摘要:

ANALYSTTHE ANALYTICAL JOURNAL OF THE CHEMICAL SOCIETYEDiTORlAL ADVISORY BOARD"Chairman: J. M. Ottaway (Glasgow, U.K.)R. Belcher (Birmingham, U.K.)L. J. Bellamy, C.B.E. (Waltham Abbey, U.K.)L. S. Birks (U.S.A.)E. Bishop (Exeter, U.K.)L. R. P. Butler (South Africa)"H. J. Cluley (WemMey, U.K.)E. A. M. F. Dahmen (The Netherlands)A. C. Docherty (Bi/lingham, U.K.)D. Dyrssen (Sweden)'P. Gray (Leeds, U.K.)J. Hoste (Belgium)H. M. N. H. Irving (South Africa)M. T. Kelley (U.S.A.)W. Kemula (Poland)'J. H. Knox (Edinburgh, U.K.)"J. N. Miller ( L oughborough, U.K.)G. W. C. Milner (Harwell, U.K.)G. H. Morrison (U.S.A.)H. W. Nurnberg (West Germany)"G. E. Penketh (Wilton, U.K.)"T. B. Pierce (Harwell, U.K.)E. Pungor (Hungary)D. I. Rees (London, U.K.)P.H. Scholes (Middlesbrougy, U.K.)'W. H. C. Shaw (Greenford, U.K.)S. Siggia (U.S.A.)"D. Simpson (Thorpe-le-Soken, U.K.)A. A. Smales,O.B.E. (Thornaby, U.K.)'A. Townshend (Birmingham, U.K.)A. WaIsh (Austraiia)T. S. West (Aberdeen, U.K.)"J. Whitehead (Stockton-on- Tees,A. L. Wilson (Medmenham, U.K.)P. Zuman (U.S.A.)U. K. )"Members of the Board serving on The Analyst Publications CommitteeREGIONAL ADVISORY EDITORSDr. J. Aggett, Department of Chemistry, University of Auckland, Private Bag, Auckland, NEW ZEALAND.Professor G. Ghersini, Laboratori CISE, Casella Postale 3986, 201 00 Milano, ITALY.Professor L. Gierst, Universite Libre de Bruxelles, Facult4 des Sciences, Avenue F.- D. Roosevelt 50,Professor R. Herrmann, Abteilung fur Med.Physik., 63 Giessen, Schlangenzahl 14, W. GERMANY.Professor W. A. E. McBryde, Faculty of Science, University of Waterloo, Waterloo, Ontario, CANADA.Dr. W. Wayne Meinke, KMS Fusion Inc., 3941 Research Park Drive, P.O. Box 1567, Ann Arbor,Dr. I. RubeGka, Geological Survey of Czechoslovakia, Kostelni 26, Praha 7, CZECHOSLOVAKIA.Professor J . R&i6ka, Chemistry Department A, Technical University of Denmark, 2800 Lyngby,Professor K. Saito, Department of Chemistry, Tohoku University, Sendai, JAPAN.Dr. A. Strasheim. National Physical Research Laboratory, P.O. Box 395, Pretoria, SOUTH AFRICA.Bruxelles, BELGl U M.Mich. 481 06, U.S.A.DENMARK.Published by The Chemical SocietyEditorial: The Direcror of Publications, The Chemical Society, Burlington House,London, W1V OBN. Telephone 01 -734 9864. Telex No. 268001Advertisements: Advertisement Department, The Chemical Society, Burlington House, Piccadilly,London, W1 V OBN. Telephone 01 -734 9864Subscriptions (non-members) : The Chemical Society, Distribution Centre, Blackhorse Road,Letchworth, Herts., SG6 I HNVolume 104 No 1241 August 1979cQ The Chemical Society 197

ISSN:0003-2654    

DOI:10.1039/AN97904FX029

出版商:RSC    

年代:1979

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ANALAO 104 (1 241 ) 705-800 (1 979)ISSN 0003-2654August 1979THE ANALYSTTHE ANALYTICAL JOURNAL OF THE CHEMICAL SOCIETYCONTENTS705 Electrochemical Studies of Strongly Chelating Anthraquinone Derivatives-G. Ali Qureshi, G. Svehla and M. A. Leonard723 Direct Differential-pulse Polarographic Determination o f Mixtures o f theFood Colouring Matters Tartrazine - Sunset Yellow FCF, Tartrazine - GreenS and Amaranth - Green S in Soft Drinks-A. G. Fogg and K. S. Yo0730 Ion-selective Polymeric-membrane Electrodes with lmmobilised lon-exchangeDevelopment of a Calcium Electrode-L. Ebdon, A. T. Ellis Sites.and G. C. CorfieldPart I.739 High-frequency Microtitrimetric Determination o f Acidic and Basic Constituentsin Lubricating Oils. Part II. Determination o f Total Base Number-T.FernBndez, J.M. Rocha, N. Rufino, A. Garcia Luis and F. Garcia Montelongo750 Continuous Solvent-extraction Method f o r the Spectrophotometric Deter-mination o f Cationic Surfactants-J iro Kawase and Makoto Yamanaka756 Automatic Emission Spectrometer f o r the Determination of Nitrogen-I 5-J. D. S. Goulden and D. N. Salter766 Studies in Gas Chromatography - Chemical Ionisation Mass Spectrometry o fSome Silicate Anions-J. B. Addison771 Internal Standardisation and i t s Value in the Assessment o f the Suitability o fthe Co I u m n f o r Qua n t i t a t i ve H i g h - perform an ce L i q u i d C h r o ma t o g ra p hy-R. A. MooreREPORT BY THE ANALYTICAL METHODS COMMITTEEDetermination of Small Amounts o f Selenium in Organic Matter 778SHORT PAPERS788 Hydrocarbon Analysis of Naphtha Using Capillary-column Gas Chromato-graphy Under Isothermal Conditions-Pradeep Kumar, S. L. S. Sarowha andP. L. Gupta792 The Impact Cup: A Simple Aid in Flame Spectrometric Analysis a t High AnalyteConcentrations-Malcolm S. Cresser797 Book ReviewsSummaries o f Papers in this Issue-Pages iv, vi, vii, x, x i iPrinted by Heffers Printers Ltd Cambridge EnglandEntered as Second Class a t New York, USA, Post Offic

ISSN:0003-2654    

DOI:10.1039/AN97904BX031

出版商:RSC    

年代:1979

数据来源: RSC

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1v SUMMARIES OF PAPERS I N THIS ISSUE August, 1979Summaries of Papers in this IssueElectrochemical Studies of Strongly Chelating AnthraquinoneDerivativesAnalytically important, strongly chelating anthraquinones and their deriva-tives were studied by d.c. polarography, cyclic voltammetry and micro-coulometry to investigate their redox characteristics. All 18 substanceswere reduced in a two-electron reversible or quasi-reversible process in bothaqueous and 75% ethanolic solutions. Depending on pH and the medium,single or double polarographic reduction waves appeared, which were diffusioncontrolled, although in some instances adsorption pre-waves were alsoobserved. This behaviour is similar to the known behaviour of simplerquinone systems. The variation of the IlkoviC coefficient and half-wavepotential with pH was studied in detail to investigate the acid - base behaviourof the species involved in the reduction process.As a result, it was possibleto describe the reduction mechanism of the anthraquinones involved.A number of new pK values were determined and others confirmed.Attempts to find linear free energy relationships were generally unsuccessful.Keywords ; Anthraquinone derivatives ; polarography ; cyclic voltammetry ;half-wave potentials; PIC. valuesG. ALI QURESHI, G. SVEHLA and M. A. LEONARDDepartment of Analytical Chemistry, The Queen’s University, Belfast, NorthernIreland, BT9 5AG.Analyst, 1979, 104, 705-722.Direct Differential-pulse Polarographic Determination of Mixturesof the Food Colouring Matters Tartrazinq - Sunset Yellow FCF,Tartrazine - Green S and Amaranth - Green S in Soft DrinksTartrazine and Sunset Yellow FCF can be determined directly in orangeadeby differential-pulse (d.p.) polarography on the addition of pH 9 Britton -Robinson buffer and tetraphenylphosphonium chloride. The tetraphenyl-phosphonium chloride removes the large polarographic maximum obtainedwith tartrazine a t pH > 4 and causes the d.p.polarographic peaks of the twocolouring matters to be separated.Tartrazine in limeade can be determined in a similar supporting electrolytebut these conditions are not suitable for the determination of Green S, whichis usually present a t a low concentration relative to the tartrazine and forwhich the d.p.polarographic peak is depressed by the addition of tetra-phenylphosphonium chloride. Green S can be determined after adding pH 4Britton - Robinson buffer and tetramethylammonium chloride to the limeade :the addition of tetramethylammonium chloride gives a better base line inthe presence of tartrazine. The solution is then re-adjusted to pH 9 andtetraphenylphosphonium chloride is added in order to determine thetartrazine.At pH 4 the sugar present in blackcurrant syrup gives a small d.p. polaro-graphic peak at the same potential as Green S. At pH > 6 the peak of thesugar disappears but amaranth gives a broad polarographic maximum. Thismaximum is suppressed at pH 7.8 by the addition of tetramethylammoniumchloride. Under these conditions the Green S peak is separate but the smallconcentrations of Green S normally present in blackcurrant drinks can onlyjust be detected.The procedures have been tested on soft drinks prepared with knownconcentrations of colouring matter.Keywords : Difleerential-pulse Polavogeraphy ; food colouring matters ; tavtvazine ;Green S ; amaranthA.G. FOGG and K. S. YO0Chemistry Department, Loughborough University of Technology, Loughborough,Leicestershire, LE11 3TU.Analyst, 1979, 104, 723-729vi SUMMARIES OF PAPERS I N THIS ISSUEIon-selective Polymeric-membrane Electrodes with ImmobilisedIon- exchange SitesPart I. Development of a Calcium ElectrodeAugust, 19 79A new type of ion-selective electrode is described in which ion-exchange sitesare immobilised in a polymeric membrane by covalent bonding.Membraneswere prepared by cross-linking a styrene - butadiene - styrene triblockcopolymer with triallyl phosphate. After subsequent hydrolysis thesemembranes were evaluated as Ca2+ sensors. The electrodes formed exhibitedan extended Nernstian response to Ca2+ ( 10-1-10-6 M) and good selectivityover other alkaline earth and alkali metals. Such electrodes offer very fastresponse times, extended lifetimes and a wide pH working range. Thepossibilities of this new class of ion-selective electrode are also discussed.Keywords : Ion-selective electrode ; calcium analysis ; electroanalytical chemistry ;polymeric membrane ; immobilised ion-exchange groups.L. EBDON, A. T. ELLIS and G. C. CORFIELDDepartment of Chemistry, She ffield City Polytechnic, Pond Street, Sheffield, S 1 1 WB.Analyst, 1979, 104, 730-738.High-frequency Microtitrimetric Determination of Acidic andBasic Constituents in Lubricating OilsPart 11.Determination of Total Base NumberA high-frequency microtitration method is described for the determinationof total base number in new and used lubricating oils. The sample, dissolvedin toluene - propan-2-01 - water (5 + 4.95 + 0.05), is titrated with standardalcoholic hydrochloric acid solution. A study of solvents, titrants, samplesize and other experimental parameters is reported. This method providessharp breaks at the end-points of the titration graphs and more repeatableand faster results than those obtained with the standard potentiometricmethod. The method can also be used for macrotitrations, with a com-mercially available oscillator, and can be suggested as an alternative toASTM and IP methods for the determination of total base number.Keywords : Total base number determination ; lubricating oils ; high-frequencytitrationT.FERNANDEZ, J. M. ROCHA, N. RUFINO and A. GARCfA LUISCompaiiia Espaiiola de Petrbleos, S.A., Research Laboratory, Santa Cruz de Tenerife,Canary Islands.and F. GARCfA MONTELONGODepartment of Analytical Chemistry, University of La Laguna, Tenerife, CanaryIslands.Analyst, 1979, 104, 739-749August, 1979 SUMMARIES OF PAPERS I N THIS ISSUEContinuous Solvent-extraction Method for the SpectrophotometricDetermination of Cationic SurfactantsviiA rapid and automated spectrophotometric method for the determination ofcationic surfactants, using the AutoAnalyzer, has been developed.Thismethod is based on the continuous solvent extraction of the ion-pair complexformed in the reaction of Orange I1 with a cationic surfactant. Good tracesand identical molar responses were obtained with seven different types ofsurfactant using methanol in the Orange I1 reagent. Fatty amines inmixtures of the amines and quaternary ammonium surfactants were deter-mined by changing the pH of the aqueous phase. The proposed method wasapplied to the determination of cationic surfactants in several commercialproducts. The results agreed with those obtained by the two-phase titrationprocedure.The detection limit is 5 p~ and the capacity is 10-20 samplesper hour, with a relative precision of better than 1.5%.Keywords : A utomatic analysis ; cationic surfactant determination ; fattyamine determination ; spectrophotometry ; Orange I IJIRO KAWASE and MAKOTO YAMANAKATochigi Research Laboratories, Kao Soap Co. Ltd., 2606 Akabane, Ichikai-machi,Tochigi, Japan.Analyst, 1979, 104, 750-755.Automatic Emission Spectrometer for the Determinationof Nitrogen- 15An automatic nitrogen-15 analyser, employing a novel use of a rhodium -platinum catalyst for the generation of nitrogen and capable of analysing 60samples per hour, is described. Nitrogen compounds of biological origin arefirst converted into ammonium chloride by conventional Kj eldahl digestionand distillation methods.The ammonium chloride sample (about 5 p1containing about 10 pg of nitrogen) is injected into a soda-lime reactor at590 "C through which flows a stream of purified helium. Ammonia that isreleased passes directly into the catalyst tube and the generated nitrogenand hydrogen are separated by passage through a gas-chromatographiccolumn, which also retains the water.After passing through a pressure restrictor the nitrogen flows in thehelium stream through a Spectrosil discharge tube located in a microwavecavity. The emitted radiation is analysed by means of a specially con-structed dual-wavelength monochromator and the intensities of the 14N14N(297.7 nm) and 14N15N (298.3 nm) bands are measured simultaneously bytwo photomultipliers.Amplified signals, proportional to the peak intensities,are fed through phase-sensitive detectors into a ratiometer, the output fromwhich is fed to a digital voltmeter and printed out in terms of nitrogen-15abundance. A peak detector indicates the total nitrogen content of eachsample and actuates the nitrogen-15 print-out.The response of the instrument is slightly curvilinear but may be regardedas linear over limited ranges. Calibration can therefore be achieved byrunning suitably chosen standards to fix upper and lower set points. Carry-over between samples is very small and is eliminated by running duplicates.Standard deviations of replicate measurements of natural abundance andenriched standards are less than 0.01 atom-%, while determinations ofnitrogen-15 in biological samples were shown to be accurate to 50.01atom- yo by comparison with a Statron NOI-4 nitrogen- 15 analyser.Keywords : Nitrogen- 15 determination ; catalytic nitrogen generation ; auto-mated emission spectrometerJ. D. S. GOULDEN and D. N. SALTERNational Institute for Research in Dairying, Shinfield, Reading, RG2 9AT.Analyst, 1979, 104, 756-765

ISSN:0003-2654    

DOI:10.1039/AN97904FP061

出版商:RSC    

年代:1979

数据来源: RSC

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X SUMMARIES OF PAPERS I N THIS ISSUEStudies in Gas Chromatography - Chemical IonisationMass Spectrometry of Some Silicate AnionsAugust, 1979Chemical ionisation mass spectrometry of trimethylsilyl derivatives of theanions SO,*-, Si,O,6-,Si,O06- and Si, OIo8- has been studied using isobutaneas the reagent gas. The protonated molecular ion was observed in all recordedspectra. Also, the chemical ionisation spectra showed much fewer fragmentpeaks when compared with the corresponding electron impact spectra.Keywords : Chemical ionisation mass spectrometry ; silicate anionsJ. B. ADDISONAtlantic Regional Laboratory, National Research Council of Canada, Halifax, NovaScotia, Canada, B3H 321.Analyst, 1979, 104, 766-770.Internal Standardisation and its Value in the Assessment of theSuitability of the Column for Quantitative High-performanceLiquid ChromatographyThe requirement that the best precision be obtained for a pharmaceuticalraw material assay using automated high-performance liquid chromatography,with valve injection, over long runs, suggested that the inclusion of an internalstandard would be beneficial in allowing an additional method for the calcula-tion of results.The accurate volume injection of the valve and loop systemthus permitted standard calibration and subsequent sample evaluation bythe peak-height ratio method as well as from absolute peak heights alone.After a number of experiments, column deterioration rendered the assaysinvalid, and retrospective analysis of the data demonstrated the merits ofinternal standard related parameters in showing the decline in performanceof the column.The major benefits derived from the approach adopted were criteria forjudging the acceptability of the assay results, without excessive replicationfor a particular sample, and prior-warning of the need to re-pack the column.Keywords : Quantitative high-performance liquid chromatography ; internalstandardisation ; column assessment ; Pharmaceutical analysis ; automaticinjectionR.A. MOORELilly Research Centre Ltd., Erl Wood Manor, Windlesham, Surrey, GU20 6PH.Analyst, 1979, 104, 771-777SUMMARIES OF PAPERS IN THIS ISSUEDetermination of Small Amounts of Selenium in Organic MatterAugust, 1979Report prepared by the Metallic Impurities in Organic Matter Sub-committeeMethods for determining small amounts of selenium in organic matter havebeen examined.Determination first requires oxidative destruction of organicmatter. The usual oxidising systems employing sulphuric acid initially,with hydrogen peroxide, nitric and/or perchloric acids, gave very low recoveriesof selenium when high proportions of fats were present in the samples. Con-tinuous combustion and oxygen flask methods were also unsatisfactory. Awet-oxidation procedure in which nitric and perchloric acids (5 + 1) werefollowed by nitric and sulphuric acids gave satisfactory ,results. It wasimportant to ensure that selenium was present as selenium(1V) by boiling thesolution with hydrochloric acid after the oxidation.Colorimetric, gas - liquidchromatographic, fluorimetric and atomic spectroscopic methods were con-sidered for the selenium determination; of these, the last two were selectedfor collaborative trials. For the fluorimetric finish, the solution, after oxida-tion, was treated with 2,3-diaminonaphthalene reagent, the complex wasextracted with cyclohexane or dekalin and the fluorescence in the organicphase was measured in a spectrofluorimeter with excitation a t 369nm andemission reading a t 525 nm. Atomic-absorption or fluorescence measure-ments were made after hydrogen selenide generation and atomisation. Theprecisions of analyses for samples with selenium contents from less than0.1 to 10 pg g-l by the fluorimetric and hydride generation methods areillustrated.Keywords : Selenium determination ; wet oxidation ; fluorimetry ; atomicspectroscopyANALYTICAL METHODS COMMITTEEThe Chemical Society, Burlington House, London, W1V OBN.Analyst, 1979, 104, 778-787.Hydrocarbon Analysis of Naphtha Using Capillary-columnGas Chromatography Under Isothermal ConditionsShort PaperKeywords : Hydvocarbon analysis ; naphtha ; capillary-column gas chromato-graphy ; Kovdts retention indicesPRADEEP KUMAR, S. L. S. SAROWHA and P. L. GUPTAIndian Institute of Petroleum, Dehra Dun-248005, India.Analyst, 1979, 104, 788-792.The Impact Cup: A Simple Aid in Flame SpectrometricAnalysis at High Analyte ConcentrationsShort PaperKeywords : Impact cup ; flame spectrometry ; high analyte concentrations ;pneumatic nebuliserMALCOLM S . CRESSERDepartment of Soil Science, University of Aberdeen, Meston Walk, Aberdeen,AB9 2UE.Analyst, 1979, 104, 792-796

ISSN:0003-2654    

DOI:10.1039/AN97904BP067

出版商:RSC    

年代:1979

数据来源: RSC

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AUGUST 1979 The Analyst Vol. 104 No. 1241 Electrochemical Studies of Strongly Chelating Anthraquinone Derivatives G. Ali Qureshi," G. Svehla and M. A. Leonard Department of Analytical Chemistvy, The Queen's University, Belfast, Northern Ireland, BT9 6AG Analytically important, strongly chelating anthraquinones and their deriva- tives were studied by d.c. polarography, cyclic voltammetry and micro- coulometry to investigate their redox characteristics. All 18 substances were reduced in a two-electron reversible or quasi-reversible process in both aqueous and 75% ethanolic solutions. Depending on pH and the medium, single or double polarographic reduction waves appeared, which were diffusion controlled, although in some instances adsorption pre-waves were also observed. This behaviour is similar to the known behaviour of simpler quinone systems.The variation of the IlkoviC coefficient and half-wave potential with pH was studied in detail to investigate the acid - base behaviour of the species involved in the reduction process. As a result, it was possible to describe the reduction mechanism of the anthraquinones involved. A number of new pK values were determined and others confirmed. Attempts to find linear free energy relationships were generally unsuccessful. Keywords : A nthraquinone derivatives ; polarography ; cyclic voltammetry ; half-wave potentials; PK values Strongly chelating anthraquinones are selective and sensitive reagents suitable for the spectrophotometric determination of a number of metallic and non-metallic substances. Among them, alizarin fluorine blue [3-NN-di (carboxymethyl)aminomethyl- 1,2-dihydroxy- anthraquinone] was introduced first by Belcher et aZ.l for the determination of fluoride.Leonard and co-workers2-11 have synthesised and studied a number of new reagents, all related to alizarin fluorine blue, and applied them for various analytical purposes. Because of their quinoidal structure, these substances display interesting electrochemical behaviour, which we undertook to study by polarography and cyclic voltammetry. Some anthra- quinone derivatives (including alizarin) have been studied polarographically in the past ,12--19 but these did not involve most of the compounds studied by ourselves. In our study we tried to avoid buffers with complex-forming characters as far as possible, in order to be able to compare the polarographic behaviour of these anthraquinones with that of their com- plexes.Experimental Reagents Some of them (I-XI and XIV) were commercially available; others (XII, XIII, XV and XVI) were prepared by Mannich condensation of alizarin with the appropriate agent, and XVII and XVIII were prepared earlier by A1 Ani20 in this Department. The purity of each compound was checked by electrophoresis, thin-layer chromatography and by elemental analysis. Compounds which, when analysed, gave more than 3% divergences from the theoretical carbon or hydrogen values were rejected. For the study 5 x lO-*molI-l aqueous solutions were prepared of each compound; sometimes a few drops of 0.1 mol 1-1 sodium hydroxide solution were added to assist rapid dissolution.* Present address: Department of Analytical Chemistry, University of Uppsala, S-75121 Uppsala 1, Sweden. 705 The compounds involved in this study are listed in Table I.706 were insoluble in water at certain pH values. used; their composition will be mentioned in the appropriate section of the text. from analytical-reagen t grade reagents. QURESHI et al. : ELECTROCHEMICAL STUDIES OF Analyst, Vol. 104 Studies were also made in solutions containing 75% V/V ethanol, as some anthraquinones In some instances special solutions were All buffer solutions, supporting electrolytes and maximum suppressors were prepared White-spot nitrogen gas was used for deaeration. TABLE I COMPOUNDS INCLUDED IN THE STUDY No. I I1 111 IV V VI VII VIII IX X XI XI1 XI11 XIV xv XVI XVIl Name of compound Substituents 1-Hydroxyanthraquinone R, == OH 2-H ydrox yanthraquinone R, == OH Alizarin R,, K, = OH 2,3-Dihydroxyanthraquinone R,, K3 = OH Alizarin-3-sulphonic acid Alizarin-5-sulphonic acid 3-Nitroalizarin (alizarin orange) 1,2,3-Trihydroxyanthraquinone 1,2,4-Trihydroxyanthraquinone 1,2,7-Trihydroxyanthraquinone 1,2,5,8-Tetrahydroxyanthraquinone (alizarin red) R,, K, = OH; R, = SOaH R,, R, = OH; R, = S03H R,, SR, = OH; R, = NO, R,, R,, R3 = OH R,, R2, R4 = OH R,, R,, R, = OH (anthragallol) (purpurin) !quinalizarin) Rip R,, R,, R, = OH R,, R, = OH; R3 = CH,-NH-CH,COOH Alizarin-3-methylglycine Alizarin-3-methylsarcozine R,, R, = OH; Ra = CH2-N<~~~coOH Alizarin fluorine blue R,, R, = OH; R3 = CH,-N(CH,COOH), R,, R, = OH; R3 = CH,-N(CH,COOH),; R5 = S03H Alizarin fluorine blue 5-sulphonic Alizarin-3-methyldiethylamine 1,2-Dihydroxy-3-( 2,4-dihydroxy- acid R,, R, = OH; R3 = CH,-N(C,H5), benzeneazo) anthraquinone R,, R, == OH; R3 = -N=N HO XVIII 1 -H ydrox y-2- (2,4-dihydroxy- benzeneaz0)anthraquinone R, = OH; R, = -N=N Apparatus All polarographic studies were carried out with the Radiometer PO4 polarograph, equipped with the E65 dropping-mercury electrode (IIME) in conjunction with the drop life timer and current sampler unit DLT1.Cyclic voltammetric measurements were made with an instrument, built in this Depart- ment, consisting of a Hewlett-Packard 3310B function generator, 1201B 100-pV dual-trace oscilloscope and a home-made potentiometric circuit with IR-drop compensation.A Radiometer P985B hanging-mercury electrode served as the working electrode, a platinum foil was used as the indicator electrode and a saturated calomel electrode as the common reference electrode. The three electrodes were mounted in such a way that the referenceA ugust ) 1979 STRONGLY CHELATING ANTHRAQUINONE DERIVATIVES 707 electrode was shielded from the IR drop between the other two electrodes. In some instances the hanging-mercury electrode was replaced with an amalgamated platinum-pin electrode. Results were evaluated on the spot using the oscilloscope in its storing mode, but whenever the sweep rates allowed, voltammograms were also recorded with a Hewlett- Packard 7035B X - Y recorder. For microcoulometric experiments a Manousek cell was used in conjunction with the PO4 polarograph. Current veys‘szcs time curves (at constant potentials) were obtained with an instrument designed and built in the Department.The stabilised potential source had a tolerance of &l mV and the current veysus time curves were displayed on an oscilloscope and photo- graphed. The currents were measurable with a relative standard deviation of less than 1%) and the system had a response time of a few milliseconds. pH measurements were carried out with an EIL 7030 pH meter with its glass and saturated calomel electrodes. Procedures All polarographic measurements were carried out with a drop lifetime of 2 s with 70% blanking time and a 50-cm mercury column height (corrected for back-pressure), unless otherwise stated.The anthraquinones were mixed with the appropriate buffer and a few drops of 0.1% Triton X solution and water to yield a 2.5 x mol 1-1 concentration in the cell (unless otherwise stated). Solutions were deaerated with a stream of nitrogen for 10 min. Polarograms were recorded with the slowest possible speed between 0 and -1.8 V. The polarograms were evaluated by the “point method”21 for both the half-wave potential and limiting current. The pH of each solution was measured after the polarographic experiment. The cyclic voltammetric experiments were carried out with electrolytes similar to those used for polarography. Sweep rates were between 10 and 1000 mV s-l, with current sensitivities adjusted appropriately. With a 10 mV s-l sweep rate a full-scale sensitivity of 1 pA was usually sufficient.Microcoulometric experiments were carried out on 2-ml solutions, which contained initially 5 x moll-1 of the alizarin together with the supporting electrolyte. The solution was placed in the Manousek cell and the capillary was positioned in such a way that its end was just below the surface of the solution. The deaerated solution was kept under a nitrogen atmosphere and was mixed after each 5 min by passage of a burst of nitrogen. A polaro- gram was then recorded every 30min. The electrolysis was carried out at a constant potential corresponding to a well defined limiting diffusion current. After 20-25y0 con- version the electrolysis was discontinued and the solution replaced with the pure supporting electrolyte to measure the residual current, which was then used as a correction when evaluating the results.This was done both on the basis of Faraday’s law directly, and with Gilbert and Rideal’s logarithmic method. 22 Current versus time curves were measured only if adsorption phenomena were suspected. In such instances the electrocapillary curve was first obtained by counting the number of drops falling naturally from a capillary per minute, at various electrode potentials. From the electrocapillary curve appropriate potential values were then selected, covering both distorted and undistorted regions, at which the current versus time curves were then measured. These curves were evaluated according to the guidelines given by Heyrovsky and K ~ t a . ~ ~ As the main task was to elucidate the electrochemical properties of strongly chelating anthraquinones, we studied compounds XII-XVIII in more detail than the others, which were examined first for the sake of comparison, mainly to investigate substitution effects. Our report will therefore be concentrated on the former group of substances. Results Polarography each compound.solution, single, double or triple waves were obtained for all the substances involved. The polarographic behaviour was investigated within the widest possible pH range for Depending on the medium and the pH of the medium and the pH of the The708 QURESHI et al. : ELECTROCHEMICAL STUDIES OF Autalyst, Vol. 104 single waves were proved to be diffusion controlled. The double waves consisted either of two diffusion-controlled waves or of an adsorption pre-wave followed by a diffusion wave.The two diffusion-controlled waves usually appeared at higher pH values, and they were well separated (see Figs. 1 and 2). The adsorption pre-wave appeared at lower pH values and usually disappeared, or became less pronounced as the pH was increased, although in some instances it reappeared again at high pH. values. This adsorption pre-wave always appeared close to the main diffusion wave. The triple waves always consisted of an adsorption pre-wave followed by two diff usion-controlled waves. Triple waves appeared only at high pH values (see Fig. 3). a 3 ov ov ov ov -0.2 v -0.2 v (pH 5.0) (6.0) (7.0) (8.0) (10.1) (12.1) Fig. 1. Polarograms of 2.6 x mol 1-l IX at various pH values in aqueous medium.The polarograms of VIII in Fig. 3 display such waves at and above pH 4.6; in more acidic solutions the two waves consist of an adsorption pre-wave and a diffusion-controlled wave. The over-all heights of the waves were found to be independent of the pH for most o the compounds. Fig. 4 shows the variation of the Ilkovie K coefficients (in amps per mole) for these anthraquinones In certain instances (e.g., I, 111, VI, XI11 and XVI) there was a significant drop in th coefficient as the pH of the solution increased and in other instances (V, XIII, XIV an1 again XVI) significant minima were observed on the K venus pH curve, which cannot b attributed to inconsistencies in the evaluation of the polarograms. The K coefficient depended, as expected, mainly on the medium-in aqueous solutions they averaged betwee 1 and 4 x 10-3 A mol-l and in 75% ethanol between 4 and 7 x 10-3 A mol-1.In some instances, however, significant variations were observed. 0.5 9 2 0 t t t + m - t t t t t -0.1 v-0.1 v-0.1 v -0.3V -0.4V -0.45V -0.5V -0.6V (pH 1.5) (2.2) (3.1) (4.3) (5.6) (7.0) (9.1) (11.1) Fig. 2. medium. Polarograms of 2.6 x lo-* mol 1-l XV at various pH values in aqueousAugust, 1979 STRONGLY CHELATING ANTHRAQUINONE DERIVATIVES 709 9 1.0 2 1 t t t t t t t +'0.2v.- ov ov ov ov ov ov ov Fig. 3. Polarograms of 3 x mol 1-1 VIII at various pH values in 76% ethanol. n XIV(W) 3.5 x 10-3 ____t I I I I I I I 0 2 4 6 8 10 12 PH I Fig. 4. Variation of the Ilkovit K coefficient with pH for selected anthraquinone derivatives in aqueous (W) and 75% ethanol (alc) solutions.710 QURESHI et al.: ELECTROCHEMICAL STUDIES OF . Analyst, VoZ, 104 The variation of half-wave potentials with pH was investigated with the greatest care, as these relationships provide valuable information on the acid - base equilibria in which the oxidised and reduced species are involved and, in an indirect way, also on the reduction process itself. Figs. 5 and 6 refer to aqueous solutions and Fig. 7 to alcoholic solutions. From the theoretical point of view the half-wave potentials themselves are not so important, but rather the slopes of the Ei versus pH plots, as well as those pH values where these slopes change. The expected values for these slopes are simple multiples of the Nernstian pre- logarithmic constant of 60 mV pH-l with factors of 0, 0.5, 1 and 1.5, that is 0, 30, 60 and 90 mV pH-l.Hence, the lines fitting the experimental points on these figures were drawn deliberately with such slopes-as is clear from the figures themselves, such lines do fit the experimental points reasonably well, although in some instances least-squares fits would produce slopes of different values. In the single instance of XI1 the experimental points could not be joined with a continuous set of lines. I 3 5 7 9 11 13 PH Fig. 5. Variation of the half-wave potentials of compounds 11-IX with pH in aqueous medium. The nature of diffusion-controlled waves has been verified by measurements taken at various mercury column heights and plotting the currents as the function of the square root of the (corrected) height.Such an operation was undertaken for each compound at several pH values, covering all sections of the E, versus pH curves. In addition to the linearity of these plots, we had various indirect evidence for diffusion control; thus, the absence ofAugust, 1979 STRONGLY CHELATING ANTHRAQUINONE DERIVATIVES 711 irregularities on the electrocapillary curves and the shape of the instantaneous current vwms time curves indicated clearly that diffusion currents were involved. Finally, waves were accepted to be diffusion controlled only when the current vucysza concentration plots proved the expected proportionality. xv -0.95 V ( X ) -0.9 v (XI) -0.9 v (XI I ) -0.7 V (XIII) -0.8 V ( X W -0.8 V (XV) - - c-- c-- - -0.85 V (XVI) t-- -0.9 v (XVII) - -0.9 v (XVIII) - 1 3 5 7 9 11 13 PH Fig.6. Variation of the half-wave potentials of compounds X-XVIII with pH in aqueous medium. The adsorption waves were investigated by several methods. Firstly, polarograms were obtained at different concentrations. Plotting the wave height against concentration a curve typical for adsorption currents was obtained (Fig. 8 ) . Temperature effects were studied. As can be seen from Fig. 9, the over-all wave height increased with temperature, but the adsorption pre-wave disappeared gradually as the temperature was increased. Instantaneous current veysus time curves were also studied whenever an adsorption wave was suspected. The oscillograms obtained at various potentials of the DME always showed distortions near to the half-wave potential of the adsorption wave (Fig.10). Finally, electrocapillary curves were obtained in the presence of the depolariser. Plotting the average drop lifetime as a function of the potential of the DME, characteristic incisions occurred at the appropriate potentials, whereas in the absence of the depolariser the curve was smooth at these potentials (Fig. 11). The cyclic voltammetric response of those substances which displayed adsorption waves was also interesting. In the first cycle, distortions were observed approximately at the expected potentials, but these disappeared when a second cycle was imposed immediately712 QURESHI et al. : ELECTROCHEMICAL STUDIES OF Analyst, Vol. 104 Fig. 7. Variation of the half-wave potentials of selected anthraquinones with pH in 76% ethanol.I I 0 1 2 c x 104/moi I-' Fig. 8. Variation of the height of the total wave (id) and of the adsorption pre-wave (ia) with con- centration of XV at pH 4.30 in aqueous medium.A ugust, 1979 STRONGLY CHELATING ANTHRAQUINONE DERIVATIVES 713 after the first. These adsorption waves displayed irreversible or, at very low scan rates, quasi-irreversible behaviour, to some extent contrary to expectation^.^^ This unusual behaviour would merit a more detailed investigation and electrochemists with more theoretical outlooks than that of the present authors might find this a rewarding area of research. t t t t t t t -0.56 V +H- 0.1 v Temperature dependence of the shape and height of the polarographic wave of a 2.6 x lo-* moll-' solution of XV at pH 6.6 in aqueous medium.Fig. 9. Adsorption pre-waves were noted in connection with compounds 111, VI, VII, IX, X, XI, XII, XIII, XIV, XV, XVI and XVIII. As indicated before, their appearance is dependent on the experimental conditions, of which pH, concentration and temperature are the most important. Further details on these adsorption pre-waves are available in Qureshi's thesis.25 All purely diffusion-controlled, single or double waves were subjected to logarithmic analysis in order to calculate the value of the pre-logarithmic factor in the Nernst equation. To our surprise, but in good agreement with results obtained on simpler quinone - hydro- quinone systems,26 the number of electrons (for a coefficient u = 1) was almost always found to be nearly unity, even with the single waves, where a value of 2 would be expected.This behaviour will be dealt with under Discussion. Microcoulometry To obtain a reliable value for the number of electrons taken up by a single molecule of the anthraquinones, we decided to carry out microcoulometric experiments. As the time H -0.1v -0.2 v- -0.3 V 4s ww w -0.35 V -0.4 V -0.5 V Fig. 10. Instantaneous current versus time curves of 2 x 10-4 moll-' solutions of XV at pH 4.30 in aqueous medium at different D.M.E. potentials.7 14 QURESHI et at. : ELECTROCHEMICAL STUDIES OF Analyst, ~ O L ? . 104 0 -0.5 -1.0 -1.5 EIV Fig. 11. Electrocapillary curve of mercury in a 10-l mol 1-1 acetate buffer (pH 4.30) in (A) the absence and (B) the presence of 2 x 10-4 moll-’ XV. duration of a single experiment is considerable, we omitted compounds VII and IX, carried out the measurement with most other compounds in neutral medium only, and only with the four key compounds, XIII-XVI, did we carry out three runs at three different pH values, involving acidic, neutral and basic media.With the three compounds displaying two separate reduction waves (VIII, XVII and XVIII), the pH was adjusted to 9, where the two waves are well established. A separate run was carried out for each wave. All results were evaluated both by graphical integration of the current versus time curves directly through Faraday’s law, and also by using Gilbert and Rideal’s equation.22 The two methods gave almost exactly the same results for the number of electrons: the difference between the corresponding figures was less than 0.5% in all instances.We feel that by evaluating results in both ways the figures are more reliable. The method based directly on Faraday’s law eliminates the apparent weakness of the method of Gilbert and Ridea1,22 which takes into account only the initial and final current, but disregards the over-all shape of the current verszts time curve. All of the compounds with single waves gave values between 1.97 and 2.30 for the number of electrons involved. The values for the double waves were in all instances higher than 1 (for each wave), but never exceeded 1.42. Fig. 12 shows the results obtained for XV. Through the points (which, theoretically, should I 0 120 240 flmin Fig. 12. Current veysus time curve obtained during the microcoulometric reduction of 2 ml of 5 x 10-4 M XV solution.August, 1979 STRONGLY CHELATING ANTHRAQUINONE DERIVATIVES 715 follow an exponential function) a straight line was drawn and the graph was integrated as a trapezium.From this integration, a value of 2.100 is obtainable for the number of electrons, while Gilbert and Rideal’s method gives 2.105. Cyclic Voltammetry The purpose of the cyclic voltammetric measurements was to find out whether the reduc- tion of these compounds is reversible or not. Up to a sweep rate of 10mVs-1 (which is 2-10 times higher than those used in the polarographic measurements) the reduction was either reversible or quasi-reversible, whereas, as pointed out, the adsorption was quasi- reversible at very low scan rates, or irreversible. In Fig.13 three cyclic voltammograms are reproduced that show these different responses. All of these curves were obtained with a 10 mV s-l sweep rate in buffered aqueous solutions at pH 10. Fig. 13(a) shows the cyclic voltammogram of 2.5 x lo4 mol 1-1 of XIV. The cathodic (positive) and anodic (negative) peak currents are virtually identical; the peak separation - is 33 mV, not far from the theoretical value of 30mV. Hence, one can say that the reduction of XIV is reversible under such conditions. A 10“ mol 1-1 solution of VI [Fig. 13(b)] displays quasi- reversible reduction, the cathodic and anodic peak currents differing by more than 10% and the peak separation being more than double the theoretical value. The adsorption peak, obtained in the first cathodic half-cycle, is barely distinguishable as a faint shoulder on the anodic half-cycle.The adsorption peak, obtained in the first cathodic half-cycle in a 3 x 10-4mo11-1 solution of VIII [Fig. 13(c)], does not show up on the first anodic half- cycle, indicating that the adsorption, under these circumstances, is irreversible. However, the two reduction peaks are reproduced with much reduced definition on the anodic half- cycle, so the reduction of VIII is still a quasi-reversible process. Reduction peak potentials [ED(c1], contrasted with the polarographic half-wave potentials (E*), together with a note on reversibility, are shown in Table 11. t o . l 4 2 0 -0.1 -0.6 -0.7 -0.8 +0.4 0 -0.4 ‘%,d(a) I I I -0.5 -0.7 -0.9 E N to. 1 0 -0.1 -0.3 -0.5 -0.7 -0.9 Fig. 13. Cyclic voltammograms of various anthraquinone derivatives.For explanation, see text. Discussion The purpose of this discussion is to explain the redox and acid - base processes in which the examined aiithraquinones are involved, and to correlate these with the experimental facts. Quinones (q), when reduced by taking up a single electron, form semiquinones (sq), while the uptake of a further electron results in the formation of hydroquinones (hq). In all of the redox processes, therefore, these three states of oxidation are expected to be involved.716 QURESHI et al. : ELECTROCHEMICAL STUDIES OF TABLE I1 CYCLIC VOLTAMMETRIC REDUCTION PEAK POTENTIALS r~,,,,] AND POLAROGRAPHIC Analyst, VoZ. 104 HALF-WAVE POTENTIALS (E,) AT P H 10 OF ANTHRAQUINONES Experimental conditions given in the text.E,(dIV E&/V f - t r \ Compound q/hq 4% sq/hq q/hq qlsq sq/hq I* -0.77 -- 0.74 I1 -0.78 -0.73 I11 -0.85 --0.82 I V -0.86 --0.81 V -0.78 -0.74 V I -0.71 --0.67 V I I -0.60 -0.87 -0.66 -0.83 V I I I -0.60 -0.76 -0.67 -0.72 IX -0.61 -0.81 -0.68 -0.78 X -0.87 -0.83 XI -0.84 --0.81 XI1 -0.82 --0.79 XI11 -0.81 --0.77 XIV -0.77 - 0.73 xv -0.78 - 0.74 X V I -0.79 -- 0.76 XVII -0.77 -0.86 -0.73 -0.83 XVIII -0.76 -0.85 -0.73 -0.81 * In 76% ethanolic solution. t R = reversible; Q = quasi-reversible. Reductiont R R Q Q Q R Q Q aR Q Q QR R R Q Q Each oxidation state, however, can attain different degrees of protonation; some of the substances involved in the study may contain as many as seven protons, which could, under certain circumstances, dissociate.Thus, the uptake of each electron may (or may not) be accompanied by the uptake of one or more protons. Fig. 14 shows all of the possible oxidation and protonation states of the substances involved; boxes drawn with solid lines represent those states which were experimentally verified (the existence of the other states, in extremely acidic or alkaline media, cannot be ruled out, however). Capital, lower-case and Greek letters are used to denote different states of oxidation and protonation, and will be applied later to describe the processes involved. Let us consider for example the reduction of the quinone H5q (c). This may take place in one step (Cy reaction) : at the potential E&t (which is close to the observed Eiqlhq half-wave potential). As a result, a single wave with the uptake of two electrons is obtained. The reduction may, of course, take place in two steps.First the semiquinone is formed (Cc reaction) : (2) H5q + e-- f H,sq- .. .. .. to which a one-electron wave of Ei91sq half-wave potential corresponds. by a second reduction step (cy reaction) : This is followed .. .. (3) H5sq- + e-- + H,hq” . . .. causing the occurrence of a second one-electron wave with Eaeqpg half-wave potential. Whether under given experimental circumstances a single two-electron wave or two single one-electron waves are observed, depends on the stability of the semiquinone. Combining equilibria (1)-(3), one can describe the disproportionation (or dismutation) of the semi- quinone with the equilibriumAugust, 1979 STRONGLY CHELATING ANTHRAQUINONE DERIVATIVES 717 Double waves 7 Ewsq .-\ f r---------- 1 I I I H7q2+ 1 1 A I r---------- 1 I I H6 9' I I I B I r------ ----i I I I q5- I 1 H I 7 r---------- 1 I I I H 7 sq+ I I a I r-------- -1 I I H3sq3- I I e I -1 r-- ------ I 1 H*sq4-- I I f I r--------- 1 Hsq5- I I 9 I r--------- 1 I I I h sq6- I I I H3 hq4- EII 1 r -.- ---- --- I 1 I Hhq6- I 77 I I r - - - - - - - - - 1 I I I hq7- I I v I L - _ _ _ _ - _ _ _ J f I EXqlhc, Single waves Fig.14. Reduction and protonation of anthraquinones.718 QURESHI et al. : ELECTROCHEMICAL STUDIES OF Analyst, Vole 104 The stability of the semiquinone can be measured through the value of the semiquinone formation constant, K,,, defined from reaction (4) as Heyrovsky and Kuta26 applied such considerations when describing the equation of the polarographic waves of quinones, and came to the conclusion that: (a) if K,, = 0 a single wave with a slope corresponding to a two-electron reduction is obtained; (b) if K,, > 16 two separate waves are obtained, each with a slope corresponding to a single-electron uptake ; and (c) if 0 < Ksq < 16 a single wave is obtained, with a slope corresponding to anything between These considerations explain the curious fact, mentioned under Results, that in many instances single waves were observed, for which the logarithmic analysis gave for the number of electrons a value close to 1, while microcoulometry always resulted in a number close to 2. The reduction of quinones may or may not be accompanied by the uptake (and in some instances by the release) of one or more protons.Although individual polarographic waves do not reveal such processes, the evaluation of the half-wave potential versus pH curves makes it possible to find out about them. To see this clearly let us again examine the reduction of a quinone (H5q). Let us suppose that Ks, = 0, so that there is no semiquinone formation; the reduction can therefore be described as and 2 electrons per molecule. * * (6) .. .. .. H5q + 2e- :+ H,hq2- Depending on the pH of the medium, the quinone may undergo stepwise dissociation. our purpose it will be sufficient to consider the first two steps only: For H5q f H+ -+ H,q- .. .. * . (7) .. and .. .. * * (8) H,q- + H+ + H,q2- .. with the dissociation equilibrium constants .. .. * ' (9) ..and Further, the hydroquinone formed in process (6) may undergo stepwise protonation. sidering the first two of these steps, we can have Con- . . (11) H,hq2- + Hi- + H6hq- .. . . .. and H6hq- + Hr + H,hq .. .. .. . . (12) With the symbols used in Fig. 14 these processes can be characterised with the following dissociation equilibrium constants :Azlgust, 1979 STRONGLY CHELATING ANTHRAQUINONE DERIVATIVES 719 .. * . .. . . (13) [H+l [H5hq2-1 CH6W-l K P Y = and The oxidation - reduction potential of the system described in reaction (6) can be expressed as x log [H5hq2-l ~ .. .. . . (15) 0.059 2 [H&l E = Eqlhq - - where E+q is the standard oxidation - reduction potential. The two concentrations included in the logarithmic term are in general not known, but can be calculated from the mass balance equations : and where chq and cq are the analytical concentrations of the hydroquinone and quinone species, respectively.Combining equations (9), (10) and (13)-(17) we can express the oxidation - reduction potential as Provided that the diffusion coefficients of the oxidised and reduced forms are equal, the half-wave potential is measured at the point where the analytical concentrations of chq and cq are equal. Thus, from equation (18) we can express the dependence of half-wave potentials on the hydrogen-ion concentration with the equation The experimentally obtained E, v e n m pH curves should conform to this equation. The correlation of theory with experimental results is simple if one considers firstly such extreme cases where equation (19) can be drastically simplified.For example, in very acidic solutions we have and also Applying such simplifications in equation (19) we arrive at the expression X log Kap KO, - 0.059pH . . .. . . (22) 0.059 2 E+ = EQpq - - AS long as the conditions described in the inequalities of (20) and (21) prevail, we therefore720 expect the decrease in the half-wave potential with pH with a 60-mV slope. words, under these conditions the reaction QURESHI et al. : ELECTROCHEMICAL STUDIES OF Analyst, VoZ. 104 In other H,q + 2H+ + 2e- + H,hq . . .. .. . . (23) (using the symbols of Fig. 14, a c 3 a process) is taking place. If these conditions are not more valid, one can describe others that fit the case and with an analogous treatment one can calculate the expected slope of the E+ versus pH curves.For the model chosen one can combine altogether nine different conditions (some of which are less likely to prevail in practice than others), which are shown in Table 111. TABLE I11 SLOPES OF 224 - PH CURVES AND REDUCTION PATHS FOR THE TWO-ELECTRON SINGLE-WAVE REDUCTION OF A QUINONE 1st condition increasing pH According to such considerations, the E, versus pH curves should consist of linear sections with slopes of 0, -30, -60 or -90mVpH-l (the occurrence of a -120mVpH-1 slope being most unlikely), for two-electron, single-wave processes. The experimental results, as shown in Figs. 5, 6 and 7, do in fact correspond to such predictions. It can also be shown that the pH values corresponding to the intersections of the linear portions with different slopes are in fact equal to the appropriate pK values of the substances involved in the process.In a corresponding manner, one can involve other species with different degrees of protonation i n such considerations, and carry out calculations in a similar fashion. Also, it is easy to extend such considerations to each of the double waves (when KBs > 16) with one- electron uptake. Based on such considerations, we evaluated au of the available 224 versyszcs pH curves and, taking into account all other experimental evidence and data obtained from the literature, have summarised our findings in Table IV. Our aim was to find (a) which species and how many electrons are involved in the electrochemical reduction and (b) what are the pK values of the quinones, semiquinones and hydroquinones involved.Table IV also contains information on the E+ versus pH curves themselves and all pK values available in the literature. The symbols and abbreviations are the same as those used in Fig. 14. It is TABLE IV REDUCTION OF ANTHRAQUINONE DERIVATIVES Parameters of the El vs. pH line pK values Reduction mechanism gxperimentally \ Compound Wave pHrange V V pH-' (cf., Fig. 14) obtained Taken from literature (a) Aqueous solutions- I1 qlhq 5.7-11.9 -0.125 -0.060 C; -+ a 111 q/hq 7.2-11.9 -0.220 -0.060 C * a ~ K C D = 8.35 6.9-8.35 -0.160 -0.060 C -., a IV qbq { 8.35-12.1 +0.096 -0.090 D -.,aAugust, 1979 STRONGLY CHELATING ANTHRAQUINONE DERIVATIVES 721 Compound V VI VII VIII IX X XI XI1 XI11 XIV xv XVI XVII XVIII pH range 3 .6 - ~ 6 -6-mll -1 1-1 3.2 3.5-6.0 6.0-6.9 6.9-9.1 9.1-12.4 6.0-13.0 6.0-13.0 7.7-11.9 7.7-11.9 4.0-7.5 7.5-1 3.1 7.5-12.2 7.4-12.9 6.6-12.2 2.4-4.0 4.9-6.25 8.8-11.8 1.8-5.5 5.5-11.2 11.2-12.5 6.25-8.8 2.8-5.5 (0.75) 5.5 (or 7.5)-8.0 8.0-10.7 10.7-12.8 1.6-2.5 2.5-5.6 5.6-8.2 8.2-9.7 9.7-11.4 2.2-4.67 4.67-10.65 10.65-11.6 11.6-12.9 7.1-8.6 8.6-12.4 7.1-8.4 8.4-11.9 7.0-8.4 8.4-11.9 7.0-8.9 8.9-11.9 (b) Solutions containing 75% ethanol- 3.4-9.2 'I1 ¶lhq { 9.2-11.6 IV ¶Ihq { 7.95-1'1.7 6 9-7 95 V qlhq 5.1-1 3.2 2.6-11.9 vll* $h9q 2.6-11.9 xvlll $zq 4.9-12.1 5.6-12.1 TABLE IV (continued) Parameters of the E ) vs. pH line Reduction Intercept/ Slope/ mechanism V V pH-' (cf., Fig. 14) - 0.140 - 0.060 - 0.160 -0.060 -0.340 -0.030 - 0.065 - 0.060 + 0.045 - 0.060 - 0.225 - 0.060 + 0.025 - 0.060 - 0.120 - 0.060 + 0.245 - 0.090 4-0.020 -0.060 - 0.185 - 0.060 -0.235 -0.060 - 0.195 - 0.060 - 0.070 - 0.060 -0.720 0 - 0.190 - 0.060 - 0.165 - 0.060 -0.845 0 -0.185 -0.060 - 0.430 - 0.030 - 0.110 - 0.060 - 0.230 - 0.060 - 0.400 - 0.030 -0.155 - 0.060 - 0.445 - 0.030 - 0.155 - 0.060 -0.500 -0.030 -0.640 0 - 0.125 - 0.060 -0.735 0 -0.230 -0.060 -0.620 0 - 0.120 - 0.060 -0.740 0 - 0.210 - 0.060 -0.145 -0.060 -0.435 -0.030 -0.155 - 0.060 - 0.130 - 0.090 - 0.240 - 0.060 -+ 0.045 - 0.090 -0.185 -0.060 - 0.190 - 0.060 - 0.130 - 0.060 -0.130 -0.060 -0.085 -0.060 - 0.225 - 0.060 pK values obtained Taken from literature r A 1 Experimentally PKuCj = 6 ~ K C D = 6.67''; 5.60:' pKfiy= 11 ~ K D S = 9.1 P K U ~ = 6.0 PKfjyW PKCD ~ K D E = 11.0P PKDE = 10.7827; 11.16 *@ ~ K C D = 6.91'l p K q ~ pKpy% 4.5 ~ K D E ~ ~ K E P W 8.80 pKYsw ~ K O D pKaD = 6.25'0 ~ K F O = 12.2"' PKDE = 11.2 pK,D = 5.50"; 5.12" PK&Z PKy8 3 P K 8 ~ N PKDE ~ K E F = 13.1OZ8 P K D ~ " PKDE pKvs = 8.0 )I< I E = 11.25a8; 11.20*' pKcn = 2.40"; 4.894 p K ~ 8 = 5.54"; 7.55' pKEF = 10.7 PKEB = 10.07"; 10.43' pKao = 11.98"; 11.19' pKpYw PKDE PKEF = 6.6 pKys w pK6€= PKEF ~ K F O = 10.O1O; ~ K O H = 12.31° pKna = 0.5"; pKaD = 1.41° ~ K D E = 2.5"; p K ~ a = 5.810 pKy8 = 11.6 PK+W ~ K C D PKDE = 10.65** pKpy= ~ K D E ~ K C D = ~ K D E = 8.6 pKcd = 8.4 ~ K C D = 4.67a8 ~ K C D = pKm = 8.4 pKcd = 8.9 ~ K C D = 8.5*O pK,p = 9.8 ~ K C D = 10.8" ~ K O D = 9.6 pKcn = 9.2 pKan = 7.86P' ~ K D E - 1427 ~ K C D = 7.95='; ~ K D E = 13.31'' p K q W pKan worthwhile pointing out that the experimentally obtained, "polarographic" pK values agree well with the values taken from the literature, which were mostly obtained by spectro- photometry.The agreement is very good for the aqueous solutions of XIII, XIV, XV and722 QURESHI, SVEHLA AND LEONARD XVIII and is acceptable even with the alcoholic solution of 111. Only with the aqueous solution of VI do the data differ substantially. In only these instances do we possess parallel polarographic and spectrophotometric data. When evaluating these results we also tried to find linear free-energy relationships among certain derivatives. It must be said that the original Hammett30 equation cannot be applied here (nor are the appropriate reaction and substituent constants available), and none of the more recent e q u a t i o n ~ ~ l , ~ ~ would really be valid.Still, it seemed to be reasonable to attempt tofind some correlation between the half-wave potentials (at a fixed pH) and the various pK values available. When doing so we hoped to extend the available polarographic free- energy correlation data for a n t h r a q u i n o n e ~ . ~ ~ , ~ ~ ~ ~ ~ We constructed separate diagrams for the P K ; ~ and pK,, values (only these are available in significant numbers) using the data in Table IV. For half-wave potential values we used the intercept figures (which correspond to the half-wave potential values extrapolated to pH 0). Although a number of points fall on a well defined straight line, as expected, we have not published such plots, because in both instances there were a number of unexpected deviations, which we were unable to explain.Also, there are parallel, sometimes greatly differing, values for the same pK value available in the literature, and a choice among them cannot easily be made. Although one can select a number of points that would show a reasonable straight-line relationship, such a selection would be arbitrary and not scientifically justified. A more detailed account of these attempts is available in a thesis.25 1. 2. 3. 4. 6. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. References Relcher, R., Leonard, M. A., and West, T. S., J . Chem. SOC., 1959, 2390. Leonard, M. A., in Johnson, W. C., Editor, “Organic Reagents for Metals and Certain Radicals,” Volume 2, Hopkin and Williams, Chadwell Heath, 1964, p.1. Leonard, M. A., and Nagi, F. I., Chem. Commun., 1968, 20, 1224. Leonard, M. A., and Nagi, F. I., Tuluntu, 1969, 16, 1108. Leonard, M. A., and Nagi, F. I., Analyt. Lett., 1969, 2, 15. Laird, C. K., and Leonard, M. A., Talanta, 1970, 17, 173. Al-Ani, K., and Leonard, M. A., Analyst, 1970, 95, 1039. Al-Ani, K., and Leonard, M. A., Proc. SOC. Analyt. Chem., 1971, 8, 190. Leonard, M. A., and Murray, G. T., Analyst, 1974, 99, 645. Leonard, M. A., Analyst, 1975, 100, 275. Deane, S. F., and Leonard, M. A., Analyst, 197?, 102, 340. Furman, N. H., and Stone, K. G., J . Am. Chem. SOC., 1948, 70, 3056. Wiles, L. A., J . Chem. SOC., 1952, 1368. Edsberg, R. I., Eichlin, D., and Garis, J. J., Arzalyt. Chem., 1953, 25, 798. Starka, L., Vystrcil, A., and Starkova, B., Chem. Listy, 1957, 51, 1440. Broadbent, A. D., and Sommermann, E. F., J . Chem. Suc. (B), 1967, 376. Broadbent, A. D., and Sommermann. E. F., J . Chern. SOC. (B), 1968, 519. Gill, R., and Stonehill, H. I., J . Chem. SOC., 1952, 1845. Gill, R., and Stonehill, H. I., J . Chem. SOC., 1952, 1857. Al-Ani, K., Ph.D. Thesis, Queen’s University, Belfast, 1971. Willard, H. H., Merritt, L. L., Jr., and Dean, J. A., “Instrumental Methods of Analysis,” Fourth Gilbert, G. A., and Rideal, E. K., Trans. Farallay Soc., 1951, 47, 369. HeFovsky, J., and Kuta, J ., “Principles of Polarography,” Czechoslovak Academy of Sciences, Prague, and Academic Press, New York, 1966, pp. 311 et seq. Brown, E. R., and Large, R. F., in Weissberger, A., and Rossiter, B. W., Editors, “Physical Methods of Chemistry,” Volume 1, Part ILA, “Electrochemical Methods,” Wiley, New York, 1971, pp. 502 et seq. Qureshi, G. A., Ph.D. Thesis, Queen’s University, Belfast, 1975. Heyrovsky, J., and Kuta, J ., “Principles Of Polarography,” Czechoslovak Academy of Sciences, Prague, and Academic Press, New York, 1966, p. 181. Radcliffe, Bl., Ph.D. Thesis, Queen’s University, Belfast, 1973. Murray, G. T., Ph.D. Thesis, Queen’s University, Belfast, 1973. Ingman, F., Talanta, 1973, 20, 135. Hammett, L. P., J . Am. Chem. Soc., 1937, 59, 96. Jaffe, H. H., Chem. Rev., 1953, 53, 191. Wells, P. R., Chern. Rev., 1963, 63, 171. Crawford, R. J., Levine, S., Elofson, R. M., and Sandin, R. B., J . Am. Chem. SOC., 1957, 79, 3153. Wiles, L. A., J. Chem. SOC., 1952, 1958. Edition, Van Nostrand, New York, 1965, p. 692. Received June 3rd, 1977 Amended October 19th, 1978 Accepted January 30th, 1979

ISSN:0003-2654    

DOI:10.1039/AN9790400705

出版商:RSC    

年代:1979

数据来源: RSC

摘要:

Analyst, August, 1979, Vol. 104, pp. 723-729 723 Direct Differential - pu Ise Pola rsg ra p h ic Determination of Mixtures of the Food Colouring Matters Tartrazine - Sunset Yellow FCF, Tartrazine - Green S and Amaranth - Green S in Soft Drinks A. G. Fogg and K. S. Yo0 Chemistry Department, Loughborough University of Technology, Loughborough, Leicestershire, LE 11 3T U Tartrazine and Sunset Yellow FCF can be determined directly in orangeade by differential-pulse (d.p.) polarography on the addition of pH 9 Britton - Robinson buffer and tetraphenylphosphonium chloride. The tetraphenyl- phosphonium chloride removes the large polarographic maximum obtained with tartrazine at pH > 4 and causes the d.p. polarographic peaks of the two colouring matters to be separated. Tartrazine in limeade can be determined in a similar supporting electrolyte but these conditions are not suitable for the determination of Green S, which is usually present at a low concentration relative to the tartrazine and for which the d.p.polarographic peak is depressed by the addition of tetra- phenylphosphonium chloride. Green S can be determined after adding pH 4 Britton - Robinson buffer and tetramethylammonium chloride to the limeade : the addition of tetramethylammonium chloride gives a better base line in the presence of tartrazine. The solution is then re-adjusted to pH 9 and tetraphenylphosphonium chloride is added in order to determine the tartrazine. At pH 4 the sugar present in blackcurrant syrup gives a small d.p. polaro- graphic peak at the same potential as Green S.At pH > 6 the peak of the sugar disappears but amaranth gives a broad polarographic maximum. This maximum is suppressed at pH 7.8 by the addition of tetramethylammonium chloride. Under these conditions the Green S peak is separate but the small concentrations of Green S normally present in blackcurrant drinks can only just be detected. The procedures have been tested on soft drinks prepared with known concentrations of colouring matter. Keywoevds : Diffeevential-pulse polarography ; food colowing mattevs ; tartrazine ; Green S ; amaranth The UK Statutory Instrument concerned with colouring matter in food1 lists permitted colouring matters and gives specifications for purity of colouring matter samples; colour is to some extent self-limiting and no limits are set on the amounts of these colouring matters that can be added to foods. Most analytical publications on food colouring matters are concerned with the identification of colouring matters, usually by thin-layer chromatographic method^.^-^ Spectrophotometric quantification can be applied after thin-layer chromato- graphic ~eparation.~ The continuing need for the use of food colouring matters, together with some concern about their widespread use, has aroused interest in their determination in foods.Food colouring matters and their intermediates have been determined by liquid - solid, ion- exchange and steric-exclusion forms of high-performance liquid chromatography (HPLC) ,* but more recently better results have been obtained by using the newly introduced paired- ion chromatographic m ~ d e .~ . ~ For thin-layer chromatographic - ultraviolet spectrometric or HPLC determination, the food colouring matters are first extracted from the foodstuff .3 With beverages and water-soluble foods the colouring matters are first adsorbed on wool or polyamide and are then re-extracted into an organic solvent. Colouring matters in more intractable foods undergo a more rigorous extraction with an organic solvent or liquid ion- exchange resin, before being adsorbed on wool or polyamide.724 FOGG AND YO0 : DIRECT DIFFERENTIAL-PULSE POLAROGRAPHIC Analyst, VOZ. 104 This work was undertaken in order to assess the value of the differential-pulse polarographic method for the determination of food colouring matters. Most colouring matters are reducible at the dropping-mercury electrode and give distinct polarographic waves.' Clearly, samples of colouring matters obtained using the clean-up procedures described above could be determined by using a diff erential-pulse polarographic finish.The peak potential observed could be used as partial confirmation of the identity of the colouring matter. Colouring matters with the same reducible group, e.g., the azo group, do tend to be reduced at similar potentials, however, so that identification cannot usually be made unequivocally and the analysis of mixtures of colouring matters can be difficult without prior separation. The ion-pair extraction method has been used extensively in the determination of ionisable pharmaceutical compounds,* and is now being adapted increasingly to paired-ion HPLC.9 We decided to try the ion-pair extraction approach with food colouring matters, and found that the acidic food colouring matters tartrazine.and Sunset Yellow FCF could be extracted into chloroform from orange squash using tetraphenylphosphonium chloride. After evaporating the chloroform and dissolving the colouring matters in pH 9 Britton - Robinson buffer, two distinct polarographic waves were obtained. Subsequently, the extraction step was found to be unnecessary; the addition of tetraphenylphosphonium chloride to orange squash buffered at pH 9 alters the potential at which tartrazine is reduced. Whereas in the absence of tetraphenylphosphonium chloride the two colouring matters are reduced at similar potentials (EP values: Sunset Yellow FCF -0.64 V; and tartrazine -0.73 V), in its presence two separate waves are obtained. Direct differential-pulse polarographic procedures for the determination of three mixtures of colouring matters have been developed.Experimental and Results Apparatus Polarographic measurements were made with a PAR 174 polarographic analyser (Princeton Applied Research). For differential-pulse operation, the forced drop time was 1 s, the pulse height 50 mV and the scan rate 2 mV s-l. Two-electrode operation was used with a dropping-mercury electrode and a saturated calomel reference electrode. A water-jacketed polarographic cell was used and the temperature was maintained at 25 "C. Reagents and Samples Britton - Robinson buffer (PH 1.9; 0.04 M in each constituent) was prepared by dissolving 2.47 g of boric acid in 500 ml of distilled water containing 2.3 ml of glacial acetic acid and then adding 2.7 ml of orthophosphoric acid and diluting to 1 1 with water.The pH of the buffer was adjusted as required by means of 0.2 or 4 M sodium hydroxide solution. Tetraphenylphosphonium chloride (0.01 M) and tetramethylammonium chloride (1 M) solutions were prepared from laboratory-grade reagents. Samples of amaranth, Sunset Yellow FCF, tartrazine and Green S, and of a blackcurrant health drink syrup and a basic syrup, were kindly provided by Beecham Products Ltd. The concentrations of colouring matters quoted in this paper assume that the samples of colouring matters are pure, i.e., that they contain 100% m/m of the colouring matter.This assumption was adequate for carrying out the recovery tests made here, but for normal routine analytical use calibration ' graphs should be obtained by using assayed samples of the colouring matters. Determination of Sunset Yellow FCF and Tartrazine in Sparkling Orangeade An orangeade containing no colouring matter was prepared by diluting 15 ml of the basic syrup to 100ml with distilled water that had previously been carbonated by using dry-ice. A blank polarographic solution was prepared by mixing 5ml of this orangeade, 5ml of 0.01 M tetraphenylphosphonium chloride and 20 ml of Britton - Robinson buffer (pH 1.9), adjusting the pH to 9 with sodium hydroxide solution and diluting to 50ml. An aliquot (20 d) of this solution was pipetted into the polarographic cell, deoxygenated for 10 min and polarographed.When carrying out development work, successive aliquots of con- centrated solutions of Sunset Yellow FCF and tartrazine were added to the blank solution by means of a 100-p1 syringe, the solution being deoxygenated for 1 min after each addition.Aztgust, 1979 DETERMINATION OF FOOD COLOURS IN SOFT DRINKS 725 a -? E Y C 3 0 t 0.5 PA I 11 PA L I I I I -0.90 -0.50 -0.90 -0.50 PotentialN Fig. 1. Effect of tetraphenylphosphonium chloride on the d.p. polarogram of a mixture of Sunset Yellow FCF and tartrazine using the recommended procedure (A) with the addition of tetraphenylphosphonium chloride and (B) without its addition. The effect of adding the tetraphenylphosphonium chloride on the polarograms of a mixture of Sunset Yellow FCF and tartrazine is shown in Fig.1. The effect on the peaks of the individual colouring matters is illustrated by the results given in Table I. On the addition of small amounts of tetraphenylphosphonium chloride, the peak potential of tartrazine is shifted to a more negative potential and the peak current is increased by 140y0. The peak potential of Sunset Yellow FCF remains unchanged but the peak current is decreased by 50%. TABLE I EFFECT OF TETRAPHENYLPHOSPHONIUM CHLORIDE (TPPC) CONCENTRATION ON THE D.P. POLAROGRAPHIC PEAKS OF TARTRAZINE AND SUNSET YELLOW FCF Tartrazine concentration = 0.4 p.p.m. ; Sunset Yellow FCF concentration = 1.84 p.p.m. TPPC concentration, p.p.m. . . . . 0 9 20 40 60 90 100 380* 900 1200 i , ~ (Sunset Yellow F c F ~ ~ ~ A .. 5.4 4.5 3.20 1.60 2.10 2.50 - 2.60 - 2.56 i,? (tartrazine) /PA . . . . 0.70 - 0.67 1.49 1.65 - 1.65 1.68 1.67 - * TPPC concentration used in recommended procedure. 1 E, = - 0.65 V (in presence or absence of TPPC). Calibration graphs for both colouring matters deviate slightly from rectilinearity at higher concentrations owing to adsorption effects at the dropping-mercury electrode but are not affected by the presence of the other colouring matter in the proportions normally found in soft drinks. The latter effect is illustrated in Fig. 2, which shows typical polarograms obtained to produce a calibration graph for tartrazine in the presence of Sunset Yellow FCF. The recommended procedure for the determination of Sunset Yellow FCF and tartrazine in sparkling orangeade is as follows.Pipette 10 ml of 0.01 M tetraphenylphosphonium chloride solution into a 50-ml beaker. Add 5 ml of 0.01 M tetraphenylphosphonium chloride solution and 20 ml of pH 1.9 Britton - Robinson buffer. Adjust the pH to 9 with sodium hydroxide solution and dilute to 50 ml in a calibrated flask. Deoxygenate a portion of this solution in a polarographic cell and polarograph it between -0.3 and -1.0 V. E, = -0.73 V (in absence of TPPC) or -0.80 V (at all TPPC concentrations > O p.p.m.1.726 FOGG AND YO0 : DIRECT DIFFERENTIAL-PULSE POLAROGRAPHIC Analyst, VOZ. 104 The procedure was tested by using a sparkling orangeade (42 p.p.m. of Sunset Yellow FCF and 20 p.p.m. of tartrazine) prepared from the basic syrup and the samples of colouring matter.The result obtained for ten determinations was 41.4 p.p.m. of Sunset Yellow FCF (coefficient of variation = 1.4%) and 20.5 p.p.m. of tartrazine (coefficient of variation = . ~. 1 .O%). I c C B A!. -0.80 -0.50 PotentialN Fig. 2. D.p. polarograms produced in obtaining a cali- bration graph for tartrazine in the presence of Sunset Yellow FCF (1.2 p.p.m. in measured solution). Tartra- zine concentration in mea- sured solution: (A) 0, (B) 1.0, (C) 2.9, (D) 5.1 and (E) 7.5 p.p.m. Determination of Tartrazine and Green S in Sparkling Limeade Tartrazine is best determined under the solution conditions above for sparkling orangeade, viz., at pH 9 in the presence of tetraphenylphosphonium chloride. Under these conditions Green S is reduced at a similar potential to tartrazine but fortunately, because only a small proportion of Green S relative to tartrazine is normally present in sparkling limeade and because the Green S wave is depressed in the presence of the phosphonium salt, interference of Green S in the determination of tartrazine is negligible.Green S is best determined at pH 4 and the addition of tetramethylammonium chloride was found to improve the base line considerably. This improvement seems to arise owing to a partial suppression of the polarographic maximum of the tartrazine. Calibration graphs for Green S in the presence of tartrazine are rectilinear. Tartrazine can be determined after re-adjusting the pH to 9 and adding tetraphenylphosphonium chloride. The recommended procedure for the determination of Green S and tartrazine in sparkling limeade is as follows.Add 5 ml of 1 M tetramethylammonium chloride and 10 ml of pH 1.9 Britton - Robinson buffer and adjust the pH to 4 with sodium hydroxide solution. Dilute to 50 ml in a calibrated flask. Deoxygenate a portion of this solution in a polarographic cell and polarograph it between -0.45 and -0.85 V. Pipette 10 ml of sparkling limeade into a 50-ml beaker.A %gust, 1979 727 Carefully re-adjust the pH of the solution in the cell to 9 with several drops of 4 M sodium hydroxide solution and add 10 mg of tetraphenylphosphonium chloride. Pass nitrogen through the solution to aid dissolution of the solid and to deoxygenate the solution, and polarograph the solution between -0.6 and -1.0 V. The procedure was tested using a sparkling limeade (20 p.p.m.of tartrazine and 2 p.p.m. of Green S) prepared from the basic syrup and the samples of colouring matter. Typical polarograms are shown in Fig. 3, DETERMINATION OF FOOD COLOURS I N SOFT DRINKS 1 / 0.2 1-1 A Green S 1 B 0.5 p A IL I I I I -0.60 -0.20 -0.90 -0.50 PotentialIV Fig. 3. Typical d.p. polarograms of Green S and tartrazine in limeade: (A) pH 4.1 and (B) pH 9.2. The result of ten determinations was 2.1 p.p.m. of Green S (coefficient of variation = 3.4%) and 19.6 p.p.m. of tartrazine (coefficient of variation = 1.9%). At pH 9 the Green S gives a small peak a t -0.75 V that is masked by the tartrazine peak. At the levels of Green S in the limeade this Green S peak is negligible; at very high concentrations of Green S a shoulder appears on the tartrazine peak.Determination of Amaranth and Green S in Blackcurrant Health Drink The sugar in the blackcurrant health drink syrup gives a small peak at pH 4 at the same potential as Green S. At pH > 6 this peak is absent. Amaranth gives a broad polaro- graphic maximum at pH > 6 but this can be suppressed by the addition of tetramethyl- ammonium chloride; tetraphenylphosphonium chloride, tetraethylammonium chloride and Triton X-100 do not suppress the maximum as effectively. The recommended procedure for the determination of amaranth and Green S in black- currant health drink is as follows. Pipette 5 ml of blackcurrant health drink into a 50-ml beaker. Add 5 ml of 1 M tetramethylammonium chloride solution and 10 ml of pH 1.9 Britton - Robinson buffer.Adjust the pH to 7.8 with sodium hydroxide solution and dilute to 50 ml in a calibrated flask. Transfer a portion of this solution to a polarographic cell, deoxygenate it for 10 min and polarograph the solution between -0.30 and -0.80 V. The procedure was tested using a blackcurrant health drink (250 p.p.m. of amaranth and 4 p.p.m. of Green S) prepared from blackcurrant health drink syrup and the samples of colouring matter. Clearly, the Green S is being polarographed near its detection limit for the amount of amaranth present but the procedure can be used as a limit test for this colouring matter. The result of ten determinations for amaranth was 245 p.p.m. with a coefficient of variation of 1.3%. A typical polarogram is shown in Fig.4.728 FOGG AND YOO: DIRECT DIFFERENTIAL-PULSE POLAROGRAPHIC Artalyst, VoZ. 104 A P- 0.2 PA Green S -0.80 -U.30 PotentiaVV Fig. 4. (A) Typical d.p. polaro- gram for Green S and amaranth in blackcurrant health drink. (B) Polarographic peak of 0.6 p.p.m. of Green S in the measured solution in the presence of a smaller pro- portion of amaranth (amaranth concentration = 2.5 p.p.m.). Discussion The established method of identifying food colouring matters is thin-layer chromato- graphy. This excellent method has the advantage of being inexpensive and the disadvantage of being difficult to quantify. Although relatively costly equipment is required, HPLC is now the nearest approach to an ideal method for the identification and determination of food colouring matters, combining efficient separations with precise quantification. Both thin- layer chromatographic and HPLC methods, however, generally require separation of the food colouring matters from even simple food matrices before application to the plate or column.The diff erential-pulse polarographic procedures described here are applied directly to the soft drinks without the need for prior separation of the colouring matters on wool or poly- amide. The differential-pulse polarographic peaks are sharp and afford some measure of identification. A 600-mV scan takes 5 min at 2 mV s-l but a scan rate of 5 mV s-1 might be acceptable for routine use; deoxygenation takes about 10min but can be effected on a batch basis in advance. Therefore, the procedures can be considered for routine use. Micro- processor-controlled polarographs are available that have full sample and data handling facilities after the solution-preparation step. The authors thank Mr. B. A. Saturley of Beechams Products for the provision of samples They also thank Dr. N. T. Crosby of the Laboratory of the and for helpful discussions. Government Chemist and Dr. W. P. Hayes for helpful discussions.August, 1979 DETERMINATION OF FOOD COLOURS IN SOFT DRINKS 729 References 1. 2. 3. 4. 5. 6. 7. 8. 9. “Food and Drugs: Composition and Labelling: The Colouring Matter in Food Regulations 1973,” Macek, K., Editor, “Pharmaceutical Applications of Thin-layer and Paper Chromatography,” Pearson, D., “The Chemical Analysis of Foods,” Seventh Edition, Churchill Livingstone, Edinburgh, Venkataraman, K., Editor, “The Analytical Chemistry of Synthetic Dyes,” John Wiley, New York, Wittmer, D. P., Nuessle, N. O., and Haney, W. G., Jr., Analyt. Chem., 1975, 47, 1422. Knox, J. H., and Laird, G. R., J . Chromat., 1976, 122, 17. Zuman, P., “Organic Polarographic Analysis,” Pergamon Press, Oxford, 1964. Schill, G., Ion Exchange Solvent Extraction, 1974, 6 , 1. Johansson, I. M., Wahlund, K. G., and Schill, G., J . Chromat., 1978, 149, 281. SI 1973 No. 1340, HM Stationery Office, London. Elsevier, Amsterdam, 1972, p. 618. 1976, p. 50. 1977, p. 466. Received November 29th, 1978 Accepted March 23rd, 1979

ISSN:0003-2654    

DOI:10.1039/AN9790400723

出版商:RSC    

年代:1979

数据来源: RSC

摘要:

730 Analyst, August, 1979, VoL 104, PP. 730-738 Ion-selective Polymeric-membrane Electrodes with Immobilised Ion-exchange Sites Part I. Development of a Calcium Electrode L. Ebdon, A. T. Ellis and G. C. Corfield Department of Chemistry, She$eld City Polytechnic, Pond Street, Shefield, S1 1 WB A new type of ion-selective electrode is described in which ion-exchange sites are immobilised in a polymeric membrane by covalent bonding. Membranes were prepared by cross-linking a styrene - butadiene - styrene triblock copolymer with triallyl phosphate. After subsequent hydrolysis these membranes were evaluated as Ca2+ sensors. The electrodes formed exhibited an extended Nernstian response to Ca2+ (10-'-10-.6 M) and good selectivity over other alkaline earth and alkali metals. Such electrodes offer very fast response times, extended lifetimes and a wide y H working range.The possibilities of this new class of ion-selective electrode are also discussed. Keywords : Ion-selective electrode ; calcium analysis ; electroanalytical chemistry ; polymeric membrane ; immobilised ion-exchange groups Ion-selective electrodes offer considerable advantages in analytical chemistry. They are simple and inexpensive to operate, sensitive and suited to on-line measurement. Thus, in addition to their application to laboratory analyses, applications to pollution monitoring, in vivo biological measurements and process control have been proposed. The major problems with ion-selective electrodes seem to be that certain of them lack selectivity and mechanical strength. The range of ion-selective electrodes until about 10 years ago was limited to classical glass electrodes or electrodes using certain crystalline membranes. The introduction of electrodes based on liquid ion exchangers, e.g., the calcium electrode first reported by Ross1 in 1967, greatly extended the range of available electrodes.Ross used rather impure calcium didecyl phosphate (DDP) as sensor in conjunction with dioctylphenyl phosphonate (DOPP) solvent mediator supported on a Millipore filter. This type of electrode has been marketed com- mercially (e.g., Orion 92-20). Such electrodes were recognised as a significant advance and were the first of a range of liquid ion exchanger ion-selective electrodes. In their design it was necessary to keep three liquids, the internal filling solution, the ion exchanger and the test solution, in electrolytic contact but to prevent mixing.This posed some constraints as did the limited lifetime of the electrodes, caused by leaking of the DDP and DOPP. This leaking also tends to raise the obtainable limit of detection. Severe interference froin H+ and some heavy metals and the expense of construction have also been cited as drawbacks of the Ross-type calcium ion-selective electrode. In 1970, Moody et aZ.2 reported a modification of the Orion system in which they used pure DDP and DOPP solvent mediator incorporated into a poly(viny1 chloride) (PVC) matrix. This convenient construction had the benefits of cheapness and ease of fabrication over the Ross system, but such electrodes still had a limited lifetime as the exchanger was entangled in the PVC matrix and so could still be leached out.The advantages of the PVC system were, however, obvious and it can be simply constructed3 or purchased from commercial suppliers, finding widespread use for many applications. Since this early work, the sensor and solvent-mediator systems have been the subject of much research, resulting in further improvements in the Ca2+ ion-selective electrode. In particular, it is now recognised that the solvent mediator performs an essential function in controlling the selectivity of the liquid ion exchanger electrodes. As an example, the use of calcium bis [di(2-ethylhexyl) phosphate] as sensor results in a Ca2+ ion-selective electrode if DOPP is used as solvent and as a divalent (water hardness) Ca2+/Mg2+ test electrode if decan-1-01 is used.4 From work carried out by RdiiCka et aL5 and later by Moody et aZ.,6 a better sensor for Ca2+ ion-selective electrodes seems to be calcium bis[di-4-(1,1,3,3-tetra- methylbutylphenyl) phosphate] with DOPP.Work has also been carried out on the effectsEBDON, ELLIS AND CORFIELD 73 1 of nitrating7ss the sensor and/or the mediator, but although giving useful information on the synergistic sensor - mediator system, nitration gave no real improvement in electrode perf onnance. In addition to the use of the organophosphoric acid type of sensor, good results have been reported for a Ca2+ ion-selective electrode using the non-cyclic neutral carrier NN'-di [ (1 1- ethoxycarbonyl)undecyl]-NN'-4,5-tetramethyl-3,6-dioxaoctanediamide as sensor and 2- nitrophenoxyoctane as solvent rnediat~r.~ Although this system also shows high Ca2+ selectivity, it is also of the PVC matrix type and so the system will be leachable and subject to limited lifetimes, particularly when used in flowing systems.A neutral carrier system has also been used in which the tetraphenylborate salts of a calcium adduct of poly(propy1ene glycol),l0 in conjunction with DOPP, is entangled in a PVC matrix. The resulting electrodes showed, in general, poorer Ca2+ selectivity than the phosphate systems and had shorter lifetimes. I t can be seen that there has been much work directed towards understanding and therefore improving the sensor exchanger - mediator system, but very little work has been carried out with different matrices.Schultz et aZ.ll made an effort in 1968 to incorporate a dialkyl phosphate sensor into collodion, with little success, and in 1972 Griffiths et aZ.12 reported poor electrode quality for some cellulose, collodion and pyroxylin entangled membranes. They suggested that the hydrophilicity of these materials is undesirable as leaching of the exchanger takes place very quickly. Schafer13 used poly(viny1 isobutyl ether) as a matrix material to prepare a divalent ion electrode, which showed no advantage over the PVC system and was again a polymer-entangled system from which the exchanger could be leached. The advantages of PVC for electrode fabrication arise from its ready availability and lack of hydrophilicity. PVC is, however, stabilised by organometallic compounds that can block the ion-exchange sites and when these are removed its stability, e.g., to ultraviolet light, is poor.More significantly, the ion exchanger in the membranes described above is not covalently bound. Thus, eventually the ion exchanger will migrate from the membrane and the resultant deterioration in electrode response as this leaching proceeds limits the useful lifetime of the membrane. We are therefore studying different polymer systems and the possibility of immobilising ion-exchange sites into polymer matrices by covalent bonding. The immobilisation of the active ion-exchange groups should overcome the problems associated with leaching referred to above and lead to the development of ion-selective electrodes with extended lifetimes and sensitivity.Additionally, the careful selection of polymer systems should perrnit optimisa- tion for enhanced mechanical and electrochemical properties. This new approach to the fabrication of ion-selective electrode membranes could lead to greater control over steric properties and hence to a significant improvement in selectivity. Two other groups of workers have also directed attention towards covalently bonding ion-exchange groups to a polymer, although their work became known to us only after we had started the study described here. Keil et aZ.14 phosphorylated a vinyl chloride - vinyl alcohol copolymer with decyl dihydrogen orthophosphate. The copolymer was then mixed with PVC in which DOPP was entangled, to produce a Ca2+ ion-selective electrode.Extended electrode lifetimes were not obtained in their study. bound ion-exchange groups to the ends of PVC chains both by the use of an amine as chain transfer agent during polymerisation and by the use of the 'SO3- radical anion as polymerisation initiator. The electrodes produced exhibited selectivity towards anionic or cationic surfactants and, although the electrode lifetimes were extended in comparison with the liquid ion exchanger surfactant ion-selective electrodes, the plasticiser used (tricresyl phosphate) was leached from the polymer and this limited the lifetime of the electrodes. In the study reported here, membranes were prepared by cross-linking a styrene- butadiene - styrene (SBS) triblock copolymer with triallyl phosphate using free-radical initiation.After subsequent hydrolysis to give covalently bound pendant dialkyl phosphate exchanger groups, these membranes were evaluated as sensors for Ca2+ ion-selective electrodes. Cutler et Experimental Chemicals Tetrahydrofuran (THF) was freshly distilled from aluminium lithium hydride to dry it732 EBDON et al. : ION-SELECTIVE POLYMERIC-MEMBRANE Anallyst, VoZ. 104 and to remove stabilisers. Triallyl phosphate (TAP) was used as received (Aldrich Chemical Co.) but the initiator a,a-azobisisobutyronitrile (ABIN) was recrystallised from methanol. Analytical-reagent grade chemicals were used during electrode evaluation. The polymer used was Cariflex SBS 1101 (Shell Chemicals, London), which was purified by dissolving it in THF and re-precipitating it in cold, well stirred methanol.Gel permeation chromato- graphy showed the number-average relative molecular mass (Mn) as 9.8 x lo4 g mol-l with 70% SBS triblock poly(styrene-h-butadiene) , 26% SB diblock and 4% homopolystyrene content. The polydispersity (Mw/Mn) was 1.51 and 100-MHz nuclear magnetic resonance spectroscopy indicated 27% m/m of polystyrene in the polymer, and hence 73% m/m as polybutadiene ; 300-MHz nuclear magnetic resonance spectroscopy showed the butadiene units to be 90% 1,4- and 10% 1,Zconfiguration. Preparation of Polymeric Membranes Re-precipitated SBS (4 g) was dispersed in the solvent (40 cm3) and allowed to dissolve overnight, after which the required amounts of TAP and ABIN were added with mixing. The polymer solution was then poured into a glass ring (100 mm i.d.) resting on a Cellophane sheet on a glass plate.A heavy weight and a front-silvered mirror were arranged such that a seal was maintained with the plate and ultraviolet radiation could be reflected normally on to the curing membrane. Membranes were usually cured after 6 h, depending on solvent volatility, forming strong, clear membranes about 1 mm in thickness. The membranes were removed from the Cellophane and portions hydrolysed to give the master membrane. Evaluation of Electrodes Discs, 10- in diameter, were cut from the master membranes and stuck to the end of clear, plasticised PVC tubing by using a cyanoacrylate adhesive (IS 12, Loctite, Dublin). Calcium chloride solution (10-1 M) saturated with silver chloride was used as the internal reference solution, the electrodes being mounted on modified pH electrodes in a similar fashion to PVC membrane electrodes3 The mounted electrodes were soaked overnight in M Ca2+ solution in order to replace the Na+ or K+ form of the exchanger with the Ca2+ form.The time required for soaking was again dependent on the degree of cross-linking, but 12 h was the maximum required by any electrode. EMF measurements were made using the cell - - Hg.HgC1, I KCl (sat.) 11 test solution I membrane I CaC1, (0.1 M) I AgC1.Ag using varying test solutions. A single-junction reference electrode (Orion 90-01, Orion Research Inc., Cambridge, Mass., USA) and a digital voltmeter (Orion 701) reading to h O . 1 mV were used. The test solutions were stirred magnetically and maintained at 25 & 0.5 "C.Calcium-ion activities were calculated from ac,2+ = cy where y, the activity coefficient, was calculated using an extension of the Debye - Huckel equation: -log y(Q+ = z2 (;yd7 -- - 0.H) where I , the ionic strength, is given by I = &2cizi2. Potentiometric selectivity coefficients were determined by a mixed solution method using an interferent-ion level of 10-3~and changing the calcium-ion concentration. Coefficients were evaluated using the IUPAC recommended rnethodl6 : where z = 2 and n = charge of interference ion M, a,*+ = M and a,,2+ is taken at the point when the mixed solution calibration differs from the extrapolated linear portion by l8/2 mV (Le., 9 mV for Ca2+). Hydrogen-ion interference is expressed as the pH range over which there is no measurable change in electrode potential at a constant calcium-ion activity.The pH was changed by using 0.1 M hydrochloric acid and 0.1 M sodium hydroxide solution in conjunction with a combination glass pH electrode (Activion Ltd., Halstead, Essex) and a pH meter (Pye, Model 290, Pye Unicam, Cambridge).August, 1979 ELECTRODES WITH IMMOBILISED ION-EXCHANGE SITES. PART I Results and Discussion 733 Incorporation of Ion-exchange Groups The ion-exchange group chosen for this study was the classical dialkyl phosphate group, as this offered a well tried system that could be incorporated into a polymeric structure by a simple copolymerisation mechanism. A poly(styrene-b-butadiene) triblock elastomer (SBS) was chosen as the polymer because it contains the necessary C=C unsaturation for cross- linking, has mechanical behaviour similar to natural rubber vulcanisates without requiring cross-linking, can be dissolved in solvents and is easy to process.In order to incorporate the phosphate exchanger groups, the unsaturated C=C bonds are cross-linked by a free- radical initiated addition mechanism using triallyl phosphate (TAP). This mechanism can be represented by the scheme shown in Fig. 1. uv (CH,),CN=NC(CH,), - 2(CH,),C' + N, I CN 1 1 CN CN ABlN R' R'+ - C H 2 ~ H - C H 2 C H = C H C H , - - ~ ~ ~ c ~ - ~ ~ , ~ ~ I I - S B S -S-CH,CH=CHCH,-CH,CH-S- I ),CH R-CH, (CH2=CHCH20)3P=0 I I -S-CH,CH=CHCH,-CH,CH-S - CH / \ R-CH, CH,-'CHCH o 2 \ ,P=O (3) (C H 2=C H CH , 0) , S-€34 I 0 II I ,CH2 I I -S-CH, CH=CHCH2-CH2CH- S CH A\ R-CH, CH,-CHCH20-P(OCH2CH=CH,), ( 4 ) 'CH -S-CH,CH=CHCH,--CH,CH-S - Fig.1. Immobilisation of phosphate groups. This simplified mechanism shows that initiation of SBS should occur [(l) and (2)], leading to incorporation of phosphate groups in the structure via covalent bonding (3) and cross- linking (4). In order to obtain the dialkyl phosphate sensor unit, it is necessary to hydrolyse the trialkyl phosphate system with alkali (Fig. 2). The stability of the resonance-stabilised dialkyl phosphate salt is such that further hydrolysis to remove other alkyl groups is very difficult and will not proceed under the conditions used. This leaves the dialkyl phosphate734 EBDON et aZ. : ION-SELECTIVE POLYMERIC-MEMBRANE Analyst, VoZ.104 group covalently bound to the polymeric membrane. Strong alkaline hydrolysis results in attack on the polymer itself, presumably at the residual unsaturation, and at a sodium hydroxide concentration of approximately 25% m/m the polymeric material breaks down completely. One disadvantage of this hydrolysis procedure is that the K+ or Na+ salt of the acid is formed and so the electrodes must be conditioned overnight in 1 0 - 2 ~ Ca2+ to create the Ca2+ form of the exchanger. OH - + +-OK Fig. 2. Hydrolysis of trialkyl phosphate grouping. The initial stage of this work was concerned with optimising the polymerisation and hydrolysis steps. The first consideration was the amount of cross-linking in the membrane, which is controlled primarily by the amount of TAP and ABIN present, with factors such as temperature and light intensity of lesser importance.Table I shows the effect of changing the amount of TAP and ABIN on the physical and electrochemical properties of the cast membrane. From these results, it can be seen that the best membrane (B) resulted from 5% m/m of TAP with 2.5% m/m of ABIN. High levels of TAP resulted in oily, non- functional membranes, and larger amounts of ABIN reduced the response and the clarity of the membrane. TABLE I EFFECT OF MEMBRANE COMPOSITION ON PHYSICAL AND ELECTROCHEMICAL PROPERTIES TAP, ABIN, THF Physical properties of Membrane % m/m yo m/m solubility* cast membranes A .. . . 2.5 0 Soluble Elastic, clear B . . .. 6 2.5 Insoluble Elastic, clear, slightly c . . . . 2.5 5 Insoluble Elastic, opaque D .. . . 5 10 Insoluble Clear with opaque E . . . . 5 15 Insoluble Bubbled, opaque F .. . . 15 2.5 Insoluble Elastic, clear, yellow G .. . . 50 5 Insoluble Oily, yellow, rigid H .. .. 100 6 Insoluble Thick, rigid with oily yellow inclusions inclusions surface * Before hydrolysis. t After hydrolysis for 6 h in 1% sodium hydroxide solution under reflux. Electrochemical propertiest Short linear range, low Functional membrane slope No response No response No response Very short, linear range, low slope Poor response Poor response Table I1 shows the effects of both the hydrolysis conditions and the degree of cross-linking on the properties of the membrane. The use of mild aqueous alkaline conditions for hydrolysis gave membranes that showed near-Nerns tian slopes but with short lifetimes.Those with longer lifetimes had significantly sub-Nernstian calibration slopes. Aqueous hydrolysis produced membranes that were more soluble in THF than the unhydrolysed membranes, presumably owing to further attack on the polymer structure in addition to hydrolysis of phosphate groups. Hydrolysis with 5% m/m methanolic potassium hydroxide solution proceeded smoothly, resulting in an insoluble membrane with a Nernstian response and an extended lifetime (membrane 5 ) . Reductions in the amounts of TAP and ABIN eventually gave membranesAugust, 1979 ELECTRODES WITH IMMOBILISED ION-EXCHANGE SITES. PART I TABLE I1 EFFECT OF MEMBRANE COMPOSITION AND HYDROLYSIS CONDITIONS ON THE PHYSICAL AND ELECTROCHEMICAL PROPERTIES OF POLYMERIC ION-EXCHANGE ELECTRODES TAP, ABIN, Membrane yo m/m % m/m Hydrolysis* 1 5.0 2.5 5 h, 1% aq.NaOH 2 4.5 2.5 3h,1% aq.NaOH 3 4.86 2.6 1h,1% aq.NaOH 4 4.54 2.5 20 h, H,O 5 4.4 2.5 5h,5y0 KOH-MeOH 6 4.4 2.5 6h,10% KOH-MeOH 7 2.2 1.26 6h,6% KOH-MeOH 8 2.2 1.26 5h, 10% KOH-MeOH 9 1.0 0.55 6h,5% KOH-MeOH 10 1.0 0.55 5 h, 10% KOH - MeOH 11 0 0 5h,6%KOH-MeOH Solubility Slope/ in THF mV decade-' Soluble + 16 Soluble (12 h) + 24.2 Slightly soluble + 29.5 Slightly soluble + 29 Insoluble + 30 Slightly soluble + 35, decreasing Insoluble + 24. decreasing to +17 after 20 d to +is after 3 ;i Soluble - 19 Soluble + 25 Soluble - 12 Soluble - 17 735 Lifetime >6 months >6 months 10 days -3 days >6 months 20 days 5 days 3 days 3 days 3 days > 3 months Appearance Milky, opaque Clear, becoming yellow - brown on hydrolysis Mainly clear, yellow on hydrolysis Dark brown and brittle Clear, pale yellow, tough and elastic Clear, yellow - brown Clear, pale yellow Brown, becoming brittle Clear and colourless Brown Clear and colourless.Pure SBS membrane * Conditions refer to reflux. After hydrolysis. that, after hydrolysis under similar conditions to those used with membrane 5, did not appear to be strongly cross-linked and showed a calibration slope below Nernstian with a short life- time. When the hydrolysis was more vigorous, a similar increase in solubility and a marked tendency to age became apparent (membrane 6). Membranes 8 and 10 showed negative calibration slopes, which may have been due to the passage of small anions through these membranes, which were not cross-linked.Porous membranes would be expected to respond to C1- ions because of the internal reference electrode. The properties of membrane 5 provide clear evidence for the immobilisation of the phosphate sensor in the polymer structure by covalent bonding. The insolubility of the polymeric material indicates that cross-linking has occurred and the presence of C-0-P and P=O absorption bands in the infrared region, using both transmission and attenuated total reflectance methods, shows the incorporation of phosphorus into the membrane. Quantitative phosphorus analysis was carried out by using acid digestion followed by a molybdenum blue spectrophotometric method. Tests on membrane 5 showed no leaching of phosphorus even after 10 d in THF, confirming the covalent binding of the phosphate group to the polymer structure.Evaluation of Electrode Response In all instances, the range of linear response is wide and in some instances extends below 1 0 - 6 ~ , although the use of diluted standards rather than a calcium-buffered system at this level is unlikely to give reliable results. This significantly extended performance on the low concentration side TABLE I11 RESPONSE AND SELECTIVITY DATA FOR POLYMERIC ION-EXCHANGE ELECTRODES Table I11 shows the electrode responses using the membranes prepared. Effective linear Membrane range/M Slope a t 25 OC/mV decade-' 1 10-6-10-2 + 16 2 10-6-10-1 +24.2 3 10-6-1 0-1 + 29.5 4 10-6-10-1 + 29 5 lo-a-lo-' 4- 30 6 10-6-10-1 + 35, decreasing to 7 10-0-10-1 + 24, decreasing to $17 after 20 d +15 after 3 d 8 10-'-10-' -19 9 10-0-10-' + 25 10 10-~-10-' - 12 11 10-~-10-' - 17 For decade change of 10-6-10-4 M Caa+ concentration.t Concentration of interferent ion 10-8 M. Static response time*/s 5 5 -120 5 2 2 5 5 6 5 6 Selectivity coefficient,t $gtM Ba Mg K Na pH range 0.3 2 00 30 5-10 0.80 0.82 0.35 0.17 4.5-10 0.4 -lo2 -10' Major interference from all cations 0.8 0.3 2.5 6 4-10 0.65 0.2 12 6 4-10 0.8 Initially negligible but 20 mV pH-I increases with ageing Responding to anions 0.17 7 20 4.5-10 Responding to anions Responding to anions736 EBDON et al. : ION-SELECTIVE POLYMERIC-MEMBRANE Analyst, Vol. 104 compared with the Orion 92-20 type is thought to be due to the non-leachable ion-exchange groups and a calcium-buffered system might be expected to extend the range below M.The range above 10-1 M Ca2+ has not been investigated, the range of concern in this study being 10-5-10-2 M Ca2+. Some initial comments on the calibration slopes of some electrodes have been made above. The results obtained confirmed that optimum electrochemical properties were obtained with electrodes that were cross-linked and contained a high density of bonded phosphate groups in the membrane. It was not possible to increase the phosphate content indefinitely without producing membranes with poor physical characteristics. Early results showed that it was necessary to use a sufficient intensity of ultraviolet irradiation during polymerisation to ensure cross-linking. Membranes 1 and 2 gave electrode slopes of +16 and +24.2 mV decade-l, and this was attributed to insufficient radiation, which resulted in poor initiator efficiency and hence lack of cross-linking.Membrane 11 was a pure SBS membrane and when treated under conditions identical with those used with membrane 5 gave a THF- soluble membrane with a negative calibration slope. Again, we would conclude that this pure SBS membrane is porous to small anions. Response times were generally very short, with static times being in the region of 5 s for the range 10-1-10-4 M Ca2+, increasing to 15 s for the and lo4 M solutions. Dynamic response times were measured by making 10-fold changes in Ca2+ concentration by addition of concentrated calcium chloride solution from a piston syringe to 100 cm3 of well stirred solution.The whole range from 10-6 to 10-1 M Ca2+ was covered in this manner and gave response times of 1 s for most solutions, only increasing to about 5 s for 10-6 M solutions. The reason for these fast response times compared with other organophosphorus-based electrodes is thought to lie in the mechanism of the electrode. In the liquid ion exchanger and PVC electrodes, the response relies on an ion-transport mechanism, whereas in this type of exchanger the mechanism might in some ways be analogous to the ion-exchange mechanism found in glass electrodes. Selectivity studies were initially performed on a limited number of cations showing the general order of selectivity to be Ca2+ > Ba2+ > Mg2+ > M+. Potentiometric selectivity coefficients for the divalent ions were kzFBa = 0.8 and kglt,, = 0.3, whilst interference from monovalent cations was less, thus making it possible to generate the K+ or Na+ form of the exchanger on hydrolysis and then to convert this into the Ca2+ form by soaking over- night in 10-1 M calcium chloride solution. These selectivity data are also presented, perhaps more meaningfully, in calibration graphs as in Fig.3, where the interferent level is maintained at The influence of increasing sodium concentration on kzitN, and electrode response is presented in Fig. 4, showing this type of electrode to be functional in the presence of 10-2 M Na+, but with loss of Ca2+ response at the 10-1 M Na+ level. The H+ interference was evaluated by varying the pH at a constant Ca2+ concentration M) (see Fig.5). Table I11 includes the pH range over which there is no measurable change in the electrode potential for different electrodes. With the electrodes reported M. > E ‘3 .- *.’ 0) Q a3 0 *.’ - - - 10-6 10-5 10-4 lo-’ Activity, aCa2+ Fig. 3. Calibration graphs for a calcium ion- selective polymeric-membrane electrode with immobilised ion-exchange sites. The inter- ferent effect of Ba2+, Mg2+, K+ and Na+ solutions ( l o - 3 ~ as chloride in each instance) on this calibration is also shown.August, 1979 ELECTRODES WITH IMMOBILISED ION-EXCHANGE SITES. PART I 737 50 > E 1. .- *I’ C 8 0 n - s -50 ’ 1O-’M Na Activity, aCa2+ Fig. 4. Effect of Na+ concentration on the calibration graph for a calcium ion-selective membrane electrode with immobilised ion-exchange sites.here, for well cross-linked electrodes the pH range was wide (pH 4-10) and showed none of the “dips” seen with earlier organophosphate-based systems. This is perhaps to be expected as these “dips” were attributed to the solvent mediator and the system here has no such mediator. The useful alkaline end is limited analytically by the formation of calcium hydroxide, although the electrode may still be capable of giving the true Ca2+ activity. Poorly cross-linked membranes showed narrower pH working ranges and were unstable at low pH values; there was, however, no poisoning of the electrodes by Hf. 2 3 4 5 6 7 8 9 10 11 PH Fig. 5. Graph of electrode response against pH in 10-3 M calcium chloride solution. In terms of general analytical behaviour, the electrodes yielded a reproducibility of & 1 mV, but the poorly cross-linked membranes resulted in poorer reproducibility, especially when they also had a limited lifetime.Drifts with the better electrodes were of the order of 1 mV d-l, but again the poorer cross-linked electrodes gave higher drifts (about 5 mV d-l) and increased noise levels. The lifetime of this type of electrode was found, as predicted, to be significantly longer than those of polymer entrapped liquid ion exchanger electrodes. Poorly cross-linked membranes lasted only a matter of days, but the well cross-linked membranes gave electrodes with lifetimes in excess of 6 months. These latter membranes showed no physical deteriora- tion and the electrode response remained Nernstian with good selectivity. In the less cross-738 EBDON, ELLIS AND CORFIELD linked membranes, shorter lifetimes of less than 1 month are accompanied by a noisy response, decreasing calibration slope and increasing monovalent cation interference.The lifetimes quoted are for electrodes stored in 1 0 - 2 ~ Ca2+ solution and calibrated at least once per week. Conclusion It is possible to form immobilised ion-exchange sites in an unsaturated polymer matrix by cross-linking SBS with triallyl phosphate and hydrolysing the resulting membrane to give pendant dialkylphosphoric acid exchange groups. The best membrane was prepared by cross-linking 4 g of SBS with 4.5% m/m TAP and 2.5% m/m ABIN initiator and hydrolysing the polymer with methanolic potassium hydroxide solution ; the resulting membrane gave an ion-selective electrode of high physical strength and stability.This ion-selective electrode gave a Nernstian response to Ca2f ions with selectivity over Ba2+, Mg2+ and alkali metal cations. It showed a wide pH working range (pH 4-10) and a lifetime in excess of 6 months. Further work on the polymer system and ion-exchange sites is expected to result in a new family of ion-selective electrodes extending to other ions. Early indications are that this type could combine improved mechanical properties, such as robustness and lifetime, with advantageous electrochemical properties, including fast response speeds and lower limits of detection. We are grateful to the Trustees of the Analytical Chemistry Trust Fund for the award of an SAC studentship to one of us (A. T. E.), which has made this work possible. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. References Ross, J. W., Science, N.Y., 1967, 156, 1378. Moody, G. J., Oke, R. B., and Thomas, J. D. R., Analyst, 1970, 95, 910. Craggs, A., Moody, G. J., and Thomas, J. D. R., J . Chew Educ., 1974, 51, 541. Craggs, A., Keil, L., Moody, G. J., and Thomas, J. D. R., Talanta, 1975, 22, 907. RbiiCka, J., Hansen, E. H., and Tjell, J. C., Analytica Chim. Acta, 1973, 67, 155. Moody, G. J., Nassory, N. S., and Thomas, J . D. R., Analyst, 1978, 103, 68. Jagner, D., and 0stergaard-Jensen, J. P., Analytica Cham. Acta, 1975, 80, 9. Keil, L., Moody, G. J., and Thomas, J. D. R., Analytica Chim. Acta, 1978, 96, 171. Amrnann, D., Giiggi, M., Pretsch, E., and Simon, W., Analyt. Lett., 1975, 8, 709. Jaber, A. M. Y., Moody, G. J., and Thomas, J. D. R., Analyst, 1977, 102, 943. Schultz, F. A., Petersen, A. J., Mask, C. A., and Buck, R. P., Science, N.Y., 1968, 162, 267 Griffiths, G. H., Moody, G. J., and Thomas, J. D. R., Analyst, 1972, 97, 420. Schafer, 0. F., Analytica Chim. Acta, 1976, 87, 495. Keil, L., Moody, G. J., and Thomas, J. D. R., Analyst, 1977, 102, 274. Cutler, S. G., Meares, P., and Hall, D. G., J . Electroanalyt. Chem., 1977, 85, 145. IUPAC, Pure Appl. Chem., 1976, 48, 129. Received October 31st, 1978 Accepted Muvch 13th, 1979

ISSN:0003-2654    

DOI:10.1039/AN9790400730

出版商:RSC    

年代:1979

数据来源: RSC

摘要:

Analyst, August, 1979, Vol. 104, pp. 739-749 739 High-frequency Microtitrimetric Determination of Acidic and Basic Constituents in Lubricating Oils Part ll.* Determination of Total Base Number T. Fernhdez, J. M. Rocha, N. Rufino and A. Garcia Luis Compafiia Espafiola de Petrdleos, S.A . , Research Laboratory, Santa Cruz de Tenerife, Canary Islands and F. Garcia Montelongo Department of Analytical Chemistry, University of La Laguna, Tenerife, Canary Islands A high-frequency microtitration method is described for the determination of total base number in new and used lubricating oils. The sample, dissolved in toluene - propan-2-01 - water (5 + 4.95 + 0.05), is titrated with standard alcoholic hydrochloric acid solution. A study of solvents, titrants, sample size and other experimental parameters is reported.This method provides sharp breaks a t the end-points of the titration graphs and more repeatable and faster results than those obtained with the standard potentiometric method. The method can also be used for macrotitrations, with a com- mercially available oscillator, and can be suggested as an alternative to ASTM and IP methods for the determination of total base number. Keywords Total base number determination ; lubricating oils ; high-frequency titration In a previous paper the determination of the total acid number of lubricating oils by use of a high-frequency titrimetric method was described.1 In this paper we report the application of the high-frequency titration method to the determination of the total base number of these oils.The basicity of an oil is caused by nitrogen compounds, salts of weak acids (soaps), heavy metal salts and additives, such as inhibitors and dispersants. Basicity is an important property of lubricating oils as bases neutralise acidity, and therefore offset corrosion, which can be caused by oxidation of the oil during use. The total base number (TBN) of an oil is defined as “the amount of acid, expressed in milligrams of potassium hydroxide, that is needed to neutralise all basic constituents present in one gram of ~arnple.”~,~ The ASTM and IP method^^,^ of measuring TBN describe the potentiometric titration of a solution of the oil in toluene - propan-2-01 with a standardised solution of hydrochloric acid in propan-2-01. If a manual instrument is used these methods are rather slow, because of the large number of points that must be recorded and the slow indication of the changing pH.At present, this situation is improved because most laboratories working with these methods use automatic titrators that do not require too much operator time. However, the graph obtained does not always show a satisfactory end-point, particularly with used oils. The use of a standard buffer solution, 2,4,6-trimethylpyridine, and hydrochloric acid in propan-2-01, improves the results somewhat, but it is not certain that these results are meaningful and actually measure TBN as previously defined. Recently, potentiometric methods using glacial acetic acid as solvent have been introd~ced.~.~ The acceptance of the results obtained is wider than for those obtained by other methods.However, they are not completely satisfactory for used oils. In service, the replacement of a lubricating oil is determined by several factors, amongst which is the TBN. For heavily used oils the time for replacement may be difficult to deter- mine because of errors in the measurement of TBN, if this is the only factor considered. High-frequency techniques have been applied successfully to the titration of bases in non-aqueous media6-9 and can help to solve the difficulties mentioned previously. In this connection a method for TBN determination has been described,’ using a high-frequency * For details of Part I of this series, see reference list, p. 749.740 FERNANDEZ et al. : HIGH-FREQUENCY MICROTITRIMETRIC Analyst, VoZ.104 titrator adapted for use with a specially designed cell, but its specific design has created difficulties in its practical application. This paper shows that the high-frequency titrators that are commercially available can be applied to macro- and micro-determinations of TBN. Procedures have been developed by using solvent and titrants similar to those used in the ASTM methods.2-5 The proposed method reduces the time of analysis to about 10 min per sample, eliminates the contami- nation of electrodes, always shows sharp breaks at the end-point and provides greater precision. In addition, more meaningful results are obtained. Experimental Apparatus High-frequency oscillator for microtitrations, digital ammeter, pH meter, electrodes and microburet te have been described previous1y.l An Oscimhometer OK-105 (Metallurgical Research Institute, Budapest),lo equipped with a 500-ml (350 ml effective) inductive cell, was used for macro-titrations. This instrument was obtained from Metrimpex, Budapest 62 (P.O.Box 202) and the cost in 1977 of the manual instrument was about fT450. According to reference 9 the Oscimhometer can be classified as being Group 1 response type, and although it is equipped with its own response meter, a Hewlett-Packard digital multimeter, Model 3465A, (1-100 pA & 0.2%) measuring instru- ment was used. Samples In order to prepare synthetic samples a paraffinic base oil [diO 0.866 g ml-l, kinematic viscosity 10 cSt (10-5 mez s-l) at 100 "C], zero TBN value is used. The additives and trade lubricating oils are listed in each table. Reagents In the following paragraphs volumes referring to macrotitrations are given in parentheses.Analy tical-reagent grade chemicals were used throughout without further purification. Titrants Hydrochloric acid, 10-1 M solution in propan-2-01. This solution is made up according to ASTM D-664.2 Standardise it frequently by use of the high-frequency technique, using 50 p1 (3 ml) of 10-1 M standard potassium hydroxide solution in propan-2-01 or 10-1 M Tris [2-amino-2-(hydroxymethyl)propane-l,3-diol] standard solution, and toluene - propan- 2-01- water as the solvent. The maximum deviation allowed must be less than &0.2% ( 5 2 X 10-'M). Perchloric acid, 10-1 M solution in glacial acetic acid. This is made up according to ASTM D-2896.4 Standardise the solution frequently by use of the high-frequency technique, using 100 pl (3 ml) of standard potassium hydrogen phthalate solution in glacial acetic acid and toluene - propan-2-01 - acetic acid as the solvent.Sodium acetate solution in glacial acetic acid, 10-l M. This is made up according to ASTM D-2896.4 Standardise the solution by using 1OOpl (3 ml) of standard perchloric acid solution and toluene - propan-2-01 - acetic acid as the solvent. Standard 10-1 M Tris aqueous solution. Standard 10-l M potassium hydrogen phthalate solution in glacial acetic acid. Dissolve the phthalate according to ASTM D-2896,4 but diluting only with glacial acetic acid. Titration solvents prepared according to ASTM D-664.2 Toluene - propan-2-01 - water (5 + 4.95 + 0.05).This is the main solvent, which is Toluene - propan-2-01 - glacial acetic acid (3.5 + 3.5 + 3). Toluene - ethanol - glacial acetic acid (4 + 3 -t 3). Procedure Method 1. Samples with TBN more than 5. The test sample, 5-50 mg (1-5 g), is weighedAugust, 1979 DETERMINATION OF CONSTITUENTS IN LUBRICATING OILS. PART 11 741 into the titration cell, dissolved with 6 ml (350 ml) of the toluene - propan-2-01 - water solvent mixture (5 + 4.95 + 0.05) and titrated with the 10-1 M alcoholic hydrochloric acid standard solution. Method 2. Samples with TBN less than 5. The test sample, 50mg ( 2 4 g), is weighed into the titration cell, 50 pl (1-2 ml) of standard 10-1 M sodium acetate solution are added, and the sample is dissolved with 6 ml (350 ml) of toluene - propan-2-01 - acetic acid solvent mixture (3.5 + 3.5 + 3).The solution is then titrated with the 1 0 - l ~ perchloric acid standard solution. Volumes and masses given in parentheses are for macrotitration with the Oscimhometer. The working technique with this instrument is similar to that already reported for micro- titrati0ns.l The titrant was added in 0.5-ml increments from a 10-ml burette, using about 3 ml of titrant to reach the end-point. Calculation The TBN value can be calculated from the equation TBN (A & B) x M x 56.1 m where A is the volume, in microlitres (millilitres), of standard acid titrant used to reach the end-point minus the volume of acid titrant corresponding to the sodium acetate solution added (Method 2); B is the volume of basic titrantl or acid titrant, molarity M , used to titrate 6ml (350ml) of solvent mixture and is to be added in Method 1 and subtracted in Method 2 (the solvent of Method 1 may become slightly acidic and that of Method 2 slightly basic, see under Sample size below); M is the molarity of the acid standard solution; and m the mass of the oil sample in milligrams (grams).Effect of the Experimental Parameters on the High-frequency Titration In order to avoid duplication and presenting a study similar to that previously reported on the determination of TAN,l we prefer to discuss only the experiments carried out by use of microtitration. However, the main studies of TEN determination were also carried out by macrotitration. As will be seen later, the shape of experimental graphs, the results and the conclusions agreed well with those obtained by use of the micro-method.Titrant standard solution In order to use, as far as possible, the reagents specified in ASTM methods D-664 and D-2896, we used only solutions of perchloric acid in glacial acetic acid and hydrochloric acid in propan-2-01 as titrants for direct titrations and sodium acetate in glacial acetic acid for back-titrations. Titration solvents Solvents used as media for the titration of weak bases in petroleum products must dissolve the sample completely, enhance the basicity of the species to be titrated and have a low solvating power for cations. Further, the relative permittivity of the solvents used in conductimetric titrations should be less than 20,* but not too small as they must ionise compounds to be titrated, at least partially.Solvents such as nitromethane, 4-methyl- pentan-2-one or trifluoroacetic acid, which are widely used in the potentiometric determina- tion of weak bases, cannot be used in conductimetric titrations because they do not satisfy some of the stated requirements. Acetic acid or toluene - alcohol - acetic acid mixtures give better results, but these solvents do not always improve the titration of weak bases. This can be related to the fact that the conductimetric end-points are generally located at the intersection of two straight, ascending lines. In order to enhance the localisation of the end-point, we tried to alter the slope of the graphs by using mainly toluene - alcohol mixtures as solvents, with or without the addition of acetic acid.The behaviour of the selected solvents was studied by reference to the response graphs of the oscillator with the acidic titrants and verified later by use of the titration graphs.742 FERN~NDEZ et at. : HIGH-FREQUENCY MICROTITRIMETRIC AaaZyst, VoZ. 104 Non-acidic solvent mixtures In our first trials a number of binary mixtures, 1 + 1 toluene-X (X being ethanol, propan-2-01, acetone, 1,4-dioxan, butan-%one or 4-methylpentan-2-one), were used as representative solvents and tested in the titration of a synthetic sample with hydrochloric acid solution in propan-2-01. Toluene - ethanol and toluene - propan-2-01 mixtures give the greatest slope change at the end-point. Further experience with such alcoholic solvents showed that a solvent with a relative permittivity of approximately 11 and a 40-60% alcoholic component should be the most satisfactory.The work was continued with toluene - ethanol (6.5 + 3 4 , (6 + 4) and (5.5 + 4.5) and the solvent described in the ASTM method D-664, toluene - propan-2-01- water (5 + 4.95 + 0.05), was chosen for comparative purposes. Response graphs of the oscillator (Fig. 1) show that toluene - ethanol (6.5 + 3.5) is the best solvent because of its high sensitivity, but that the ASTM D-664 solvent, which has less sensitivity, can be recommended as it originates a more linear response graph over a wide range of titrant concentrations. Both solvents can dissolve up to 200mg of sample under the experimental conditions. Volume of lo-' M acid reagent solution/pl Fig. 1.Instrument response graphs for 10-1 M hydrochloric acid and several solvent mixtures: A, toluene - ethanol (6.5 + 3.5); B, toluene - ethanol (6 + 4) ; C, toluene - ethanol (5 + 5) ; D, toluene - ethanol (5.5 + 4.5) ; E, toluene - propan-2-01- water (5 + 4.95 + 0.05), M perchloric acid as titrant; and F, toluene .. propan-2-01- water (5 + 4.95 + 0.05). Acidic solvent mixtwes Acetic acid was selected as the acidic component, because of its well known behaviour, as the solvent and species to be titrated are similar to those described in ASTM D-2896. From the mixtures tried, after reference to their response graphs, toluene - ethanol - acetic acid (4 + 3 + 3) and toluene - propan-2-01- acetic acid (3.5 + 3.5 + 3) were selected. Toluene - acetic acid (1 + l), chlorobenzene - acetic acid (6.7 + 3.3) and the solvent specified in ASTM method D-2896 were also studied for comparative purposes.These solvents were further tested in the titration of a synthetic sample by direct and back-titration (Table I). From the experimental graphs shown in Fig. 2, it can be deduced that toluene - propan-2- ol- acetic acid mixture is the best solvent for the direct titration and toluene-ethanol- acetic acid for the back-titration. Fig. 2 also shows that the solvent in the ASTM D-2896 method and toluene - acetic acid are not suitable for either titration method. Table I summarises the recommended solvent - titrant pairs.Aztgzl.St, 1979 DETERMINATION OF CONSTITUENTS I N LUBRICATING OILS. PART I1 TABLE I 743 SOLVENTS AND TITRANTS RECOMMENDED FOR THE DETERMINATION OF TBN VALUES BY HIGH-FREQUENCY TITRATION Solvent Titrant ( -10-1 M) Toluene - propan-2-01- water (5 + 4.95 + 0.05) Toluene - propan-2-01- acetic acid (3.5 + 3.5 + 3) Toluene - ethanol - acetic acid (4 + 3 + 3) * Total volume added less than 60 pl.t Perchloric acid previously added, back-titration. Hydrochloric acid - propan-2-01; perchloric acid - Perchloric acid - glacial acetic acid Sodium acetate - glacial acetic acidf glacial acetic acid* .llO1 (a' A A 100 - 90 - 80 - 0 20 40 60 80 100 120 140 Volume of lo-' M acid reagent so I u t ion /p I I , I 0 30 60 90 120 150 Volume of lo--' M Cti,COONa sol u t ion/;r 1 Fig. 2. Titration graphs for a synthetic sample of ECA-5830, TBN value 74, in several solvent mixtures.(a) Direct titration with 10-1 M perchloric acid; ( b ) back- titration with 10-1 M sodium acetate solution (150 pl of 10-1 M perchloric acid pre- viously added). A, Toluene - propan-2-01- acetic acid (3.5 + 3.5 + 3) ; B, toluene - propan-2-01- water (5 + 4.95 + 0.05), 10-1 M hydrochloric acid as titrant; C, chloro- benzene - acetic acid (6.7 + 3.3) ; D, toluene - acetic acid (5 + 5) ; and E, toluene - ethanol - acetic acid (4 + 3 + 3). Saqble size The response graphs showed that the total amount of titrant added should be limited to about 1OOpl. The sample size chosen was adequate for the expected TBN value and the deviation of the results was studied. Table I1 shows the sample sizes recommended for different expected TBN values in micro- and macrotitration, and Table I11 shows the deviation obtained in titrating solutions of pure additives.The so-called pure additives are not pure substances but mixtures of different chemical species (e.g., zinc dialkylphosphoro- TABLE I1 SAMPLE SIZE RECOMMENDED FOR VARIOUS EXPECTED TBN VALUES Microtitration A r > I Expected Approximate Sensitivity TBN value sample masslmg of weighinglmg 0-5 100 0.2 5-10 50 0.1 10-20 25 0.05 20-50* 20 0.02 50-100* 15 0.02 Macrotitration Approximate Sensitivity sample mass/g of weighing/g 5-3 0.01 3-2 0.005 2- 1 0.005 1-0.5 0.002 0.5-0.2 0.001 A > * Alternatively use a preparation of the lubricating oil in a base oil, which allows an amount in one of the first three ranges to be weighed out.744 FERNANDEZ et aZ. : HIGH-FREQUENCY MICROTITRIMETRIC Analyst, Vd.104 dithioate - basic barium dinonylnaphthalene sulphonate, polyamino monoalkenylsuccinimide - alkaline calcium petroleum sulphonate, barium thiophosphonate) . Further experiences with a synthetic sample made from ECA-5830 and the base oil, TBN value 7.4, and amounts of sample ranging from 20 to 100 mg, showed that maximum deviation from the mean is 0.2 TBN unit (Table IV). This result is the same as was found for pure Lubrizol 864 solution, which has the same TBN value but a very different composition. TABLE I11 TBN VALUES DETERMINED FOR SAMPLES OF ADDITIVES TBN value Lubrizol 864 Anglamol 99 Lubrizol 6610 Determined . . . . . . . . .. 8.95 19.2 61.4 8.91 19.3 62.3 8.69 18.7 62.6 Mean . . .. . . . . . . .. 8.85 19.1 62.1 Maximum deviation (from mean) ..0.2 0.4 0.7 Maximum deviation (among results) . . 0.3 0.6 1.2 TABLE IV INFLUENCE OF SAMPLE SIZE ON THE DETERMINATION OF THE TBN VALUE FOR A SYNTHETIC SAMPLE OF ECA-5830 DISSOLVED IN BASE OIL Sample mass/mg 24.13 40.09 57.3 67.5 76.7 92.7 111.6 10-1 M HCl added/pl 33 54 77 90 101 120 143 TBN value 7.67 7.56 7.64 7.48 7.39 7.26 7.19 Mean .. . . . . .. .. 7.44 Mean deviation (among results) . . .. 0.5 Mean deviation (from mean) . . . . 0.2 Standard deviation .. .. . . 0.2 Relative standard deviation . . . . 2.3% Relative error11 . . . . . . . . 2.1% Similar experiments were carried out with the same synthetic sample, toluene - propan- Fig. 3 shows the experimental graphs and Table V the results obtained. Fig. 4 relates 2-01 - acetic acid solvent and perchloric acid in acetic acid as the titrant.100 2 90- W 80- f! 70- - a m C .- L s 50b ;o 40 $0 E;o lA0 1;o 1 2 Volume of l O - ’ M perchloric acid sol u t i on/p I Fig. 3. Titration graphs for the deter- mination of synthetic samples of ECA-5830 dissolved in base oil, according to Table V (graphs A, B, C, D and E correspond to titrations 1, 2, 3, 4 and 5 in Table V).Augast, 1979 DETERMINATION OF CONSTITUENTS IN LUBRICATING OILS. PART 11 745 TABLE V INFLUENCE OF SAMPLE SIZE ON THE DETERMINATION OF THE TBN VALUE FOR A SYNTHETIC SAMPLE OF ECA-5830 DISSOLVED IN BASE OIL, IN TOLUENE -PROPAN- 2-OL - ACETIC ACID AS SOLVENT AND 10-l M PERCHLORIC ACID AS TITRANT Volume of 10-1 M perchloric acid Test Sample masslmg added/$ 1 20.36 31 2 32.49 49 3 48.03 69 4 71.3 101 5 89.2 125 TBN value r Uncorrected Corrected* 8.54 7.60 8.46 7.88 8.06 7.65 7.95 7.67 7.86 7.64 A \ Mean value .. . . .. .. .. .. 8.17 7.69 Maximum deviation (among results) . . . . 0.7 0.3 Standard deviation . . .. .. . . . . 0.3 0.1 Relative standard deviation . . .. . . 3.8% 1.4% Relative error1' .. .. .. . . . . 4.7% 1.8% Maximum deviation (from mean) . . .. , . 0.4 0.2 * 3 pl blank correction. titrant consumptions to sample mass, and extrapolation reveals the need for a blank correction for the solvent, which must be taken into account in calculations. In this way the standard deviation and relative error are improved and the maximum deviation among results becomes less than 0.3 TBN unit. This correction can be related to the basicity of the acetate ions in the solvent.Sample mass/mg Fig. 4. Relationship between titrant consumption and sample mass according to Table V (on the magnified scale the blank value is shown). Addition of sodium acetate or Tris As increasing sample size above 200 mg leads to a loss in sensitivity and disturbs readings, the addition of a pure, typical, weak base to the sample was studied, in order to allow titration of those with a very low TBN value. Sodium acetate and potassium hydrogen phthalate were tested in the acidic solvent with perchloric acid as titrant. The results shown in Table VI suggested that the use of sodium acetate is better than potassium hydrogen phthalate. However, according to the results in Table VI, in a medium of toluene - propan-2-01- water with hydrochloric acid in propan-2-01 as titrant, the addition of Tris can be convenient; this base must be used cautiously, keeping the amount added to a limit of 2.5 pmol.In all of the experiments the consumption of the 10-1 M acid titrant was kept below 150 pl so that the net consumption of titrant was about 30 p1.746 FERNANDEZ et al. : HIGH-FREQUENCY MICROTITRIMETRIC Analyst, VoZ. 104 TABLE V I INFLUENCE OF THE ADDITION OF SODIUM ACETATE, POTASSIUM HYDROGEN PHTHALATE OR TRIS ON THE DETERMINATION OF THE TBN VALUE FOR A SYNTHETIC SAMPLE OF ECA-5830 DISSOLVED IN BASE OIL TBN value Volume of 10-l M base added/pI* 0 25 25 50 50 75 100 Acetate? 7.72 7.79 7.78 7.24 7.81 7.87 7.97 Phthalatet 7.72 8.07 8.10 8.73 8.38 7.83 7.28 TrisS 7.67 7.69 7.62 8.02 8.12 8.70 8.72 * Sample size -20 mg.t In toluene - propan-2-01 - acetic acid as solvent with 10-l M $ In toluene - propan-2-01 - water as solvent with 10-1 M hydro- perchloric acid as titrant (3 pl blank solvent, Table V) . chloric acid as titrant. Results and Discussion In order to check the proposed methods, synthetic samples with compositions corres- ponding approximately to trade samples were prepared from a base oil and a set of additives widely used in the lubricating oil industry. The TBN values of the additives were known from the manufacturer or could be determined by the standard ASTM potentiometric method. Table VII summarises the TBN values obtained in the microtitration of several synthetic samples. TABLE VII TBN VALUES DETERMINED FOR SEVERAL SYNTHETIC SAMPLES OF ADDITIVES DISSOLVED I N BASE OIL Lubrizol TBN value Oloa , A \ 851 4266 4084 6610 3882 3882 56 56 219 4084 1360 1360 3715 Determined .... 14.5 5.48 6.85 7.02 9.35 11.1 15.8 64.7 14.6 5.46 6.81 6.96 9.38 11.4 15.7 63.8 14.5 5.66 6.83 7.16 9.44 11.6 16.2 64.1 Mean . . 14.5 5.53 6.83 7.05 9.39 11.4 15.9 64.2 Expected’;alue;. . . 13.9 5.4 6.8 7.1 9.4 11.4 16.0 64.3 Potentiometrict . . . . 5.25 6.91 10.7 15.1 (5.11-5.47) (6.63-7.20) (10.3-11.3) (14.8-15.4) Difference$ .. .. $0.6 -0.13 $0.03 -0.05 -0.01 0.0 -0.1 -0.1 * Calculated from % m/m and TBN of additive added as known from the manufacturer. -f Determined by ASTM D-664, mean of three determinations; range given in parentheses. 3 Difference: mean - expected value. TABLE VIII DETERMINATION OF TBN VALUES ON SAMPLES OF NEW CEPSA TRADE LUBRICATING OILS Sample trade name ‘ Rodaje Extra Multigrado Teseo S-3 Serie 3 Troncoil RCgulo Chrysler HD-10 20-40W SAE-30 SAE-30 1530 HD-50 Determined .. . . . . 5.94 6.27 7.10 9.72 11.3 15.1 16.8 5.72 6.21 6.96 9.68 11.4 15.2 15.9 5.78 6.20 7.13 9.83 11.2 15.1 15.9 Mean . . . . . . 5.81 6.23 7.06 9.74 11.3 15.1 16.9 Expected Val&* . . . . 5.4 6.8 7.1 9.4 11.4 15.0 16.0 Potentiometrict . . . . 6.10 9.14 14.9 (5.92-6.27) (8.99-9.34) (14.6-1 5.3) Difference$ . . . . . . +0.41 -0.57 -0.04 +0.34 - 0.1 + 0.1 - 0.1 *, t and 2 as in Table VII. Rdgulo Super 650 63.2 64.6 64.4 64.1 64.3 60.4 - 0.2 (59.8-61.6) Other titrations were performed on two used oils from a Chrysler C-24s plant motor afterA$tgust, 1979 DETERMINATION OF CONSTITUENTS IN LUBRICATING OILS.PART I1 747 Fig. 5. Typical graphs obtained in the deter- mination of TBN values of two used lubricating oils removed from a Chrysler C-24s plant motor. (a) Motor oil MIL-L-2104B after working time: A, 48 h (TBN 4.01); B, 96 h (TBN 2.79) ; C, 192 h (TBN 2.24); D, 312 h (TBN 1.26); E, 408 h (TBN 0.30); and F, 0 h (TBN 6.23). (b) Motor oil MIL-L-46152 after working time: A, 50h (TBN 5.07); B, 100 h (TBN 3.97); C, 225 h (TBN 1.80) ; D, 300 h (TBN 1.36) ; and E, 0 h (TBN 7.06). various periods of working (Figs. 5 and 6). on samples of new CEPSA trade lubricating oils [Table VIII and Fig. 7(a)]. X; the latter table summarises TBN values determined on used oils from marine engines. Replicate determinations were also carried out Finally, the results obtained by macrotitration are shown in Fig.7 ( b ) and Tables IX and L 100 200 300 400 500 600 Timelh Fig. 6. Variation of TBN value with working time for two lubricating oils from a Chrysler C-24s plant motor: (a), motor oil MIL-L-2104B; ( b ) , motor oil MIL-L-46152. The repeatability obtained (Tables VII and VIII) is very good, as the differences from the mean are less than 0.1 TBN unit; such values are obtained with the ASTM standard method only when the potentiometric graphs show good inflection points2 The precision was established (Table IV) for a synthetic sample of ECA-5830, TBN value 7.4, and shows a standard deviation of 0.2 and 5.6% relative error (95% probability). The estimated accuracy of the method can be deduced from the results recorded in Table VII. In this wide range of TBN values (5-64), the differences between the obtained and expected values are less than or equal to 0.1, except for one sample.We must emphasise the need748 FERNANDEZ et al. : HIGH-FREQUENCY MICROTITRIMETRIC Analyst , VoZ. 104 - ( a ) 20 10 - - 00 90 80 70 60 50 0 20 60 100 140 180 Volume of lo-' M hydrochloric acid solution/pI / a / Fig. 7. Typical graphs obtained in the determination of TBN for synthetic, new and used lubricating oils by (a) micro- and (b) macro- titration: A, synthetic Lubrizol 56 (TBN 64.2-65.7); B, new Teseo S-3, SAE-30 (TBN 9.74-9.553 ; C, synthetic Lubrizol 56 + Lubrizol 1360 (TBN 15.9) ; D, used marine oil (TBN 13.0-14.2, vessel D, Table IX) ; and E, used marine oil (TBN 6.58-5.51, vessel A, Table IX). for a good determination of the blank correction for the solvent, particularly with solvent mixtures containing acetic acid (Table V).Samples with low TBN values can be resolved by cautious additions of Tris or, better, by adding sodium acetate. Nevertheless, this method is not totally satisfactory from the standpoint of the definition of TBN value according to ASTM method D-664, as basic constituents with a pKb of about 10 cannot be titrated. I t must be taken into account that the high-frequency conductimetric end-points are generally located at the intersection of two ascending lines. The slope of the titration line before the end-point becomes closer to the slope of the excess acid line as the basic strength of the sample decreases and the end-point becomes increasingly difficult to locate accurately.In this sense, the differences observed in titrating samples of different sizes can be related to their content in very weak species. TABLE IX COMPARISON OF TBN VALUES DETERMINED BY MACRO- AND MICROTITRATION ON SYNTHETIC AND NEW LUBRICATING OILS Sample TBN value Microtitration Macrotitration r A I Lubrizol4266* . . .. .. .. 5.66 5.60 Lubrizol 56* .. .. 64.2 65.7 Lubrizol 56 + LubAzol 1360* . . .. 15.9 15.9 Lubrizol 3882 + Lubrizol 1360* .. 9.39 9.40 ECA-5830* .. .. .. .. 7.96 7.99 Rodaje Chryslert . . .. .. .. 5.81 5.96 Teseo S-3 SAE-307 . . .. .. 9.74 9.65 * Synthetic samples: additives dissolved in base oil. t CEPSA samples, trade name. In spite of the difficulties described, high-frequency titration appears to be a very encouraging technique, as by using mixed solvents it is possible to differentiate weak bases in mixtures.6 A limitation in sample size can be avoided by use of the OscimhometerfoAzqpst, 1979 DETERMINATION OF CONSTITUENTS IN LUBRICATING OILS.PART 11 TABLE X DETERMINATION OF TBN VALUES FOR SAMPLES OF USED OILS FROM MARINE ENGINES Vessel A Vessel B Vessel C Vessel D TBN value r * - 7 - - - - t t t t Determined . . 5.61 5.50 6.99 6.51 6.55 6.77 13.0 14.4 5.48 5.63 6.88 7.01 6.66 7.82 12.8 14.0 5.64 5.42 6.94 6.89 6.98 6.47 13.1 14.1 Mean . . .. 5.58 6.54 6.94 6.80 6.73 7.02 13.0 14.2 New motor oil .. 8.00 9.74 15.13 25.64 * Microtitration. t Macrotitration. 749 according to the procedures described. In connection with this, Tables IX and X also report results obtained in tests performed by use of macrotitration using this oscillator, and one can see that the mean TBN values obtained with both techniques are virtually equal, except for one sample.Likewise, the shapes of the titration graphs are very similar [Figs. 7(a) and (b)]. With used oils, Table X suggests that the repeatability obtained by use of microtitration is better than that obtained with macrotitration. However, this suggestion cannot be substantiated firmly because there are too few results. Workers in CEPSA labora- tories are now establishing a co-operative testing programme using macrotitration in order to solve this problem. Compared with the standard potentiometric method, the use of high-frequency analysis avoids the possibility of electrode contamination and damage, as the electrodes are outside the titration cell.A sharp break is always obtained at the end-point of the titration graph, and no buffer is needed. Once the apparatus has been set up, a single TBN determination can be performed in about 10 min, instead of about 1 h, which is required for the potentio- metric determination when a manual instrument is used. The outstanding advantages of the high-frequency method of TBN determination are its good accuracy, the validity of its end-points and the smaller spread of results. As a com- parison, four synthetic and four trade oil samples were analysed by the standard potentio- metric method; results shown in Tables VII and VIII prove that TBN values obtained by use of the potentiometric method are not as consistent as those obtained with the high- frequency method. In back-titration, the high-frequency method (Table I) is an improve- ment on the equivalent potentiometric method, ASTM D-2896. The method developed for TBN determination can be suggested as an alternative to ASTM and IP methods. A similar conclusion was reached in our paper on TAN determination by high-frequency titrati0n.l This similarity can be used as an argument for the acceptance of €ugh-frequency titration in both determinations. References 1. 2. 3. 4. 6. 6. 7. 8. 9. Fernkndez, T., Rocha, J. M., Rufino, N., Garcia Luis, A., and Garcia Montelongo, F., Analyst, 1978, “1977 Book of ASTM Standards,” Part 23, American Society for Testing and Materials, Philadelphia, “Institute of Petroleum Standards 1978,” Part I, Volume 1, Institute of Petroleum, London, 1978, “1977 Book of ASTM Standards,” Part 24, American Society for Testing and Materials, Philadelphia, “Institute of Petroleum Standards 1978,” Part I, Volume 2, Institute of Petroleum, London, 1978, McCurdy, W. H., Jr., and Galt, J., Analyt. Chem., 1958, 30, 940. Caughley, B. P., and Joblin, M. V., Analyt. Chem., 1969, 41, 1211. Pungor, E., “Oscillometry and Conductometry,” Pergamon Press, London, 1965. Blaedel, W. J., and Petitjean, D. L., “High-Frequency Method of Chemical Analysis,” in Bed, W. G., Editor, “Physical Methods in Chemical Analysis,” Volume 111, Academic Press, New York, ” ‘Oscimhometer’ Type OK-105 Instrument Manual,” Research Institute for Non Ferrous Metals, Budapest, 1972. Lacroix, Y., “Analyse Chimique (Interpretation des RCsultats par le Calcul Statistique),” Masson, Paris, 1962. 103, 1249. 1977, Methods D664-58. Method 177/64. 1977, Method D2896-73. Method 276175. 1956, pp. 107-134. 10. 11. NOTE-Reference 1 is to Part I of this series. Received December 4th, 1978 Accepted March lst, 1979

ISSN:0003-2654    

DOI:10.1039/AN9790400739

出版商:RSC    

年代:1979

数据来源: RSC

摘要:

750 Analyst, August, 1979, Vol. 104, $9. 750-755 Continuous Solvent-extraction Method for the Spectrophotometric Determination of Cationic Su rfa cta n ts Jiro Kawase and Makoto Yamanaka Tochigi Research Laboratories, Kao Soap Co. Ltd., 2606 Akabane, Ichikai-machi, Tochigi, Japan A rapid and automated spectrophotometric method for the determination of cationic surfactants, using the AutoAnalyzer, has been developed. This method is based on the continuous solvent extraction of the ion-pair complex formed in the reaction of Orange I1 with a cationic surfactant. Good traces and identical molar responses were obtained with seven different types of surfactant using methanol in the Orange I1 reagent. Fatty amines in mixtures of the amines and quaternary ammonium surfactants were deter- mined by changing the pH of the aqueous phase.The proposed method was applied to the determination of cationic surfactants in several commercial products. The results agreed with those obtained by the two-phase titration procedure. The detection limit is 5 p~ and the capacity is 10-20 samples per hour, with a relative precision of better than 1.5%. Keywords : Automatic analysis ; cationic swfactant determination ; fatty amine determination ; spectrophotometry ; Orange II Recently, new kinds of commercial products containing cationic surfactants have been extensively developed, in addition to the well known hair rinse, fabric softener and sanitiser types. The determination of the cationic surfactants has hitherto been effected by two- phase titration pro~edures.l-~ which has hitherto been conducted only manually by spectrophotometry, can be automated successfully by means of the Auto- Analyzer.We report here that the Orange I1 Experimental Reagents Benzethonium chloride (A), hexadecylpyridinium chloride (B) , benzylhexadecyldimet hyl- ammonium chloride (C) and trimethyloctadecylammonium chloride (D) were purchased from Tokyo Kasei Co. Ltd. Hexadecyl(2-hydroxyethyl)methyloctadecylammonium chloride (E) and hexadecylbis(2-hydroxyethyl)octadecylammonium chloride (F) were purchased from ICI Australia. Hexadecyldimethyloctadecylammonium chloride (G) , hexadecylamine (H) , octadecylamine (I), dialkylamine (R, and R, = C14-Cl8, relative molecular mass 499.83) (J) and alkyldimethylamine (R = C8-Cl8, relative molecular mass 293.69) (K) were produced by Kao Soap Co.Ltd. These reagents were purified by recrystallisation from acetone or an acetone - ethanol mixture. Stock solutions (3 mM) of A-G were prepared by dissolving the purified compounds in distilled water and standardised according to the direct two-phase titration procedure3 just before use. Amine surfactant (H-K) solutions ( 7 0 ~ ~ ) were prepared by weighing purified compounds and acidifying the amines with an adequate amount of dilute sulphuric acid. Dissolve 50 mg of Orange 11, 3.78 g of potassium dihydrogen orthophosphate, 1.99 g of disodium hydrogen orthophosphate and 520 ml of methanol in 300 ml of distilled water, dilute the solution to 1 1 with distilled water and adjust the pH to 7.3 with 1 N sodium hydroxide solution.Dissolve 50 mg of Orange 11, 2.24 g of potassium chloride, 3 ml of concentrated hydrochloric acid and 520 ml of methanol in 300 ml of distilled water, dilute the solution to 1 1 with distilled water and adjust the pH to 1.6 with 1 N hydrochloric acid. All other reagents were of analytical-reagent grade. Orange I I reagent, pH 7.3. Orange 11 reagent, pH 1.6.KAWASE AND YAMANAKA 751 Methylene blue solutiolz. Dissolve 0.03 g of methylene blue, 50 g of sodium sulphate (anhydrous) and 12 g of concentrated sulphuric acid in 500 ml of distilled water and dilute the solution to 1 1 with distilled water. Standardisation of Cationic Surfactants3 To 10 ml of sample solution (approximately 3 mM), measured in a measuring cylinder, add 25ml of the methylene blue solution and 15ml of chloroform. Titrate the sample solution with 0.004~ sodium lauryl sulphate solution by the usual method.At the same time, test a blank by titrating 10 ml of distilled water in the same manner. The end-point is when the blue colour is completely discharged from the aqueous layer. ,Wethod of calculation c = (v, - ‘vb) x 0.004 x 100 where C mmol is the concentration of the cationic surfactants, Vs ml is the volume of 0.004 M sodium lauryl sulphate solution used in the titration of the sample and v b ml is the volume of 0.004 M sodium lauryl sulphate solution used in the titration of the blank. Apparatus and Procedure The apparatus is a Technicon AutoAnalyzer, Model 11, consisting of the following com- ponents : automatic sampler, peristaltic pump, manifold, spectrophotometer equipped with a flow cell (light path length 15 mm) and 485-nm filter and recorder [Fig.l ( a ) ] . When necessary, a dilution loop [Fig. l ( b ) ] was used for analyses of commercial products. Pumping rate/ t mI min-’ ler wash recevtacle * PTFE tubing f’ Polyethylene tubing 0 PTFE LC three-way joint Fig. 1. (a), Diagram for extraction of cationic surfactants with Orange 11. (b), Flow diagram of a dilution loop. Except for Acidflex pump tubing, 2-mm bore PTFE tubing, 0.8-mm bore polyethylene tubing, mixing coils made of 2-mm bore PTFE tubing wound round a polyethylene pipe (25-mm diameter) and PTFE liquid chromatography (LC) three-way joints (Nihon Seimitsu JTO2U) are used for lines that come into contact with chloroform. All connections, especially between the phase separator and the flow cell, should be as short as possible.Before beginning a run, the tubing should be washed out by pumping methanol through the lines used to introduce chloroform. To prevent the inflow of the aqueous phase into the flow cell, it was found necessary to have the chloroform phase present in the tubing prior to intro- ducing the aqueous phase. As complex formation with fatty amines depends on the pH of752 Analyst, VoZ. 204 the aqueous phase,5 Orange I1 reagent at pH 1.6 was used for the over-all determination of quaternary ammonium and amine surfactants, and that at pH 7.3 for the determination of quaternary ammonium surfactants alone. The determination is effected automatically by passing a sample solution through a dilution loop, reacting the surfactant with the anionic dyestuff to form the ion-pair complex, extracting it into the chloroform phase and measuring the absorbance at 485 nm after phase separation.The sampling rate is 10-20 samples per hour. All calibration graphs were calculated with a custom-built microprocessor (Tachibana Electronic Co. Ltd.) connected to the recorder. The signal output of the recorder was monitored continuously at 1-s intervals over the run time. The absorbance, the equation of the straight line calibration graph, the regression coefficient and the calculated concentration of the sample were obtained from the print-out. KAWASE AND YAMANAKA : SOLVENT-EXTRACTION METHOD FOR Results and Discussion Effect of Methanol The slow extractability of a cationic surfactant in the Orange I1 method is well known.6 Ion-pair extraction in the continuous-flow analysis differs from that in the manual method, because it proceeds in the limited interfacial region between alternative aqueous and chloro- form segments in a definite length of mixing coil, and is seldom allowed to reach equilibrium.Thus, in the continuous-flow analysis the relative molar extractability of the ion-pair complex would be influenced by the corresponding extraction rate, which differs slightly for each type of cationic surfactant. However, in practical analysis it is necessary to obtain identical molar responses that are independent of the type of surfactant. The role of the solvating agent and its affinity for the ion-pair complex have been considered by Higuchi et al.6 The addition of methanol to the water - chloroform extraction system in the continuous-flow analysis would be expected to enhance markedly the extraction process, through solvation of the ion pairs that are fornied, and to be a powerful means of varying the relative molar extractability in the analysis.Investigation has shown that the relative molar extractability of the ion-pair complexes formed by the reaction of Orange I1 with cationic surfactants A-G, in the continuous-flow analysis, differs slightly and varies with the methanol content of the Orange I1 reagent to different extents (Fig. 2). The rate of extraction of the complex is also different for each surfactant. It should be noted that the rate of extraction for the surfactants is much more rapid with a higher methanol concentration and that the molar extractability of the complex increases with increasing methanol concentration (up to 40% V/V).0.26 ’ I I 1 I 0 10 20 30 40 50 f Methanol concentration, % V/V 3 Fig. 2. Effect of methanol concentration on the absorbance measured at 485nm, of: 0, surfactant B; A, surfactant C; A, surfactant D; and 0, surfactant G. Each cationic surfactant concentration was 200 p ~ . The pH of the Orange I1 reagent was 7.30.A zigmt, 1979 THE SPECTROPHOTOMETRIC DETERMINATION OF CATIONIC SURFACTANTS 753 There is also a distinct variation in the peak shapes with a change in the methanol con- centration (Fig. 3). A longer period of steady state was attained with a higher concentration of methanol; the peak shape for each type of surfactant deteriorates as the hydrophilicity increases.A,G AIG B,C DIE F Original peak. (methanol not added) Most improved peak shapes (methanol content, 52% V/V) Fig. 3. Variation in the peak shapes with the methanol content and the type of Sampling cam: 10 samples per hour, 3 parts of sample to 1 part surfactant, A-G. wash. The effect of methanol can be explained in terms of both solvating and dissolution effects. With increase in the concentration of methanol in the Orange I1 reagent, the methanol content of the chloroform phase also increases and this contributes to both the extraction rate and the extractability of the complex by its solvating effect. With 40% V/V or more of methanol in Orange I1 reagent, the ion-pair dissolution effect of methanol in the aqueous phase may predominate over the solvating effect and result in a decrease in the absorbance.In this work, 52% V/V of methanol in the Orange I1 reagent was chosen as the best concentration to give good peak shapes and almost identical molar responses for the seven different types of surfactant (Table I). TABLE I RELATIVE MOLAR EXTRACTABILITIES OF CATIONIC SURFACTANTS All values are normalised relative to the molar extractability of surfactant G. pH of buffer I A \ soh tion* A B C D E F G 7.30 103.3 102.0 100.0 98.9 101.1 97.9 100.0 1.60 103.3 102.0 100.2 99.1 101.6 98.2 100.0 Cationic surfactantt * Methanol content of the Orange I1 reagent was 52% V/V. t Each surfactant concentration was 200 p ~ . Calibration Graph and Precision The proposed method is suitable for determining up to 200 p~ of cationic surfactant, and provided that the dilution loop [Fig.l ( b ) ] is used, a linear calibration graph covering the range up to 3 mM can be realised. The detection limit was found to be approximately 5 p ~ . The resulting calibration graphs for each cationic surfactant were linear at pH 7.30 and 1.60. The precision of the method was evaluated by determining 1.5mM of each cationic surfactant (A-G). The results proved to be within 1.5% of the given concentration with a coefficient of variation of o.3-1.5y0. Cationic surfactant D serves as a representative example (Fig. 4). The cationic surfactant most frequently used in commercial products seems to be a dialkyldimethylammonium type, and therefore cationic surfactant G was used as a standard material for the commercial products analyses.Interferents likely to be present in commercial products were examined. Inorganic salts, acids and non-ionic surfactants found in the usual formulations had only a small effect on the determination.754 KAWASE AND YAMANAKA : SOLVENT-EXTRACTION METHOD FOR Analyst, Vol. 104 '"fl 172.7 180.7 172.7 172.7 r- I 0 3 6 9 12 15 18 21 24 27 30 33 36 Time/m in I Fig. 4. Determination of surfactant D. Values on peaks Rate of sampling is 20 per represent the concentration in p ~ . hour. Application to the Determination of Fatty Amines and Commercial Products Analyses Solutions (70 PM) of the fatty amines (H-K) were analysed at pH 1.60 using the calibration graph for surfactant G.The results were satisfactory (Table 11). Several commercial products containing cationic surfactants were analysed by the proposed method, after adequate dilution of the aqueous solutions. Orange I1 reagent at pH 1.6 was used for the over-all determination of quaternary ammonium and amine surfactants and that at pH 7.3 for the determination of quaternary ammonium surfactants alone. The results were com- pared with those obtained by the direct two-phase titration procedure,3 which determines TABLE I1 RECOVERY OF FATTY AMINES Amine Presentlpf Found/pM Recovery,* yo H 70.0 67.2 96.0 I 70.0 69.2 98.9 70.0 69.9 99.9 70.0 71.4 102 * The pH of the Orange I1 reagent was 1.60. The dilution loop [Fig. l(b)] was not used. TABLE I11 COMMERCIAL PRODUCTS ANALYSES AutoAnalyzer method r Sample* 1 2 3 4 5 6 7 8 9 pH 7.30: found/mM 2.78 2.03 1.90 2.10 1.82 3.33 1 .oo 1.08 3.33 pH 1.60: found/mM 2.85 2.06 2.14 2.13 1.80 3.52 - ._ Two-phase titration procedure3: found/mM 2.87 2.08 2.24 2.19 1.84 3.59 1.00 1.05 3.29 * Samples 1-3 were fabric softeners and samples 4-9 were hair rinses.August, 1979 THE SPECTROPHOTOMETRIC DETERMINATION OF CATIONIC SURFACTANTS 755 over-all cationic surfactants (Table 111). Good agreement was obtained between the auto- mated and conventional determinations. References 1. 2. IS0 Standard, IS0 2871-1973. 3. 4. 5. 6. Epton, S. R., Trans. Faraday SOL, 1948, 44, 226. Japanese Industrial Standard, K 3362, 1970. Few, A. V., and Ottewill, R. H., J . Colloid Sci., 1956, 11, 34. Scott, G. V., Analyt. Chem., 1968, 40, 768. Higuchi, T., Michaelis, A., Tan, T., and Hurwitz, A., Analyt. Clzem., 1967, 39, 974. Received October l l t h , 1978 Accepted February 27th, 1979

ISSN:0003-2654    

DOI:10.1039/AN9790400750

出版商:RSC    

年代:1979

数据来源: RSC

摘要:

756 Analyst, August, 1979, Vol. 104, $9. 756-765 Automatic Emission Spectrometer for the Deter m i nation of N i trog en -1 5 J. D. S. Goulden and D. N. Salter* National Institute for Research in Dairying, Shinfield, Reading, RG2 9A T An automatic nitrogen-15 analyser, employing a novel use of a rhodium- platinum catalyst for the generation of nitrogen and capable of analysing 60 samples per hour, is described. Nitrogen compounds of biological origin are first converted into ammonium chloride by conventional Kj eldahl digestion and distillation methods. The ammonium chloride sample (about 5 pl containing about 10 pg of nitrogen) is injected into a soda-lime reactor at 590 "C through which flows a stream of purified helium. Ammonia that is released passes directly into the catalyst tube and the generated nitrogen and hydrogen are separated by passage through a gas-chromatographic column, which also retains the water.After passing through a pressure restrictor the nitrogen flows in the helium stream through a Spectrosil discharge tube located in a microwave cavity. The emitted radiation is analysed by means of a specially con- structed dual-wavelength monochromator and the intensities of the 14N14N (297.7 nm) and 14N15N (298.3 nm) bands are measured simultaneously by two photomultipliers. Amplified signals, proportional to the peak intensities, are fed through phase-sensitive detectors into a ratiometer, the output from which is fed to a digital voltmeter and printed out in terms of nitrogen-15 abundance. A peak detector indicates the total nitrogen content of each sample and actuates the nitrogen-15 print-out. The response of the instrument is slightly curvilinear but may be regarded as linear over limited ranges.Calibration can therefore be achieved by running suitably chosen standards to fix upper and lower set points. Carry- over between samples is very small and is eliminated by running duplicates. Standard deviations of replicate measurements of natural abundance and enriched standards are less than 0.01 atom-%, while determinations of nitrogen-15 in biological samples were shown to be accurate to fO.01 atom-% by comparison with a Statron NOI-4 nitrogen-15 analyser. Keywords ; Nitrogen- 15 determination ; catalytic nitrogen generation ; auto- mated emission spectrometer The analysis of nitrogen-15 (15N) by the classical method of Rittenbergl using a mass spectrometer is a tedious and demanding operation, which for many years severely limited the use of nitrogen-15 as a tracer in biological research.Subsequently, the technique of emission spectrometry was applied to the analysis of nitrogen isotopes by Broida and Chapman,2 and the developments of Meier and Muller3 and Faust4 led to the introduction of reliable and relatively cheap commercial emission spectrometers. Further developments by Lloyd- Jones and co-workersk7 have resulted in improved accuracy and speed of analysis. The methods for the preparation of nitrogen for isotopic analysis have, however, continued to be time consuming and proved to be a limiting factor in nutritional studies.8 The application of optical and mass-spectrometric techniques has been discussed in a recent review.9 .In this paper an automatic nitrogen-15 analyser, capable of analysing 60 samples per hour, is described. The generation of pure nitrogen gas from a solution of ammonium chloride prepared from the test material is carried out automatically and the result printed out as nitrogen-15 atom per cent. Total nitrogen content of the sample can also be indicated simultaneously and displayed as a peak on a chart recording. The method is used routinely for samples containing 10 pg of nitrogen, and with some loss in accuracy may be used for samples containing as little as 2 pg of nitrogen. It is therefore ideally suited to the deter- mination of nitrogen-15 in compounds available only in very small amounts, e.g., amino acids prepared by ion-exchange chromatography of protein hydrolysates or physiological fluids.A preliminary report of this apparatus has been published.10 * To whom all correspondence should be addressed.GOULDEN AND SALTER 757 Principle of the Method With the manual nitrogen-15 analyser (e.g., the Statron NOI-4 or NOI-5), part of the emission spectrum of nitrogen in the 300-nm region is recorded and the peak heights for 14N14N and 14N15N in a series of replicate scans are measured. Nitrogen-15 isotopic abundance is then calculated from the mean ratio of these peak heights, after correction for background emission. In the National Institute for Research in Dairying (NIRD) automatic nitrogen-15 analyser, the two peaks at 297.7 and 298.3 nm are measured simultaneously, thus eliminating errors in the ratio due to variations of source intensity during the time needed for a wavelength scan.For both the manual and the automatic methods the nitrogen-15 labelled biological material is first subjected to a Kjeldahl digestion; alkali is added to the digest and the ammonia released is steam distilled into dilute hydrochloric acid, from which ammonium chloride is recovered by evaporation. From this stage onwards analysis with the NIRD analyser is fully automatic apart from the manual injection of samples by syringe, avoiding the troublesome vacuum technique involved in the preparation of emission tubes containing nitrogen. Ammonium chloride solution (approximately 10 pg of nitrogen) is injected into a stream of helium and passes immediately through a heated reactor tube, the front portion of which contains soda-lime.Ammonia is generated and is dissociated by a catalyst in the rear portion of the tube. The generated gases are separated by a gas-chromatographic column, which also retains the water. They are then eluted sequentially through a flow restrictor into a discharge tube located in a microwave cavity. The emitted radiation is analysed by means of a dual-wavelength monochromator. Signals proportional to the intensities of the bands at 297.7 and 298.3 nm, measured simultaneously, are amplified and fed through a pair of phase-sensitive detectors into a ratiometer whose output is fed t o a digital voltmeter and printed out as nitrogen-15 abundance in atom per cent.As the nitrogen flows through the discharge tube a peak detector actuates the printer and gives an indication of the total mass of nitrogen present in the sample. Materials and Methods The Statron NOI-4 emission spectrometer, the preparation of reagents and samples for analysis and the operation of the equipment have been described previously.6 Standard nitrogen-15 labelled ammonium chloride samples (measured in a mass spectrometer) were obtained from C.Z. Scientific Ltd. For analysis in the NIRD automatic nitrogen-15 analyser, samples of biological material (normally containing 200-1 000 pg of nitrogen, but for amino acid fractions from a chromatography column sometimes as little as 20-50 pg of nitrogen) are digested with 3 ml of concentrated sulphuric acid (low nitrogen, Aristar grade, BDH, Poole, Dorset) and one tablet of mercury(I1) oxide catalyst (Kjeltabs F.M., Thompson & Capper Ltd.), heating being continued for at least 1.5 times the clearing time.The ammonia released from the digest, by addition of an excess of 10 M sodium hydroxide solution containing 25 g 1-1 of sodium thiosulphate, is distilled in a Markham's apparatus and collected in 10 ml of 0.025 M hydrochloric acid. Between distillations of different nitrogen samples, 20-25 ml of ethanol are distilled to remove trace amounts of adsorbed ammonia and prevent cross-contamination. The resulting ammonium chloride solution is evaporated to dryness at 70 "C and the solid re-dissolved in glass-distilled water to give a solution containing approximately 2 pg pl-l of nitrogen.Alternatively, the microdiffusion method of Conwayll can be used for samples of low nitrogen content Ammonia released from portions of the diluted digest with 10 M potassium hydroxide solution containing 100 g 1-1 of sodium thio- sulphate is collected in 1 ml of 0.05 N hydrochloric acid in the centre well of a 60-mm Conway unit. The nitrogen-15 labelled ammonium chloride solution is evaporated to dryness and reconstituted to approximately 2 pg pl-1 of nitrogen. Components of the NIRD Nitrogen-15 Analyser in Fig. 1. Helium supply passing it through a BOC Rare Gas Purifier. A block diagram of the main components of the automatic nitrogen-15 analyser is shown Cylinder helium, first dried by passing through a molecular sieve (grade 5A), is purified byAmp1 ifier B .Phase- sen sit ive Peak - Chart detector - detector recorder B Wide slit mirror I - 1 ' EHT power unit - Microwave generator Ratiometer Phase- 1 sensitive detector A I I Vacuum pump I Photo- mu1 ti pl ier / I ' Discharge tube Dual wavelength monochromator 11 Furnace Soda- I i me Printer 'z FH-1 Calibration voltmeter Digital n Helium Helium purifier I 1 Oveh < I Nitrogen Pressure regulators controller I I I Valve Sample injection Fig. 1. Block diagram of main components of automatic nitrogen-15 analyser. bAugust, 1979 SPECTROMETER FOR THE DETERMINATION OF NITROGEN-15 769 Reactor tube In an earlier version of the NIRD nitrogen-15 analyser,12 nitrogen was generated from the ammonium chloride solution by reaction with copper and copper(I1) oxide, but a “memory effect” became apparent and produced serious cross-contamination of successive nitrogen-15 samples.Tests showed that the memory effect was most. probably caused by the formation of oxides of nitrogen as by-products of the Dumas reaction. These reacted with water already adsorbed on the chromatographic column to produce nitric acid, which could take part in exchange reactions with nitrogen. With the catalytic method of nitrogen generation, the front portion of a fused silica reactor tube (about 300 mm long, 14 mm 0.d.) is packed with soda-lime (BDH, 10-16 mesh). The rear 100-mm portion (7mm 0.d.) of the tube is packed with about 1.4g of the catalyst (Engelhard Industries Ltd., Chemical Group, Cinderford, Gloucestershire, Code No.99966, 0.25% rhodium - 0.25% platinum coated on alumina spheres, diameter approximately 0.2 mm) and the whole reactor tube is enclosed in a furnace at 590 “C. As the alumina spheres on which the catalyst is deposited are known to sinter at temperatures above 600 “C, the furnace temperature is reduced to about 500 “C during overnight regeneration. The sample of ammonium chloride solution (about 10 pg of nitrogen for normal operation) is injected manually through a silicone-rubber septum into the soda-lime tube and the liberated ammonia is dissociated. Because the dissociation reaction is endothermic, it is promoted by high temperatures. Ammonia dissociation is also accompanied by an increase in volume so that decomposition is favoured by high dilution in the helium carrier gas and small sample size.Exhaustion of the soda-lime (indicated by an erratic decrease in the intensity of the nitrogen peak on the chart record) leads to poisoning of the catalyst with the consequent presence of undissociated ammonia and the possibility of a memory effect between successive samples. Several thousand samples can be analysed before replacement is necessary. Gas-chromatographic column The column removes water and readily separates the nitrogen from attendant hydrogen. The nitrogen peak emerges 36 s after the hydrogen peak and has a pulse time of 2.4 s for half-peak maximum. The molecular sieve (13X, 30-60 mesh, Chromatography Services Ltd.) is packed into a stainless-steel tube 1.3 m long x 4 mm i.d., which has a water capacity equivalent to several hundred samples before regeneration.As water is absorbed the effective column length is reduced so that the elution time of about 1 min for nitrogen on a freshly regenerated column is progressively reduced during the analysis of a series of samples. The column is regenerated overnight by reversing the helium flow and setting the oven temperature surrounding the column to about 220 “C. Sintered stainless-steel filters are fitted at both column and reactor outlet connectors as a precaution against blockage of the flow restrict or. Discharge tube As the discharge operates at 10-15 Torr and the remainder of the preparation unit is at a positive helium pressure (approximately 30 p.s.i., which gave an elution time of just under 1 min for a 1.3-m column), it is necessary to introduce a throttle between the column exit and the discharge tube.This consists of a short stainless-steel tube flattened sufficiently to give the desired pressure drop. To avoid fluorescence effects the discharge tube was made from Spectrosil, its dimensions being 250 mm over-all length x 3 mm i.d. Light output is affected only slightly by changes in the discharge tube internal diameter, operating temperature, microwave power and pressure. It is convenient to operate the discharge at room temperature at about 22 W. Optical system A Rank-Hilger D331 double monochromator fitted with a 2400 lines per millimetre grating blazed for 300 nm was converted into a dual- wavelength monochromator. At 300 nm, the stated aperture and reciprocal dispersion were j75.5 and 1.3 nm mm-1, respectively.The transmitted beam enters a second compart- ment through a relatively wide slit that enables a narrow range of wavelengths to fall on the An optical diagram is shown in Fig. 1.760 GOULDEN AND SALTER: AUTOMATIC EMISSION Analyst, Vol. 104 wavelength change mirror. This mirror can be rotated through a small angle by a long lever working between a pair of adjustable stops. Choice of optimum slit widths and electronic parameters proved to be particularly difficult as all of these factors are interrelated, and a compromise had to be made between optical resolution and electronic noise in relation to sample size. Satisfactory performance was obtained with 0.10-mm entrance and exit slits, giving a spectral band width of 0.13 nm. The intermediate slit was sufficiently wide to allow both the 14N14N and 14N15N bands to be transmitted.The nitrogen supply was used only for setting up the optical system. Electronics system The electronics system was assembled from commercially available units with a minihum amount of modification. A Microtron 200 Mk I11 [Electro-Medical Supplies (Greenham) Ltd.] microwave generator is used with about 80% depth of modulation at approximately 1 kHz to excite the discharge. To avoid troublesome earth-loop problems, the screen on the modulation lead from the case is disconnected, and the 3A/4 cavity (Electro-Medical Supplies, Type 215L) insulated from the monochromator chassis. A pair of EM1 9789 QB photomultipliers (EMI, Hayes, Middlesex) are fitted into Rank-Hilger mounts and the electronics altered for ax.operation with the cathode earthed, supplied by an EM1 Type PM 25A EHT unit. The measuring system consists of a pair of precision amplifiers (Brook- deal Electronics, Model 9452), and a pair of phase-sensitive detectors (Brookdeal Electronics, Model 941 1) supplying the two input channels of a ratiometer (Brookdeal Electronics, Model 9547). The A lead (signal beam, see Fig. 1) is connected directly to the ratiometer whilst the B lead (reference beam) is connected through a potentiometer circuit (not shown), which serves as a fine control for balancing the two signals. An adjustable signal is taken off this circuit to operate a 1-mV chart recorder by means of which the approximate nitrogen concentration of the sample is monitored. A specially designed electro-mechanical peak detector built into the recorder actuates the printer at the peak maximum.13 The output of the ratiometer supplies the input to the printer (Anadex DP 501) via a calibration box (providing variable scale expansion and back-off) and a digital voltmeter (Fenlow Type 701-BCD).This allows the output to be printed as either millivolts or atom per cent. nitrogen-15 with appropriate adjustments for the slope and intercept of the straight- line calibration relationship. Tem$erature control For satisfactory reproducibility it was found necessary to house the nitrogen-15 analyser in a temperature-controlled room. A number of different components contribute to the temperature sensitivity of the instrument, e.g., wavelength shifts at the monochromator, photometric imbalance due to the differing temperature coefficients of the photomultipliers and temperature drifts of the electronic units. Tests have shown that provided the mono- chromator temperature is held to 30.2 O C , the errors in recorded nitrogen-15 are less than hO.01 atom-%. Regular temperature checks and occasional adjustments of the setting of the room thermostat enabled the temperature to be held within these limits without difficulty. Vacwm and @essure system The vacuum pump is not required to have a high performance and even a small rotary pump, such as the Edwards ISP 30, needs several feet of 2-mm bore tubing to restrict pumping capacity. Calibration and Correction for the Background Effect Even with no nitrogen present, the discharge shows some emission in the 300-nm region.Increasing the helium flow-rate, however, reduces the background emission , possibly owing to a change in pressure in the discharge zone. When the total amount of nitrogen in the sample decreases, it is necessary to increase the amplifier gain or photomultiplier EHT. This leads to an increase in the relative error due to background emission. The signal at 298.3nm (14N15N band) was backed off by a voltage corresponding to the backgroundAugust, 1979 SPECTROMETER FOR THE DETERMINATION OF NITROGEN-15 761 emission. This enables a two-wavelength measurement to provide results that are substantially independent of the nitrogen content. In the diagram the heights of the 14N14N and 14N15N bands relative to the signal with an opaque shutter are represented by x and y, respectively.The corresponding background levels are a and b. The true intensity ratio (R) is given by R = ( x - a)/(y - b) and hence x = Ry - Rb + a. The signal ratio (R’) observed during normal signal measurement is given by The theoretical basis for this technique is illustrated in Fig. 2. X Rb a (Rb - a) Y Y Y Y R ‘ = - = R - - + - = R - and for R’ to approach the true value R, the condition to make uncorrected background errors negligible is therefore that y > (Rb - a). This does not necessarily hold at low nitrogen-15 enrichments when R is fairly large, hence the need to make a background correc- tion by subtracting a constant signal (c, the back-off) from each channel.The measured ratio then becomes R’ = ( x - c)/(y - c). By making the back-off equal to the background emission (b) at the wavelength of the 14N15N peak, which is the smaller signal at low nitrogen- 15 enrichments, we minimise errors, and R’ = ( x - b)/(y - b). At low nitrogen-15 enrich- ments b < x and b = a, so that R’ = ( x - a)/(y - b) = R. In a practical test it was found that signals a and b were 0.6 and o.5y0, respectively, of the signal x for a natural abundance sample. At very high nitrogen-15 abundance, i.e., if y > x , it may be more accurate to put c = a, but this is seldom necessary in practice. 1 4 ~ 1 4 ~ 297.7 nm - f- :/ .1 r------ ~a /Opaque shutter zero Fig. 2. Theoretical basis of correction for bacl Wave1 ength/nm ground effect.For normal operation, a 5-pl sample of an ammonium chloride solution containing 10 pug of nitrogen is injected into the helium stream and the EHT adjusted so that the nitrogen peak on the recorder reaches about 70% of full-scale deflection. Tests have shown that using the zero off-set procedure, errors in nitrogen-15 atom per cent. for the low enrichment samples are within hO.01 when the peak height is within the range 60-9070 deflection. Adjustment of the volume injected from the 10-pl syringe enabled sample concentrations within the range 1 4 pg pl-l to be accommodated. Samples containing as little as 2 pg of nitrogen may be measured if the photomultiplier EHT is increased and the instrument calibration is set up at this level, but the error then increases to about 0.1 atom-%.The calibration procedure is carried out using a known low nitrogen-16 abundance sample (usually natural abundance) and one other of known enrichment chosen in relation to the enrichment of the samples of interest. For normal use, the instrument is first conditioned762 GOULDEN AND SALTER: AUTOMATIC EMISSION Analyst, Vol. 104 by injecting 10 samples of ammonium chloride solution each containing 10 pg of nitrogen in 5 p1. After checking EHT, zeros and balance, the low nitrogen-15 sample is injected and the intercept (back-off) potentiometer adjusted to show the correct reading. The high nitrogen-15 sample is then injected and the slope (variable scale expansion) potentiometer adjusted to give a correct reading. These two potentiometers are housed in the calibration box (Fig.1). When the upper set point is fixed (usually at 1.88 atom-%) and the lower at natural abundance (0.365 atom-%), readings between 0.365 and 2 atom-% are correct to hO.01 atom-% nitrogen-15. As the true relationship between millivolts and nitrogen-15 atom per cent. is curvilinear, print-out values for samples above 2 atom-% can be taken as approximate, and a first-order correction made using an error curve. More accurate results for such samples can be obtained by running the samples with an appropriate series of standards and carrying out a regression analysis on the printed nitrogen-15 readings. Alternatively, if a number of samples to be run are expected to have high enrichments, the instrument may be calibrated with two standard nitrogen-15 samples with values on either side of the mean expected, using the lower enrichment to set the intercept.The results can then be taken directly from the print-out. Re+eatability and accuracy chloride solutions are shown in Table I. The results of measurements of isotopic enrichments in three standard [15N] ammonium Six samples from each solution were measured in TABLE I REPEATABILITY SAMPLES OF MEASUREMENTS OF STANDARD [15N]AMMONIUM CHLORIDE WITH THE NIRD AUTOMATIC NITROGEN-15 ANALYSER Measured nitrogen-15, atom-% I A \ Enrichment of standard NIRD automatic used for calibration.* r A \ atom- % nitrogen- 15 Values 0.37 0.364 0.373 0.386 0.379 0.378 0.361 1.44 2.54 1.430 1.441 1.427 1.432 1.424 1.424 2.536 2.541 2.545 2.527 2.548 2.554 * Supplied by C.Z.Scientific Ltd. t S.D. = Standard deviation. Mean S.D.? 0.374 0.009 J -l Manual emission spectrometer Values 0.398 0.393 0.352 0.395 0.411 0.360 1.468 1.433 1.430 1.400 1.389 1.404 2.556 2.566 2.525 2.523 2.618 2.530 Mean 0.385 J 1.421 1 J i 2.553 S.D..f 0.023 0.029 0.036 the Statron NOI-4 and a further six samples from each of the same solutions were measured in the NIRD automatic nitrogen-15 analyser. These results show that the automatic analyser gave acceptable values for the standards (within 0.01 atom-%) and also indicate a higher reproducibility using the automatic method. The accuracy of the NIRD auto- matic nitrogen-15 analyser is illustrated in Table 11, which shows the results of measurements of nitrogen-15 in samples of the duodenal contents of a steer that had been given a feed containing [15N]urea.In each instance two samples of the biological material were digestedAagust, 1979 SPECTROMETER FOR THE DETERMINATION OF NITROGEN-15 TABLE I1 ACCURACY OF MEASUREMENT OF NITROGEN-15 IN BIOLOGICAL SAMPLES The nitrogen-15 enrichment of the same ammonium chloride solutions prepared from various samples of duodenal contents and rumen bacteria from a steer were measured both in the NIRD automatic analyser and in the Statron NOI-4 emission spectrometer. Sample Sample No. Duodenal contents . . .. 1 2 3 4 5 6 Rumen bacteria, C375/1 . . 1 2 3 4 5 6 and a sample of the ammonium chloride NIRD automatic Aliquot 1 Aliquot 2 0.59 0.58 0.79 0.81 1.11 1.12 1.40 1.38 1.67 1.67 1.51 1.52 w- 0.37 - 0.93 - 1.81 - 2.06 I 1.93 - 1.69 - Manual emission spectrometer Aliquot 1 Aliquot 2 0.59 0.55 0.79 0.76 1.10 1.10 1.36 1.37 1.73 1.76 1.52 1.55 7 0.39 - 0.92 - 1.83 - 2.06 - 1.94 - 1.68 - 763 prepared from each was measured, and also measured again using the manual emission spectiometer. Results from the two instruments agreed well, the automatic analyser showing less variation between duplicates than the manual analyser.The standard deviation obtained when 10 measurements were made with the NIRD analyser on a sample with a mean nitrogen-15 enrichment of 0.608 atom-% was 0.006 atom-%, similar to that obtained with standard ammonium chloride solutions. Memory efect The memory effect with the NIRD automatic analyser is very small and for practical purposes can be ignored.To test the effect four natural abundance samples, injected at l-min intervals, were followed by four samples of exceptionally high nitrogen-15 abundance (19.5 atom-%) and these were followed by five samples of natural abundance. The results (Table 111) show that even under these extreme conditions an increase in the natural abundance sample immediately following the highly enriched sample was only just detected. TABLE I11 TEST OF THE MEMORY EFFECT IN NIRD AUTOMATIC NITROGEN-15 ANALYSER Four natural abundance samples of ammonium chloride were injected at 1-min intervals and immediately followed by four samples containing 19.5 atom-% nitrogen-16 and a further five samples of natural abundance, at the same rate. Each sample contained 10 pg of nitrogen. Sample No.1 2 3 4 5 6 7 8 9 10 11 12 13 Nominal atom-% 15N 0.365 0.365 0.365 0.365 19.5 19.5 19.5 19.5 0.365 0.365 0.365 0.365 0.365 Print-out value atom-% lfiN 0.365 0.359 0.364 0.360 19.519 19.513 19.413 19.514 0.407 0.372 0.362 0.362 0.364764 GOULDEN AND SALTER : AUTOMATIC EMISSION Analyst, Vol. 104 Discussion The most important feature of the NIRD automatic nitrogen-15 analyser is the high rate of analysis that is possible (one sample per minute) compared with manually operated optical emission spectrometers. This can be achieved without sacrificing the high chemical sensi- tivity obtainable manually, and with at least as high accuracy (hO.01 atom-% in the normal operating range). It is also simple to operate, completely dispensing with time- consuming and tedious vacuum techniques.The essential development that has enabled this high throughout is the use of a rhodium - platinum catalyst for the generation of pure nitrogen. At the time when this work was started the use of such a catalyst in nitrogen-15 analysis had not been reported. Subse- quently, however, a report of the use of a rhenium-filament catalyst for the batchwise conversion of ammonia to nitrogen for the mass-spectrometric determination of 14N to l5N ratios has appeared.14 Under the conditions maintained in the analyser this method does not suffer significantly from interfering side-reactions that hinder the use of traditional methods such as that of Dumas, using copper - copper(I1) oxide, or that of Rittenberg, using alkaline hypobromite solution. The latter method has been used in an East German auto- matic nitrogen-15 analyser (Statron Isonitromat) , which was announced after the work reported here was started, and the procedures required to correct for the memory effect in this instrument result in a slower rate of analysis (18 samples per hour).I t is quoted to have a similar reproducibility (hO.01 atom-%) to that of the NIRD analyser. However, in the Statron system, measurements are made at three different wavelengths, enabling an automatic correction to be made for background emission. The use of triple wavelength measurement in conjunction with the NIRD nitrogen preparation system might reduce background drift. This drift is probably caused by changes in the background level due to emission from residual water molecules.As the effective length of the molecular sieve column becomes reduced, owing to saturation with water molecules, the drift tends to increase towards the end of a run. Similarly, a further improvement in stability with a consequent extension of the limits of accuracy below kO.01 atom-% might be achieved by replacement of the double photomultiplier measurement system with a single photo- multiplier and suitably modified electronics to allow simultaneous measurements at three wavelengths. Within the limits used, however, the gradual downward drift observed in calibration is not a serious problem, as it shows up as a change in intercept and, to a first approximation, affects all readings equally. Correction can be readily made either by adjustment of the intercept potentiometer or by analysing a natural abundance sample a t intervals throughout a run and applying the correction to the printed results.Although mass spectrometry remains the most accurate means of measuring nitrogen-15, with a reproducibility of &0.001 atom-% possible in modern apparatus, automatic optical emission spectrometry offers a number of advantages. Not least among these are the ease of operation and the much higher rate of analysis possible over extended periods. The very small amount of nitrogen required (10 pg), compared with the mass spectrometer (200- 2000 pg for highest accuracy), permits measurement of nitrogen-15 in nitrogen metabolites obtainable only in small amounts. We are grateful to Dr. E. W. Evans of the Physics Department, NIRD, for helpful criticism of the manuscript, and to the NIRD Instrumentation Section Workshop for assistance in the construction of the instrument. References 1. 2. 3. 4. 5. 6. 7. 8. 9. Rittenberg, D., in Wilson, D. W., Nier, A. 0. C., and Reimann, S. P., Editors, “Preparation and Broida, H. P., and Chapman, M. W., Analyt. Chew., 1958, 30, 2049. Meier, G., and Miiller, G., Isotopenpraxis, 1965, 1, 53. Faust. H., Isotopenpraxis, 1965, 1. 62. Lloyd-Jones, C. P., Hudd, G. A., and Hill-Cottingham, D. G., Analyst, 1974, 99, 580. Lloyd-Jones, C . P., Adam, J.. and Salter, D. N., Analyst, 1975, 100, 891. Lloyd-Jones, C. P., Adam, J., Hudd, G. A., and Hill-Cottingham, D. G., Analyst, 1977, 102, 473. Salter, D. N., and Smith, R. H., Br. J . Nutr., 1977, 38, 207. Hauck, R. D., and Bremner, J. M., Adv. Agron., 1976, 28, 219. Measurement of Isotopic Tracers,” Ann Arbor, Mich., 1947, pp. 31-42.Aztgast, 1979 SPECTROMETER FOR THE DETERMINATION OF NITROGEN-15 765 10. 11. 12. 13. 14. Goulden, J. D. S., and Salter, D. N., Proc. Nutr. Soc., 1977, 36, 132A. Conway, E. G., “Microdiffusion Analysis and Volumetric Error,” Fourth Edition, Crosby Lockwood, Goulden, J. D. S., and Salter, D. N., UV Spectrom. Gr$ Bull., 1975, 3, 74. Goulden, J. D. S., and Salter, D. N., J . Chromat., submitted for publication. Walker, R. L., Walton, J. R., Carter, J. A., and Matthews, D. R., in “Proceedings of the I.A.E.A. Symposium on Isotope Ratios as Pollutant Source and Behaviour Indicators, 18-22 November, 1974, Vienna,” I.A.E.A., ‘Vienna, 1975, p. 429. Received November 17th. 1978 Accepted February 16th, 1979 London, 1957, p. 138.

ISSN:0003-2654    

DOI:10.1039/AN9790400756

出版商:RSC    

年代:1979

数据来源: RSC