. The method involves theuse of the catalytic effect shown by the uranyl cation on the photo-decolorisa-tion of a naturally occurring carotenoid-type pigment.The interfering effectsof several common anions and cations are also discussed.GLENN PETER WOODDepartment of Chemistry, University of San Andres, La Paz, Bolivia.Analyst, 1973, 98, 525-528.An Improved Plasma Jet System for Spectrochemical AnalysisThe stability of a plasma jet has been improved by replacing the direct-injection nebuliser with a pre-mixed chamber - nebuliser arrangement of thetype used in flame spectrometry. This arrangement has alleviated manyof the operational problems that were experienced with the original system.As a result of the improved performance, the unit can be operated in con-junction with instantaneous photoelectric read-out. Details of the modifica-tion are given and the stability obtained is demonstrated.The sensitivityof the modified plasma jet has been investigated and detection limits for themost sensitive lines of sixty-seven elements are presented. These detectionlimits are compared with those obtained by using a nitrous oxide - acetyleneflame with the same optical arrangement and detection equipment.J. F. CHAPMAN, L. S. DALE and R. N. WHITTEMAustralian Atomic Energy Commission, Research Establishment, Lucas Heights,New South Wales, Australia.Analyst, 1973, 98, 529-534.A Method for Determining Free Azide Ions by AutomaticAnalysis in the Presence of a Covalent Cephalosporin AzideAn assay was devised in order to follow quantitatively the release ofazide as free ions, N3-, from a cephalosporin azide when attacked by p-lacta-mase enzymes produced by specific strains of bacteria. Experiments werearranged to ascertain whether or not the azide cleavage occurred a t thesame rate as the rupture of the p-lactam ring.The assay was required todetermine free azide a t concentrations between 2 and 20pg ml-1; otherexperimental limitations were imposed by the requirements of enzymolysis.The procedure adopted was based on the complete oxidation of azide ionsto nitrogen by an excess of a standard aqueous solution of nitrite ions a tpH 4.6. The residual nitrite was removed by diazotisation with 4-amino-salicylic acid, followed by coupling of the product with a second moleculeof the 4-aminosalicylic acid. The reaction mixture was then rendered alkalineby the addition of tetramethylammonium hydroxide solution.The finalyellow colour was stable and had an absorption maximum at 440nm. Itwas not possible, however, to achieve sufficient operational reproducibilitymanually, and an AutoAnalyzer system was therefore used.The results show that cleavage of azide from the parent molecule occursa t the same rate as the rupture of the p-lactam ring and parallels the declinein microbiological potency.R. E. WALLERGlaxo Laboratories Ltd., Greenford, Middlesex.Analyst, 1973, 98, 535-541July, 19731 THE ANALYST xiiiPARTICLE SIZEANALYSIS1970THE Society for Ana ytical Chemistryhas published in this book all paperspresented at the Second Particle SizeAnalysis Conference, held in Bradfordin September 1970, and the full discus-sions on them.The 35 papers cover all aspects ofresearch into the subject, basicallycovering the 4-year period since the firstconference was held in Loughboroughin 1966, and include plenary lectures bythe late Professor H.Heywood and byProfessor K. Leschonski. The volumeis a companion to “Particle SizeAnalysis” - the report of the First Con-ference, also published by the Society.Pp. x + 430Price E7.750 btai nable from-THE SOCIETY FOR(Book Department),9/10 Savile Row, London, WIX 1AFANALYTICAL CHEMISTRY,Members of The Chemical Society may buypersonal copies at the special price of €6.25TR .ACE ELEMENTANALYSISP RO BLEMS?Activation Analysis could be the answer. Thistechnique offers the advantages of high sensitivityand specificity, good accuracy and precision, even atsub-p.p.m. concentrations.A bulk analysis isobtained, often non-destructively. The maindisadvantage (the need for a nuclear reactor) can beovercome by making use of the Activation AnalysisService provided by the Universities Research Reactor.If you would like to discuss your particular problem,or would like to receive further details, please contact:Dr. G. R. Gilmore (Activation Analyst),Activation Analysis Service,Universities Research Reactor,Risley,Nr. Warrington,Lancs.Phone: Warr. 32680,33114BUREAU OF ANALYSEDSAMPLES LTD.announce the issue of the followingNEW SAMPLESChemical StandardsNo. 387 Nimonic 901 Alloy (12% Cr,6% Mo) Also available asspectroscopic standard.No. 389 High Purity Magnesite stan-dardised for B203No.390 High Tensile Brass (BS 2874CZ 114)Spectroscopic StandardsNos. 651-655 Malleable Irons contain-ing increments of C,Si, Mn, S and PNos. 666-670 Nodular Irons contain-ing increments of C ,Si, Mn, S, Ni and MgFor further details please write to:-NEWHAM HALL, NEWBY,MIDDLESBROUGH, TEESSIDE, ENGLAND.TS8 9EAor Telephone 0642 3721xiv SUMMARIES OF PAPERS I N THIS ISSUEThe Histochemical Detection of Soya “Novel Proteins” inComminuted Meat ProductsThe enforcement of the regulations governing meat and meat productsrequires the determination of meat content. Meat content is assessed fromthe total nitrogen content, from which suitable deductions are made for thenitrogen contributed by the other ingredients of significant nitrogen contentpresent in meat and meat products.The availability of “novel proteins”and the possibility of the addition of these proteins to meat products necessi-tates the detection and determination of “novel proteins” in such productsfor the true assessment of their meat content. A microscopical method thatindicates the presence of “novel protein” of soya origin in meat products hasbeen examined. This method involves the use of a specific technique todemonstrate the presence of carbohydrate material and is diagnostic forthe cellular fraction of many processed soya products.M. COOMARASWAMY and F. OLGA FLINTProcter Department of Food and Leather Science, The University, Leeds 2.Analyst, 1973, 98, 542-545.Interference of Carbon Dioxide, Resulting from the SchonigerFlask Combustion of Organofluorine Compounds, in theTitrimetric Determination of FluorineCarbon dioxide produced during the Schoniger flask combustion oforganofluorine compounds has been found to interfere in the titrimetricdetermination of fluorine. When thorium nitrate that had been standardisedagainst sodium fluoride solution was used, the fluorine content found wasconsistently 93 per cent. of theory. This interference was overcome by addingsodium carbonate to the solution used for standardisation before titration.For detection of the titration end-point, an improved indicator, methylthymolblue, was used.By using this procedure, compounds containing between 4 and 30 percent. of fluorine, either in a CF, or C F group, have been assayed satisfactorily.No interference from nitrogen, sulphur or chlorine contained in some of thecompounds was observed.WILLIAM F. HEYESInternational Development Laboratory, E. R. Squibb & Sons Ltd., Moreton, Cheshire.Analyst, 1973, 98, 546-549

ISSN:0003-2654    

DOI:10.1039/AN97398BP079

出版商:RSC    

年代:1973

数据来源: RSC

摘要:

JULY, 1973 THE AhTALYST Vol. 98, No. 1168 Mass and Charge Transfer Kinetics and Coulometric Current Efficiencies Part V.* Comparison of Pattern Theory, Tafel, Allen and Hickling and Lewartowicz Methods, and Apparatus and Procedures for Ramping Voltammetry BY E. BISHOP AND P. H. H1TCHCOCK-J. (Chemistry Department, University of Exeter, Stocker Road, Exeter, EX4 4QD) A comparison is made of the provenance and application of pattern theory, Tafel, Allen and Hickling and Lewartowicz methods for the deter- mination of charge-transfer kinetic parameters. A general-purpose electro- chemical cell is described together with a ramping potentiostat - galvanostat for slow single-sweep voltammetry. The mass-transfer characteristics, which are governed by the geometrical placing of the various appurtenances that dip into the solution and by the stirring speed, were examined by determining the apparent diffusion layer thickness at various stirring speeds for a system of known mass-transfer properties, copper(I1) - copper(1).It was further established that the limiting current of reduction of hexacyanoferrate(II1) is reproducible and linearly related to concentration. IN the derivation of pattern theory,1,2 the thermal diffusion coefficient, Dx, of the species X and the diffusion-layer thickness, ax, were retained as discrete entities. The diffusion-layer thickness in a turbulently stirred solution is a pure fiction, Mass transport is nevertheless a definite and, if conditions such as the stirring speed, solution volume and viscosity and the geometrical placing of the various appurtenances in the cell are held constant, reproducible phenomenon.Although no hydrodynamic treatment of turbulently stirred solutions is possible, L e ~ i c h ~ - ~ has shown that mass transport is proportional to the concentration, [XI,, of the active species in the bulk of the solution. It is therefore proper to use an over-all conditional mass-transfer rate constant, J2,,,,,7 which can be determined in situ from a voltammetric scan to the limiting current, ILx, and absorbs all indeterminate quantities, such as the roughness factor, r, of the electrode and the transport number, tx, of the species X, so that, for a projected area, A , of the electrode This relationship can be directly substituted in the behaviour equations and solutions for the charge-transfer rate constant, k , and the charge-transfer coefficients, a, for cathodic processes, and (1 - a), or /3, for anodic processes.A realistic treatment of mass transfer in electrode processes is thus secured in the practical context. There are many methods in use for the determination of charge-transfer rate parameters. A small selection of the simpler and more popular of these methods will be reviewed together with pattern theory in the context of stirred solutions and coulometric current efficiencies. PATTERN THEORY Pattern theory is entirely rigorous when the conditions are satisfied,1,2 viz., that the charge-transfer overpotential, ri)a, is substantial so that the backward reaction is without effect, and [ nqn I 2 0.059 log,, (lOO/.r), where .r is the percentage experimental error in the measurement of the current in a voltammetric scan.The method for the determination of k and a (or /3) is extremely simple and rapid.2 A single voltammetric scan is made in the appropriate direction, cathodic or anodic, and the substitution of two points on each wave, including the background for the calculation of current efficiencies, into the appropriate * For details of Parts I11 and IV of this series, see reference list on p. 474. For Part VI, see p. 475. t Present address: Ever Ready Co. (G.B.) Ltd., Central Research Laboratory, St. Ann’s Road, @ SAC and the authors. London, N.15. 465466 BISHOP AND HITCHCOCK: MASS AND CHARGE TRANSFER KINETICS [Analyst, VOl. 98 will give a quick check. In a careful study, as many points can be taken as desired and subjected to statistical appraisal.The calculation is almost as quickly performed manually as by computer when the time taken to punch the data cards is taken into account. The scan speed should be slow enough to avoid maxima and to allow a reasonable approach to quasi-equilibrium, but not so slow as to produce any significant change in [XI,. Scan speeds between 0-2 and 2.0mVs-l are suitable. When the process is applied during the course or at the start of a coulometric determination, the quantity of electricity passed and performing desired reactions is easily integrated from the corrected (for unwanted reactions) area under the voltammogram. The method has been thoroughly tested experimentally, as will be shown in later papers, and gives results in concordance with other methods when they have been duly corrected for their deficiencies.One application not mentioned earlier1s2 is the determination of n', the number of electrons involved up to and including the rate- determining step.7 This determination simply involves back-computation of the experimental voltammogram with different integral values of n, excluding those which give values of a greater than unity. Non-integral values of n can be diagnostically useful in working out reaction mechanisms that involve chemical steps. Pattern theory takes full cognisance of mass transfer, but applies only to slow reactions. It applies also to background reactions2 and therefore to current efficiency calculation^.^ It is not restricted to cases for which the limiting currents and conditional potentials are known.TAFEL PLOT METHOD In this well known method,* the logarithm of the total current is plotted against the working electrode potential. All too frequently it is applied ignorantly, or with the bland assumption that mass transfer is insignificant; the current is often taken beyond the limiting current of the reaction under examination. Extrapolations of the linear parts, if there are any, of the cathodic and anodic wings of the plot cross at the equilibrium zero-current potential and the logarithm of the exchange current, i0,7 from which k can be calculated. The slopes of the cathodic and anodic wings of the plot give a and p. The fallacy lies in the dismissal of mass-transport and mass-transfer overpotential.For extremely fast reactions in solutions that contain a high concentration of the active species, it is possible to have mass-transfer overpotential without detectable charge-transfer overpotential. However, unless a net current passes, i, + i a # 0, there can be no charge-transfer overpotential, and if a net current, however small, flows, there must always be mass transport to carry the current, and therefore mass-transfer overpotential. A charge-transfer overpotential can never arise without an accompanying mass-transfer overpotential, which cannot be ignored. If the potentials in the Tafel plot are corrected for mass transfer, which can be easily done but involves more calculation, then the plot and the parameters derived therefrom will be valid; the plot will have straight parts of the wings unless there are other complications.Without this correc- tion, the wings of the plot can never be linear, and the extrapolations will be of dubious validity. Fundamentally, the Taf el relationship depends on the same assumptions as pattern theory, that I nva I 2 log,, (lOO/n), so that one of the exponential terms in equations (20) and (22) in reference 7, and equation (1) in reference 1, can be neglected with respect to the other. The Tafel method is therefore restricted to the pattern region. Furthermore, the part outside the exponentials is falsely identified as the exchange current, io, which is where the subscript B refers to conditions in the bulk of the solution, and equation (3) is true only when Ta = 0.To avoid confusion, the relevant section in equation (2) is identified in pattern theory as 10s and the subscript S identifies concentrations at the plane of closest approach to the electrode surface : Only when this correction is made do Tafel plots become valid, and it should be noted that equation (4) contains the unknown parameters that the plot is intended to reveal. This i, = nFAk [Ox]g-") [Redlg . . .. .. ' * (3) IOS = nFAK [Oxlg-.) [Red]: .. . . * (4)July, 19731 AND COULOMETRIC CURRENT EFFICIENCIES. PART V 467 difficulty is resolved by using equation (5) in reference 7 in order to separate qa and to correct the potential for mass transport : where the concentration term, qc, refers to the plane of closest approach’: .. - * (5) y/a = Ewe - Eo’ - .... .. . . (6) I I n FAhnass ox [ox]B - I + [RedIB -- n FAk mass red 2.303 RT q c = nF log,, This procedure is possible only if values for the two limiting currents IL,, and I L r e d can be obtained and the conditional mass-transfer rate constants calculated. Then q a becomes a logarithmic function of the current or current density [equations (16) and (41) in reference 13. The corrected Tafel relationships become equation (7) for cathodic and equation (8) for anodic reactions : a n F .. . (7) In I = (In 10s) -- RT ya m e . . The Tafel method is therefore restricted to slow reactions, and is not helpful with background reactions. It requires knowledge of limiting currents and conditional potentials. ALLEN AND HICKLING METHOD Allen and Hicklingg expanded the second exponential in equation (Z), extracted the common term in cc and, like Tafel, identified the non-exponential terms as the exchange current, io, writing then rearranging and taking logarithms : They then plotted the left-hand function in equation (10) against qa, the slope of the line giving cc and the intercept giving io, from which k could be calculated.This method does not, therefore, neglect one of the exponential terms, and is therefore applicable to faster reactions, in the categories fast and m0derate.l However, it makes the assumption that the non-exponential terms are constant and equal to i, (which is true only when I = q a = 0), whereas the second term on the right-hand side of equation (10) is properly I O S [equation (4)], which is not constant.In evaluating the functions in equation (lo), it is again necessary to know qa, which can be calculated from equation (5) and the mass-transfer equation (6), but again only if the limiting currents can be measured for the calculation of the mass-transfer rate constants. The further assumption is made that a + p = 1, although a change in slope as the plot passes from positive (cathodic) to negative (anodic) currents will reveal any change in the charge-transfer coefficient. Even at extremely low currents, the “constant,” In io, in equation (10) is not constant unless the reaction is very slow, which vitiates the retention of both exponential terms, and [ox]B and red]^ are large and equal. Otherwise, the “constant,” In (nFA k OX]^ [RedIE), requires evaluation and therefore pre-knowledge of the unknowns, k , cc and p.LEWARTOWICZ METHOD Reviewing earlier work,l0 Lewartowicz took as his starting point the work of Audu- bert,llJ2 and developed m e t h ~ d s l * J ~ - ~ ~ for linearising the Tafel plots of the logarithm of the current versus potential with or without making a correction for diffusion, and with or468 BISHOP AND HITCHCOCK: MASS AND CHARGE TRANSFER KINETICS [Analyst, VOl. 98 without making the concentrations of Ox and Red equal. He referenced his potentials to the equilibrium, zero-current potential of the solution of Ox and Red, so that 7 = Ewe - Eeq, the total overpotential arising from the passage of current, and then corrected for diffusion by subtracting yes, for the evaluation of which anodic and cathodic limiting currents must be known: The current in equation (2) is then split into partial currents, cathodic I , and anodic I , (which is negative), and their values are calculated by using the value of 7 a calculated from equation (11) : ... . (12) I and I , = 1-exp [ _____ --;g%] e x p [ $ F ] - - l I I c - Anodic currents being negative, the logarithm of the modulus of the partial current is plotted against Ewe. The slope of the cathodic wing gives cc, that of the anodic wing ,B, and the intercept of the two curves gives the corrected exchange current, io, from which k can be calculated and the equilibrium potential of the particular solution, Eeq. Lewartowicz took the process a step further by calculating the “ideal” partial currents, which sum to the total “ideal” current Ii; using his notation of a single prime for cathodic and a double prime for anodic reactions, Ii = I’ + I” .. . . .. .. . . (13) Plots of the logarithm of the “ideal” partial currents against Ewe give more effective linearisa- tion than the use of the “real,’ currents, I , and I,. Both current and potential are therefore “corrected” for mass transport, and there is no neglect of either exponential term in equa- tion (2), so the treatment is applicable to moderate and fast reactions. Knowledge of the limiting currents of both anodic and cathodic directions and of the equilibrium, zero-current potential of the system is required, but if the electrode process is fast it is usually possible to measure these quantities experimentally.Lewartowicz’s approach deserves more attention : it has tended to be neglected in view of similar approaches by other workers a t about the same time. CURVE-FITTING METHODS Of the foregoing methods, only pattern theory can be used on background reactions,2 which come into the pattern region at moderate charge-transfer overpotentials because of the larger number of electrons. Before pattern theory had been developed for background reactions, and in default of better methods, curve-fitting methods involving the use of curves computed from the full rigorous theory’ were perforce used. A family of curves for a series of values of k and a first guess for a, or (1 - a), was plotted by computer using the program VOLTAMMETRY 9 G/P.’ These curves were then matched with the experimental curves, and the best value of k was chosen.The process was repeated for a series of values of cc, the computed curves were offered to the experimental voltammograms and the best value of cc was chosen. The process could be repeated as often as necessary in order to refine the charge- transfer parameter values further, but in practice no more than two such essays were required. After development of pattern theory for background reactions, curve fitting was no longer required, because background waves usually came well into the pattern region at solid electrodes. SUMMARY It cannot be said that the problem of measuring the rate parameters for all the reactions , including the solvent molecule and ion reactions, in vigorously stirred solutions under the conditions of coulometry has been solved for fast reactions.However, the method of patternJuly, 19731 AND COULOMETRIC CURRENT EFFICIENCIES. PART V 469 theory is the simplest of all, involves the least calculation, can be applied at any time before, during and after a coulometric determination thus to detect any change in rate parameters and therefore current efficiencies, and is completely rigorous at adequate charge-transfer overpotentials. It does deal with background reactions,2 and can be applied without know- ledge of limiting currents or equilibrium or conditional potentials. The implementation of the method involves merely a single-sweep voltammetric scan in the appropriate direction using the working electrode and the electrolyte and species under the conditions pertaining to an ensuing coulometric determination, which additionally allows selection of the working potential for potentiostatic determinations and the working current for amperostatic deter- minations productive of the maximum current efficiency.Equipment has therefore been designed and tested for the examination of coulometrically useful reactions. The design of a ramping potentiostat - galvanostat, together with certain tests, will be described in this paper, and the applications in later papers. EXPERIMENTAL The thermostatically controlled cell for voltammetry is shown in Fig. 1. Water main- tained thermostatically at 25 & 0.05 "C is pumped through the jacket. The lid is machined from Perspex and B19 cones are cemented with Perspex dissolved in chloroform into holes drilled in the lid. The stirrer paddle is a magnetic follower covered with PTFE and fitted A€ I R S WE I, J' r - Fig.1. Thermostatically controlled voltam- metric cell: AE, auxiliary electrode in filter stick ; RE, reference electrode (in thermostat tank); WE, working electrode; C, Luggin capillary; D, Perspex stirrer disc; L, machined Perspex lid ; M, PTFE-coated magnetic follower; N,, nitrogen inlet to disperser; S, salt bridge between reference electrode and Luggin capillary ; and T, jacket of cell thermostat470 BISHOP AND HITCHCOCK: MASS AND CHARGE TRANSFER KINETICS [Analyst, Vol. 98 into a Perspex disc machined so as to be a close fit in the bottom of the cell : this device gave much smoother stirring and therefore lower signal noise than the follower alone.Platinum working electrodes are made of bright platinum sheet, one side and all edges and corners of which are covered with lead-glass, so that one face alone is exposed to the solution. Electrical connection is made by platinum wire spot-welded to the back of the plate, welded in turn to tinned copper connecting wire, and sheathed with lead-glass. The dimensions were measured by means of a travelling microscope. Platinum-wire electrodes consist of 25-mm lengths of 22 s.w.g. wire sealed into soft glass. Gold working electrodes consist of lengths of 22 s.w.g. wire, 1000 fine, on to which a bead of cobalt glass (Plowden and Thomson) is sealed by winding a thread of glass round the heated wire. The hot bead is then sealed to a stem of soft glass and the whole annealed.The exposed wire is trimmed to a length of 25 mm. The auxiliary electrode is a spiral of platinum wire that dips into supporting electrolyte in a porosity 4 filter stick. The reference electrode is either a saturated calomel or a saturated mercury(1) sulphate electrode immersed in a beaker filled with salt-bridge electrolyte and placed in the thermostat, and connected to the Luggin capillary by means of a polythene tube. The Luggin capillary is placed close to the working electrode. In normal operation, the reference electrode is fully protected from polarisation. The cell contents are de-aerated with white- spot nitrogen, scrubbed first with chromium(I1) chloride solution and then with distilled water, passing through a porosity 2 disperser.The gas escapes via the rim of the lid. RAMPING POTENTIOSTAT- The essentials of a simple potentiostat are shown in Fig. 2, the heart of which is the control amplifier, CA, which operates in the manner of all operational amplifiers so as to maintain the potential of the inverting input or summing junction, S, at the same potential as its non-inverting input, which is shown as connected to earth. The summing junction is therefore a virtual earth. The negative feedback loop includes the auxiliary and reference electrodes and the cell electrolyte, and the amplifier passes a current through auxiliary and working electrodes so as to bring the potential of the working electrode with respect to the reference electrode to equality with a command potential, which is pre-set by a signal genera- tor, R,E,.The amplifier C h is therefore in the configuration of a linear combiner. In order -=_L - Y Y Fig. 2. A simplified practical potentiostat based on a linear combiner: CA, potentiostat control amplifier; S, summing junction ; VF,, voltage follower on the reference electrode; CF, current follower; X,X, output to X-axis of recorder; Y,Y, output to Y-axis of recorder; Rj (Ri’), current-measuring standard resistor ; Rf, feedback resistor of CA; R, and R,, input resistors for signal generators El and E,, respec- tively; RD, dummy load resistors ; AE, auxiliary electrode : RE, reference electrode; WE, working electrode; and CA, VF, and CF, Philbrick P65AU differential operational amplifiersJuly, 19731 AND COULOMETRIC CURRENT EFFICIENCIES.PART V 471 to prevent the reference electrode from being polarised, a second operational amplifier, VF,, is connected between the reference electrode and the feedback resistor, Rf, of CA. VFI pro- vides a low impedance output to supply current to Rf while offering a very high input impedance and therefore drawing a minimum current through the reference electrode. VF, is a voltage follower in the non-inverting mode a t unity gain. The potential of the working electrode can be measured against the reference electrode either directly with a high-impedance meter with a recorder output to the X amplifier of an X - Y recorder, or at the points XX, which can be taken direct to the recorder, or via a pH meter. The measurement of the cell current poses certain problcms, as do the earthing and placement of the signal generators.These topics have been reviewed by Schwarz and Shain.22 The current is measured in terms of a voltage drop across a standard resistor, Ri, which could be placed between the working electrode and earth. This arrangement, however, would form an extra component in the command voltage provided by the signal generator, and an unwanted voltage in the output of the control amplifier. Such a resistor could be placed in the line to the auxiliary electrode as represented by Ri’ in Fig. 2 . Neither end of the resistor is then a t earth potential and, although the input to the Y-axis amplifier of the recorder can be floated, this position of the resistor was found to be productive of excessive noise.Instead, therefore, an operational amplifier in the current follower mode, CF, is placed in the working electrode to earth path, thus holding the working electrode at virtual earth, and the calibrated resistor, Ki, forms the feedback loop of the current follower, the output of which with respect to earth is taken to the Y-axis of the recorder. Unless the cell with its electrodes and electrolyte solution are in circuit, the control amplifier will slam into saturation when trying to obey impossible commands, and so dummy load resistors, R,, are switched in in place of the cell when the latter is not in use. A second signal generator, R,E,, is included, which can be used to set the starting voltage of a scan, and forms the third input to the linear combiner, CA.In the complete seven-amplifier circuit shown in Fig. 3, a second voltage follower, VF,, is added between the output from the current follower and the X - Y recorder, because the latter is a low-impedance instrument (lo4 to lo8 Q). The amplifiers used are capable of producing currents of only 5 2 . 2 mA at kll V, and so booster amplifiers, B, are included in the feedback loops of the current-driving amplifiers, the control amplifier, CA, and the current follower, CF, so as to increase the available current to 100 mA. The pre-set starting potential signal generator is retained in the lorm of a Mallory cell and a 15-turn Helipot potentiometer. The potential command signal generator shown as R2E2 in Fig. 2 is replaced by a ramp generator, RG, which takes the simple form of an integrator, for which the output potential is given by t n .. (15) By integrating a constant, stable input voltage, the output voltage increases linearly with time. The ramp speed is controlled by R, and C ; in practice, it was found convenient to use a fixed capacitance of 10pF and a four-decade resistance box, such as was formerly used in d.c. differential electrolytic p~tentiometry.~~ The integrator can be arranged so as to hold a given command potential by open-circuiting the input, and then the instrument becomes an ordinary potentiostat. Ramping in either direction is secured by reversal of the polarity of the input voltage to the integrator; re-setting to zero is unnecessary and the ramp can be reversed at any potential. The integrator is re-set to 0 V by discharging the capacitor, C, through the shorting resistor and re-set switch.The loose circuitry, batteries, resistors, capacitors, switches, etc., are mounted on eircuit boards inside an aluminium box, and connections to the cell electrodes and the recorders are co-axial. The box is provided with a number of Cinch sockets, into which the canned operational amplifiers and boosters and the power supplies can be plugged as required. The whole assembly is very flexible and circuit alterations can be made very rapidly. OTHER EQUIPMENT- E.I.L. Vibron 39A pH meters were used for the measurement of potential at high impedance, or as impedance transducers. Honeywell-Brown 10-mV strip-chart recorders with chart speeds of 1 to 360 in h-l, and a Hewlett-Packard Moseley 7035A X - Y recorder472 BISHOP AND HITCHCOCK: MASS AND CHARGE TRANSFER KINETICS [Analyst, Vol.98 b b Y Y Fig. 3. The seven-amplifier ramping potentiostat : RG, Philbrick P65AU integrating operational amplifier ; B, Philbrick P66A current booster amplifiers; VF,, voltage follower on the working electrode; XX, output to X-axis amplifier of recorder: YY, output to Y-axis ampli- fier of recorder; C, integrating capacitor, 10 x 1 p F rf 10 per cent. 160-V d.c. polyester capa- citors, Mullard Series C296 AA/A ; El, starting potential signal generator, 5.4-V Mallory cell or 2.0-V accumulator; E, supply battery for ramp generator, as El; Rf, feedback resistor of CA = R, = 1 to 100 ksZ in five steps; R,, 100 to 1 M a resistors in seven steps; R,, integrating resistor, 100 ksZ to 100 MsZ in decades; R4, calibrated current-measuring resistor, 10, 100 or 1000 Q; other components as in Fig.2 of sensitivity 1 mV to 10 V in-l, input impedance lo4 to lo8 IR, depending on range, and a writing area of 10 by 7 inches were used. The operational amplifiers in the present phase of the work were as shown in Table I. TABLE I OPERATIONAL AMPLIFIERS Type D.c. open loop gain Philbrick P65AU 2 2 x 104 differential chopper-stabilised chopper-stabilised current booster Philbrick SP456 2 108 Solartron AA1023 2 108 Philbrick P66A - output Drift &ll V a t 50 pV d-l 12.2 mA 30 pV 'C-l f l O V a t 1 pV week-' &lo0 V at &12mA &12 V a t - A100 mA f 2 0 mA 0.1 p v "C-1 20 pV d-l Power supplies included either a Philbrick PU3, 3 1 2 to 15 V at 1 A per line, or lead - acid accumulators, k 1 4 V and 80 A h, and transformer-supplied 6.3 and 115 V, 50 Hz for the SP456 amplifiers.A Solartron AS 141 1 constant-current source2* was used for integrator calibration. The high-precision p~tentiometer~~ was a Cropico P3, and Cropico RS 1 resistance standards were used. The stroboscope was an EM1 type 6, and the thermoregulator a B.T.L. C' ircon. VOLTAMMETRIC PROCEDURE- The voltammetric cell was initially thoroughly leached so as to condition it, cleaned with chromic acid, very thoroughly washed and again conditioned with pure electrolyteJuly, 19731 AND COULOMETRIC CURRENT EFFICIENCIES. PART V 473 solution. The supporting electrolyte was placed in the cell and de-aerated for a t least 20 minutes.During this period, the working electrode was given the appropriate pre- treatment so as to “clean” or “activate” it. The Luggin capillary end of the salt bridge was filled with cell electrolyte by application of suction to the Y-piece shown in Fig. 1. The ramping potentiostat was run in to thermal equilibrium with the dummy load resistors switched in, and the integrator re-set switch closed. The balance of the differential amplifiers was checked and adjusted if necessary by means of the built-in pre-set. At the end of the de- aeration, the working and auxiliary electrodes were positioned with care so as to ensure constant geometry. The electrodes were then connected to the potentiostat, the starting potential was pre-set and the cell switched into circuit.After holding the starting potential for 15 s, the shorting switch on the integrator was opened and the Mallory cell switched into its input. The required portion of the current - voltage curve was recorded on the X - Y recorder. The scan was then repeated after addition of the de-aerated sample. If required, the working electrode was pre-treated before each scan. RESULTS OF PERFORMANCE TESTS- The characteristics of the cell and the method of stirring were investigated by scanning a M solution of copper(I1) chloride in 1.0 M potassium chloride adjusted to pH 2.0 with hydrochloric acid, because this sample gives well defined voltammetric waves at platinum electrodes, and the thermal diffusion coefficient of copper(I1) ion in this medium has been determined.25 At first, the PTFE-covered follower alone was used for stirring, but gave an uneven effect and fluctuations on the current axis of the voltammogram.When fitted into the Perspex disc, much smoother stirring was achieved. This disc was used thereafter unless the electrolyte was such as to attack Perspex. In 7 and 10 M sulphuric acid, the follower alone proved satisfactory because the high viscosity of such solutions smoothed out the stirring to acceptable noise levels. The limiting current at constant stirring speed varied with the positioning of the working electrode, and it was therefore necessary to ensure constancy of geometrical placing of the various appurtenances that dipped into the solution. The orienta- tion of the working electrode with respect to the flow of the solution was adjusted to that giving the maximum limiting current, which proved to be the condition with the exposed face of the electrode at right-angles to the direction of flow of the solution.The speed of rotation of the stirrer disc was measured with a stroboscope. The effect of stirring speed on the limiting current for the reduction of copper(I1) was investigated. Assuming that r and tx in equation (1) are constant, a value of 6x was calculated with the aid of the reported value of Dx.25 The relationship between 6% and stirring speed is shown in Fig. 4. Whatever interpretation may be placed on ax, its value approaches a minimum of about low3 cm at high stirring speeds, and is nearly constant at speeds in excess of about 6 Hz. This is fortunate in that it permits an ordinary stirrer motor to be 0.008 - 0.006 - E 1 cg % 0.004 - 0.002 - 0 1 2 3 4 5 6 7 8 9 Speed of rotation of stirrer/Hz Fig.4. Effect of stirring speed on the apparent thickness of the diffusion layer. Reduction of M copper(I1) in 1.0 M potassium chloride solution a t pH 2.0474 BISHOP AND HITCHCOCK run on an unstabilised mains supply at a sufficiently high speed without small fluctuations in speed having any material effect on the mass-transfer process. The dependence of the limiting current on the concentration of the electroactive species was examined for potassium hexacyanoferrate(II1) in 1.0 M hydrochloric acid. I t was found that the relationship was obeyed over the concentration range of indicates that a stable and reproducible mass-transfer condition is readily attainable. funds to provide a 3-year maintenance grant for this work. I L [Fe(CN)i-]1*00 i O * O o ~ to 6 x M hexacyanoferrate(III), which We are pleased to record our gratitude to Imperial Chemical Industries Limited for 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. REFERENCES Bishop, E., Analyst, 1972, 97, 761. -, Ibid., 1972, 97, 772. Levich, B., Russ. J . Phys. Chem., 1947, 21, 689. -, Ibid., 1948, 22, 575. -, Ibid., 1948, 22, 711. Bishop, E., Chemia Analit., 1972, 17, 511. Tafel, J., 2. phys. Chem., 1905, 50, 641. Allen, P. L., and Hickling, A,, Trans. Favaday SOL, 1957, 53, 1626 Lewartowicz, E., J. Chivn. Phys., 1952, 49, 557. Audubert, R., Ibid., 1924, 21, 351. -, Ibid., 1952, 49, 106. Lewartowicz, E., Ibid., 1952, 49, 551. -, Ibid., 1952, 49, 573. -, Ibid., 1954, 51, 267. -, C . R. Hebd. Acad. Sci., Pavis, 1954, 238, 1580. -, Ibid., 1948, 22, 721. -, Ibid., 1952, 49, 565. -, Ibid., 1954, 238, 1812. -, Ibid., 1959, 248, 2996. -, CITCE Colzf., Lindau, 1955, 144. -, CITCE Conf., Paris, 1957, 267. Schwarz, M. W., and Shain, I., A.Pzalyt. Chem., 1963, 35, 1770. Bishop, E., and Short, G. D., Analyst, 1962, 87, 467. Bishop, E., and Riley, M., Ibid., 1973, 98, 305. Peters, D. G., and Cruser, S. A., J . Electroanalyt. Chem., 1965, 9, 27. NOTE-References 1, 2 and 7 are to Parts 111, IV and I of this series, respectively. Received December 29tlt, 1972 Accepted Mavch 20th, 1973

ISSN:0003-2654    

DOI:10.1039/AN9739800465

出版商:RSC    

年代:1973

数据来源: RSC

摘要:

Analyst, July, 1973, Vol. 98, $$. 475-484 475 Mass and Charge Transfer Kinetics and Coulometric Current Efficiencies Part VI.* The Pre-treatment of Solid Electrodes, and a Review of the Effects of Oxidation of Platinum BY E. BISHIOP AND P. H. HITCH COCK^ (Ckemistvy Department, University of Exeter, Stocker Road, Exeter, EX4 4520) The nature and condition of working electrode surfaces are set in the context of reaction speeds and current efficiencies. The formation of films and their effects are examined for platinum and other metals and alloys. Adsorption and specific adsorption on electrode surfaces are briefly reviewed. An attempt is then made critically to appraise the current state of the art in respect of the activation and deactivation of electrodes. Methods of cleaning electrodes are canvassed.The more significant theories of activation and deactivation are reviewed with specific reference to platinum electrodes. These theories include the impurity theory, the platinisation theory and the various oxygen-containing surface theories. For the last, the formation of oxides, the measurement of film thickness, the film thickness and the nature of the oxide reducible at 0.6 V are discussed, followed by a selective review of the oxygen-bridge theory, the half-reduced oxide theory and the platinum - oxygen alloy theory. Tentative conclusions are reached. Finally, gold electrodes and their behaviour are briefly examined. WITH very few, if any, exceptions, the primary overriding consideration in the rate of the charge-transfer process at a working electrode, and therefore in the current efficiency of the desired process, is the condition of the electrode surface.The coulometric current efficiency is a function of the relative rates of wanted and unwanted electrode processes, and all rates are affected, often to greatly differing degrees, by the condition of the electrode surface. No matter how carefully the rate parameters and solution conditions are determined, or how accurately the total current, I , and working electrode potential, Ewe, are measured at one instant in time, the mere act of using the electrode can change the charge-transfer kinetic parameters by several orders of magnitude. Particularly is this true of solid electrodes, but the assumption that liquid electrodes are self-cleaning is over-optimistic.Nor are the many forms of carbon electrode free from these effects. For any particular reaction at any particular kind of electrode, there is a maximum inherent speed that is governed by the activation energies of the transition states. Commonly, this maximum speed is attained at a perfectly clean electrode, and it is not possible to generalise on how perfect cleanliness is achieved or what it means. From the point of view of current efficiency, it is well to assume this maximum speed for all unwanted reactions, including the background solvent reactions, and to monitor at frequent intervals the real speed of the desired reaction. Unless the reaction is very fast, the anodic and cathodic directions occur a t potentials that are sufficiently different to engender a change in the nature of the electrode surface, and the apparent over-all conditional rate constant, k , will seldom be the same for both anodic and cathodic reactions.Theory states that the charge-transfer coefficient for the cathodic direction is cc, and that for the anodic direction is necessarily (1 - cc), but, while this may hold for fast reactions, the difference in potential between the two directions of reaction again makes it unlikely that the cathodic and anodic charge- transfer coefficients will add up to unity, and so the designation of p for the anodic charge- transfer coefficient retains some practical usefulness. The most generally used solid electrode material is platinum. A highly subjective, and selective, review of the pre-treatment and oxidation of platinum in the restricted context of coulometry is appropriate at this stage as a preliminary to the report of practical results.It cannot be empliasised too often that it is essential in any report fully to specify the electrode pre-treatment and solution purification. The objective of electrode pre-treatment is to give * For Part V of this series, see p. 465. t Present address: Ever Ready Co. (G.B.) Ltd., Central Research Laboratory, St. Ann’s Road, @ SAC and the authors. London, N. 15.476 [Analyst, Vol. 98 a reproducible electrode surface, which can be regained at any time by exact repetition of the pre-treatment. The pre-treatment of electrolyte solutions has not only the objective of discovering and removing any electroactive impurities in them, but also of removing trace amounts of surfactant materials that may be detectable only by their malign influence on electrode behaviour.De-ionised water and surfactant detergents should be avoided at all costs. BISHOP AND HITCHCOCK: MASS AND CHARGE TRANSFER KINETICS FILMS ON PLATINUM The nature, condition and participation in reactions of the electrode surface is a com- plicated and contentious subject, extremely difficult experimentally to investigate and little productive of unequivocal interpretation, as the extensive literature attests. Noble metals are by no means inert,l and both chemical and anodic oxidation produce similar attacks2 Baker and McNevin3 found that oxidised surfaces slowed the oxidation of arsenic(III), and thereafter adopted a cathodic pre-treatment of platinum as a routine procedure.4 They ascribed the interference of trace amounts of antimony to the formation of a film of the insoluble antimony oxide Sb,O, on the electrode ~urface.~ Bard5 identified hydrated tin oxide films on cathodes.Lingane and co-workers6 detected oxidation of platinum during the oxidation of iodine to iodate, and later provided chemical evidence of anodic att.ack7 and film formation in chloride media,8 although the precise interpretation of the evidence was ~hallenged.~ DavislO observed oxidation of platinum in the chronopotentiometry of iron( 111) and cerium(IV), and found that repeated reduction of the electrode was necessary in the oxidation of hydroxylamine.ll Anson found that the reduction of iodate was much faster at an oxidised electrode,12 first postulating an oxygen-bridge mechanism and then platinisa- tion.13 Lingane found that an oxide film stopped the oxidation of 0xa1ate.l~ This effect was ascribed to prevention of the adsorption of oxalic acid, which is a necessary preliminary to its 0xidati0n.l~ Organic electrode processes are strongly affected by the condition of the electrode,16 and the rate constant of the hexacyanoferrate( 111) - hexacyanoferrate( 11) process at an oxidised electrode is only one tenth of that at a clean ele~trode.1~ Kozawals ascribed humps in voltammograms for alkaline solutions to the formation of oxides or hydroxides. Laitincii and Enkelg showed that the process of oxidation is slow, and postulated a hydroxyl radical mechanism.Feldberg, Enke and Bricker20 favoured chemisorption followed by a two-step process. Meyell and Langer21 showed that the oxide layer thickened with increasing anodisation potential until it became constant at potentials above 1.8 V. Lingane,, showed that the reduction of oxygen to water at platinum without the intermediate formation of hydrogen peroxide was fast at an electrode that had one tenth of a monolayer of oxide on its surface, but was slow at a reduced electrode. He suggested that a chemical reaction between oxygen and platinum occurred, followed by reduction of the oxide; this suggestion was attacked by Anson and King,13 but was supported by Sawyer and Interrante,23 who ascribed the decreasing speed of reaction with time to ageing of the oxide film, and found the speed at a pre-reduced electrode to be so slow that the electrode retained the characteristics of a reduced surface even in oxygen-saturated solutions.Other metals have been examined less extensively. Palladium and gold are oxidised in the same way chemically and anodically., Gold23-,6 is particularly attacked in media that contain complexing ions such as cyanide and chloride, but is more resistant than platinum to oxidation. Attack of single-crystal silver depends on the crystal face exposed to the solution.27 Alloys of gold and p l a t i n ~ m ~ 8 ~ ~ ~ show interesting properties, as do those of gold with palladium, silver and platinum.30 Iridium migrates preferentially to the surface of platinum - iridium all0ys.~1 ADSORPTION ON ELECTRODES Adsorption in general, or specific adsorption, of reactant, product or other species may be a necessity, a nuisance or a disaster, and again has an extensive literature; some has already been quoted.&l27l5 Anson has made many contributi~ns.~~-~l He ascribed activity to very light platinisation of the electrode on reduction of the oxide,32 found that perchloric acid could not replace sulphuric acid in chemical treatment with iron(II),12 and ascribed the accelerated reduction of iodate1, and vanadate at a pre-oxidised electrode to electrolytic p1atini~ation.l~ The same reason was adduced by Bard42 for the accelerated oxidation of hydrazine at a.c.biassed activated electrodes. Feldberg, Enke and Bricker20 contended thatJuly, 19731 AND COULOMETRIC CURRENT EFFICIENCIES.PART VI 477 the activity resided in a half-reduced oxide. Adsorption effects were detected in studies of the iron(II1) - iron(I1) system,33 but chronopotentiometric evidence of the adsorption of iron ions37 was traced to capillary cracks in the glass-to-metal seal of the electrodeJ36 but adsorption of and prevention of adsorption of oxalate14J5 by oxide films is confirmed. Anson and Schultzf5 postulate that adsorption of certain species, such as oxalate, iodide, hydrogen peroxide , arsenic (111) , thiocyanat e, nitrite , et hylenediaminetetraacet at ocobalt at e (I I) and methanol, is a necessary preliminary to their oxidation, and adsorption and oxidation are inhibited by oxide films. Adsorbed bromide catalyses the anodic oxidation of ethylene- diaminetetraacetatocobaltate(I1) and the product contains no br~mide.~s Adsorption of ethylenediaminetetraacetatocobaltate(II1) ion on a platinum cathode inhibits the reduction of the unadsorbed complex ion.39 Preferentially adsorbed ions, such as iodide, will displace adsorbed complex ions and so eliminate the interference.The cobalt (11) complex reduction product is also adsorbed. Mercury cathodes extensively adsorb the cobalt (11) complex,40 but not the tris(ethy1enediamine)cobalt (111) cationJ41 contrary to previous findings.43344 Inter- face and film inhibition at mercury electrodes have been reviewed by Oeder, Seiler and F i ~ c h e r . ~ ~ Unusual adsorption effects at mercury in the reduction of flavin nucleotide have been observed.46 Murray and co-worker~~~-~~ used several techniques in adsorption studies at mercury electrodes.Reduction of mercury(1) and bismuth( 111) is inhibited by adsorbed lead iodide, bromide and t h i o ~ y a n a t e ~ ~ , ~ ~ ; charge transfer is slowed by a chemical step at an adsorption-blocked surface in the reduction of copper(I1) in tartrate media by brucine. Supporting electrolyte effects in non-aqueous media have been examined ,51 and adsorption has been deliberately used in electrochemical masking with an adsorbed metal complex.52 Adsorbed lead selectively penetrable by different ions was used in the analysis of silver - mercury( 11) mixtures. Interpretative studies by Wopschall and Shai11~~-55 set out to test a theory53 by examination of the reduction of methylene blue,54 wherein the product is strongly adsorbed, and of the azobenzene - hydrazobenzene system,55 wherein there is a succeeding chemical reaction of the adsorbed reactant.Returning to platinum, thin-layer cells have been used in adsorption s t ~ d i e s . ~ ~ - ~ ~ Adsorption of EDTA complexes of cobalt and iron,57 and of iodine - iodide systems58 has been examined. Napp and Bruckenstein59 revealed the risks of unsuspected adsorption in a ring-disc electrode study of 0.5 M hydro- chloric acid at potentials from 0.25 to 1.25 V. They challenged earlier work,8 and ascribed the observed behaviour to a trace amount of copper(I1) in the medium, which is reduced to and adsorbed as copper(1) on the electrode surface. GileadiG0 has reviewed the adsorption of uncharged molecules on solid electrodes.ACTIVATION AND DEACTIVATION OF ELECTRODES A caveat against de-ionised water and detergents has already been entered. Flaming, or dipping in alcohol and flaming, will only poison the electrode. Treatment with chromic acid is strongly to be deprecated: not only does chromium(V1) oxidise the surface, but chrom- ium species are tenaciously adsorbed on platinum and block entirely such reactions as the reduction of vanadium(V).61,62 Reduction does not remove the chromium, and washing so prolonged as to remove it will result in a deactivated electrode. Chemical stripping in fresh, warm aqua regia, followed by washing with quartz-distilled waterJs3 will remove gross con- tamination ; anodisation in hydrochloric acid serves much the same purpose. This stripping stage can be followed by chemical reduction64 or cathodisation.Commonly, however, several cycles of anodisation and cathodisation in purified sulphuric acid, finishing with a protracted cathodisation, are used, and hydrogen adsorbed on or dissolved in the electrode is removed potentiostatically at 0.25 to 0.4 V. It is not possible to generalise further. In use, the electrode will be deactivated and need fresh pre-treatment. Setting aside specific adsorption, discussed above, of the very many theories of electrode behaviour, it is worth examining three, which have commanded considerable attention in the past, or appear to have present value. These theories are the impurity theory, the platinisation theory, and a group that may be termed oxygen-containing surface theories, and all are specifically directed to platinum.THE IMPURITY THEORY- The malign influence of surf ace-active impurities was demonstrated in 1937 by Frumkin’s Ion-exchange resins gave gr0up,~5--~~ and 30 years later68 was used in their determination.478 BISHOP AND HITCHCOCK: MASS AND CHARGE TRANSFER KINETICS [Analyst, l70l. 98 water of low conductance, but M for double-distillation in quartz. Charcoal columns removed most of the surfactant impurities but introduced ionic impurities, which may or may not be of importance. We have found charcoals to be in- effective for sulphuric acid purification, but both and c h a r c ~ a l ~ ~ ~ ~ ~ have been used successfully. We have found adsorption on a large electrode to be successful, but erratic, for sulphuric acid61; Bockris and c o - w ~ r k e r s ~ ~ ~ ~ ~ have used this method.BarkerTd has used a combination of column and electrolytic methods. Trial and error is the only general approach to purification of solvents and electrolytes, and little is known of the nature of surfactants; both organic and inorganic materials can be r e s p o n ~ i b l e . ~ ~ ~ ~ ~ One difficulty may be removed at the cost of introducing another, and particular care is needed in treating sample solutions.70 used solution cleaning methods in a study of the effect of impurities on the iron(II1) - iron(I.1) and hydrogen ion - hydrogen gas processes at platinum. He recorded the current at constant potential of a small, spinning platinum-wire electrode. In untreated iron(II.1) - iron(I1) in perchloric acid, an electrolytically activated electrode showed a current that decreased with time, until it became equal to the current at an untreated electrode and then remained constant.After charcoal “cleaning” of the solution, both treated and untreated electrodes behaved alike, this time in the manner of the initial behaviour of a treated electrode in an untreated solution. He concluded that the electrolytic treatment removed impurities from the surface of the electrode, which, on exposure to a cleaned solution, did not become dirty again. Similar observations were recorded for the hydrogen system, but the charcoal treatment, whether or not followed by electrosorption, did not entirely remove surfactants from the sulphuric acid. Both before and after “cleaning” of the electrolyte, the rate of decay of the current was accelerated by increasing the rotation speed of the electrode. This effect was to be expected when deactivation depended on migration of impurities to the electrode.James concluded that deactivation arose by re-adsorption of solution impurities, and not solution ions, and that it was unlikely that slow dissolution of hydrogen gas in the metal surface contributed to deactivation. Damj anovic, Genshaw and B ~ c k r i s , ~ ~ studying oxygen reduction in acidic media, supported the view that impurities exerted a profound effect. I t is notable that scrupulously clean solutions in equally clean vessels containing perfectly clean electrodes acquire impurities even when the system is completely ~ e a l e d .~ ~ s ~ ~ s ~ 8 Warner, Schuldiner and Pierama78 found that platinum electrodes kept on open circuit in the solution remained clean for several weeks, but when in continuous use, impurities built up, and they concluded that the impurities originated from the electrodes. This conclusion need not be true. An impurity theory is impossible to disprove because the level needed to interfere with electrode processes is so low that the impuritycannot be detected other than by its interference. Acceptance of a form of impurity theory does not reveal the full picture, and other effects require consideration. THE PLATINISATION THEORY- Smooth platinum gives no zero-current hydrogen-ion response, as it is unable to catalyse the rate-determining combination of hydrogen atoms.When coated with finely divided palladium-black or platinum-black, the effective surface area is multiplied several thousand- fold and contains the necessary energetic sites for the catalysis. The platinisation theory argues for an increase in surface area and catalytic power, and states that anodisation - cathodisation cycling produces light platinisation on reduction of oxide. Certainly initially bright surfaces are quickly dulled and etched by this treatment. Anson and King13 used 60-Hz a.c., probably of a very rough waveform, and produced a grey to black deposit, and Shibata79ss1 observed a similar phenomenon, claiming that the oxide was PtO,. Hoare,82 however, investigated a.c. polarisation of platinum, rhodium, palladium, iridium and gold, and found that only those metals which are capable of dissolving hydrogen, i.e., platinum and palladium, formed roughened surfaces, and ascribed the break-up of the surface to alternate dissolution and removal of hydrogen, One might ask whether roughness increases the charge- transfer rate.Anson and King13 did not purify their reagents, and the activation may simply be removal of impurities.’O Shibatasl repeatedly distilled his hydrochloric acid in a quartz still, but this is still not unequivocal in view of the work of Berezina and Nikolaeva-Fedorovich.68 Shibata cleaned platinum electrodes, aged them for 2 weeks in hydrogen, oxidised them chemically or anodically, and determined the amount of oxide chronopotentiometrically. Oxides formed in either way showed a single halt at 0.6 V and the amount increased with time M in surfactants againstJuly, 19731 AND COULOMETRIC CURRENT EFFICIENCIES.PART VI 479 of oxidation. Prolonged oxidation produced what seemed to be a second oxide, which was reducible at 0.3 to 0.2 V. All electrodes showed the same initial activity in hydrogen produc- tion, and the decay in activity was a function of the oxidation time. At the extreme, anodi- sation for 28 hours at 100 mA cm-2 gave an electrode that showed a negligible loss of activity after 30 hours of continuous use. The dependence of decay rate on oxidation time seems to exclude adsorption of impurities for deactivation, and Shibata opined that the platinised layer produced was unstable, but had a high initial activity that decreased as the platinised layer recrystallised to a stable form.Shibata’s work suggests that impurities alone cannot account for deactivation, but this neither proves the platinisation theory nor disproves the impurity theory. Reports of surface recry~tallis2tion~~-~~ have been challenged by Gilman,86$87 who found the deactivation to be dependent on stirring speed, which argues for impurity deactivation. Warner, Schuldiner and Pierama78 used clean electrodes and electrolyte and showed that well annealed platinum was more active than unannealed or drawn platinum in the reaction but annealing was without effect on simple electrochemical reactions. The work of Khazova, Vasil’ev and Bagotskiis8 would seem effectually to disprove the platinisation theory. They studied the catalytic and electrochemical activities of smooth and platinised platinum over the roughness factor range from 2 to 6500, and found the kinetic principles for chemisorption and electro-oxidation to be the same for all electrodes.The maximum rate of either type o€ reaction in relation to the true surface area decreased by an order of magnitude from smooth to highly platinised electrodes ; the sharpest change occurred in the roughness factor range from 10 to 1000, and further increase in roughness did not affect the rates. Observation of transients absolved mass transfer from the decrease in rate, so platinisation reduces the charge-transfer rate constant. Pt-0 + H2-Pt + H2O OXYGEN-CONTAIKING SURFACE THEORIES- This awkward term attempts to include several theories ranging from the formation of stoicheiometric oxide films to dissolution of oxygen in platinum.The subject is contentious in the extreme, presents enormous experimental difficulties, absorbs a huge research effort, not least in fuel and has produced a formidable literature, of which this review can do no more than skim the surface in the special context of coulometry, as previously adum- brated.64 Dispute continues over the nature of oxidised platinum, originating from the difficulty in distinguishing chernisorption from stoicheiometric phase oxides. Confusion is the more confounded by lack of reproducibility of the results (or inadequate reporting) of the cathodic-stripping determination of the amount of oxygen present. Contentions that films scarcely exceed a m o n ~ l a y e r ~ l - ~ ~ are difficult to prove.Electron and X-ray diffraction are useless, but ellipsometry assisted by coulometric stripping appear at present to offer a reason- able guide.gi19g5 At potentials cathodic to 0-9s V, there was no ellipsometric evidence of oxide; the sensitivity was about 0.01 nm. At 0.98 & @OP V, a film of 0.02 nm thickness suddenly appeared and grew linearly with increasing potential to 1.6 V, when the thickness was 0-55 nm; higher potentials were not examined. Coulometryg4 indicated that a 0-05-nm thick film was present below 0.98 V, and that the film thickened at about the rate indicated by ellipsometry up to 1.6 V, but the film was always 0.85 nm thicker than was found by ellipsometry. A complete monolayer would be about 0.3 nm thick.Ellipsometry depends on a difference in optical constants of the film and the surrounding medium, and the difference between chemisorbed osygen and water is too small to show up. Below 0.98 V, the coulometrically detectable oxygen was interpreted as being chemisorbed, and above 0-9s V a phase oxide was formed. The coexistence of chemisorbed oxygen and phase oxide had been predicted earlier.96 Film thickness-Chronopotentiometric reduction after conventional anodisation indicates a single oxid? reducible at 0.6 V,7714319978 and coulometry supports the view that no more than a monolayer is formed,7~8~14~20~78~s1~s3~s7~g8 amounting to 0.3 to 0.74 mC cm-2 depending on the roughness factor, but there is some evidence of thicker filims.1,81796 Endeavours by Cooksey and Bishop to establish precise growth-rate laws have been frustrated by poor reproducibility, but have emphasised that the competitive formation of molecular oxygen during oxidation has too often been ignored.Stronger anodisation yields thicker films,21~81~s4~99-101 and in some instances a second reduction h d t at 0.2 to 0.3 V has been noted.829101v102 Meyell and Langer21480 BISHOP AND HITCHCOCK: MASS AND CHARGE TRANSFER KINETICS [Analyst, VOl. 98 have reported many platinum to oxygen ratios, including a “tight” PtO. Their de-oxygenation procedure was inadequate, and Hoarelo3 rightly suggests that unremoved oxygen contributed to the stripping current. Visscher and DevanathanS4 reported a linear relationship between the stripping charge and the potential of oxide formation (Bishop and Cooksey find this to be a gross oversimplification); even with a roughness factor of 2.5,21 oxide formed at 1.7 V required 2 mC cm-, for complete reduction, implying a layer six to seven atoms thick.Kolthoff found that layers 0.3 to 1.4 mC cm-, thick were formed on chemical oxidation, depending on the nature of the oxidant and the time of attack. Prolonged anodisation gave second halts at 0.3 Vsl or 0-2 V,lol and the amount of low-voltage oxide increased with anodisa- tion time. Shibatagl reached 36 mC cm-2, which corresponds, at a roughness factor of 2.5, to forty oxygen atoms per surface platinum atom. The nature of the oxide reducible at 0.6 V-This has been extensively researched, as Young’s104 and Hoare’slo5 books and Hoare’s extended reviewlO3 show.The re-interpretation9 of Anson and Lingane’s chemical stripping results’ has been mentioned; the same objections to other work can be raised.loO Hoarelo3 canvasses a wide variety of likely and unlikely oxides. The most widely held view is that moderate anodisation produces PtO or Pt(OH), in hydrated form, and probably some chemisorbed oxygen as well, but drastic conditions are required in order to produce platinum(1V) oxide, which is the reverse of the situation with sulphide, where PtS, is formed but not PtS.lo6 Decreases, often drastic, in the rate of the charge-transfer process at oxide-filmed electrodes have been rep0rted.13J4J~,4~~~~~~O~ MiillerlO8 demonstrated that the rate of the charge-transfer process is proportional to the surface area of the platinum that remains unoxidised.Reports12y107-109 of reactions accelerated by oxide filming have mostly been d i s c o ~ n t e d ~ ~ , ~ ~ ; what has been observed can be interpreted as reaction at a bare platinum surface after the oxide has been chemically stripped. THE OXYGEN-BRIDGE THEORY- The analogy with the oxygen-bridge mechanism in homogeneous oxidation - reduction reactions is tempting but false. An attemptllO was made to explain an observed “reversibility” of freshly anodised platinum electrodes on a basis of an “oxygen bridge” structure of the oxide layer, claimed to accelerate the iron(II1) - iron(I1) and cerium(1V) - cerium(II1) reac- tions. Anson first lent support to this interpretation,12 but later changed to the platinisation theory.13 Davislo7 found that heavy oxide films suppressed the iron(II1) - iron(I1) reaction as earlier predicted,3 and the vanadium(V) - vanadium(1V) reaction, but claimed that a light film, less than a monolayer, facilitated oxygen-bridge formation and accelerated these reactions.He suggested that oxide formation occurred at grain boundaries, but this suggestion was conclusively challenged.lll James76 demonstrated that a simple oxygenated surface could not account for platinum activation. At a potential of 0-8 V, a t which the oxide should be stable, he found a considerable decay in activity with time ; contrarily, prolonged anodisation produced long-lived activity in hydrogen evolution at 0.0 V, at which the oxide should have been completely destroyed.THE “HALF-REDUCED OXIDE” THEORY- Studying the oxidation of platinum in perchloric acid, Feldberg, Enke and Bricker20 concluded that film formation was a two-step process, the first being slow formation of Pt(OH), and the second fast oxidation to Pt(O),. On cathodic stripping, reduction of Pt(O), to Pt(OH), was fast, but the slow reduction of Pt(OH), to bare platinum was complete only after prolonged potentiostatic reduction, while the process stopped a t Pt (OH), on ampero- static reduction, the current then being used in the generation of hydrogen. If the quantity of electricity required to form the oxide is Qa and for reduction is Qc, then on the first cycle Qa/Qc should be 2, decreasing to unity. This, by and large, is so. The arguments against this theory, that the half-reduced oxide is the active condition, are the same as for the oxygen-bridge the01-y.~~ The observations can be interpreted in several ways.Vetter and Berndt112 adduced different reactions, saying that the oxide was formed from water by a four-electron step, and reduced to hydrogen peroxide by a two-electron step. Breiter,l13 using low-frequency cyclic voltammetry, found that Qa/Qc started at greater than 2 but did not fall below 1.18, which agrees with our findings,64 and considered that the observations were contrary to previous t h e ~ r i e s . ~ O , ~ ~ ~ He suggested that Qa included oxidation of organic i m p ~ r i t i e s , l l ~ , ~ ~ ~ while Qc was little affected because reduction of the oxide occurred atJuly, 19731 AND COULOMETRIC CURRENT EFFICIENCIES. PART VI 481 potentials negative to 0.9 V, when such reactions became very slow.He also claimed that whereas oxygen atoms dissolve readily in platinum, their desorption is slow. Preferring the first explanation, he regarded Qc as a measure of the “electrochemical cleanliness” of the system. We s u g g e ~ t ~ l ? ~ ~ that the formation of molecular oxygen occurs during anodisation, and that this oxygen in part diffuses away from the electrode and is therefore not reduced during cathodisation. Gilmans6ys7 reached similar conclusions. THE PLATINUM - OXYGEN ALLOY THEORY- Breiter’s suggestion about oxidisable i m p u r i t i e ~ l l ~ J ~ ~ may well be right, but his suggestion of dissolution of oxygen in platinum is supported by other work116 in which a pulse and decay technique was used.Perhaps the most significant work at the time of writing is that of Hoare,92J03~105~117-127 who ascribes the activity of pre-anodised electrodes to oxygen dis- solved in the surface layers of the bulk Oxygen has been reported to dissolve in massive platinum to a depth of three to four layers before saturation is rea~hed.10~~11~1128-130 Schuldiner and Warner116 found a direct connection between oxygen dissolved in platinum and the catalytic activity of the metal. Hoare based his hypothesis126 on earlier work,92s119y122 and made solutions of reagent grade chemicals in triply distilled water and pre-electrolysed the solutions for a t least 24 hours before use,117 and was thus reasonably assured of impurity- free media.Further, he found that oxidation in concentrated nitric acid produced an electrode that displayed a steady potential of 1.225 V for over 24 hours when immersed in 1.0 M sulphuric acid saturated with pure oxygen, in accord with prediction.121 Such an electrode also cata- lysed the reduction of oxygen and hydrogen peroxide in acidic media.122J26 A minimum of 50 hours’ soaking was needed in order to produce an electrode of stable activity, which is much longer than is necessary in order to remove impurities or to coat the metal with oxide. This result is compatible with the postulate that oxygen dissolved in the metal to give a “platinum - oxygen alloy” electrode.92 A cathodic chronopotentiogram of one of Hoare’s activated electrodes has been quoted76 as showing a single arrest a t 0.7 V similar to that of conventionally anodised electrodes.Hoare reports that when his activated electrodes were immersed in hydrogen-saturated acidic solution, they took 40 minutes to reach the same potential as a reduced electrode, compared with 4 minutes for conventionally anodised elec- trodes.1191127 Breiter131 reported that nitrogenous species were tenaciously adsorbed on platinum soaked in nitric acid, but Hoare could not detect such species.127 Hoare’s prolonged treatment with nitric acid seems to produce electrodes with unique properties, which he attributes to the platinum - oxygen alloy, but it can equally be postulated that prolonged anodisation produces not only a surface-phase oxide but also some of this alloy or solid solution, which suffices to activate the electrodes towards oxidation - reduction systems.The oxide layer would be stripped chemically or cathodically, but the oxygen in solid solution would be removed only very slowly.126 Hoare does not offer a mechanism; this is probably not yet possible. Much of the earlier work can be re-interpreted in the light of Hoare’s suggestion. Particularly, Shibata’s resultss1 concerning prolonged anodisation could be explained by the extensive formation of the platinum - oxygen alloy, with consequent long-lived activity, rather than by platinisation. CONCLUSIONS There can be no finality yet: hot debate continues. No sooner has one read one paper that appears to be convincing and conclusive, than another of contrary view that is equally convincing and conclusive appears.Tentatively, however, desorption and adsorption of impurities can be said to contribute substantially to activation and deactivation processes, respectively, and a t the moment Hoare’s platinum - oxygen alloy theory seems to be the most attractive. Chemical oxidation1,2 seems to produce the sameoxide, which can be stripped a t 0-6 V, as anodisation, but electrochemical activation is to be preferred. There are many report~,103~10*y1~~~13~ some of considerable a n t i q ~ i t y , l ~ ~ - l ~ ~ of yellow or red films formed on platinum by means of a periodic current of 50 to 60 Hz, with a d.c. bias. Such films could well consist of oxide. In our laboratory, extremely hard, scratch-proof gold films have been produced by precisely symmetrical waveforms, but the rough waveforms from such an iron-cored inductance as a transformer gave black films that could be wiped off with a moist paper tissue.482 BISHOP AND HITCHCOCK: MASS AND CHARGE TRANSFER KINETICS [A%dySi!, Vol.98 GOLD ELECTRODES The behaviour of gold is simpler because neither hydrogen nor oxygen is soluble, and has raised less controversy. An oxidised surface renders gold a poor electrode for oxidation - reduction reaction~,~5,~~ but as soon as the oxide has been reduced, reactions proceed at normal speed. Hoare has examined gold as a possible oxygen electr0de,1~~J~~-1~5 but it seems118J40 that gold gives a mixed potential in oxygen-saturated solutions. Potentiostatic in acidic oxygen solutions indicate a partial layer of oxygen to be adsorbed between 0-9 and 1-3 V, which is a good electrical conductor. Above 1.36 V, Au,03, probably hydrated, is formed and is not a good conductor,l12 so that passivation occurs between 1.36 and 1.60 V.I R e p o r t ~ ~ , ~ ~ ~ of the formation of Au20 and AuO are not held in much regard. A trans-passive region between 1-60 and 2.0 V has been attributed to the migration of gold atoms through the loosely packed Au20, film.f42 Such migration produces an oxide film that is much thicker than a Reduction of an electrode anodised at high potentials gives considerable roughening, attesting to a thick porous oxide film.140 Baumann and recommend oxidation - reduction pre-treatment, and ascribe the improved activity to the removal of surface impurities. This improvement is probably the main consequence of electrolytic pre-treatment, although roughening may give minor benefit.No attempts have been made to explain the activity of gold electrodes on a basis of an oxygen-containing surface. Charging curves for go1d20,23J45 indicate that Qa = Qc, which accords with the insolubility of oxygen in gold. The proposal of oxygen diffusion along grain boundaries23 has been challengedlll on the grounds of lack of evidence. Gold is more resistant25 than platinum to chemical oxidation in non-complexing media, but gold dissolves readily in media that contain cyanide or halide ions. This attack starts at 0.6 V in chloride media, and so severely restricts the use of gold as an anode. We express our sincere gratitude to Imperial Chemical Industries Limited for a research grant covering 3 years.1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 16. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. REFERENCES Kolthoff, I. 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Japan, 1962, 35, 1051. Laitinen, H. A., and Enke, C. G., J . Electrochem. Soc., 1960, 107, 773. Feldberg, S. W., Enke, C. G., and Bricker, C. E., Ibid., 1963, 110, 826. Meyell, J. S., and Langer, S. H., Ibid., 1964, 111, 438. Lingane, J. J., J . Electroaizalyt. Chem., 1961, 2, 296. Sawyer, D. T., and Interrante, L. V., Ibid., 1961, 2, 310. Laitinen, H. A., and Chao, M. S., J .Electrochem. SOC., 1961, 108, 726. Baumann, F., and Shain, I., Analyt. Chem., 1957, 29, 303. Genshaw, M. A,, Damjanovic, A., and Bockris, J. O’M., J . Electroanalyt. Chem., 1967, 15, 163. Nikulin, V. N., Russ. J . Phys. Chem., 1961, 35, 40. Breiter, M. W., J . Electroanalyt. Chem., 1966, 11, 157. Damjanovic, A., and BrusjC, V., Electrochim. Acta, 1967, 12, 1171. Giner, G., Parry, J. M., and Swette, L., Abstv. Pap. 154th Meet, Amer. Chew. Soc., September 1967, L15. Chodos, A. A., and Meites, I>., Analyt. Chem., 1969, 41, 846. Anson, F. C., Ibid., 1961, 33, 934. -, Ibid., 1961, 33, 939. -, Ibid., 1961, 33, 1123. -, Ibid., 1961, 33, 1438. I -- -, Ibid., 1963, 35, 764. -, Ibid., 1961, 33, 1498.July, 19731 AND COULOMETRIC CURRENT EFFICIENCIES.PART VI 483 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 81. 82. 83. 84. 85. 86. 87. 88. 89. 90. 91. 92. 93. 94. 95. 96. 97. 98. 99. 100. 101. 102. 103. 104. 105. 106. -, J . Amer. Chem. Soc., 1961, 83, 2387. -, J. Electrochem. Soc., 1963, 110, 436. -, Analyt. Chem., 1964, 36, 520. -, Ibid., 1964, 36, 932. Bard, A. J., Ibid., 1963, 35, 1602. Laitinen, H. A., and Randles, J. E. B., Trans. Faraday Soc., 1955, 51, 54. Laitinen, H. A., and Chambers, L. &!I., Analyt. Chem., 1964, 36, 5. Oeder, D., Seiler, W., and Fischer, H., Z . analyt. Chem., 1966, 216, 256. Hartley, A. M., and Wilson, G. S., Analyt. Chem., 1966, 38, 681. Gross, D.J., and Murray, R. W., Ibid., 1966, 38, 405. Kodama, M., and Murray, R. W., Ibid., 1065, 37, 1638. Murray, R. W., and Gross, D. J . , Ibid., 1966, 38, 392. Murray, R. W., and Kodama, M., Ibid., 1965, 37, 1759. Murray, R. W., and Hiller, L. K., Ibid., 1967, 39, 1221. Murray, R. W., and McNeeley, R. L., Ibid., 1967, 39, 1661. Wopschall, R. H., and Shain, I., Ibid., 1967, 39, 1514. , Ibid., 1967, 39, 1527. , Ibid., 1967, 39, 1535. Hubbard, A. T., and Anson, F. C., J . Electroanalyt. Chem., 1965, 9, 163. -__ , Analyt. Chem., 1966, 38, 1601. Hubiard, A. T., Osteryoung, R. A., and Anson, F. C., Ibid., 1966, 38, 692. Napp, D. T., and Bruckenstein, S., Ibid, 1968, 40, 1030. Gileadi, E., J. Electroanalyt. Chem., 1966, 11, 137. Hitchcock, P. H., Ph.D. Thesis, University of Exeter, 1969.Bishop, E., and Hitchcock, P. H., Analyst, 1973, 98, in the press (Part VII of this series). Bishop, E., and Sutton, J . R. B., Analytica Chim. Acta, 1960, 22, 590. Bishop, E , and Riley, M., Analyst, 1973, 98, 416. Levina, S., and Sarinsky, V., Acta Phys-Lhim. URSS, 1937, 6, 475. -- , Ibid., 1937, 6, 491. Erschler, B., and Proskurnin, M., Ibid., 1937, 6, 195. Berezina, N. P., and Nikolaeva-Fedorovich, N. V., Soviet Electrochem., 1967, 3, 1. Hillson, P. J., Trans. Faraday Soc., 1954, 50, 385. James, S. D., Electrochim. Acta, 1967, 12, 939. Joshi, K. M., Mehl, W., and Parsons, R., in Yeager, E., Editor, “Transactions of the Symposium on Electrode Processes, 1959,” John Wiley, New York, 1960, p. 249. Bockris, J . O’M., and Conway, B. E., Trans.Faraday Soc., 1949, 45, 989. Damjanovic, A., Genshaw, M. A., and Bockris, J . O’M., J . Electrochem. SOC., 1967, 114, 466. Barker, G. C., Rep. U.K. Atom. Energy Auth., AERE-C/R 1563, 1957. -, J. Electrochem. SOG., 1966, 113, 1024. James, S. D., Ibid., 1967, 114, 1113. Bockris, J . O’M., and Huq, A. K. M. S., Proc. R. SOC., A , 1956, 237, 277. Warner, T. B., Schuldiner, S., and Pierama, B. J., J . Electrochem. SOC., 1967, 114, 1120. Shibata, S., Nippon Kagaku Zasshi, 1958, 79, 239. -, Bull. Chem. SOC. Japan, 1963, 36, 525. Hoare, J. P., Nature, Lond., 1964, 204, 71. Breiter, M. W., and Kennel, B., 2. Elektrochem., 1960, 64, 1180. Will, F. G., and Knorr, C. A., Ibid., 1959, 63, 1008. , Ibid., 1960, 64, 258. Gilman, S., J. Phys. Chem., 1963, 67, 78. -, J. Electroanalyt.Chem., 1965, 9, 276. Khazova, 0. A., Vasil’ev, Yu. B., and Bagotskii, V. S., Soviet Electrochem., 1967, 3, 915. Bagotskii, V. S., Nekrasov, L. N., and Shumolova, N. A. Buss. Chem. Revs, 1965, 34, 717. Kronenberg, M. L., J . Electroanalyt. Chem., 1066, 12, 122. Boeld, W., and Breiter, M., Electrochim. Acta, 1961, 5, 145. Hoare, J. P., Nature, Lond., 1964, 204, 71. Vetter, K. J., “Electrochemical Kinetics,” Academic Press, New York, 1967, p. 619. Visscher, W., and Dcvanathan, M. A. V., J . Electroanalyt. Chem. 1964, 8, 127. Reddy, A. K. N., Genshaw, M., and Bockris J. O’M., Ibid., 1964, 8, 406. Dietz, H., and Gohr, H., Electrochim. Acta, 1963, 8, 343. Schuldiner, S., and Roe, R. M., J . Electrochem. Soc., 1963, 110, 332. Schuldiner, S., Warner, T. B., Anson, F.C., and Osteryoung, R. A. Analyt. Chew., 1964, 36, 2510. Every, R. L., J . Electrochem. SOC., 1965, 112, 524. Every, R. L., and Grimsley, R. L., J . Electroanalyt. Chem., 1965, 9, 165. Kozawa, A., Ibid., 1964, 8, 20. Obrucheva, A. D., Z h . Fiz. Khim., 1952, 26, 1448. Hoare, J. P., “The Oxygen Electrode on Noble Metals,” in Delahay, P., Editor, “Advances in Electrochemistry and Electrochemical Engineering,” Volume IV, John Wiley, New York, 1967. Young, L., “Anodic Oxide Films,” Academic Press, New York, 1961. Hoare, J. P., “The Electrochemistry of Oxygen,” Interscience Publishers, New York, 1968. Najdeker, E., and Bishop, E., J . Electroanalyt. Chew., 1973, 41, 79. -, Ibid., 1966, 38, 54. , __- > __- -, Ibid., 1959, 80, 453. * --484 107. 108. 109. 110. 111. 112. 113. 114. 115. 116. 117. 118. 119. 120. 121. 122. 123. 124. 125. 126. 127. 128. 129. 130. 131. 132. 133. 134. 135. 136. 137. 138. 139. 140. 141. 142. 143. 144. 145. BISHOP AND HITCHCOCK Davis, D. G., Talanta, 1960, 3, 335. Muller, L., EZectrochim. Acta, 1967, 12, 557. Karp, S., andMeites, L., J , Amer. Chem. Soc., 1962, 84, 906. Kolthoff, I. M., and Nightingale, E. R., Analytica Chim. Acta, 1957, 17, 329. Mohilner, D. M., Argersinger, W. J., and Adams, R. N., Ibid., 1962, 27, 194. Vetter, K. J., and Berndt, D., 2. Elektrochem., 1958, 62, 378. Breiter, M. W., Electrochim. Acta, 1966, 11, 905. -, Ibid., 1962, 7 , 533. -, J. Electrochem. Soc., 1962, 109, 42. Schuldiner, S., and Warner, T. B., Ibid., 1965, 112, 212. Hoare, J. P., Ibid., 1962, 109, 858. -, Ibid., 1963, 110, 245. -, Ibid., 1963, 110, 1019. -, Electrochim. Acta, 1964, 9, 599. -, J. Electrochem. Soc., 1965, 112, 602. -, Electvochim. Acta, 1966, 11, 203. -, Ibid., 1966, 11, 311. -, Ibid., 1966, 11, 549. -, J. Electroanalyt. Chem., 1966, 12, 260. Hoare, J. P., Miebuhr, S. G., and Thacker, R., J . Electrochem. Soc., 1966, 113, 1078. Hickling, A., Trans. Faraday Soc., 1945, 41, 333. Kalish, T. V., and Burshtein, R. Kh., Dokl. Akad. hTauk SSSR, 1951, 81, 1093. -- , Ibid., 1953, 88, 863. Breiter, M. W., J . EZectroanalyt. Chem., 1964, 8, 230. Nagel, K., and Dietz, H., Electrochim. Acta, 1961, 4, 1. Margules, M., Wiedemanns AnnZn, 1899, 65, 629. Marie, C., C. R. Hebd. Se'anc. Acad. Sci., Paris, 1907, 145, 117. -, J . Chim. Phys., 1908, 6, 596. Ruer, R., 2. Elektrochem., 1908, 14, 309. -, Ibid., 1908, 14, 633. Watanabe, N., and Devanathan, M. A. V., J . Electrochem. SOL, 1964, 111, 615. El Wakkad, S. E. S., and El Din, A. M. S., J . Chem. Soc., 1954, 3098. Barnartt, S., J. Electrochem. Soc., 1959, 106, 722. Armstrong, G., and Butler, J. A. V., Trans. Faraday SOG., 1934, 30, 1173. Armstrong, G., Himsworth, F. R,, and Butler, J. A. V., PYOG. R. Soc., A , 1933, 143, 89. Hickling, A., Trans. Faraday Soc., 1946, 42, 518. -, Ibzd., 1965, 112, 849. -, Ibid., 1899, 66, 540. -, Ibid., 1908, 146, 475. Received January 17th, 1973 Accepted March ZOth, 1973

ISSN:0003-2654    

DOI:10.1039/AN9739800475

出版商:RSC    

年代:1973

数据来源: RSC

摘要:

Analyst, July, 1973, Vol. 98, PP. 485492 485 Ionic Polymerisation as a Means of End-point Indication in Non-aqueous Thermometric Titrimetry Part IV.* The Determination of Catecholamines BY E. J. GREENHOW AND L. E. SPENCER (Department of Chemistry, Chelsea College, University of London, Manresa Road, London, S . W.3) ( -)-Adrenaline, adrenaline hydrogen tartrate, L-noradrenaline, dopamine hydrochloride, L-dopa, DL-dopa, L-a-methyldopa, D-a-methyldopa and ( + ) - Corbasil have been determined in amounts down to 0.0001 mequiv by catalytic thermometric titration of their basic and acidic functions. Basic functions were determined by titration with 0.1, 0.01 and 0.001 M perchloric acid by using the ionic polymerisation of a-methylstyrene to indicate the end-point, while acidic functions were determined in a similar manner with tetra-n-butyl- ammonium hydroxide as the titrant and acrylonitrile as the end-point indicator.The L-dopa contents of tablets and capsules have been determined by using these techniques and the assay results have been compared with those obtained by alternative methods, namely, the recently described B. P. pro- cedure involving non-aqueous titration, and ultraviolet spectrophotometry. Magnesium stearate, which is used as a lubricant and flow promoter in tablet manufacture, is titrated as a base in the solvents used, but in titra- tions of the acidic function of catecholamines its effect is negligible. THE determination of catecholamines in biological specimens normally requires the use of trace analysis techniques such as fluorimetry and chromatography, as the concentrations usually encountered rarely exceed the microgram per gram level and are often in the sub- nanogram per gram range-l In contrast, catecholamines that form the active constituents in pharmaceutical preparations are present therein in relatively large amounts, and can be determined conveniently at the milligram per gram level or at even higher concentrations.For the routine assay of L-dopa [~-3-(3,4-dihydroxyphenyl) alanine], noradrenaline, adrenaline hydrogen tartrate and noradrenaline hydrogen tartrate, non-aqueous titration with perchloric acid is recommended.2J For the determination of catecholamines in formula- tions the more selective spectrophotometric methods are usually ~ s e d , ~ ~ ~ presumably in order to avoid interference from excipient material.However, in recent monographs,6 non-aqueous titration is the method prescribed for the assay of the L-dopa content of tablets and capsules. Thermometric procedures for the determination of organic acids and bases in amounts down to about 10 pg are described in Parts I7 and IL8 In these determinations a monomer capable of ionic polymerisation is added to a non-aqueous solution of the acid or base prior to titration. The end-point is indicated by a rise in temperature, which results from the ionic polymerisation of the monomer and which is initiated by the titrant following neutralisa- tion of the sample. The present paper describes the application of these thermometric procedures to the determination of some catecholamines of medical importance, namely (-)-adrenaline, adrenaline hydrogen tartrate, L-noradrenaline, dopamine hydrochloride, L- and DL-dopa, L- and D-a-methyldopa, and (+)-Corbasil.It is particularly concerned with the development of suitable solvent systems for these rather intractable compounds. The formulae of the compounds are shown in Fig. 1. Determinations have been carried out on the pure compounds and also on L-dopa tablets and capsules, and both the basic and acidic functions have been determined. As in the previous investigations, perchloric acid was used as the titrant and a-methylstyrene as the end-point indicator for the determination of basic functions, while tetra-n-butylammonium hydroxide was used as the titrant and acrylonitrile as the end-point indicator for the determination of acidic functions.The results obtained with L-dopa and * For details of Parts I, I1 and I11 of this series, see reference list on p. 492. 0 SAC and the authors.486 [Analyst, Vol. 98 with formulations that contain it are compared with those obtained by using the B.P. non- aqueous titration method6 and ultraviolet spectrophotometry. GREENHOW AND SPENCER: IONIC POLYMERISATION FOR END-POINT H R3 H Fig. 1. Structural formulae of the catecholamines (-)-Adrenaline OH H 2 CH, (+) -Corbasil OH CH, H H DL-Dopa H COOH H H Dopamine H H H H a-Methyldopa H COOH CH, H L-Noradrenaline OH H H H R, R2 R4 EXPERIMENTAL REAGENTS- Glacial acetic acid and propan-2-01 were of analytical-reagent grade and acrylonitrile, 1,2-dichloroethane, dimethylformamide and a-methylstyrene were laboratory-reagent grade materials.Oxalic acid (analytical-reagent grade) was dried at 110 "C for 3 hours before use. Other solvents and reagents were laboratory-reagent grade materials and were used as received. Perchloric acid, 0.1 M solution in acetic acid-Prepare this solution and standardise it by the method described in Part I.' Prepare 0-01 and 0.001 M solutions by diluting the 0.1 M titrant with 1,2-dichloroethane. Tetra-n-butylammonium hydroxide, 0.1 M in toluene - methanol-Laboratory-reagent grade material was used as received. Prepare 0.01, 0.002 and 0.001 M solutions by adding appro- priate volumes of toluene - propan-2-01 mixture (3 + 1) to the 0.1 M reagent. Standardise the solutions against benzoic acid (analytical-reagent grade) in dimethylformamide by the thermometric method.CATECHOLAMINES- (-)-Adrenaline, L-noradrenaline, L-dopa, DL-dopa, dopamine hydrochloride and adren- aline hydrogen tartrate were laboratory-reagent grade materials. All other catecholamines were gifts: L-dopa tablets (normal and sustained release) and capsules from Brocades (Great Britain) Ltd., L-a-methyldopa and D-cc-methyldopa from Merck, Sharp and Dohme, and (+)-Corbasil from Dr. S. Jones (Department of Pharmacy, Chelsea College). APPARATUS- described in Part 11.8 flask is adequate €or the present work. A. TITRATION OF THE BASIC FUNCTION- Manual method-Weigh about 0.1 mequiv of the sample into the titration vessel, add 0.5 ml of formic acid and mix well, then add 2-0 ml of acetic acid and 3 ml of a-methylstyrene.Add the titrant (0.1 M perchloric acid) at the rate of about 0.4 ml min-1 to within 0.3 ml of the end-point, noting the temperature at 15-s intervals, and complete the titration with addition of titrant at a rate not exceeding 0.2 ml min-1. Automatic method-Add the titrant at a constant rate (not exceeding 0.2 ml min-l) to a mixture of the sample, solvent and monomer in the 8-ml titration flask that is appropriate to the titrant concentration (see Table I). Record the temperature and titrant volume on a millivolt chart recorder (20-mV scale) at a chart speed of 600 mm h-l. All were dried over molecular sieve 4A before use. Manual method-Use the apparatus described in Part 1119 and the 15-ml titration flask Automatic method-Use the motor-driven syringe described in Part 111.An 8-ml titration PROCEDUREJuly, 19731 INDICATION IN NON-AQUEOUS THERMOMETRIC TITRIMETRY. PART IV Suitable amounts of sample, solvent and monomer are given in Table I. 487 TABLE I AMOUNTS OF SAMPLE, SOLVENT AND MONOMER RECOMMENDED FOR USE WITH THE DIFFERENT CONCENTRATIONS OF TITRANT Ti trant-Perchlor ic acid I A \ 0.1 M 0.01 M 0.001 M Samplelmequiv .. . . 0.1 t o 0-02 0.01 to 0.002 0.001 to 0~0001 Formic acid/ml . . f . 0.5 0.2 0.05f Acetic acid/ml . . .. 2.0 0.2 0.2 1,2-Dichloroethane/ml . . - Propylene carbonatelm1 . . - 1-1 - a-Methylstyrenelml .. 0.5 0.9 2.0 Solveats- 0-6 0.75 * With dopamine hydrochloride use the solvent mixture prepared by mixing 0.1 ml of formic acid, 0.5 ml of acetic acid and 0.4 ml of 1,2-dichloroethane.The end-point of the titration is measured, as in Part I,’ at the “upturn” temperature. This is the point where the titration curve leaves the tangent drawn to the “horizontal” portion of the curve in the vicinity of the temperature rise. B. TITRATION OF ACIDIC FUNCTIONS- Use the same general procedure as that used for titrations of the basic function but with the recorder set at 100 mV full scale. Prepare titration solutions by adding 2 ml of acrylonitrile to a solution of the sample in either 1 ml of dimethylformamide (suitable for dopamine hydro- chloride, adrenaline hydrogen tartrate and cc-methyldopa) or 1 ml of dimethylformamide containing an approximately equivalent and accurately known amount of P-toluenesulphonic acid [suitable for adrenaline, L-noradrenaline, L- and DL-dopa and (+)-Corbasil] .Appropriate amounts of sample for 0.1 M titrant (tetra-n-butylammonium hydroxide) are: 0.1 to 0.02 mmol of (-)-adrenaline, (+)-Corbasil and L-noradrenaline; 0.05 to 0.01 mmol of L- and DL-dopa, dopamine hydrochloride and cc-methyldopa; and 0.04 to 0.007 mmol of adrenaline hydrogen tartrate. With 0.01, 0.002 and 0.001 M titrant, use correspondingly smaller amounts of sample. The end-point of the titration is measured, as in Part 11,8 by the method of Vaughan and Swithenbank,lo in which it is located as the point where the tangent to the main heat rise leaves the curve at its lower temperature end. All determinations were carried out by using the automatic method. B.P. ASSAY METHOD- This method, which involves titration of a solution of the sample (about 500mg of active constituent) in a formic acid - acetic acid mixture with 0.1 M perchloric acid, with Oracet blue as the indicator, is described in detail in the British Pharmacopoeia.6 ULTRAVIOLET SPECTROPHOTOMETRY- Extract an amount of sample containing about 50 mg of L-dopa with 40 ml of 0.1 M hydrochloric acid.Filter and, by using 0.1 M hydrochloric acid as the diluent, make the volume up to 100 ml, then dilute 10 ml of this solution to 100 ml. Measure the absorbance of a l-cm layer of the solution at a wavelength of 280 nm and determine the L-dopa content by comparing the result obtained with values obtained with a “pure” sample of L-dopa. TITRATION OF MAGNESIUM STEARATE- Basic function-Add magnesium stearate (0.03, 0-06 or 0.1 mequiv) to a mixture of 0.5 ml of formic acid and 2 ml of acetic acid.Stir the mixture for 5 minutes, add 0.5 ml of a-methylstyrene and titrate the solution according to procedure A (automatic method). Basic function after precipitation of magnesium as the oxalate-Mix magnesium stearate (0.3, 0.6 or 1.0 mequiv) with 10 ml of formic acid, filter, wash the insoluble material with formic acid and make the volume of filtrate up to 20 ml with formic acid. Add 0 . 5 g of488 [Analyst, Vol. 98 dry oxalic acid, stir and allow the solution to stand for 4 hours. Filter and, by using 0-5 ml of the filtrate, determine the basic function as above. Acidic function-Add magnesium stearate (0-03, 0.06 or 0.1 mequiv) to 1 ml of a 0.1 M solution of benzoic acid in dimethylformamide and titrate the total acidity by using pro- cedure B.RESULTS AND DISCUSSION GREENHOW AND SPENCER: IONIC POLYMERISATION FOR END-POINT The sample must be partially soluble, at least, if consistent results are to be obtained by the thermometric method. Except for D- and L-a-methyldopa, which are soluble in dimethyl- formamide, the catecholamines examined in the present investigation were virtually insoluble in neutral and weakly basic organic solvents. For the determination of the basic function adrenaline and L-noradrenaline can be dissolved in acetic acid, but with L- and DL-dopa and (+)-Corbasil it is necessary first to dissolve the sample in formic acid before adding acetic acid. To avoid separation of the phases when a-methylstyrene is added to the sample solution there must be present a sufficient excess of acetic acid over the a-methylstyrene.A homogeneous solution can also be achieved by addition of propylene carbonate to mixtures of formic and acetic acids and a-methylstyrene. Formic acid and, to a lesser extent, acetic acid reduce end-point sharpness, particularly in titrations with 0.01 and 0.001 M titrants. With the weaker titrants the proportion of formic and acetic acids is kept to a minimum and 1,2-dichloroethane is used as a diluent. Details of suitable solvent systems are given in Table I. For titration of the acidic functions, dimethylformamide is a satisfactory solvent for adrenaline hydrogen tartrate and dopamine hydrochloride , as well as for the cc-methyldopas. The other catecholamines can be dissolved conveniently in dimethylformamide containing about 1 equiv of $-toluenesulphonic acid.It is, of course, necessary to subtract the volume of titrant consumed by the latter from the final titration volume. 0.1 M Tetra-n-butylammonium hydroxide/ml (1 division= 1 ml) Fig. 2. Thermometric titration of the acidic functions of catecholamines in suspension in weakly basic media Compound/mg N, 21.5 C, 16-6 A, 21.4 D, 19.0 A, 18.8 Solvent/ml F,2 F , 2 P, 1 H, 1 M,2 a b C d e Acrylonitrile/ml 1 1 2 2 1 Compounds-N, L-noradrenaline; C, (+)-Corbasil; A, (-)-adren- aline; and D, DL-dopa Solvents-F, dimethylformamide; P, NN-diethyl-3-aminopro- pionitrile ; H, hexamethylphosphoramide ; and M, N-methyl- morpholine Arrows indicate theoretical end-pointsJuly, 19731 INDICATION IN NON-AQUEOUS THERMOMETRIC TITRIMETRY.PART IV 489 It is possible to titrate these latter catecholamines in dimethylformamide alone, and also in N-methylmorpholine, NN-dimethyl-3-aminopropionitrile and hexamethylphosphor- amide, although they are almost insoluble in these solvents, but the precision of the deter- mination is not good. In the course of the titration there is initially a sharp rise in temperature until the combination of the heat evolved and added titrant brings about dissolution of the sample. The temperature then slowly decreases as neutralisation of the sample continues ; when neutralisation is complete the usual sharp temperature rise occurs. The resultant S-shaped titration curve (Fig. 2) is characteristic of the particular catecholamine.The lack of precision of the determination may be caused by the temperature fluctuations. In determinations of both the basic and acidic groups, amounts of sample were chosen so as to give titration volumes in the range 0.1 to 2.0 ml. In this range, calibration graphs were linear for the 0-1,O-Ol and 0.001 M titrants and 0-002 M tetra-n-butylammonium hydro- xide. In determinations of the acidic functions only one of the hydroxyl groups of the catechol moiety could be titrated (see Table 111, Part 11). Thus the dopa and a-methyldopa isomers and dopamine hydrochloride were determined as dibasic acids and adrenaline hydrogen tartrate was determined as a tribasic acid. Typical titration curves obtained by using the manual method for the determination of the basic function are shown in Fig.3. Curves of similar shape were obtained with the auto- matic apparatus, which was used for most of the determinations including those carried out in order to establish the precision of the method, both for basic and acidic functions. The results of the precision measurements are summarised in Tables I1 and 111. The coefficients of variation are similar in order to those obtained with the simpler bases and acids examined in previous paper^,^,^ and lie in the range 0.2 to 1-76 per cent. 0.1 M Perchloric acid/ml (1 division = 1 ml) Fig. 3. Thermometric titration curves obtained in determina- tions of the basic functions by the manual method Compound/mg r I a b C d L-Dopa, 19.8 (-)-Adrenaline, 18.0 hydrochloride, 19.1 L-Noradrenaline, 16.9 Dopamine Arrows indicate theoretical end-points With the 0.001 M titrants catecholamines could be determined in amounts down to about 20 pg.by titration of the basic function and to less than 20 pg by titration of the poly- functional acids. A comparison of the results given in Table I1 with those in Table 111 indicate that titration of either the basic or the acidic groups should be a satisfactory procedure for the assay of the catecholamines although, with compounds containing two or more acidic functions, titration of the acidic function has the advantage of higher sensitivity. The L-dopa content of formulations, namely, ordinary tablets, sustained-release tablets and capsules, has been490 [Analyst, Vol. 98 determined by titration of the basic function and of the titratable acidic functions. The same procedures as those used for titration of the pure catecholamines were used.The results of these determinations have been compared with those obtained by two methods that are currently used for the assay of L-dopa and its formulations, namely, the recently described B.P. method6 and ultraviolet spectrophotometry of a solution of the L-dopa in aqueous hydrochloric acid. It can be seen from the summary of the comparative study, shown in Table IV, that the results obtained by the B.P. method, ultraviolet spectrophoto- metry and thermometric titration, with one exception, do not differ by more than 1-6 per cent., and can be considered to be comparable. The exception is the value obtained in the thermo- metric titration of the acidic functions of capsules which, a t 95-2 per cent., is significantly higher than those obtained by the other methods.This higher value may possibly be attrib- uted to acidic excipient material, e.g., stearic acid or citric acid. GREENHOW AND SPENCER: IONIC POLYMERISATION FOR END-POINT TABLE I1 RESULTS FOR PRECISION FROM THE THERMOMETRIC TITRATION OF CATECHOLAMINES WITH 0.1 TO 0.001 M SOLUTIONS OF PERCHLORIC ACID Amount/ Catecholamine mg (-)-Adrenaline . . . . 18.36 DL-Dopa .. .. . . 19-56 L-Dopa (tablets) . , . . 29-52 L-Dopa (sustained-release tablets) . . ,. . . 31-22 L-Dopa (capsules) . . . . 20.08 (-)-Adrenaline . . . . 1.84 L-Noradrenaline . . . . 0.17 Dopamine hydrochloride . . 0.19 (-)-Adrenaline . . . . 0.18 Titrant Mean molarity* n t titrelml 0.1 4 1-04 5 1.01 5 1.04 4 1.27 3 0.94 0.01 3 1-43 0.001 3 0.86 3 0.95 3 1.24 Standard deviation 0-006 0.009 0.005 0.011 0.016 0.006 0.015 0.014 0.007 Coefficient of variation, per cent.0.56 0.88 0.50 0.91 1-23 0.40 1-76 1.49 0.58 * Nominal value. t Number of determinations. When the B.P. method is used for the assay of L-dopa formulations, care must be taken in deciding when the end-point has been reached as the indicator, titrant and L-dopa can be adsorbed on to the surface of insoluble excipient material in the sample. If such adsorption is likely, it is advisable to stir the titration solution for about 3 minutes between additions of titrant near the end-point so as to ensure that all of the titrant is consumed. This operation will, of course, increase significantly the time required for each determination. TABLE I11 RESULTS FOR PRECISION FROM THE THERMOMETRIC TITRATION OF CATECHOLAMINES WITH 0.1 TO 0.001 M SOLUTIONS OF TETRA-n-BUTYLAMMONIUM HYDROXIDE Catecholamine Dopamine hydrochloride L-Dopa (tablets) .. (-)-Adrenaline .. (-)-Adrenaline . . DL-Dopa .. .. DL-Dopa .. .. Dopamine hydrochloride Dopamine hydrochloride Amount/ 15.63 9-48 15.07 0.31 0-97 0.94 0.095 0.029 mg* p-Toluene- sulphonic acid/mg 18.98 18-98 1.90 1.90 - - - 0-057 Titrant Mean molarityt nS titrelml 0.1 3 1.93 4 1.03 3 2-50 0.01 3 0.41 3 2.18 3 1.03 0.002 3 0.84 0.001 3 1.70 * In 1 ml of dimethylformamide. t Nominal value. Number of determinations. Standard deviation 0.017 0.006 0.005 0.005 0.025 0.006 0*01.2 0.027 Coefficient of variation, per cent.0.89 0.63 0.20 1.23 1.16 0.59 1.46 1.61 Some formulations contain small amounts of magnesium stearate, e.g., from 0.5 to 2 per cent., as a lubricating agent for ease of tabletting and as a flow promoter. It was found that magnesium stearate dissolved in formic acid is titrated as a base when the thermometric method is used. An addition of 2 per cent. of magnesium stearate increases the titre byJuly, 19731 INDICATION IN NON-AQUEOUS THERMOMETRIC TITRIMETRY. PART IV 491 0.66 per cent., which is not insignificant in the assay of catecholamines in formulations. Magnesium can be precipitated as a solvated oxalate from formic acid solutions by the addition of an excess of oxalic acid. However, trace amounts are difficult to remove by precipitation because the oxalate is slightly soluble in formic acid and, after filtration, the solution still contains about 0.01 mequiv ml-l.This amount is equivalent to 10 per cent. of the titre when one is determining catecholamines at the 0-1 mequiv ml-1 level. TABLE IV COMPARISON OF METHODS FOR THE DETERMINATION OF THE L-DOPA CONTENT O F FORMULATIONS Thermometric titration method of spectro- Basic Acidic B.P. Ultraviolet Sample assay photometry function functions Levodopa tablets (normal) . . .. 67.9 68.7 67.2 67.7 Levodopa capsules . . .. .. 92.3 93.2 91-3 95.2 Levodopa tablets (sustained release) . . 74.2 74.3 74-9 73-9 L-Dopa .. .. .. .. .. 98.9 (100) * 99.5 100.9 * Calibration standard. All determinations were carried out on the same bulk sample of each formulation type, prepared by grinding together twenty dosage forms.T’alues given are per cent. m/m, and are the average of three determinations. In contrast, the effect of magnesium stearate on the titration of acidic functions is small and can be attributed partly, but not wholly, to small amounts of free stearic acid. The addition of 2 per cent. of magnesium stearate to L-dopa would increase the titre by about 0.1 per cent., and for all practical purposes this effect can be ignored. The results of a study of the titration of the basic and acidic functions of relatively large amounts of magnesium stearate are summarised in Fig. 4. 0.05 0.1 Magnesium stearate/mequiv Fig. 4. Calibration graphs for the thermometric titration of magnesium stearate : a, titration of the basic function; b, titration of the basic function after removal of precipitated magnesium oxalate ; and c, titration of the acidic function + 1 mequiv of benzoic acid.Details are given in the procedure Tables I1 and I11 show that the precision that can be obtained in the determination of the L-dopa content of formulations is similar to that achieved with the pure catecholamines. When one considers also that the thermometric method could be considerably faster than492 GREENHOW AND SPENCER the B.P. method, particularly if difficulties arise with indicator adsorption in the latter, then the method reported in this paper might be worthy of consideration as a procedure for routine assays. With the technique at its present state of development, i.e., with a lower limit of deter- mination of 10 to ZOpg, it cannot yet be considered to be suitable for the determination of catecholamines in body fluids and tissue extracts, unless a concentration step can be introduced into the analytical procedure.We thank the donors of the test compounds and formulations as indicated above. Mr. M. B. Arthur of Brocades (Great Britain) Ltd. is thanked for helpful advice on assay procedures for L-dopa. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. REFERENCES Holzbauer, M., and Sharman, D. F., in Blashko, H., and Muscholl, E., Editors, 44Catecholamines,” Handbook of Experimental Pharmacology, Volume XXXII, Springer-Verlag, Berlin, Heidelberg and New York, 1972, p. 111. “The British Pharmacopoeia 1968,” The Pharmaceutical Press, London, 1968, pp. 19, 20 and 662; “The British Pharmacopoeia 1973,” H.M. Stationery Office, London, 1973, p. 262. “The Pharmacopoeia of the United States of America, 18th Revision, 1970,” United States Pharmacopoeial Convention Inc., Washington, D.C., 1970, pp. 362, 227 and 230. Clarke, E. G. C., “The Isolation and Identification of Drugs in Pharmaceuticals, Body-fluids and Post Mortem Material,” The Pharmaceutical Press, London, 1969, pp. 176 and 322. “The Pharmacopoeia of the United States of America, 18th Revision, 1970,” United States Pharmacopoeial Convention Inc., Washington, D.C., 1970, pp. 352 and 354. “The British Pharmacopoeia 1973,” €3. M. Stationery Office, London, 1973, pp. 263 and 264. Greenhow, E. J., and Spencer, L. E., Analyst, 1973, 98, 81. , Ibid., 1973, 98, 90. -- , Ibid., 1973, 98, 98. Vauihan, G. A., and Swithenbank, J. J., Ibid., 1970, 95, 890. NOTE-References 7, 8 and 9 are to Parts I, I1 and I11 of this series, respectively. * -- Received January 30th, 1973 Accepted March 2nd, 1973

ISSN:0003-2654    

DOI:10.1039/AN9739800485

出版商:RSC    

年代:1973

数据来源: RSC

摘要:

Analyst, July, 1973, Vol. 98, @. 493-501 493 The Determination of Di-n-alkyl Phthalates in Cosmetic Preparations by Gas - Liquid Chromatography BY E. W. GODLY AND A. E. MORTLOCK (Department of Trade and Industry, Laboratory of the Government Chemist, Cornwall House, Stainford Street, London, SE 1 9NQ) An improved gss-chromatographic method for the direct determination of C,-C, di-n-alkyl phthalates in toiletry samples that contain ethanol is des- cribed and a range of perfume essential oils and perfume synthetic chemicals is examined for possible interference. MANUFACTURERS of preparations that contain ethanol are required by H.M. Customs and Excise to include denaturants in order to render the preparations unpotable. The suitability of newly proposed denaturants and their effective concentration are determined according to the recommendations of the Laboratory of the Government Chemist, which examines also the finished products to ensure that the required level of nauseousness is maintained.The list of accepted denaturants is large and increasing and, as the samples examined are drawn from the entire cosmetic range and indeed extend to any retailed preparations (other than beverages) that contain spirit, special analytical problems arise. Diethyl phthalate is commonly used as a denaturant of toiletries because it is usually compatible with other ingredients and has properties that may be convenient, e.g., as a perfume fixative or plasticiser. Its inclusion at a minimum level of 1 per cent. V/V in per- fumes made with Q-grade industrial methylated spirit to the Statutory Formula I1 is manda- tory.1 In a previous paper2 from this laboratory, three methods for the determination of diethyl phthalate were described as follows: by (i) direct gas - liquid chromatography with two alternative column systems; (ii) isolation of the diethyl phthalate by column cliromatography followed by measurement of optical density at 227 nm ; and (iii) gravimetric determination as phthalanil, which involves alkaline hydrolysis, liberation of the free phthalic acid by acidification and extraction into diethyl ether followed by treatment with aniline to give a precipitate that can be weighed.The acidification stage provides the possibility of determining the plithalic acid by back-titration. Formerly, the only comparable published gas - liquid chromatographic method for the determination of diethyl phthalate3 involved the use of 30 per cent.sodium dodecylbenzene- sulphonate as the stationary phase and a thermal conductivity detector. Although this system yielded symmetrical peaks, the time required for a single analysis was excessive. By making use of low loadings of polar phases that were stable and non-fugitive at high temperatures, Hancock, Rose and Singer2 were able to shorten the analysis time and thus provided a useful routine method. Some tailing of peaks was experienced but this was not considered unacceptable. The stationary phases were fluorosilicone oil 1;s 1265 and silicone gum rubber SE-30, which were used in each instance a t a loading of only 1.5 per cent.m/m on HMDS-treated Chromosorb W. These authors compared results obtained from a range of toiletry samples by the gravimetric method with those obtained by ultraviolet absorption measurements, and those for gas - liquid chromatographic determinations, including also the results calculated from the phthalic acid values obtained by back-titration. Agreement was generally good in both instances but some discrepancies were noted with the comment that the chroma tographic method served as a screening technique to overcome the non-specificity inherent in the other methods. It was frustrating that the FS 1265 system, although less prone to tailing, and therefore giving better chromatograms than those obtained with the SE-30 system, failed to resolve diethyl phthalate from isopropyl myristate, a fairly common toiletry ingredient, while, even with SE-30, with which this resolution was complete, it was found that commercial isopropyl myristate contained an impurity that eluted close to die thy1 phthalate .0 SAC; Crown Copyright Reserved.494 GODLY AND MORTLOCK: DETERMINATION OF PHTHALATES [Analyst, VOl. 98 In more recent work in which the determination of diethyl phthalate in small-arms propellents is de~cribed,~ the stationary phase used was 20 per cent. SE-30 on an acid-washed Chromosorb FV support at 200 "C, and the published chromatograms show that, even with such high loading of the stationary phase, the problem of tailing had not been entirely elimin- ated. The system for the determination of diethyl phthalate now proposed is, as previously ~laimed,~ free from the interference complained of and providess ymmetrical peaks at retention times compatible with routine analysis.I t is applicable to many other denaturants including dimethyl phthalate and, after slight modification, to di-n-butyl phthalate. APPARATUS- A Pye, Model 104, gas chromatograph with a flame-ionisation detector was used. CoEwumns-Usually glass, 1-52m x 3 mm i.d. (5 feet x Q inch o.d.), but stainless-steel columns have also been used without difficulty. Packiwg-This consists of 8 per cent. of nonylphenoxypoly(ethy1eneoxy)ethanol on 80 to 100-mesh acid-washed DMCS-treated Chromosorb W. The prepared column contains 5.6 to 5.7 g of packing and should have an efficiency of 1200 to 1800 plates per metre, as measured on the peak obtained from a 1-pl injection of 2 per cent.diphenylamine in ethanol. Columns are packed under gentle suction to within 11 cm of the injection head and both ends are plugged with glass string. The stationary phase, also known as Antarox CO-990 and Igepal CO-990, has been mentioned6 in connection with the determination of dimethyl phthalate in propellent plasticisers but under slightly different conditions. Its recommended temperature range is 50 to 225 "C. CoZwnn conditioni.Yzg-Conditioning overnight at 225 "C with nitrogen passing through the column a t the rate of 5 ml min-l has proved satisfactory. Carrier gas-Nitrogen at a flow-rate of 55 to 60 ml min-l. Column te.lnperntwre--For dimethyl phthalate and diethyl phthalate, 200 to 210 O C , and The temperature of both the injection block and detector oven was 250 "C, and the for di-n-butyl phthalate, 220 "C.attenuation was x 5000. The detector response is measured from a chart recorder as peak height. 1 4 I I 15 10 5 0 Time/m i nutes Fig. 1. Gas chromatogram of mix- ture A. Peaks: 1, ethanol; 2, methyl salicylate ; 3, 1-chloro-4-nitrobenzene ; 4, isopropyl myristate ; 6, dimethyl phthalate ; 6, dicthyl phthalate; 7, di-n-propyl phthalate; 8, diphenylamine; and 9, di-n-butyl phthalateJuly, 19731 IN COSMETIC PREPARATIONS BY GAS - LIQUID CHROMATOGRAPHY 495 Injections were made from a 1-pl syrinse fittcd wit11 an ll-cm needle. Uri1ms otherwise stated, p!urr:ss. grade cli~~micn!s wr.v used ; the ethanol uszd was absolute ethanol. Fig. 1 shows a c l i ~ o i i ~ ’ ohk+~cd i~r~d~ar t!jnsz conditions a t 205 “C from a 0.2-pl injcciion of ‘ ‘ I ~ ~ Y ~ Z I P C A,” a u n 01 riiiwd didkyl p:i tl,ilatcs and some otbcs compounds of interest in ethanol a t conrr-n% ions 1mgir-g i1-0m 0.1 to 2.0 g per 100 ml, ai2J in Table I are listed the retention t k c s o€ tile constitu:-xts o! this mistwe in the ordei- oT their elution.TABLE I ELUTION OF CONSTITUENTS OF “MIXTURE A” Compound Ethanol . . . . Methyl salicylate . . l-Chloro-4-nitrobenzene Jsopropyl myristate Dimethyl phthalate . . Diethyl pl-ithalate . . Di-n-propyl phthalate Diphenylamine . . Di-n-butyl phthalate Retention tirnc/miiiutes .. . . 0.4 . . .. 1.2 . . .. 1.8 . . . . 2.2 . . .. 4.0 . . .. 4.9 .. . . 7-65 ,. .. 9.0 .. .. 12.8 R* 0.08 0.24 0.37 0.45 0.82 1.53 1.84 2.61 (1.00) TEST OF LINEARITY OF DETECTOR RESPONSE- A series of standard solutions containing from 0-5 Lo 4.0 per cent.V/V of diethyl phthalate in ethanol was preparcd and each solution was then mixed with an equaI voilime of a solution containing 2 g of diphenylamine in 100 ml of ethanol. Each mixture was injected under the conditions described and the peak heights were measured for both components. In Fig. 2, the peak height ratio of diethyl phthalate to diphenylamine is plotted against the initial dietliyl phthalate concentration and is clearly linear within the limits of measurement Y m a 0 1.0 2.0 3.0 4.0 Diethyl phthalate/per cent. V/V Fig. 2 . Detector response over a range of cliethyl phthalate concentrations 0.6 0.7 0.8 0.9 Peak height ratio of dimethyl phthalate t o internal standard 13:.3. Calibralioii graph for de tel-in {nation o 1 -1 ime;liyl plitlia- late. liatio: A, of di1me“lyl phtha- late to clipheriylami~ie, :itid B, of djirlcihvl phthaLtte io I rhloro- 4-17 I I obenzene. x , ui (5 Eor first day; and and 0, rcsuitc for second day during continuous running496 GODLY AND MORTLOCK: DETERMINATION OF PHTHALATES [AndySt, VOl. 98 over the phthalate concentration range 0.2 to 2.0 per cent., which is twice the normal de- naturant level. ANALYTICAL METHODS DIMETHYL PHTHALATE- Prepare a standard solution containing 0.5 per cent. V/V of dimethyl phthalate and 1.0 g of diphenylamine per 100 ml of ethanol. Make successive injections of this solution over the volume range 0.1 to 1.0 p1 under the conditions described above.Measure the peak heights for dimethyl phthalate and diphenylamine and then plot the dimethyl phthalate peak height as ordinate against the peak height ratio of dimethyl phthalate to diphenylamine as abscissa (Fig. 3A). This peak height ratio is 0.6 to 0.7, which enables samples containing excess of dimethyl phthalate up to 50 per cent. above the legal denaturant minimum to be analysed at a single attenuation without dilution. To a known volume of each sample add an equal volume of a standard solution containing 2 g of diphenylamine per 100 ml of ethanol (for convenience, 2-ml aliquots were dispensed from pipettes into 10-ml McCartney bottles). Inject the mixture and measure the heights of the peaks due to dimethyl phthalate and diphenylamine. Determine the ratio of the former to the latter (x) and read off the ratio (y) corresponding to each dimethyl phth2late peak height from the standard graph. The concentration of dimethyl phthalate in the sample is then x/y per cent. V/V.If the standard graph is considered to be close enough to the vertical, plotting of ratios can be dispensed with and an average ratio applied to any volume injected; the calculation then becomes one of simple proportion. The same procedure can be followed with an alternative internal standard. If a solution containing 0.6 g of 1-chloro-4-nitrobenzene in 100 ml of ethanol is substituted throughout for the 2 per cent. diphenylamine solution, analytical results are obtained in exactly the same way. The only differences are that, as 1-chloro-4-nitrobenzene elutes before dimethyl phthalate, the duration of each analysis is reduced while the risk of interference from perfume constituents is increased. A typical standard graph obtained with this internal standard is shown in Fig.3B. DIETHYL PHTHALATE- The analytical procedure is exactly as described above for dimethyl phthalate and the same two alternative internal standards are used. Fig. 4 shows a standard graph with diphenylamine as internal standard, to which points were added over a period of 8 days' continuous running. DI-n-PRoPYL PHTHALATE- This ester has not yet found use as a denaturant but it has been included in order to complete the C,-C, series. The method is as described for dimethyl phthalate except that the temperature of the column oven is set at 215 "C.Diphenylamine is the appropriate internal standard and, under these conditions, the retention times are 4.9 minutes for di-n-propyl phthalate and 5.6 minutes for diphenylamine. DI-n-BUTYL PHTHALATE- The method is as described for dimethyl and diethyl phthalates but with the following modifications: the temperature of the column-oven is 220 "C; the nitrogen flow-rate is 80 ml min-l; and the two alternative internal standards are diphenylamine and methyl fi-hydroxybenzoate, each at a concentration of 2 g per 100 ml of ethanol. Under these conditions, the retention times are diphenylamine 5.4, di-n-biityl phthalate 7.3 and methyl 9-hydroxybenzoate 11.4 minutes. Fig. 5A shows a graph of di-n-butyl phthalate peak height against the peak height ratio of di-n-butyl phthalate to diphenylamine over a 2-day period, while Fig.5B shows the graph obtained over a similar period when the diphenylamine was replaced with methyl $-hydroxy- benzoate. Turbidity occasionally results when the sample is mixed with the solution of internal standard. The solution can sometimes be clarified for long enough to permit an injection of homogeneous liquid by gently warming the capped bottle, but if this method fails, dilutionJuly, 19731 IN COSMETIC PREPARATIONS BY GAS - LIQUID CHROMATOGRAPHY 497 of the mixture with a known excess of ethanol can be combined with appropriate reduction in attenuation. Alternatively, the internal standard can be replaced with the second choice. I I I 0.5 0.6 0.7 Peak height ratio of diethyl phthalate to diphenylamine Fig.4. Calibration graph for determination of diethyl phthalate. Continuous running for 8 days: dif- ferent symbols indicate results obtained on different days Peak height ratio of di-n-butyl phthalate to internal standard Fig. 5 . Calibration graph for determination of di-n-butyl phthalate. Ratio : A, of di-n-butyl phthalate to diphenylamine; and B, of di-n-butyl phthalate to methyl p-hydroxyben- zoate RESULTS RECOVERIES FROM SOLUTIONS OF PURE PHTHALATES I N ETHANOL- A range of standard solutions of each ester in ethanol was prepared; with di-n-propyl phthalate a single concentration was considered sufficient. Each ester was weighed, dissolved in and made up to volume with ethanol in calibrated flasks at 20 "C and the solution was then taken for analysis as an unknown. Three to ten determinations were made at each level by the methods described above as appropriate to each ester.Mean recoveries are given in Table 11, where the percentage of ester in the prepared solutions has been converted for comparison from grams per 100 ml into per cent. V/V by means of published density data at 20 "C. These results provided assurance that the system was functioning correctly and was capable of yielding significant results. RECOVERIES FROM TOILETRY SAMPLES- For these recovery experiments toiletry samples were chosen to represent various types of goods received for analysis. In each instance the content of the ester concerned had been found to be nil or negligible. Each sample was treated as follows: a 6-ml aliquot was transferred into each of four 14-ml McCartney bottles, 2.0, 1.5, 1.0 and 0-5 ml of ethanol were added to successive bottles followed by 0, 0.5, 1.0 and 1.5 ml, respectively, of the phthalate concerned in ethanolic solution at a concentration of 10 per cent. V / V , all additions being made by means of graduated pipettes.These mixtures were then analysed by the appropriate procedure as described above. The results are set out in Tables 111, IV and V.498 GODLY AND MORTLOCK: DETERMINATION OF PHTHALATES [A%&!@, VOl. 98 TABLE I1 RECOVERIES OF PURE ESTERS FROM ETHANOLIC SOLUTION Found by analysis, per cent. Ester dispensed, Ester per cent. V / V Dimethyl phthalate 0.18 0.45 0.75 0.99 1-38 1-77 Diethyl phthalate Mean recovery, per cent.0.22 0.47 0.80 1.25 1.51 1-38 Mean recovery, per cent. Di-n-propyl phthalate 1.10 Di-n-butyl phthalate 0.20 0.40 0-79 1.19 1.98 Mean recovery, per cent. Against l-chloro-4-nitrobenzene 0.18 & 0.01 0.47 & 0.01 0.76 f 0.02 0.99 f 0.01 1.38 f 0.02 1.74 f 0.02 102 0.21 f 0.01 0.47 f 0-00 0.80 f 0.00 1.25 f 0-02 1.52 & 0.02 2.02 0.02 100 Against methyl p-hydroxybenzoat 2 0.38 f 0.00 0.79 f 0.03 1-22 f 0.03 2-13 f 0.03 99 - 0-19 f 0.00 > Against d j phen ylamine 0.19 f 0.01 045 f 0.02 0.74 f 0.00 0.99 f 0.01 1-37 5 0.01 1.76 f 0.02 99 0.22 & 0.00 0-48 & 0.00 0.81 & 0.01 1.27 f 0.01 1.52 f 0-01 1.96 f 0-01 101 1-09 0.03 Against d jphen ylamine 0.19 f 0.00 0-38 i 0.00 0.79 f 0.00 1.21 f 0.01 2.14 & 0-03 100 TABLE I11 RECOVERY OF DIMETHYL PHTHALATE Dimethyl phthalate recovered, per cent.V / V , after addition of- 7- > Description 0.5 1.0 1.5 Internal standard of sample Nil per cent. per cent. per cent. Diphenylamine Cleansing lotion Negligible 0-44 1.03 1.48 After-shave lotion 0 0-52 0.98 1.51 Toilet water Negligible 0.51 0.96 1.52 Eau-de-cologne 0 0.49 1.08 1.50 Skin tonic 0 0.50 1.02 1.50 Mean recovery per cent. 98 101 100 1-Chloro-4-nitrobenzene Perfume 0 0.49 0.95 1.49 Friction lotion 0 0.50 1.02 1-52 After-shave lotion 0 0.5 1 1.10 1.50 Bay-rum* 0.02 0.51 (0.49) 0.99 (0.97) 1.57 (1.55) Mean recovery per cent. 100 100 101 * Figures in parentheses were used in calculation of mean recovery. EXAMINATION OF PERFUME ESSENTIAL OILS AND PERFUME SYNTHETIC CHEMICALS FOR POSSIBLE It is clear from the results that the method provides a quick and efficient analytical pro- cedure for the determination of these toiletry denaturants.However, it is appreciated that dependence on a single-column system fails to eliminate the possibility of undetected inter- ference. It was, therefore, a matter of immediate interest to examine a range of typical INTERFERENCE-July, 19731 IN COSMETIC PREPARATIONS BY GAS - LIQUID CHROMATOGRAPHY TABLE IV RECOVERY OF DIETHYL PHTHALATE Diethyl phthalate recovered, per cent. V / V , aftcr addition of- 499 Description Internal standard of sample Diphen ylamine Lavender water Hair lotion A Hair lotion B Spirit shampoo* Alter-shave lotion Vegetable hair tonic Setting lotion? Tonic cleanser Toilet water Skin tests Nil 0 0 0 0 Ncgl j gible 0 0.03 0 Negligible 0 Mean recovery per cent.l-Chloro-4-nitrobenzene Deodorant 0 Toilet water A 0 Toilet water B 0 Skin cleanser 0 After-shave lotion Negligible Friction lotion 0 Perfume Xegligible Skin tonic 0 Mean recovery per cent. 0-5 per cent. 0.5 1 0.50 0.50 0.41 0.50 0.50 0.50 (0.47) 0.48 0.50 0.51 99 0.49 0.50 0-51 0-5 1 0.50 0.49 0.50 0-49 100 1.0 per cent. 0.99 1.01 1.02 0-90 1.00 1.00 0.99 0.96 1.00 100 1 *oo 0.99 0.99 1.08 1.00 1-03 1.03 0.98 101 1.02 (0.99) 7 1-5 per cent. 1.51 1.50 1.50 1-45 1.50 1.51 1-51 (1.4s) 1.52 1-65 1.50 101 1.48 1.52 1-54 1.51 1.6 1 1.49 1.49 1.52 101 * Persistent sediment on mixing. t Figures in parentheses used in calculation of mean recovery. pErfume synthetic chemicals and some available perfume oils for this possibility. The perfume chemicals were injected as 10 per cent.V/V solutions in ethanol but the perfume oils were injected undiluted with increased attenuation in order to demonstrate trace constituents. In most instances, elution of all of the compounds was complete within 2 minutes of injection under the conditions of the method. In Table VI the perfume synthetic chemicals are listed alphabetically, giving the retention time of the principal constituent and its RDEP value (diethyl phthalate = l), together with the number of other constituents; figures in parentheses convey the number of separate peaks that can be ascribed to trace constituents. TABLE V Internal standard Diphen ylamine RECOVERY OF DI-n-BUTYL PHTHALATE Di-n-butyl phthalate recovered, per cent. V / V , after addition of- A r 7 Description 0.5 1.0 1.5 of sample Nil per cent.per cent. per cent. Toilet water 0 0.51 0.98 1-52 Hair lotion Negligible 0.47 1.02 1.48 After-shave lotion Negligible 0-53 1-02 1.55 Skin cleanser 0 0.50 1.00 1.52 Mean recovery per cent. 100 100 101 Methyl p-hydroxybenzoate Skin cleanser 0 0.49 1-04 1.56 Hair lotion Negligible 0.53 0.98 1-46 After-shave lotion 0 0.49 0.99 1-50 Perfume 0 0.53 1.04 1-53 Mean recovery per cent. 102 101 101500 GODLY AND MORTLOCK: DETERMINATION OF PHTHALATES [Analyst, VOl. 98 In some instances spectrographically pure chemicals might have been obtainable but the interest in this survey applied equally to impurities, isomers, etc., that are normally found in commercial-grade products. Comparable results for some perfume essential oils are listed in Table VII, except that no RDEP values are given because, in many instances, no single constituent predominated.Compound Pentyl acetate . . Benzyl benzoate . . Rornyl acetate . . Butyl acetate . . Cinnamyl acetate . . Citronellol . . . . Citronellyl acetate. . Citronellyl butyrate Ci tronellyl isobutyrate Citronellyl valerate Diethyl nialonate . . Diethyl phthalate . . Diethyl succinate . . Diethyl tartrate . . Diethyl sehacate . . Ethyl acetate . . Ethyl butyrate . . Ethyl octanoate . . Ethyl cinnamate . . Ethyl cyanoacetate Ethyl heptanoate . . Ethyl hexylacetate Ethyl lactate . . Ethyl palmitate . . Ethyl phenylacetate Geraniol . . . . Geranyl acetate . . Geranyl propionate Isopropyl palmitate D-Limonene .. Linalol . ... Linalyl butyrate . . Linalyl propionate Menthyl acetate . . Menthyl salicylate. . Musk (synthetic) . . Nerol . . .. Nerolidyl acetate . . Xerol idyl propionate a-Pinene . . .. Pulegol . . . . Sextol phthalate . . Terebene . . .. a-Terpineol .. Terpineol . . .. Terpinoline . . a-Terpinyl acetate. . Te trah y droger aniol .. .. .. .. . . . . . . . . . . .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . . .. .. .. . . .. .. .. .. .. .. .. .. .. .. .. .. .. .. TABLE V I PERFUME SYNTHETIC CHEMICALS Retention time of principal No. of constituent/minutes RDEP* impurities . . 0.3 0.05 1 .. 11.4 2.07 ,. 0.9 0.17 i + (2) . . 0.4 0.08 (2) .. 3- 1 0.56 1 + ( 8 ) .. 1.0 0.1s (11) .. 0.9 0.17 1 . . (14 constituents in 3.7 minutes) .. 1.0 .. 1.6 .. 0.6 .. 6.5 ..0.8 .. 4.4 .. 11.2 .. 0.2 .. 0.3 .. 0-6 .. 3-0 .. 0.9 .. 0.4 .. 0.5 .. 0 4 .. 4.6 .. 1-2 .. 1.1 .. 0.8; 1.0 .. 0.2 .. 4.6 .. 0.4 . . 0.6 .. 0.9 .. 0.S .. 0.7 .. 7.3 .. 9.7 .. 1.0 .. 0.6 .. 3.0 .. 0.3 .. 0.7 .. 14 .. 0.6 .. 0.9 .. 0.9 .. 0.4 .. 1-1 .. 0-7 * DEP denotes diethyl phthalate. 0.18 0.30 0.12 1.00 0.15 0.80 2.14 0.05 0.05 0.7 1 0.54 0.18 0-08 0.10 0.08 0.84 0.23 0.2 1 0.14; 0.18 0.04 0.83 0.09 0.S 1 0.17 0-15 0.12 1.32 1.77 0.18 0.11 0-64 0.06 0.14 1.54 0.10 0.17 0.17 0-07 0.2 1 0-14 It will be noted that, on this evidence, the risk of interference is small. The first con- stituent peak obtained from Indian sandalwood oil is the only one of all those examined which would affect the validity of the analysis. The ultraviolet absorption spectrum of a solution of diethyl phthalate in ethanol was compared with that of a solution of Indian sandalwood oil in ethanol at the same concentration.The sandalwood oil was shown to be free from diethyl phthalate and the ultraviolet method would therefore resolve any difficulty with formulations that contain both of these substances.July, 19731 the Government Chemist for permission to publish this paper. IN COSMETIC PREPARATIONS BY GAS - LIQUID CHROMATOGRAPHY 501 The authors thank Mr. A. J. Blake and Mr. G. A. Pask for technical assistance and TABLE VII PERFUME ESSENTIAL OILS Oil (with source, if known) Oil of fennel (U.S.S.R.) . . Geranium bourbon essence Juniper berry oil (U.K.) . . Lavender oil . . .. Lavender exotic . . .. Lavender . . .. .. Lavender abrialis (France) Lemon oil (Italy) . . . . Oil of limes (U.K.) .. Oil of marjoram . . . . Myrcene . . .. . . Oil of rosemary . . .. Litsea cabeba (China) . . Mace oil . . . . .. Oil of rosemary (Spain) . . Oil of rosemary (Peru) . . Sandalwood oil (India) . . Tangerine oil . . . . Oil of thyme, red . . . . Oil of thyme, white . . Oil of ylang ylang (U.K.) Oil of ylang ylang (France) Diethyl phthalate . . .. . . . . . . . . .. .. .. .. . . . . .. .. . . . . . . .. . . .. . . . . . . .. .. Retention times of principal constituents/ minutes 0.5; 1-3 0.7; 0.9; 1.1 0.4; 0.4 0.5; 0.7; 0.8 0.4; 0-6; 0.7 0.4; 0.6; 0.8 0.2; 0 . 5 ; 0.5 0.5 0.3; 0.4; 0.9; 1.0 0.4; 1.0 0.3; 0.4; 0.7; 4.1 0.5; 0-5; 0.7; 0.9 0.4; 4.1; 4.7 0.3; 0.4; 0.8 0.4; 0.5; 0.8 0.5; 0.9 5-4; 6.7 0-4; 0.5 0.6; 3.2 0.6; 3.2 0.5; 0.6; 1.0; 1.2; 1-3 0.8; 0.9; 1.1; 1.2 5.5 Number of subsidiary constituents 9 14 12 8 10 8 9 8 4 15 12 20 9 6 6 14 13 5 14 10 13 15 - Retention time of final constituent/ minutes 9.1 5.0 4.1 2.4 2.4 2.3 2.4 1.5 1.2 4.7 4.15 4.9 4.7 2.4 2.4 4.8 9.0 2.7 8.5 3.4 5.2 5.2 - REFERENCES 1. 2. 3. 4. 5. 6. “The Methylated Spirits Regulations,” S.I. 1952 No. 3320, H.M. Stationery Office, London. Hancock, W., Rose, B. A., and Singer, D. D., Analyst, 1966, 91, 449. Baines, C. B., and Proctor, K. A., J . Phavm. Pharmac., 1959, 11, 230T. Norwitz, G., and Apatoff, J. B., J , Chromat. ?ci., 1971, 9, 682. “Report of the Government Chemist, 1968, Tunstall, F. I. H., Analyt. Chem., 1970, 42, 542. H.M. Stationery Office, London, 1969, p. 12. Received December l3th, 1972 Accepted March 16th, 1973

ISSN:0003-2654    

DOI:10.1039/AN9739800493

出版商:RSC    

年代:1973

数据来源: RSC

摘要:

502 Aualyst, July, 1973, Vol. 98, $9. 502-505 Residues of Prophylactics in Animal Products Part 111." The Determination of Carbarsone in Poultry Meat BY R. A. HOODLESS AND I<. R. TARRANT (Department of Trade and Industry, Lahoratory of the Government Chemist, Cornwall House, Stamford Street, London, SE1 9NQ) A method for the determination of carbarsone in poultry meat is de- scribed. The carbarsone is extracted from the sample with methanol and, after clean-up on an ion-exchange column, hydrolysed to arsanilic acid with sodium hydroxide. The arsanilic acid is diazotised, coupled with 2-amino- ethyl-l-naphthylamine and determined spectrophotometrically. CARBARSONE (4-ureidoplienylarsonic acid) is incorporated into poultry feedingstuff s for growth- promoting or prophylactic purposes, usually at a level of about 375 mg k g l .The need exists for a method for the determination of residues of carbarsone that could occur in meat derived from poultry treated with this compound, other than by determination of the total arsenic content, which is capable of being used to determine residues of carbarsone down to at least 1 mg k g l . This level corresponds to 0.29 mg kg-I as elemental arsenic, which is well below the limit of 1 mg kg-1 specified in the Arsenic in Food Regulations.1 Although total arsenic determinations are sufficient to check compliance with the Arsenic in Food Regu- lations, the proposed method supplies additional information concerning the form in which the arsenic is present. Weston, Wheals and Kensett2 have published a method for the determination of car- barsone in animal feedingstuffs.Their method is based on the conversion of carbarsone into arsanilic acid, which is then reduced to aniline and the aniline determined by gas chromato- graphy with flame-ionisation detection. However, flame-ionisation detection was found not to be sufficiently sensitive or selective for the amounts of residues likely to occur in animal tissues and so attempts were made to prepare a derivative of aniline suitable for electron- capture detection. Derivatives were prepared by reaction with bromine, trichloroacetic anhydride, trifluoroacetic anhydride, l-fluoro-2,4-dinitrobenzene and heptafluorobutanoyl chloride. The procedure of Weston, Wheals and Kensett was applied to treated chicken tissues and the aniline obtained was converted into the heptafluorobutanoyl derivative.The results were unsatisfactory because variable recoveries resulted and co-extractives gave rise to high blank values. Various clean-up procedures were examined, the most suitable of which was thin-layer chromatography. By use of thin-layer plates coated with cellulose and a mixture of butan-1-01, glacial acetic acid and water (50 + 25 + 25) as the developing solvent, carbarsone could be separated from most of the co-extracted material, but the interfering material that gave rise to the high blank values was not removed. This failure may occur because the method is prone to interference from compounds that yield aniline or a similar amino compound on hydrolysis and reduction. To overcome this problem of interference a method was sought in which the phenyl- arsonic acid moiety of the molecule remained intact; the possibility of using fluorescence spectrophotometry was therefore examined.It was found that an aqueous solution of carbarsone fluoresced at a wavelength of 315nm when excited at 255nm. However, an aqueous extract of chicken tissue contained co-extractives that interfered, and none of the clean-up procedures tried was effective in removing this co-extracted material. These difficulties may arise because a wavelength of 255 nm is required for excitation, as Parker3 has shown that most impurities present in solvents fluoresce when excited at 250 nm. As the above approaches proved unsuccessful, methods based oil colorimetric procedures were investigated.The basis of these methods was conversion of carbarsone into arsanilic acid, diazotisation of the arsanilic acid and coupling with first 2-amin0ethyl-l-naphthylamine,~ secondly 1-naphthol5 and thirdly resorcinol.5 The diazotisation of the arsanilic acid was carried out as described by El-Dib5 as this method was found to be more effective than the procedure of Bratton and Marshall6 and gave low blank values. The procedure involving The heptafluorobutanoyl derivative was found to be the most suitable. * Part I1 of this series appeared in Analyst, 1972, 97, 254. @ SAC; Crown Copyright Reserved.HOODLESS AND TARRANT 503 the use of 2-aminoethyl-1-naphthylamine was preferred as the dye produced can be extracted into butan-1-01 and is more sensitive.Accordingly, the method proposed by the Society for Analytical Chemistry Prophylactics in Animal Feeds Sub-Committee4 has been adapted so as to enable amounts of carbarsone down to 1 pg to be determined. Before this colorimetric determination could be applied to an extract of chicken tissue, it was necessary to clean up the extract and it was considered desirable that the clean-up should also give specificity to the method. Use has therefore been made of the ionic character of the arsonic acid group. Carbarsone can be held on an anion-exchange resin and, after washing the resin with water to remove most of the interfering material, can be eluted with sodium chloride solution. Sephadex QAE anion exchanger has been found to be the most suitable material.METHOD APPARATUS- Chromatographic columns-These were made of glass, 140 mm in length and o l 19 mm i.d., fitted with a PTFE stopcock and with a 100-ml capacity reservoir. Preparation of chromatographic column-Place sufficient glass helices in the chromato- graphic column to fill the narrow tube that joins the stopcock to the column. Insert a small plug of cotton-wool on top of the helices and add 3 g of sand to form a level base. Carefully add a prepared slurry of 2 g of Sephadex QAE in water and allow it to settle to give a column about 50 mm in height. Run off the water and wash down any Sephadex adhering to the walls of the glass column, then place more sand on top of the Sephadex to form a layer about 5mm deep. Filtration a~~a~atus-This comprised a Buchner funnel fitted with a glass sinter of porosity 3, an adaptor with a side-arm for connection to a suction pump and a 150-ml conical flask.Mixer-A high-speed laboratory mixer, made by Silverson Machines Ltd., was used. REAGENTS- All reagents should be of analytical-reagent grade quality. 2-Aminoet~yl-l-naphtFY3/lamine dihyd~ochloride solution-Dissolve 50 mg of 2-aminoethyl- 1-naplztliylamine dihydrochloride in water and dilute to 50 ml. Prepare the solution freshly each day. Butan-1-ol. Carbavsone standard solution-Dissolve 10.0 mg of 4-ureidophenylarsonic acid in water The latter solution Prepare a separate column for each sample. and dilute to 100ml; then dilute 2.5ml of this solution to 200 ml. contains 2.5 pg ml-l of carbarsone. Celite 545-This was obtained from Koch-Light Laboratories Ltd.Hydrochloric acid, 5 M. illethanol. Sand, acid washed-Heat the sand at 500 "C for 20 hours, cool and store it at room temperature. Sephadex QAE ion exclzanger, Type A-25-Allow 2-g portions of the Sephadex to swell in 25 ml of water, either a t room temperature for 1 to 2 days or on a boiling water bath for 2 hours. Sod iztm chloride. Sodizm chloride solution, 0.3 M. Sodium hydroxide, pellets. Sodium nitrite solution-Dissolve 1 g of sodium nitrite in water and dilute to 50 ml. Sulphamic acid solution-Dissolve 2-5 g of sulphamic acid in water and dilute to 25 ml. Prepare this solution immediately before use. Prepare the solution freshly each day. PROCEDURE- Transfer 25 g of minced poultry meat into a 250-ml beaker, add 70 ml of methanol and macerate the mixture with a high-speed mixer for 2 minutes. Filter the extract through the Buchner funnel, which has previously been covered with a 2-mm layer of Celite.Repeat the extraction twice by macerating the sample with 70-ml portions of methanol and filtering through the Buchner funnel. Combine the filtrates and dilute to 250 ml with methanol.504 HOODLESS AND TARRANT : RESIDUES OF PROPHYLACTICS [A?ZdySt, VOl. 98 Transfer by pipette a 10 or 25-ml aliquot of this last solution to a small beaker and evaporate it to about 1 ml on a steam-bath. Add a few millilitres of water and pour the solution on to the top of a prepared Sephadex column. Allow the solution to pass through the column until the liquid level reaches the top of the column and collect the eluate in a receiver that is capable of measuring a volume of 100 ml.Wash out the beaker with small volumes of water and transfer the washings to the column, then pass water through the column at a rate of 1 to 2 ml min-l until 100 ml of eluate have been collected. Discard the eluate. Pass 0.3 M sodium chloride solution through the column at a rate of 1 to 2 ml min-l, reject the first 10 ml of eluate and collect the next 30 ml in a 150-ml flask. To the contents of the flask add 5 g of sodium hydroxide pellets and, when these have dissolved, boil the solution gently for 1.25 hours under a reflux condenser. Wash down the condenser with a few millilitres of water and cool the flask in a cold water bath. Add 30 ml of 5 M hydrochloric acid, mix, again cool in a cold water bath and transfer the acidified solution to a 100-ml separating funnel with the aid of a small volume of water.Add 1 ml of sodium nitrite solution, mix thoroughly and, after 3 minutes, add 1 ml of sulphamic acid solution. Allow the mixture to stand for 15 minutes, shaking it frequently so as to ensure complete destruction of the excess of sodium nitrite. Add 1 ml of the 2-aminoethyl-1-naphthylamine dihydrochloride solution and mix well. After 10 minutes add 10 ml of butan-1-01 and shake the mixture, then add 10 g of sodium chloride, again shake well and allow the layers to separate. Run the lower, aqueous layer into a second separating funnel and the butan-1-01 into a 25-ml calibrated flask. Extract the aqueous layer twice more, first with 10 ml and then with 5 ml of butan-1-01, add the extracts to the 25-ml calibrated flask and dilute to the mark with butan-1-01.Measure the absorption of this solution at a wavelength of 542 nm in a 4-cm cell against butan-1-01 as reference solution. Ascertain the amount of carbarsone present in the sample solution by reference to a standard graph. PREPARATION OF STANDARD GRAPH- Transfer 1, 2, 3 and 4-ml portions of the carbarsone standard solution, equivalent to 2.5, 5.0, 7.5 and 10.0 pg of carbarsone, to separate 150-ml flasks. Add 0.3 M sodium chloride solution to the contents of each flask until the volume of solution is 30 ml in each instance, and then proceed as described above, commencing at“. . . add 5 g of sodium hydroxide pellets.” The standard solutions should be run through the procedure at the same time as the sample with use of the same reagents.RESULTS AND DISCUSSION Carbarsone is soluble in water and methanol but is only sparingly soluble in other organic solvents. Methanol was found to be the most suitable solvent for the extraction of carbarsone from chicken tissue because with water or dilute acid increased problems were encountered in the filtration step, and no other suitable extraction procedure could be found. The recovery of carbarsone from chicken tissue was simulated by adding known volumes of a solution of carbarsone in methanol to weighed amounts of minced tissue and leaving them overnight before extraction as described under Procedure. The results are shown in Table I. The recovery appears to depend on the procedure used for spiking the samples, because recoveries varied with the length of time that elapsed between spiking and extraction.Recoveries of over 90 per cent. were obtained when carbarsone was added to a methanolic extract of chicken tissue or when the sample of tissue was extracted immediately after spiking, whereas, if the spiked sample was kept for 3 to 4 days before extraction, the recoveries were about 60 per cent. Consequently, it is important that samples are extracted immediately after being minced and the carbarsone content of the extract determined without delay. As is usual in residue work, it is difficult to obtain standard samples with known amounts of the compound under investigation incorporated in the tissue so as to enable absolute determinations to be made. Most chicken samples tested gave a small blank value, corresponding to 0-4 mg kg-l of carbarsone.Total arsenic determinations were carried out on chicken sample B (Table I) and the arsenic content was found to be less than 0.1 mg k g l . The history of these samples was not fully known, so it is possible that the blank value may be the result of an additive fed to the birds at some time during their lifetime.July, 19731 I N ANIMAL PRODUCTS. PART I11 TABLE I RECOVERY OF CARBARSONE FROM CHICKEN TISSUE Sample Carbarsone addedlmg kg-1 Carbarsone found/mg kg-l Recovery, per cent. A B C B D 0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0 3.0 3.0 3.0 3.0 3.0 3.0 0 3.0 3.0 3.0 0 6.0 6.0 6.0 6.0 6.0 6.0 0 6.0 6-0 6.0 0 0.75 0.7 0.7 0.75 0.7 0.7 0.7 0.7 0.75 0.45 2.5 2.5 2-6 2.6 2-6 2.6 0.4 2-8 2.6 2.35 0-45 4.75 4.5 4.75 4-65 4.75 4.5 0.4 4.4 4.7 4.4 - 75 70 70 75 70 70 70 70 75 Mean .. .. 72 68 68 72 72 72 72 Mean . . .. 71 80 73 65 Mean . . . . 73 72 68 72 68 72 68 Mean .. . . 70 67 72 67 Mean .. . . 69 - - - - 505 The yield of arsanilic acid from carbarsone is dependent on the concentration of sodium hydroxide and the hydrolysis time. Conversion into arsanilic acid was complete when 5 g of sodium hydroxide were added to the eluate from the Sephadex column and the solution heated for 1.25 hours. After formation of the dye by the coupling of diazotised arsanilic acid with 2-aminoethyl-l-naphthylamine, it was found necessary to extract the aqueous phase more than once with butan-1-01 in order to ensure complete extraction of the dye.Once the dye had been extracted into butan-1-01 it was stable for at least 3 hours. There was no interference from amprolium, dinitolmide, ethopabate or sulphaquinoxaline. Of the other arsenicals used in animal feedingstuffs, nitarsone (4-nitrophenylarsonic acid) and roxarsone (4-hydroxy-3-nitrophenylarsonic acid) did not interfere. However, arsanilic acid is eluted with carbarsone from the ion-exchange column and is determined by the proce- dure, but theinterference from this source can be overcome by applying the method to a second aliquot of the sample extract with which the hydrolysis stage has been omitted. The car- barsone content of the sample can then be calculated by subtracting the absorbance obtained without hydrolysis from the value obtained after hydrolysis. We thank the Government Chemist for permission to publish this paper. 1. 2. 3. Parker, C. A., “Photoluminescence of Solutions,” Elsevier Publishing Company, Amsterdam, 4. 6 . 6. REFERENCES “The Arsenic in Food Regulations,” S.I. 1959 No. 831, H.M. Stationery Office, London. Weston, R. E., Wheals, B. B., and Kensett, M. J., Analyst, 1971, 96, 601. London and New York, 1968, p. 419. Analytical Methods Committee, Analyst, 1971, 96, 817. El-Dib, M. A., J . Ass. 08. Analyt. Chevn., 1971, 54, 1383. Bratton, A. C., and Marshall, E. K., J . Biol. Chem., 1939, 128, 537. Received February 8th, 1973 Accepted Mawh 12t12, 1973

ISSN:0003-2654    

DOI:10.1039/AN9739800502

出版商:RSC    

年代:1973

数据来源: RSC

摘要:

506 Analyst, July, 1973, Vol. 98, pj4. 506-511 The Determination of Microgram Amounts of Sulphate by Emission Spectroscopy of Barium with a Nitrous Oxide - Acetylene Flame BY E. A. FORBES (Ruakura Soil lPesearch Station, Pvivate Bag, Hamilton, New Zealand) The sulphate content of aqueous solutions has been determined in- directly, in the ranges 0.5 to 5-0 and 1.0 to 10-0 p.p.m., from barium emission measurements. By using a slight excess of barium, the sulphate is precipi- tated in a 50 per cent. solution of propan-2-01 and its concentration is calcu- lated from the decrease in the barium content of the solution. The amount of barium in solution is determined from its emission at 553.55 nm in a nitrous oxide - acetylene flame. The only major flame interference detected is that from the band emission of calcium; 410 p.p.m.of calcium gave an emission intensity equal to that of 1 p.p.m. of barium. Sulphate has been determined in both pure solutions and in synthetic sample solutions containing other electrolytes. Major interferences were noted for potassium and ammonium oxalate, sodium orthovanadate, nickel chloride and to a lesser extent for sodium fluoride and perchloric acid. The sulphur content of biological material, digested by oxygen-flask combustion, has been determined satisfac- torily by using this method. THE determination of barium as a method of indirectly measuring microgram amounts of sulphate has, in the past, had limited application owing to interferences and the low sensitivity of the techniquesl-7 available for determining barium, which have made the method relatively unattractive.Some p i - ~ c e d ~ r e ~ , ~ ~ g while possessing sensitivity, have not been amenable to rapid routine analysis because of the need to use radioisotopes. However, recent worklo with the nitrous oxide - acetylene flame has errabled a sensitive, non-radiochemical method for measuring barium to be produced, which is comparatively free fron interferences. A simple procedure is dcscrihed here, which depends on the precipitation of barium sulphate in a 50 pcr cent. solrit.,on or" proym-2-o!. This nicdium was chosen because it reduces the solubility of barium sulphrzee to an aciep-tablte levcl, and because it can be sprayed into a nitrous oxide - acetykn.: urcd by sclecting an amount of barium sligiiliy in excess 0:' that which is r,)tyrilvnlcnt to ti1 : mctximum amount of sulphate.Following precipkLtion xiid cent\ Zfugation, Ltx unr d II.LI iurn content of the solution can be determined from its erni5s;on intensity at 553-35 n:n in the nitrous oxide - zcetylene flame. It has b x n found that this method can be US::^ l o - the measurement o€ sulphate concentrations in the prcsenice of an excess of many electrolytes. ithout c~usin:; institb.1 i ty. conn=eali-a, ;0.1s can be rn A particul:ar r~ngi: of s EXPERIMENTAL I K STR u M E N TA I,- A Techtmn AA4 spectrophotometer, with a reciproc:il clisymion o€ 3.3 nrn mmn-l, was operated with a flame-emission accessory and a slit width of 50 pm. The nitrous oxide - acetylene flame was formed on a Techtroa AB50 b w ncr.'Xhe ni trous oxide pressure was maintained at 26 p.s.i. whil:: the acety:?.lit: flow was adju;'i?d to give maximum barium emission. Measztvemcnt of cmissZo;?,-Aikr a &minute w:ixn-im rci i d the tcst solutions were introduced into the dame through a 'Techtron nebuliser and spray chamber. The flame was viewed lengthwise m d the emission intensity of the atomic i-e'anance line at 553-55 nm was measured without noise s u p p ~ ~ i o r . The ccnission signal iroin barium in a 50 per cent. solution of prop;)n-Z-ol was as stable as that originating from aqueous solution. However, the approach to a steady rcading following sample changes was considerably slower than that for an aqueous solution. @ SAC and the author.FORBES 507 REAGENTS- Analytical-reagent grade chemicals were used throughout.Standavd barium solution-Dissolve 0.9782 g of barium chloride diliydrate in distilled water and dilute to 1 litre to give a stock solution containing 550.0 p.p.m. of barium. Dilute 5 and 10-ml volumes of the stock solution to 100 ml so as to give working solutions containing 27.5 and 55.0 p.p.m. of barium, respectively. .Stock sulphate solution, 1000 p.p.m.-Dissolve 1.814 g of potassium sulphate in distilled water and dilute the solution to 1 litre. Prepare working solutions in the concentration range 0.5 to 10 p.p.m. of sulphate by appropriately diluting the stock solution. Chloroacetic acid - potassium hydroxide solution-Dissolve 47-25 g of chloroacetic acid in distilled water. Add 3.40 g of potassium hydroxide and dilute the solution to 250 nil.Filter the solution through Whatman No. 542 filter-paper before use. ProPaia-2-ol. RECOMMENDED PROCEDURE- Quickfit conical flasks with ground-glass stoppers were used as reaction vessels. Reaction volumes were prepared by mixing 10 ml of aqueous sulphate solution, 15 ml of propan-2-01, 2 ml of chloroacetic acid - potassium hydroxide solution and 3 ml of standard barium chloride solution. (The barium concentration of the standard solution is chosen so as to accommodate the required sulphate range.) For the ranges 0-5 to 5.0 and 1.0 to 10.0 p.p.m. of sulphate it is recommended that solutions containing 27.5 and 55-0 p.p.m. of barium, respectively, be used. The flasks were agitated for 15 hours, and the barium sulphate suspension was centrifuged at 3500g for 20 minutes.Aliquots of the supernatant liquid were then taken for emission analysis. Although the two ranges of sulphate concentration given above overlap considerably, the use of the weaker barium solution is advantageous for smaller sulphate concentrations, particularly at and below the 1 p.p.m. level, because of the nature of the difference method used. DEVELOPMENT OF THE METHOD ENHANCEMENT OF BARIUM EMISSION BY POTASSIUM- Results similar to those reportedlo for 1 p.p.m. of barium in aqueous solution were obtained for 1 p.p.m. of barium in a 50 per cent. solution of propan-2-01. The barium emission intensity obtained in the presence of 600 p.p.m. of potassium (the concentration of potassium obtained from the chloroacetic acid - potassium hydroxide solution) was 97 per cent.of that obtained in the presence of 1000 p.p.m. of potassium. EMISSION INTENSITY AS RELATED TO BARIUM CONCENTRATION- over a wide concentration range (Table I). for a zero addition of barium, relative to 2.42 and 22.3 p.p.m. of barium. The relative emission intensity was found to be linearly related to barium concentration The background emission at 553.55 nm originating from the flame and reagents is shown TABLE I VARIATION OF EMISSION INTENSITY WITH BARIUM CONCENTRATION Barium concentration, p.p.m. 0 0.17 0.34 0.81 1-20 1.61 2.42 Relative emission intensity* 6 13.5 20.2 37.5 54.5 70 100 Barium concentration, p.p.m. 0 1.72 3.44 5.15 8.58 12.0 17.2 22.3 Relative emission intensityt 0.8 9-8 17-5 26 43 58 80 100 * Relative t o 2.42 p.p.m.of barium = 100. t Relative to 22-3 p.p.in. of barium = 100.508 FORBES: DETERMINATION OF MICROGRAMS OF SULPHATE BY EMISSION [A?Zdyst, VOl. 98 ENHANCEMENT OF BARIUM EMISSION BY PHOSPHATE- The effect of phosphate (as potassium dihydrogen orthophosphate) on the emission intensity produced by 2.7 and 5.3 p.p.m. of barium was measured for amounts containing 0 to 10mg of phosphorus. The presence of 4mg of phosphorus in solution produced an enhancement of the order of 2 per cent. for both barium concentrations. A maximum enhancement of 8 per cent. was obtained from 10 mg of phosphorus in a solution containing 2.7 p.p.m. of barium. The total volume of each solution was 30 ml. The results given later in Table VII for added potassium chloride (22 mg) show that this enhancement is not due to the suppression of barium ionisation by potassium.CALCIUM EMISSION AT 553.55 nm IN THE NITROUS OXIDE - ACETYLENE FLAME- The emission intensity from calcium in a 50 per cent. solution of propan-2-01 in the presence of potassium was linearly related to concentration, but the intensity was reduced by a factor of 2.6 when the Techtron burner head AB40 was replaced with the Model AB50. While using the latter the net emission from 410 p.p.m. of calcium was identical with that produced by 1 p.p.m. of barium. EFFECT OF REACTION TIME ON PRECIPITATION- A factor that greatly affects the measurement of sulphate concentration is the rate of growth of barium sulphate crystals during precipitation. By using sulphate labelled with sulphur-35 it was possible to measure the efficiency of removal of sulphate from solution as barium sulphate.The efficiencies obtained by centrifuging at 3500 g are given in Table I1 as functions of sulphate concentration, reaction time and degree of agitation of the solution. It is evident from Table I1 that, by agitating the solutions during precipitation, the efficiency of sulphate separation at low concentrations was increased. A high efficiency of separation a t all concentrations in the range 0.5 to 5.0 p.p.m. of sulphate was obtained when the reactants were agitated for 15 hours. Maximum efficiency was achieved within 15 hours for all concentrationi in PERCENTAGE Reaction time/hours the range 1.0 to 10.0 p.p.m. of sulphate. TABLE I1 OF SULPHUR-% ACTIVITY REMOVED BY CENTRIFUGATION FOLLOWING PRECIPITATION Unstirred solution Agitated solution A r 7 15 * .... 14 90 0.5 2.5 Sulphate concentration, p.p.m. 0.5 75 92 44 67 94 1.01 88 95 58 84 96 2.01 91 97 74 93 98 4-03 95 97 -* -* 97 5-04 97 97 77 94 96 * Not determined. RESULTS SULPHATE DETERMINATION IN STANDARD SOLUTIONS- The sulphate contents of dilute potassium sulphate solutions were measured by using standard barium chloride solutions containing 24.9 and 49.7 p.p.m. of barium. The more dilute barium solution was used for the range 0-5 to 5.0 p.p.m. of sulphate. Table I11 gives the emission from the unreacted barium for various sulphate concen- trations. The theoretical emission intensities, calculated for stoicheiometric reactions in which all of the sulphate is precipitated, are given in Table I11 for comparison.As barium sulphate has an acceptably small but nevertheless finite solubility in a 50 per cent. solution of propan-2-01, the measured emission values (adjusted for flame background) should, in principle, be slightly greater than the theoretical values given in Table 111. EFFECT OF PHOSPHATE ON PRECIPITATION- The presence of phosphate (as potassium dihydrogen orthophosphate) during the pre- cipitation reaction caused, in some instances, a reduction in the residual emission intensity.July, 19731 SPECTROSCOPY OF BARIUM WITH A NITROUS OXIDE - ACETYLENE FLAME TABLE I11 RESIDUAL EMISSION INTENSITY AS A FUNCTION OF SULPHATE CONCENTRATION 509 Emission Sulphate* 7- Sulphatet concentration, Measured concentration, p.p.m. (adjusted) Theoretical p.p.m.0 100 100 0 0-81 84.5 84.6 1.22 1.22 76.5 76.6 2-03 2.03 60.0 61.1 4-05 4.05 19.5 22.4 4.87 4.87 6.2 6.7 6-07 9.33 * Precipitated with a solution containing 24.9 p.p.m. of barium. t Precipitated with a solution containing 49.7 p.p.rn. of barium. Emission 1 Measured (adjusted) Theoretical 89-0 88.3 82.0 80.5 62.5 61.1 54.5 53.3 42-5 41.7 11.0 10-4 100 100 By precipitating in the presence of 2.5 mg of phosphorus, the effect of the phosphate on the determination of sulphate in the ranges 0.5 to 5.0 and 1.0 to 10.0 p.p.m. of sulphate, by using solutions containing 26.6 and 53.2 p.p.m. of barium, respectively, was ascertained. The results are given in Table IV. TABLE IV EFFECT OF 2.5 mg OF PHOSPHORUS AS PHOSPHATE ON RESIDUAL EMISSION INTENSITY Residual emission intensity Sulphate* - Sulphatet concentration, Measured concentration, p.p,m.(adjusted) Theoretical p.p.m. 0 100 100 0 2.5 76.0 77-6 0.5 5.0 51.0 55-2 1.0 6.0 41.3 46.2 2.0 7.5 27.3 32.7 3.0 9.0 16.9 19.3 5.0 * Precipitated with a solution containing 53-2 p.p.m. of barium. t Precipitated with a solution containing 26.6 p.p.ni. of barium. 10.0 9.9 10.3 Residual emission intensity r------7 Measured (adjusted) Theoretical 90.3 91.0 81.2 82.1 62.8 64.1 45.6 46.2 12.5 10.3 100 100 The presence of 2.5 mg of phosphorus reduced the residual barium emission intensity obtained for the concentration range 1 to 10 p.p.m. of sulphate, the maximum reduction in intensity occurring at 6.0 p.p.m. of sulphate. In contrast, a negligible effect was observed for the range 0-5 to 5 p.p.m.of sulphate. The reduction in residual barium emission following a precipitation is in contrast with the enhanced barium emission in the presence of phosphate referred to earlier. It is believed that phosphate becomes involved in the precipitation process and that a small amount of barium, in addition to that removed stoicheiometrically with sulphate, is lost from solution. The influence of different amounts of phosphate on the residual emission intensity obtained for a single sulphate concentration in the range 1 to 10 p.p.m. of sulphate is given in Table V. In order to obtain the maximum variation in results an aliquot containing 6-15 p.p.m. of sulphate was allowed to react with 3 ml of a solution containing 55.21 p.p.m. of barium (see Table IV).Table V shows that 0.125 mg of phosphorus produces an effect of about 2 per cent. An increase from 0.125 to 1.00 mg of phosphorus resulted in a further reduction in emission intensity, while the change from 1.00 to 2-63 mg had no additional effect. By reducing the residual barium emission intensity, phosphate causes an overestimation of the sulphate concentration. At the sulphate concentration that gives maximum phosphate interference the presence of as much as 0.125 mg of phosphorus as phosphate can be tolerated. Even when 2.63 mg of phosphorus as phosphate is present the sulphate concentration is overestimated by only 9 per cent. The effect of phosphate on the determination of sulphate can be partially eliminated by preparing a standard graph from sulphate solutions containing phosphate.However, this standard graph will not be linear.510 FORBES: DETERMINATION OF MICROGRAMS OF SULPHATE BY EMISSION [Analyst, Vol. 98 TAUI-E V VARIATION OF RESIDUAL EMISSION INTEKSITY wIm PHOSPHATE CONTENT Phosphate contentlmg of phosphorus . . 0 0.125 0.25 0-50 1.00 2-63 Residual emission intensity . . . . 50.3 48.7 47.9 46-9 45.7 45.7 INFLUENCE OF OTHER ELECTROLYTES ON PRECIPITATION- Microgram amounts of sulphate were precipitated in the presence of different electrolytes in order to determine the possible effect of each on the measurement of sulphate concentrations by this procedure. The effect of each salt, with the exception of calcium nitrate, was examined for only one sulphate concentration (see Table VI). In Table VII the effect of several other salts, at a single concentration, is given so as to indicate interferences arising from reaction with barium alone or from an effect on the precipitation reaction.TABLE VI EFFECT OF ELECTROLYTES ON RESIDUAL BARIUM EMISSION INTENSITIES OBTAINED FROM THE PRECIPITATION OF 4.62 p.p.m. OF SULPHATE Sodium Residual barium fluoridefmg emission intensity 0 62.8 1.5 61.7 3 62-3 6 56.6 9 51.5 15 40.8 Perchloric Residual barium acid/mmol emission intensity 0 61.4 0.24 60.4 0.47 61.4 0.93 81.9 1.39 67-5 2.32 75.6 Potassium oxalatelmg 0 1.0 2.1 4.1 6.15 10.25 Residual barium emission intensity 61.4 59.4 57.9 55.8 54.3 51.S Calcium ztitvate-The effect of 2 mg of calcium [as Ca(N0,),.4H20] on the precipitation was determined for several sulphate concentrations in the range 1 to 10 p.p.m.of sulphate. It was found that, after subtracting the calcium component of the emission, the residual barium emission intensity obtained for each sulphate concentration was identical, within experimental error, with that given in Table 111. TABLE VII INDICATIONS OF THE EFFECT OF OTHER SALTS ON SULPHATE DETERMINATIONS Relative effect on Relative effect on barium alone* sulphate precipitation? Salt Amount/mg (emission units) (emission units) None .. . . .. . . - 50 62.3 Magnesium chloride hexahydrate 16 49 61.7 Cobalt chloride hexahydrate . . 20 49.6 61.2 Nickel chloride hexahpdrate . . 20 44 50.0 Sodium orthovanadate . . . . 6 (approx.) 46 51.0 Sodium acetate trihydrate . . 24 48.5 61-53 Sodium citrate dihydrate . . . . 8 48 58.7 Potassium chloride .... 22 50 62-8 Potassium iodide . . .. .. 6 49.7 59.7 Potassium carbonate . . .. 10 51 61-7 Potassium hydrogen tartrate . . 6 50.5 60.2 Ammonium chloride . . .. 15 49 59.7 Ammonium molybdate . . .. 6 49 60.7 Aluminium chloride hexahydrate 45 44-5 54.1 Citric acid . . .. .. .. 22 48-5 57.6 Ammonium oxalate monohydrate 20 48 47.5 * The salts were present in 50 per cent. solutions of propan-2-01 containing chloroacetic acid - potassium hydroxide and 5.52 p.p.m. of barium. The solutions were agitated for 14 hours and centrifuged a t 3500g. Aliquots of the supernatant solution were then analysed. ? The emission intensity values were obtained from the analysis of a 10-ml aliquot of a 4.62 p.p.m. sulphate solution by using 3 ml of 55.21 p.p.m.barium solution. The same procedure as that described in the above footnote was used. Sodium cfEuoride-Amounts of sodium fluoride up to 3 mg had no effect on the deter- mination of 4.62 p.p.m. of sulphate in the range 1 to 10 p.p.m. of sulphate. With amountsJuly, 19731 SPECTROSCOPY OF BARIUM WITH A NITROUS OXIDE - ACETYLENE FLAME 51 1 of sodium fluoride above 3 mg the residual barium emission intensity decreased linearly with concentration (see Table VI). Perchloric acid-The residual barium emission intensity was unaffected by the presence of 0-93 mmol of perchloric acid during the precipitation reaction. Greater amounts of perchloric acid apparently increase the solubility of barium sulphate in the medium (see Table VI). Potassiztm oxalate-A progressive decrease in the residual barium emission intensity was obtained for amounts of potassium oxalate increasing from 1 to 10mg (see Table VI).Sodi.um chloride and sodium nitrate-Amounts of these salts up to 20 mg of each had no effect on the residual barium emission intensity obtained for 4-62 p.p.m. of sulphate. ANALYSIS OF BIOLOGICAL MATERIAL- The sulphur content of sixteen samples of biological material including mixed pasture, lucerne, grass, onion, fruit tree leaves, dried blood and faeces has been determined. The samples were digested by oxygen-flask combustion followed by absorption in dilute sodium hydroxide solution. Aliquots of this solution were analysed by the above procedure. Eight samples were analysed in duplicate, six in triplicate and two in quadruplicate. The replicates were prepared from fresh sub-samples of material (not from the same solution) and at least one replicate was produced by a different analyst. The sulphur content of the dry matter ranged from 0.135 to 0-804 per cent. The over-all coefficient of variance was 3-3 per cent.and the standard deviation was 0.0126. CONCLUSION Barium emission in the nitrous oxide - acetylene flame provides a satisfactory means of determining microgram amounts of sulphate indirectly. Smaller solution volumes can be analysed by this procedure simply by reducing by a constant factor the sample volume and the reagent volumes given in this paper. The minimum volume is limited only by the amount required for an accurate emission reading. The sulphur content of biological material, digested by oxygen-flask combustion, has been satisfactorily determined by using this method.An alternative digestion procedure, which involves the use of nitric and perchloric acids, can be used provided the perchloric acid concentration in the digest is reduced sufficiently by dilution prior to the precipitation of barium sulphate. In principle, atomic-absorption spectroscopy could be applied in a similar way to determine the residual barium in solution after precipitation of the sulphate, but this possibility has not been explored in the present work. The author gratdully acknowledges the assistance given by the late Mr. J. E. Allan, who also suggested the idea on which this paper is bascd. The author also thanks Dr. J . H. Watkinson for providing the results for the determination of sulpliur in l~io’logical material. REFEREKES 1. 2. 3. -- - , Ibid., 1960, 85, 688. 4. 5. 6. 7. 8. 9. 10. Burriel-Marti, F., Rarnirez-Mufioz, J., and Rexach-M. cle Lizarduy, M. L., Analytica C h k . Acta, Cullutn, D. C., and Thomas, D. R., Analyst, 1959, 84, 113. Alt, b., Laadzo. Forsch., 1964, 16, 278. Strickland, R. D., and Maloney, C. M., Amer. J . Clin. Path., 1954, 24, 1100. Roe, D. h., Miller, P. S., and Lutwak, L., AylaZyt. Rimhem., 1966, 15, 313. Dunk, R., Mostyn, R. A., and Hoare, H. C . , Atom. Absovptmn Newsl., 1969, 8, 79. Picou, D., and Waterlow, J. C., Nature, Lond., 1963, 197, 1103. Goode, E. V., Analyst, 1968, 93, 663. Koirtyohann, S . R., and Pickett, E. I<., Spectvochim. Acta, 1968, 23B, 673. 1957, 17, 559. Received January 22nd, 1973 Accepted March Znd, 1973

ISSN:0003-2654    

DOI:10.1039/AN9739800506

出版商:RSC    

年代:1973

数据来源: RSC