ISSN:0003-2654    

DOI:10.1039/AN97196FX005

出版商:RSC    

年代:1971

数据来源: RSC

ISSN:0003-2654    

DOI:10.1039/AN97196BX007

出版商:RSC    

年代:1971

数据来源: RSC

摘要:

i V THE ANALYST [February, 1971THE ANALYSTED IT0 RI AL ADVl SO RY BOARDChairman: A. A. Smales, O.B.E. (Harwell)*T. Allen (Bradford)*L. S. Bark (Solford)M. T. Kelley (U.S.A.)W. Kemula (Poland)R. Belcher (Birmingham)L. J. Bellamy, C.B.E. (Waltham Abbey)L. S. Birks (U.S.A.)E. Bishop (Exeter)*R. C. Chirnside (Wembley)A. C. Docherty (Bi/lingham)D. Dyrssen (Sweden)*W. T. Elwell (Birmingham)*D. C. Garratt (London)*R. Goulden (Sittingbourne)J. Hoste (Belgium)D. N. Hume (U.S.A.)H. M. N. H. Irving (Leeds)*G. F. Kirkbright (London)*G. W. C. Milner (Harwell)G. H. Morrison (U.S.A.)*G. Nickless (Bristol)S. A. Price (Tadworth)D. 1. Rees (London)E. B. Sandell (U.S.A.)W. Schoniger (Switzerland)H. E. Stagg (Manchester)E. Stahl (Germany)A. Walsh (Australia)*T.S. West (London)P. Zuman (U.S.A.)*A. G. Jones (Welwyn Garden City)*Members of the Board serving on the Executive Committee.NOTICE TO SUBSCRIBERSSubscriptions for The Analyst, Analytical Abstracts and Proceedings should be(Other than members of the Society)sent through a subscription agent or direct t o :The Chemical Society, Publications Sales Ofice,BI ack horse Road, Letc h wo rt h Her ts.(a) The Analyst, Analytical Abstracts, and Proceedings, with indexes . . ..index), and Procecdings . . .. .. .. .. .. ..index), and Proceedings . . .. .. .. * . .. ..(b) The Analyst, Analytical Abstracts printed on one side of the paper (without(c) The Analyst, Analytical Abstracts printed on one side of the paper (withThe Analyst and Analytical Abstracts without Proceedings-(d) The Analyst and Analytical Abstracts, with indexes .. .. .. ..index) . . . . . . . . . . . . . . . . .. ..index) . . . . . . . . . . . . . . . . . . . .(e) The Analyst and Analytical Abstracts printed on one side of the paper (without(f) The Analyst and Analytical Abstracts printed on one side of the paper (with€27.50 $66.00€28.50 $69.00€34.75 $84.00€25.00 $60.00€26.00 $63.00f 32.25 $78.00(Subscriptions are NOT accepted for The Analyst and/or for Proceedings alone)Members should send their subscriptions to the, Hon. Treasurevi SUMMARIES OF PAPERS IN THIS ISSUE [February, 1971Summaries of Papers in this IssueMetal - Metallochromic Indicator Complexes as Acid - BaseIndicatorsIVhen the formation of metal - metallochromic indicator complexcs isaccompanied by the release of more than one proton per indicator moleculecomplexed, the colour changc a t the end-point is sharper than that of aconventional acid - base indicator, when only one proton is released.Thelarger the number of protons released per indicator molecule, the sharper thecolour change. Because complexes of differcnt stabilities are formed betweenmetals and metallochromic indicators, one such indicator can provide awhole range of p€I indicators by variation of the metal ion used. The pKvalue for the indicator system is a conditional constant, and can be Ioweredby increasing the amount of free metal ion present or by increasing theconcentration of the indicator complex.The nature of the metal - indicatorcomplex can be deduced from a study of the absorbance - pH curve fordifferent ratios of metal ion and indicator concentrations. Either componentof the indicator system can be determined by potentiometric (pH) titrationwith the other. Limitations are imposed by the necessity to avoid thepresence of species that form more stable complexes with the metal ion,by hydrolysis, and by thc acid - base characteristics of the free indicator.In the most favourable instances it is possible to achieve a. complete colourchange over 0.5 pH unit or less. The pK value is relatively indifferent tochanges in ionic strength.ROBERT A. CHALMERS and FRANK I. MILLERDepartment of Chemistry, University of Aberdeen, Old Aberdeen, Scotland.AI/alJ)st, 1971, 96, 97-105.The Influence of the Formation of Metal - Indicator Complexes ofthe M21 Species on the Accuracy of Complexometric Micro Titrationswith Photometric End-point DeterminationWhen microniolar amounts of tervalent and quadrivalent metals such asbismuth, thorium and iron are bound to triphenylmethane dyes and titratedcomplexometricall y, large systematic deviations are found.There are reasons to suppose that M,I type complexes are formed.it is shown that if M21 predominates in solution, the large systematic devi-ations can be accounted for theoretically.The use of azo-type indicators,such as PAR, TAR and TAN, gives more accurate results.J. KRAGTENNatuurkundig Laboratorium, University of ihsterdam, The Netherlands.A~zaZyst, 1971, 96, 106-109.The Use of Partial-pressure Mass Spectrometry in the Study ofthe Thermal Desorption and Oxidation of Carbon and GraphiteA partial-pressure mass-spectrometer system is described for measuringthermally desorbed species from solid surfaces.The system evaluates notonly the ratio of masses present in the gaseous phase but also relates thespecific mass (mg g-1 of solid) desorbed or decomposed during thermal treat-ment. The extension of the method toinclude oxidation studiesis also described.A study has been made of the initial evolution of gases from graphitic andnon-graphitic carbons. These range in properties from a ground graphiteof specific surface 103 ma g-l to a nuclear-type graphite of 0.6 m2 g-l.Astudy of a non-graphitic carbon, saran charcoal, of molecular-sieve typeis also included.The formation of surface oxide on a clean surface at low pressures isevaluated together with the resultant thermal decomposition of the surfaceoxide. The results from this paper together with other published work ongraphites are reviewed and used to illustrate the application of the resultsof thermal desorption to oxidation studies on carbons and graphites.F. E. AUSTIN, J. G. BROWN, J. DOLLIMORE, C. M. FREEDMAN andB. H. HARRISONDepartment of Pure and Applied Physics, University of Salford, Salford 5, Lanca-shire.Analyst, 1971, 96, 110-116[February, 1971 ...Vlll SUMMARIES OF PAPERS I N THIS ISSUEThe Determination of Fluorine in Rock Materials by y- Activationand Radiochemical SeparationA technique is described for the determination of fluorine in rockmaterials involving irradiation in a source of high energy y-photons to inducethe I9F (y,n) I8F reaction.Fluorine-18 is then separated from the radioactivematrix by distillation and its activity measured either in the distillate oras precipitated calcium fluoride and compared with that of irradiated calciumfluoride standards. The technique has been applied to the analysis ofstandard rock materials G1 (635 p.p.m.), W1 (221 p.p.m.), T1 (476 p.p.m.)and to Apollo 11 lunar fines (76 p.p.m.). Precautions are taken to eliminateinterferences. The results obtained for the standard rocks are in good agree-ment with those of conventional methods but disagree with other activationresults.J. S.HISLOP, A. G. PRATCHETT and D. R. WILLIAMSAnalytical Sciences Division, Atomic Energy Research Establishment, Harwell,Berks.Analyst, 1971, 96, 11 7-123.The limit of detection of the method is 0-002 pg.Pyridine-2,3- diol as Metal Indicator in the ChelatometricDetermination of Iron(II1) with EDTAPyridine-2,3-diol can be satisfactorily used as an indicator in the chelato-metric determination of iron(II1) over the pH range of 1 to 4, the end-pointbeing sharp and distinct. The indicator is effective in the presence of commonbivalent metal ions. Interferences from some quadrivalent, tervalent andbivalent metal ions have been prevented by using masking agents, b u t oxalateand thiocyanate seriously interfere.Borate, tartrate and citrate do notinterfere. However, the presence of a large excess of acetate ions mustbe avoided.D. P. GOEL and R. P. SINGHDepartment of Chemistry, University of Delhi, Delhi-7, India.Arialyst, 1971, 96, 123-126.Spectrophotometric Determination of Vanadium withN- Benzoyl- 0- tolylhydroxylamineThe method prescribed by Maj umdar and Das for the spectrophotometricdetermination of vanadium(V) with N-benzoyl-o-tolylhydroxylamine has beenre-examined. The validity of the method originally reported is supportedby the further results presented.A. K. MAJUMDAR and S. K. BHOWALDepartment of Inorganic and Xnalptical Chemistry, Jadavpur University, Calcutta-32,India.Analyst, 1971, 96, 127-129.The Analysis of Tin Stabilisers Used in Poly(viny1 chloride)CompositionsSchemes are described for the analysis of commercial tin stabiliserscommonly used in poly(viny1 chloride) compositions.Methods are givenfor the chemical breakdown of a sample and subsequent separation andidentification of the breakdown products, which may include dialkyltin oxides,alcohols, thiols, carboxylic acids and thioacids. Separations are carried outby precipitation, solvent extraction and thin-layer chromatographic tech-niques, and the components identified by gas-chromatographic, infrared,nuclear magnetic resonance and mass-spectroscopic methods.Manual and potentiometric titration procedures for quantitative examina-tion together with recommended methods for the determination of tin areeither described in this paper or in the literature cited, and methods foridentification of additives and procedures for identification of tin stabilisersin the presence of excess of plasticiser, by means of column chromatographyand ion-exchange procedures, are also included.J. UDRISResearch Department, Imperial Chemical Industries Limited, Plastics Division,Welwyn Garden City, Herts.Analjvst, 1971, 96, 130-130

ISSN:0003-2654    

DOI:10.1039/AN97196FP021

出版商:RSC    

年代:1971

数据来源: RSC

摘要:

February, 19711 THE A4NALYST xiAPPOINTMENTS VACANT LECTURES AND COURSESPUBLIC SERVICE OF TASMANIA, AUSTRALIACHEMIST - MINES DEPARTMENTApplications are invited for appointment to a position of Chemistin the Chemical/Metallurgical Division of the Department of Mines,Tasmania.Locntion: La uncestonSalary Range: $A3768-$A7749 per annumCommencing salary will be according to qualifications andexperience, and will be payable from date of embarkation forAustralia.Duties: Chemical research and analyses of ores and metallurgicaland mineral products and rocks using chemical and instrumentalmethods, particularly AAS and XRF.Qtralifications: Degree or Diploma in Chemistry with acceptableexperience.Passages from the United Kingdom are provided under the Common-wealth Migration-Assisted Passage-Scheme, arranged throughthe Agent General for Tasmania.In addition, further assistancewith passages and freight chargrs, etc., is available.Recreation leave, sick leave and long service leavc are providrd.Superannuation is compulsory for permanent officers.Enquiries for Application Forms and publications should be addressedto the Agent General for Tasmania, 455/9 Strand, London, W.C.J.UNIVERSITY OF STRATHCLYDEDEPARTMENT OFPHARMACEUTICAL CHEMISTRYMAY AND BAKER STUDENTSHIPINPHARMACEUTICAL ANALYSISApplications are invited from Honours grad-uates in Pharmacy or Chemistry, and fromundergraduates now in their final year for theMay & Baker Studentship, which is availablefor award to students enrolled on the courseleading to the University’s M.Sc.Degree inPharmaceutical Analysis, commencing inOctober, 1971. The course which includesinstruction in a wide range of modern instru-mental methods and a project in pharmaceu-tical analysis is specifically orientated to therequirements of the pharmaceutical industryfor specialists in analytical research and qua-lity control of pharmaceutical products usedin human and veterinary medicine.The Studentship covers University fees and amaintenance award of E550 per annum.Applications should be addressed to ProfessorJ. B. Stenlake, Department of PharmaceuticalChemistry, University of Strathclyde RoyalCollege Building, George Street, Glasgow,C.1., from whom further particulars of thecourse can also be obtained.Analytical Flame SpectroscopyEdited by R.MavrodineanuUnder the general editorship of R. Mavrodineanuthis volume presents a number of contributions byindividual experts in the general field of atomicabsorption, thermal emission and atomic fluorescenceflame spectroscopy. This is an important and rapidlyexpanding field of analysis and each of the individualauthors has already made a considerable researchcontribution to it. A l l aspects of the subject arecovered from sample introduction to method ofmeasurement and interpretation of results.8OOpp S14Mac m i I I axiv SUMMARIES OF PAPERS IN THIS ISSUE [February, 1971Revision of a Field Method for the Determination ofTotal Airborne LeadAn improved field method is described for determining airborne leadat concentrations up to 0-8 mg m-3 of lead. After collection on a filter thelead is dissolved in acid and complexed with dithizone.The lead dithizonateis extracted into 1,1, l-trichloroethane and the colour intensity of the complexis compared visually with standards. The apparatus used is simple to operateand the time required for a complete determination is about 15 minutes.D. M. GROFFMAN and R. WOODDepartment of Trade and Industry, Laboratory of the Government Chemist, CornwallHouse, Stamford Street, London, S.E.l.Analyst, 1971, 96, 140-145.zThe Spectrophotometric Determination of the Purity of CommercialDithizone and the Purification of Small Amounts of theReagent by ChromatographyThe purity of commercial dithizone is determined quantitatively bycomparing the optical density of a solution a t wavelength 620 nm with thatof a pure sample.Qualitatively, the purity is estimated from the patternobtained by chromatographing the crude dithizone on Whatman SG81(silica-impregnated) paper.Small amounts of the pure reagent are separated by chromatography onlarge sheets of Whatman SG81 paper or on a column consisting of a mixture ofequal parts of acid-washed silica gel (Kieselgel N) and Celite 545, with benzeneas eluting agent.H. G. C. KING and G. PRUDENRothamsted Experimental Station, Harpenden, Herts.Analjist, 19i1, 96, 146-148.The Determination of Dimethylpolysiloxane in Beer and YeastAn infrared method that provides an estimate of the silicone contentof beer and yeast has been developed.Because of the increased sensitivityover other reported methods i t is possible to use smaller samples, which couldbe an advantage with biological material. The rccommendcd workingconcentration range is 0.2 to 3 nig 1-1.A. SINCLAIR and T. R. HALLAMAllied Breweries Process Research Dcpartment, Burton upon Trent, Staffordshire.AnaZyst, 1971, 96, 149-154.Determination of Cyanogen Chloride in Activated Niacin Test StripsA siniple paper-strip method for the determination of niacin in culturesof human tubercle bacteria based on the reaction with cyanogen chlorideis described. Although a yield of only about 5 per cent. of cyanogen chlorideis obtained, it represents an excess of from 5 to 50 times that needed to reactwith the 5 x lo-' to 5 x Thechemistry of the system, the determination of cyanogen chloride and anevaluation of the procedure are presented.MORTON S.LEFAR, ALVIN MASLANSKY, WILLIAM D. YOUNG andDONALD P. KRONISHDepartments of Analytical/Physical Chemistry and Diagnostics Research, Warner-Lambert Research Institute, Morris Plains, New Jersey 07950, U.S.A.Analyst, 1971, 96, 155-158.mole of niacin expected in samplesxvi SUMMARIES OF PA4PERS I N THIS ISSUE [February, 197 1The Use of NN- Dimethylcasein in the Determination of ProteolyticEnzymes in Washing Products and Airborne Dust SamplesNN-Dimethylcasein is used as substrate in an automatic method forthe determination of proteolytic enzymes in washing products and airbornedust samples.Amino-acids formed by reaction with the enzyme are causedto react with 2,4,6-trinitrobenzenesulphonic acid to form stable, colouredMeisenheimer complexes. As NN-dimethylcasein does not react n-ith trinitro-benzenesulphonic acid there is no need to remove excess of substrate beforecolour development, and the enzyme digestion and colour reactions can beconducted simultaneously. This leads to high sensitivity, which is of par-ticular value in dust analysis and allows the use of a simple trouble-freemanifold.E. DUNN and R. BROTHERTONProcter & Gamble Limited, Newcastle Technical Centre, Newcastle upon Tyne.Atzalyst, 1971, 96, 150-163.A Simple Cutting, Holding and Back-flushing ArrangementA simple cutting, holding and back-flushing arrangement, involving onlyon - off gas valves, is described, which can be fitted to commercial dual flame-ionisation gas chromatographs and can be operated from a single pressure-controlled carrier-gas supply.It does not interfere with the normal inde-pendent operation of either column. Any peak or portion of a peak emergingfrom the first column can be transferred either directly or via an interceptivetrap to the second column for further examination (cutting operation). Simul-taneously, by equilibrating the pressure across the first column, materialremaining on this column can be held (holding operation) for subsequentelution or examination on the second column or, by releasing the pressure atthe inlet to the first column, can be eluted in a reverse direction (back-flushingoperation). The arrangement has proved particularly useful for determiningtrace impurities that are not completely separated from the tail of a majorcomponent.NORMAN MELLORImperial Chemical Industries Limited, Dyestuffs Division, Hexagon House, Blackley,Manchester 9.Aizalyst, 1971, 96, 164-171.for Dual Flame-ionisation ChromatographFebruary, 19711 THE ANtiLYSTAn ‘impossible’ analytical problem?sviitwo minutes from now ~~-you could be on the way to solving itActivation analysis is a fast-developing technique particularlyhelpful in solving difficult problems of trace element analysis.It offers a unique combination of extreme sensitivity with unam-biguous identification of an impurity.Sample contamination andreagent ‘blank’ errors are avoided and the technique can often beusednon-destructively. An Activation Analysis Unit has now beenestablished a t Harwell in collaboration with the Analytical Re-search & Development Unit. If you would like further details, orwould like the opportunity to discuss ways in which we can helpt o solve your particular problems, complete and post the couponor ring Abingdon 4141, Ext.3085.To :Activation Analysis Unit, Harwell, Didcot, Berks.I am interested in the services of the Activation Analysis Unit.I should like to :Receive further information by post Discuss my problem with YOUI am also interested in assistance with :IR spectrometry [7 mass spectrometry 0 NMR spectrometry[3 computer applicationsother analytical techniques (tick as appropriate)Name .Position ---Addressc] on-line analysisTel No.AA 1

ISSN:0003-2654    

DOI:10.1039/AN97196BP029

出版商:RSC    

年代:1971

数据来源: RSC

摘要:

FEBRUARY, 1971 THE ANALYST Vol. 96, No. I139 Metal = Metallochromic Indicator Complexes as Acid - Base Indicators* BY ROBERT A. CHALMERS AND FRANK I. MILLER (Department of Chemistry, University of A berdeen, Old Aberdeen, Scotland) When the formation of metal - metallochromic indicator complexes is accompanied by the release of more than one proton per indicator molecule complexed, the colour change at the end-point is sharper than that of a conventional acid - base indicator, when only one proton is released. The larger the number of protons released per indicator molecule, the sharper the colour change. Because complexes of different stabilities are formed between metals and metallochromic indicators, one such indicator can provide a whole range of pH indicators by variation of the metal ion used.The pK value for the indicator system is a conditional constant, and can be lowered by increasing the amount of free metal ion present or by increasing the concentration of the indicator complex. The nature of the metal - indicator complex can be deduced from a study of the absorbance -pH curve for different ratios of metal ion and indicator concentrations. Either component of the indicator system can be determined by potentiometric (pH) titration with the other. Limitations are imposed by the necessity to avoid the presence of species that form more stable complexes with the metal ion, by hydrolysis, and by the acid - base characteristics of the free indicator. In the most favourable instances it is possible to achieve a complete colour change over 0.5 pH unit or less.The pK value is relatively indifferent to changes in ionic strength. ALTHOUGH the theory of metal - metallochromic indicator complexes is well understood (see, for example, Schwarzenbach and Flaschkal), and the possible use of these indicators as acid - base indicators is implicit in the theory, they do not seem to have been applied hitherto for this purpose. The idea of investigating this possibility arose when demonstrating how to adjust the pH of a solution of bismuth and lead for consecutive EDTA titrations of the two com- ponents. The bismuth must be titrated at pH 1, at which lead does not interfere, and the pH is then raised to about 5 for titration of the lead. The sample solution must be acidic (or the bismuth would hydrolyse) but its pH is not known.If xylenol orange is added to the sample solution and a purple colour is produced, then the pH must be above 1, but its true value will not be known. If acid is now added until the yellow colour of the acid form of the free indicator is produced, the pH will be below 1 (as when the yellow colour appears in the first place). It follows that simple dilution with water to ten times the original volume will raise the pH value by 1 unit, and a colour change will be seen during the dilution. If the original solution was more acidic than 0.1 N, then the metal - indicator will not appear until the solution is sufficiently diluted to raise the pH by the necessary amount. After titration of the bismuth a buffer is used to raise the pH value to 5 for the lead titration, and a colour change again occurs when a particular pH value is reached (dilution to, say, 10 litres is not a practical proposition).Investigation has shown that the colour change occurs almost completely during a 2-fold dilution of the solution, corresponding to a change of 0.3 pH unit. In a conventional acid - base indicator, the colour change arises because the release of one proton from the indicator molecule results in a species of different colour from the original. The result is that for the transformed fraction of the indicator to change from 10 to 90 per cent. (the limits usually regarded as necessitated by the response of the human eye to mixtures of colours) the pH value must be increased by about 2 units. The form of the equilibrium * Paper presented at a meeting of the Society for Analytical Chemistry, Birmingham, May '7th and 8th, 0 SAC and the authors.' 1970. 9798 CHALMERS AND MILLER: METAL - METALLOCHROMIC [Analyst, Vol. 96 equation shows that if n protons could be released at the same time, the pH interval for the complete colour change would be 2/n. This is precisely the situation with many metal - metallochromic indicator complexes, especially those which have iminodiacetic acid groups substituted into an acid - base indicator molecule. The purpose of this paper is to explore the possibilities of that situation. THEORY The general equation for a metal reacting with a metallochromic indicator to form a complex containing only one ligand molecule may be written- mMn+ + H,I s MmH+-mh) Im(n--h)+ + mhH+ .. .. . * (1) which allows for the possibility of formation of polynuclear and protonated complexes (h is the number of protons released per metal ion complexed). It follows that (omitting charges for simplicity)- - m log [MI - mh pH .. * * (3) log K=log [MmH ( 2-mh ,I1 [HXII When the transformed fraction (TF) of the indicator is 50 per cent.- so and Similarly, For fixed initial concentrations of metal ion (CM) and indicator (CI), with ratio r = CM/CI and the condition CM > mCI, if the stability constant of the indicator complex is reasonably high and all m metal ions are complexed simultaneously [MI,,, = CM - 0.5 mCI = (Y - 0-5 m)CI . . .. e . [MIlo% = Ch1 - 0.1 mC, = (r - 0.1 m)CI . . .. .. [MIgo, = CM - 0.9 mCI = ( I - 0.9 m)CI .. .. .. and Theref ore .. 1 1 h mh " log K - - log (Y - 0.1 m)C, - - 1 mh PHlo% = -- and .. 1 1 h mh * ' log K - -log (r - 0.9 m)CI + - 1 mh PH90% = --February, 19711 INDICATOR COMPLEXES AS ACID - BASE INDICATORS TABLE I EFFECT OF VARIOUS PARAMETERS ON APH,O/lo AND ApKMI A PH9om h Y* 1 1 2 3 4 5 10 Limiting value 2 1 2 3 4 5 10 Limiting value 3 1 2 3 4 5 10 Limiting value r m = l 2.95 2.24 2.15 2.10 2.08 2.04 2.0 1.48 1.12 1.07 1.05 1.04 1.02 1.0 0.99 0-75 0.72 0.70 0.70 0.68 0.67 1 m = 2t 1.95 1-37 1.24 1.18 1.0s 1.0 - - 0-98 0-68 0.62 0.59 0.54 0.5 - 0.65 0.45 0.41 0.39 0.36 0.33 A pKm$ m = l 0 - 0.3 - 0.4 - 0.6 - 0.7 - 1.0 0 - 0.2 - 0.3 - 0.5 0 -0.1 - 0.2 - 0.3 - 0.1, - 0.3, -0.1, - 0.2, 99 * CI = 1 x 1 0 - 5 ~ . t Calculated on the assumption that both metal ions are complexed simultaneously. APKM = PKYI(r-s) - PKYI(r-1) For a fixed value of m, the difference in ApHgOI,, at two Y values can be calculated (SApH), and comparison of experimental and calculated values of 6ApH will permit evaluation of m and h.The values of ApH,,~,, are shown in Table I. If an absorbance - pH curve is plotted for r = 1 and 10, it will at once be obvious from the pH,,, and pH,,% values ( i e . , the pH values for absorbances equal to 10 and 90 per cent. of the maximum net absorbance) whether m is 1 or 2 and whether the complex is protonated or not. It also follows that addition of an increasing excess of metal ions narrows the transition interval, with a limiting value of 1.0 for m = 1 and h = 2, and of 0.7 for m = 1 and h = 3; the limits become narrower if a binuclear complex is formed with both metal ions complexed simultaneously.If there is successive formation of MI and M,I, the latter will be formed at a higher pH value and the pH - absorbance curve will show two steps (cf. Fig. 1). A graph of pKMI versus Y shows that the former decreases as the latter increases (which is to be expected because of the increased competition by metal ion for the indicator anion). This effect permits fine control of the pH range covered by the indicator colour change. The concentration ratio of metal - indicator complex to free indicator is constant at pH equal to pKMI (equation 6) but the free metal-ion concentration will depend on the total indicator concentration and the concentration ratio (Y); because the value of pKm for a given con- centration ratio decreases with increasing free metal-ion concentration (see Table I) , it follows that pKm should decrease with increasing total metal - indicator complex concentration.The situation in which complexes of the type M,Ii are formed, where i is greater than unity, has not been considered, for two reasons: it is not amenable to simple logarithmic treatment, and if i is greater than m the indicators would be no better than conventional acid - base indicators.100 CHALMERS AND MILLER: METAL - METALLOCHROMIC [Analyst, Vol. 96 APPLICATIONS AND LIMITATIONS By varying the metal and metallochromic indicator, and their concentrations, indicators can be obtained for almost any pH range. Limitations are imposed, however, by the nature of the system.The solution to be tested must not contain ligands that form more stable complexes than the indicator with the metal ion, nor must it contain metal ions that form more stable complexes with the indicator than the metal ion chosen unless the difference in stability is small enough for the change in pK to be tolerated. The metal -indicator complex must be sufficiently stable to avoid hydrolytic effects. To determine the pH value of a system, the metallochromic indicator is added first, and then different metal ions are added in order of increasing stability of metal - indicator complex, until one is found which gives a colour change. The pH value can then be assessed more accurately by adding a known amount of free indicator and “titrating” with metal ion (see Table I).For example, for CI = 1 x M, the zinc - xylenol orange complex is orange at pH 4-5 and r = 1, but the orange colour does not form until 7 = 17 if the pH value is 3.9. For acid - base titrations the appropriate end-point pH is selected and the corresponding indicator - metal combination is used. For titration of polyprotic acids it is best to begin with the normal salt and titrate it with a strong acid, by using a single metallochromic indicator and a series of metals in order of increasing stability of indicator complexes. More than one metallochromic indicator can be used, provided the colours of the free indicators and their metal complexes do not give a masking effect. If the successive dissociation constants of the acid are sufficiently well separated, it should be possible to titrate stepwise in this way.When metal ion is added to free metallochromic indicator, the pH value will decrease rapidly as protons are released, then gradually as Y increases. A graph of pH against volume of metal solution added gives two straight lines that intersect at the equivalence point. Either component of the indicator system can therefore be determined by titration with the other, provided the initial pH value of the indicator solution is about 7 and the metal ion does not undergo strongly hydrolytic interaction with water. For example, xylenol orange can be determined by potentiometric titration with zinc sulphate solution (Fig, 4). EXPERIMENTAL M) of metal ions and indicators were made from the purest materials readily available.No attempt was made to purify the indicators, because those indicators prepared by condensation of iminodiacetic acid groups on to conventional acid - base indicators would function equally well whether they contained one or two iminodiacetic acid groups (see Results and Discussion below). Suitable volumes were mixed and diluted to give an initial indicator concentration of about 10-5 M, and these solutions were titrated with acid or base, pH and spectrophotometric measurements being made to determine the pH range over which the indicators changed colour, and the sharpness of the change. RESULTS AND DISCUSSION INDICATOR COMPLEXES- The pH ranges for traverse of the acid - base end-point are shown in Tables I1 to VI for various metallochromic indicator - metal combinations.The ranges are those for the colour changes shown, which were judged visually. The volume of base required to effect the colour change is also shown, and serves as an estimate of the sharpness of the transition. The total volume of solution used was usually 20 ml. Figs. 1 and 2 show the pH- absorbance curves for the xylenol orange complexes of bismuth, lead and zinc, the bismuth curve being calculated from results given by KotrlS; and VEeSt&12 for other purposes, and the other two being obtained experimentally. The curves for zinc clearly show evidence of stepwise formation of the zinc - xylenol orange and (zinc), - xylenol orange complexes shown to exist by Murakami, Yoshino and Hara~awa.~ The effect of changing the concentration ratio of metal to indicator is shown for zinc and lead, and the ApH,,,,, values are 1.0 0.1 forgall four curves, which are in agreement with those in Table I for a mononuclear unprotonated complex of a bivalent metal.The effect of in- creasing the charge on the metal ion and hence the number of protons released per indicator molecule completed is clearly shown by comparison of the bismuth and zinc curves. Stock solutions (2 xFebruary, 19711 Metal Nickel . . .. Bismuth . . Lead . . . . Calcium . . Zinc . . .. Cadmium . . Molybdenum . . Aluminium . . Thallium . . Copper .. INDICATOR COMPLEXES AS ACID - BASE INDICATORS TABLE I1 METAL - PYROGALLOL RED COMPLEXES PH cM/10-5 M C I / ~ O - ~ M 0-k .. 1.03 0.98 5.5 5.9 .. 1.00 0.98 1.7 2.0 ..1-05 0.98 5-0 5.3 .. 1-06 0-98 4.2 4.6 .. 1-13 0.98 5.7 6.1 .. 1.06 0.98 5.9 6.1 .. 1.05 0.98 4.5 4.8 .. 1.08 0.98 3.2 3.9 .. 1-02 0.98 3.5 3.9 .. 1-07 0.98 4-9 5.6 101 V/ml 0.04, 0.03, 0.05, 0.03, 0.80 0.37 - 0.080 0.02, 0.05, CM is the concentration of metal before titration. CI is the concentration of indicator before titration. V is the volume of M sodium hydroxide required to traverse the end-point for 10 ml of titration solution, TABLE I11 METAL - METHYLTHYMOL BLUE COMPLEXES Metal Lead .. Zinc .. Copper .. Bismuth . . Cadmium . . Nickel . . PH -7 Almost CM/~O-~ M c1/1O-~ M Yellow colourless Blue .. 1.05 0.99 4.2 4.6 4.8 .. 1.13 0.99 4.7 4.9 5.1 .. 1.05 0.99 4.5 5-1 5.4 .. 1.00 0.99 1.2 1-3 1.5 PH r > Yellow green Green Blue Very pale . . 1.06 0.99 5.5 5.5 6.3 6.7 0.02, ..1.03 0.99 4-0 4.8 5.1 5.7 0.01, TABLE IV METAL - ALIZARIN RED s COMPLEXES PH V/ml 0.02, 0.00, 0~01, - r 1 Almost Metal c~/lO-5 M CI/~O-~ M colourless Pink Iron . . .. .. 1-05 0-99 5.2 5-5 Nickel . . .. .. 1.03 0.99 5.6 5-8 Zinc . . . . . . 1.13 0.99 5-7 6.1 Copper . . .. .. 1-05 0.99 4.6 5-0 Lead . . .. .. 1.05 0.99 5.2 5-6 V/ml 0.02, 0.01, 0.02, 0.02, 0.01, TABLE V METAL - XYLENOL ORANGE COMPLEXES PH -7 Metal CM/~O-~M C I / ~ ~ - ~ M Yellow Orange Red V,/ml v,/ml Copper . . . . 1-05 0.92 4-5 4.7 5-0 0.02, 0.01, Cadmium .. . . 1-06 0.92 5-5 5.6 5.7 0-OO, 0.00, Nickel I . . . 1.03 0.92 4.2 4.5 4.8 0.07, 0.03, Lead . . .. . . 1.05 0.92 3.6 3.7 3.9 0.12 0.06, Zinc . . .. . . 1-13 0.92 4-4 4.5 4.7 0.01, 0-02, - - Bismuth . . . . 1.00 0-92 1.1 1.2 1-3 V, is the volume of 10-2 M sodium hydroxide required to traverse the yellow t o orange end-point.V, is the volume of 10-2 M sodium hydroxide required to traverse the orange t o red end-point.102 80 60 m -f2 0, n ; .I tJ - Q) U 20 CHALMERS AND MILLER : METAL - METALLOCHROMIC TABLE VI METAL - PYROCATECHOL VIOLET COMPLEXES [ArtuZyst, Vol. 96 PH - Pale Metal CM/~O-~M CI/lO-% Yellow green Blue VJml V,/ml - - - Bismuth . . . . 1.0 0.93 - 5.3 6.2 - 0.06, Calcium . . . . 1.06 0.93 4.7 5-2 5.5 0-08, 0-03, Copper . . . . 1-05 0.93 5.2 5.4 6.0 0.01, 0-03, Lead . . .. . . 1-05 0.93 5-8 5.9 6.7 0.01, 0-04, Aluminium . . . . 1.02 0.93 - 4.8 5.2 - 0.07, Zinc . . .. . . 1.13 0.93 5.9 6.5 6.9 0.02, 0-05' V , is the volume of lo-* M sodium hydroxide required to traverse the yellow to pale green end-point.V , is the volume of lo-, M sodium hydroxide required to traverse the pale green to blue end-point. I I I I I 0 2 4 6 8 PH Fig. 1. Absorbance versus pH curves for A, bismuth - xylenol orange; B, zinc - xylenol orange, r = 2; C, zinc - xylenol orange, r = 11 ; and D, a conventional acid - base indicator 1 I I I I I 0 2 4 6 .8 10 PH Fig. 2. Absorbance versus pH curves for A, lead - xylenol orange, Y = 10; B, lead - xylenol orange, r = 2; and C, free xylenol orange The results shown in Fig. 3 for the pyrogallol red complexes of copper(I1) and nickel indicate that protonated complexes are formed. Some curves for free indicators and a conventional acid - base indicator are given for comparison purposes. Table VII shows the effect of increasing the total indicator concentration, and the results are in accordance with theory (Table I) provided the decrease in the pH value resulting from the excess of metal ions is also taken into account (cf.results 4 and 5 for nickel). EFFECT OF CARBON DIOXIDE- Carbon dioxide should have no effect on the pH indicated, provided that the latter is below 5 , for the same reason that it does not affect conventional indicators that change colour at pH values below 5. The results in Table VIII, for pyrogallol red complexes, show that this is generally true. At higher pH values the free metal-ion concentration could be reduced by complex formation with hydrogen carbonate, carbonate or hydroxide ions, thus giving a positive shift in pK,, as exemplified in Table VIII.DETERMINATION OF THE COMPONENTS OF THE INDICATOR COMPLEX- The end-point obtained by extrapolation is within 1 per cent. of the equivalence point. Fig. 4 shows the pH - titration curve for xylenol orange titrated with a zinc solution.February, 19711 INDICATOR COMPLEXES AS ACID - BASE INDICATORS 6.6 6-2 .- 5 5-8 I I I 1 I 0 2 4 6 8 10 5.4 PH Fig. 3. Absorbance versws pH curves for A, copper - pyrogallol red (py-rogallol red con- 5.0 103 - - - - - - - I 1 ' 1 " 1 ' 1 ' ' TABLE VII EFFECT OF CMI ON pKm OF PYROGALLOL RED Metal C~/lo-' M C1/10-' M cm/10-6 M Nickel . . .. .. 6.2 5 5-0 4.13 3.92 3.92 3-10 2.94 2-94 2.48 2.0 2.0 2.07 1.96 1.96 1.24 1.0 1.0 Copper .. .. 5-27 4.90 4-90 4.22 3.92 3.92 3.16 2.94 2.94 2.11 1.96 1.96 1.05 0.98 0.98 PKm 6.4, 6.4 6-77, 6-71 7-05, 7.01 7.12, 7.10 7.17, 7-25 7.63, 7.50, 7.57 4.16, 4.06 4.24, 4-24 4-32, 4-36 4.51, 4.47 4-69, 4.73, 4-69 TABLE VIII EFFECT OF CARBON DIOXIDE ON METAL - PYROGALLOL RED COMPLEXES Metal Nickel .. .. .. Calcium .. .. Bismuth .. .. Lead . . .. .. Zinc . . .. .. Cadmium .. .. Iron . . .. .. Aluminium . . .. Copper . . .. .. Carbon dioxide present Carbon dioxide boiled out PH PH Orange Pinl; 5-0 5.4 5.5 5.95 4.2 4.6 4.1 4.5 1.7 2.0 1-8 2.1 5.0 5.35 5.05 5.35 5.7 6.1 5.5 5-8 5.9 6.1 5.3 5.8 3.8 4.3 4.1 4.5 4.5 4-83 4.1 4.4 3.5 3-9 3.5 4.0 Orange -104 CHALMERS AND MILLER: METAL - METALLOCHROMIC [Analyst, Vol. 96 EFFECT OF IONIC STRENGTH- In the theoretical part of this paper the effect of ionic strength on pKMI was not considered, but from inspection of equation (2) it appears likely that the influence of change of ionic strength on the activities of the various species involved would to a large extent be self- compensating.[Apply the Debye-Huckel approximation to equation (2) for k = ut; the effects on {M”+} and {H+} will largely cancel out, and so will those on the activities of the two forms of the indicator.] Tests were made with the ionic strength varied by addition of potassium chloride, and for the zinc - xylenol orange complex the value of pKMI was 4.67, 4.75, 4-82 and 4-83 for added salt concentrations of 0, 0.1, 0.5 and 0.85 M, respectively, thus confirming the predictions made. DETERMINATION OF STABILITY CONSTANTS- If we write M2+ + H,I + MH(,-2) I + 2H+ then The stability constant of the complex is given by and because for the species [H,,,) 12--3 we can write then .... .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. As the value of K:;:-2, can usually be found in the literature and K,, can be calculated experimentally from equations (6) and (15), the stability constant for the metal - metallo- chromic indicator complex can be obtained. As examples, the stability constants for the lead and zinc complexes of xylenol orange were determined. For xylenol orange in the pH range of these indicator complexes, log K:;:-z)I is given4 as 9.6. The results obtained are given in Table IX. TABLE IX DETERMINATION OF STABILITY CONSTANTS Y ~ K M I [Mz+] at pKu, 10-5 M log Keq log KMH(z-z)l M Metal Zinc . . . . 2-27 4.45 1-77 -4.14 5.46 11.35 4.07 10.85 -4.18 5.43 Lead .. . . 2.09 3.70 1.59 - 2.60 7-00 10-45 3.30 9-95 - 2.60 7-00 SINGLY AND DOUBLY SUBSTITUTED INDICATORS- It is known that in the preparation of such indicators as xylenol orange and methyl- thymol blue condensation of two iminodiacetic acid groups an to the parent acid - base indicator may not have occurred, and the singly substituted products, known as “semi- xylenol orange,” etc., can still act as metallochromic indicators but without the facility to form binuclear complexes of the type M2L. For the present purposes it is immaterial whether there is contamination with the “semi” product, provided the doubly substituted product gives stepwise formation of ML and ML,, as there will be the same number of protons released per molecule of L in each step and in the “semi” reaction. If the stability constants forFebruary, 19711 INDICATOR COMPLEXES AS ACID - BASE INDICATORS 105 the “semi” and “full” complexes of the metal are fairly similar, there will be no over- all effect on the pKM1 of the complex, and if they are different then the more stable of the two will determine the pH of colour change in a titration from low to high pH and the less stable will do so for titration in the reverse direction, provided that the molar absorptivities of the complexes are similar and there is a t least about 20 to 30ger cent. of the minor component present in the mixture. We thank Dr. I. L. Marr for helpful discussion throughout the work. REFERENCES 1. 2. 3. 4. Schwarzenbach, G., and Flaschka, H., “Complexometric Titrations,” Second English Edition, Kotrl9, S., and VieStbl, J., Colln Czech. Chem. Commun., 1960, 25, 1148. Murakami, M., Yoshino, T., and Harasawa, S., Talanffc, 1967, 14, 1293. Ringbom, A., “Complexation in Analytical Chemistry Translated by Irving, H. M. N. H., Methuen, London, 1969. Interscience Publishers Inc., New York, Received July 3rd, 1970 Accepted September 3rd. 1970 1963, p. 368.

ISSN:0003-2654    

DOI:10.1039/AN9719600097

出版商:RSC    

年代:1971

数据来源: RSC

摘要:

106 Analyst, February, 1971, Vol. 96, $9. 106-109 The Influence of the Formation of Metal - Indicator Complexes of the M,I Species on the Accuracy of Complexometric Micro Titrations with Photometric End-point Determination* BY J. KRAGTEN (Natuurkundig Laboratorium, University of Amsterdam, The Netherlands) When micromolar amounts of tervalent and quadrivalent metals such as bismuth, thorium and iron are bound to triphenylmethane dyes and titrated complexometrically, large systematic deviations are found. There are reasons to suppose that M21 type complexes are formed. It is shown that if M21 predominates in solution, the large systematic devi- ations can be accounted for theoretically. The use of azo-type indicators, such as PAR, TAR and TAN, gives more accurate results. IN a previous paper1 the complexometric micro titration of metal ions in the presence of an approximately equivalent amount of indicator is described for the situation in which a 1 : 1 complex is formed between the metal M and the indicator I.In practice azo dyes and triphenylmethane dyes are regularly used. These indicators form MI and MI, complexes, although M,I complexes are also known. Kotr13f2 has already considered the stepwise formation of metal - indicator complexes of the type MI, (m = 1, 2 and 3) and the influence of a system of these three simultaneously occurring species on the shape of the titration curve. If MI, predominates in solution, the combination of Kotrlf's consideration, and the theoretical considerations in our previous paper1 leads to the conclusion that the systematic error can be kept below 0.5 per cent.if the following titration conditions are satisfied- and log- Z M - log (S+E) > 3.5 .. .. Z M I m ZMI, When both MI and MI, occur in solution, the systematic error can be kept below 0.5 per cent. if the above conditions are satisfied for the complex that predominates, because immedi- ately before the equivalence point the concentration of both MI and MI,, and therefore the absorbance of the solution, will be linearly dependent on the volume of added titrant. In a few instances the accuracy was unexpectedly poor, namely when triphenyl- methane dyes, e.g., xylenol orange (XO), pyrocatechol violet (PCV) and chromazurol S (CAS), were used in combination with tervalent and quadrivalent metals.It is known that the combinations iron - CAS, thorium - XO, bismuth - XO, thorium - PCV and bismuth - PCV3y4Jj form M,I complexes. For this reason the influence of the formation of M,I complexes on the shape of the titration curve has been investigated. It will be shown that the systematic deviations found in practice can be explained theoretically. Terminology and symbols used follow present practice in this field of in~estigation.~ It is assumed that M,I is the only complex formed between M and I. CM, CL and CI are the total concentrations of M, L and I, respectively, present in any form; m,i = -2- [M I1 equals CM * Paper presented at the Symposium "On the Accurate Methods of Analysis for Major Constituents," London, April 3rd and 4th, 1970. (6 SAC and the author.KRAGTEN 107 “reduced’ concentration; the absorbance is a linear function of m,i.The reduced concen- trations, mi, mi,, m, i, 1 and ml, are defined similarly. The relative amount of indicator, ,f3, is equal to I . The mathematical treatment is simplified by using reduced concentrations and by using the dimensionless conditional constants 2 instead of the conditional constants K. (3 Now, ~ M = c , K , , = ( ml ) .. .. . . .. m x l and &,I = C12 KM21 = /32(*.) . . .. .. m x i The following mass balances hold- 1 = m + ml + 2m,i .. .. .. .. .. / 3 = i +m2i .. .. .. .. .. .. 0.30 ._ 0.20 E N 0.10 f Fig. 1. Theoretical titration curve constructed from B = CI/CM = 2, log ZM = 8 and z M 2 I = 10. The term fa is represented ten times enlarged. f is found by adding f,, f2 and f, in the “horizontal” direction: A, f, = (1 - 2m2i); B, f = fl + fi + f3; C, f, M 1; and D, fa = (-m) ..1 f 0.94 Fig. 2. Practical titration graph of thorium with EDTA. The thorium solution has been stan- dardised microgravimetrically and by back titration with lead and cerium (xylenol orange indicator). The dotted line is the curve found with commercial quality xylenol orange (85 per cent. pure). The drawn line has been found with chromatographically purified xylenol orange. The difference is presum- ably caused by the presence of semi-xylenol orange (SXO) in the commercial quality XO. SXO may be assumed to form only a 1 : l complex with thorium. For convenience the absorbance is plotted versus f. Thorium taken = 0.507 pmole; /? = 0.5; pH = 2.5.The pH adjustments have been made with monochloroacetic acid to prevent the formation of thorium hydroxide, which dissolves slowly108 From these equations the titration parameter f is found as a function of m,i- KRAGTEN : INFLUENCE OF THE FORMATION OF METAL - INDICATOR [Analyst, Vol. 96 = f, + f, + f,. Comparison of the analogous relationships for the cases in which solely 1 : 1 or 1 : 2 complexes are formed (ZMI = C, KMI and Z M I ~ = C12 KmJ- for 1 : 1 complexes f = (1 - mi) - .. .. mi and for 1 : 2 complexes (9) shows that a marked difference in the corresponding f, terms exists. The importance of this for the shape of the titration curve and for the accuracy of the determination is shown for the titration of iron(II1) with EDTA, with CAS as indicator.If CFe is approximately lo-* (1 pmole in a 10-ml cell) and log ZFe - EDTA = 10 (pH = 3 ; log K = 14), from the pH dependence of the colour formed between iron and CAS it can be assumed that at pH 3 the Z value of the metal - indicator complex is between 10 and 50 regardless of whether M,I, MI or MI, is formed in the solution. When Fe - CAS or Fe - CAS, is formed in the solution, substitution of the data shows that equations (1) and (2) are satisfied. The systematic deviation should be negligible ( N 10-7) and the end-point is easily found from the intersection of the linear portions of the titration graph [see equations (9) and (lo)]. In practice, however, the change in the slope of the titration graph is not very abrupt, and the systematic deviations appear to be between 2 and 4 per cent.This can be explained by assuming that Fe, - CAS has been formed. Substitution of the above Z values in equa- tion (8) shows that the term f, can be neglected up to the equivalence point (m2i > 10-6). The dissociation of ML, therefore, makes no contribution to the curvature of the graph near the equivalence point. f, does not depend linearly upon m,i near the equivalence point. It makes a negative parabolic contribution to the m,i - f curve for small values of m,i (see Fig. 1). In practical titration graphs the curvature near the equivalence point cannot be dis- tinguished from a possible curvature originating from f,. Therefore, the curvature will normally be attributed to the dissociation of ML or to the interference of impurities from the commercial quality indicators (see broken line in Fig.2 ) . If the end-point is determined by extrapolation, or if the intersection of a tangent with the f-axis is taken as the end-point, a systematic deviation will occur (see Figs. 1 and 2). To give an idea of the magnitude of this systematic deviation the tangent procedure has been adopted for the determination of the end-point. A value for Afe can be calculated from equation (8), and from the equation for the tangent at the point [(m2i)o; foJ, f, = fo - (m2i10 (L) .. .. d(m,i) 0 . . (11) we get .. . . (12) In general the tangent is drawn at m,i = 0.l.l Furthermore, the amount of indicator is approximately equal to the amount of metal, so /3 12: 1. Substitution together with log ZM = 10 and ZM~I = 30 gives the theoretical result Afe = -3 per cent.for the Fe, - CAS - EDTA ti tration. In practice systematic deviations of this magnitude are found for other combinations also, e.g., bismuth and thorium with XO and PCV (Table I). For comparison, also, some practical results are given for titrations with azo dyes.February, 19711 COMPLEXES ON COMPLEXOMETRIC MICRO TITRATIONS 109 Metal Iron . . .. Thorium . . Bismuth . . .. Thorium . . .. TABLE I Indicator Triphenylmethane dyes .. CAS .. xo xo PCV Bismuth . . .. Azo dyes PAR Thorium . . TAN .. PAR Mole taken 1.0 0.6 0.2 0.5 0.5 0.6 0.5 0.5 0.5 0.5 0.5 1.0 1.0 2.0 0.5 1.0 2.0 2.0 0-5 0.5 0.5 B 1.0 1.0 1.0 0.5 0.5 1.0 1-0 2.0 1.0 1.0 1.0 1.0 1.0 0.5 1.0 3.0 3.0 3.0 1.0 2.0 1-5 PH 2.8 3.0 2-5 2.5 2.5 4-0 4.0 2-6 2.6 4.0 4.0 1.6 1.6 1.8 1.6 1.5 1.6 1-8 3.5 3.5 3.5 A fe, per cent. - 2 - 4 - 5 - 6 - 8 - 1 - 1.5 - 3 - 10 - 3 - 3 + 0.5 + 0-4 - 0.1 - 0.9 + 0.1 + 0.5 + 0.1 + 0.8 + 0.3 - 0.7 A- detailed comparison with the theory is impossible because of a lack of knowledge of accurate values of KM~I.Some transition-point values have been determined experimentally for xylenol orange.3 The value of ZM,I can be estimated from these results but the accuracy is too poor for our pur- poses. From preliminary investigations under titration conditions ZM~I = 10 30 per cent. was found at pH 2.5 for (thorium), - XO; this value largely accounts for the experimentally found systematic deviations. It can be noted that according to the theory the systematic deviation will be negligible when (ZM,X//~) is greater than 500.This condition, however, could not be satisfied by increasing the pH, probably because of a simultaneous increase in the side-reaction coefficient aM(OH), which prevents a large increase in KM,I and ZM~I. The purpose of this paper was to seek a theoretical explanation for the large systematic deviations found in practice from the formation of M21 alone. In practice, mixtures of M21, MI and MI, can be expected. The absorbance - titrant volume curve depends upon the concentrations of these compounds and their molar absorptivities. It may be expected that when M,I predominates, large deviations will occur; therefore, the dominant formation of M21 should be avoided in micro titrations. The formation of M21 can be expected for combinations of tervalent and quadrivalent metals with indicators such as PCV, XO, CAS and, presumably, methylthymol blue; these indicators have two distinct groups with which metal bonds can be formed. The use of azo- type indicators, e.g., PAR, TAR and TAN, is preferable in these cases. I thank Dr. K. Pypers for the chromatographically purified xylenol orange, and Dr. G. den Boef for critically reading the manuscript. REFERENCES 1. 2. 3. 4. 5. Kragten, J., Tulanta, in the press. Kotrlf, S., Analyticu Chim. Acta, 1963, 29, 552. Ringbom, A., “Complexation in Analytical Chemistry,” Interscience Publishers, New York and Ryka, O., and Cifka, J., Colln. Czech. Chem. Commun., 1958, 23, 71. Horiuchi, Y., and Nishida, H., Jaflan Anulyst, 1967, 16, 769. London, 1963, p. 368. Received April 29th, 1970 Accepted June 26th. 1970

ISSN:0003-2654    

DOI:10.1039/AN9719600106

出版商:RSC    

年代:1971

数据来源: RSC

摘要:

110 Analyst, February, 1971, Vol. 96, $9. 110-116 The Use of Partial-pressure Mass Spectrometry in the Study of the Thermal Desorption and Oxidation of Carbon and Graphite* BY F. E. AUSTIN, J. G. BROWN, J. DOLLIMORE, C. M. FREEDMAN AND €3. H. HARRISON (Department of Pure and Applied Physics, University of Salford, Salford 5, Lancashire) A partial-pressure mass-spectrometer system is described for measuring thermally desorbed species from solid surfaces. The system evaluates not only the ratio of masses present in the gaseous phase but also relates the specific mass (mg g-l of solid) desorbed or decomposed during thermal treat- ment. The extension of the method to include oxidation studies is also described. A study has been made of the initial evolution of gases from graphitic and non-graphitic carbons.These range in properties from a ground graphite of specific surface 103 m2 g-l to a nuclear-type graphite of 0-6 m2 8-l. A study of a non-graphitic carbon, saran charcoal, of molecular-sieve type is also included. The formation of surface oxide on a clean surface at low pressures is evaluated together with the resultant thermal decomposition of the surface oxide. The results from this paper together with other published work on graphites are reviewed and used to illustrate the application of the results of thermal desorption to oxidation studies on carbons and graphites. THERMOGRAVIMETRIC methods have be,en developed, with quartz springs and vacuum micro- balances, to investigate the kinetics of decomposition. In terms of orders of magnitude, however, this is the least sensitive parameter.Measurements of evolved gas pressure provide a sensitivity of at least an order of magnitude greater than that obtained from conventional weight-loss methods. Methods involving total pressure change have produced results of kinetic interest when one gas is evolved.lS2 The logical extension of this approach is to use a partial-pressure mass spectrometer when multiple evolution occurs and to calibrate the expansion volumes so that conventional weight loss results can still be obtained. The authors have shown that an exact correlation can be established (Table I)3 between conventional microbalance results and the weight loss calculated from such a calibrated mass spectrometer system. TABLE I CUMULATIVE WEIGHT LOSS* ; A COMPARISON OF SIMULTANEOUS MASS SPECTROMETER AND MICROBALANCE DATA FOR AN ACHESON GRAPHITE OF 103 m2 g-l SURFACE AREA 400°C 600°C 600°C 65OOC 700°C 750°C 800°C 900°C to to to to to to to to Temperature range 500°C 600°C 650°C 700°C 750°C 800°C 900°C 950°C Carbon monoxide/mg .. . . 0.063 0-199 0.335 0.473 0.663 0-823 1.010 1.085 Carbon dioxide/mg . . . . 0.052 0.094 0.111 0.121 0.128 0.128 0.128 0.128 Cumulative total/mg g-l (MSIO mass spectrometer) . . . . 0.432 1.100 1.675 2.231 2-971 3-573 4.275 4.557 Cumulative total/mg g-1 (Cahn RGvacuum microbalance) . . 0-451 1.146 1.705 2.261 3.043 3.610 4.320 4.624 at 250°C and 2 torr oxygen pressure. * From sample of weight 0.266 2 g. The oxygen complex was adsorbed on the clean surface * This paper is based on a lecture given by J .Dollimore to the Thermal Analysis Group on November 0 SAC and the authors. 13th, 1969.AUSTIN, BROWN, DOLLIMORE, FREEDMAN AND HARRISON 111 An example of a complex evolved-gas analysis is the thermal oxidation and desorption of carbons and graphites when conventional thermobalance methods often give unreliable results because of the small weight losses involved, so pressure-rise systems have to measure separately both the evolved carbon monoxide and dioxide if the correct value for the amount of carbon burn-off is to be obtained. The above problem was solved by Walker, Laine and Vastola,* who placed a partial- pressure mass spectrometer in a constant-volume reactor system.Their work showed that two reactions occurred, vix., thermal decomposition and subsequent desorption of stable oxygen complex from the surface in which the fractional coverage (8) of the active surface is reduced with increased temperature; and an oxidation reaction with carbon burn-off from (1 - 8) of the active surface (As).The equation = kP (1 - 0) A, dt has been shown to describe the oxidation kinetics of Graphon at low pressures. An initial transient period occurs with variable 8 (surface complex formation) followed by a logarithmic oxidation law with 8 constant. The carbon - oxygen system is of great interest for two reasons. In the first place many aspects of the mechanism are still in doubt,5 and secondly, the desorption of the surface oxygen complex is a “thermal decomposition” experiment that presents difficulties in interpretation when conventional thermal-analysis systems are used.This paper is concerned with a review of the results of the authors’ work on the carbon - oxygen system with the mass-spectrometer method, together with some previously unpublished data on the carbon monoxide and dioxide evolution. The carbons studied are an Acheson graphite (surface area 103 m2 g-l), a nuclear reactor graphite (surface area 0.60 m2 g-l) and poly(viny1idene chloride) char (surface area 1000 m2 g-l). The wide range of surface area and gas evolution involved leads to a considera- tion of the sensitivity, utility and difficulties in the application of this type of thermal analysis. APPARATUS The apparatus consisted of a constant-volume reactor system with the gas phase moni- tored by an A.E.I.MSlO mass spectrometer and has been described in detail el~ewhere.~ The volumes and ion currents were calibrated to enable the conversion of partial pressures to specific mass. The mass spectrometer was connected to the reactor volume by a variable leak valve. This gave a convenient working pressure range of 2 000 mtorr down to mtorr. a I . 150 R I complex - formation I Oxidation 4 Transient formation 100 n 0 40 80 120 0 40 80 120 160 Time/minutes Time/minutes Fig. 2. Oxygen chemisorption on Fig. 1. Oxygen chemisorption on poly- Acheson graphite plotted as log pressure (vinylidene chloride) char a t 300 “C veYsus time a t 400°C112 AUSTIN et al. : USE OF PARTIAL-PRESSURE MASS SPECTROMETRY [Analyst, Vol. 96 PROCEDURE Consideration was given to the relationship between the mass of sample and the volume of the reactor in order that the gas evolution would be detectable over the residual pressure.Thermal-desorption studies from carbons require stringent vacuum conditions over a wide range of temperatures, ensuring that any carbon gasification is due solely to decomposition of the surface oxygen complex and not to the presence of residual oxygen in the system. Fortunately an independent check is possible. Fig. 1 shows the pressure - time graph of an oxygen chemisorption on a clean carbon surface (poly(viny1idene chloride) char). The surface oxygen complex as an equivalent (0,) pressure can be calculated with an oxygen mass balance. If this is followed by a desorption to 950 "C, then the surface oxygen complex can be calculated independently from the total evolution.In the course of a large number of experiments we have come to expect an agreement of approximately 5 per cent. between these values. This is rather an exacting condition and if it is not achieved because of a small leak or, more probably, by de-gassing of the system components, this does not entirely invalidate the results. Oxidation experiments in which the system pressure is of the order of 200mtorr may not require such careful technique. In thermal desorption, however, residual oxygen is rapidly taken up as additional surface oxygen complex, with the carbon acting as a scavenger in the system. Provided this effect is small the desorption results can still be analysed for mechanisms. The authors consider, however, that this precludes a direct comparison with the preceding chemisorption experiment in which the surface complex was formed.The actual experimental technique used for the desorption analysis was an isothermal incremental method with the gases removed after each 100 "C step to reduce the possibility of any secondary gas phase and gas - solid reactions. ACHESON GRAPHITE- Acheson graphite can be regarded as a plate-like graphite with additional edges produced by grinding. Before cleaning in a high vacuum to 950°C it would have been expected to permit enhanced carbon dioxide evolution because of ground-in defects in the basal plane.6 The gas evolution from the original surface (0 to 950°C) had been shown to be a coverage of 10 m2 g-l for carbon dioxide and 14.8 m2 8-l for carbon monoxide.Subsequent oxygen chemisorption on this clean surface at 250 mtorr and 300 "C followed by thermal desorption confirmed the reduction in carbon dioxide evolution, as the carbon dioxide coverage was 1.3 m2g-l and the carbon monoxide 36.9m2g-l. The above calculations were based on a carbon atom occupying an area of 0.83 nm2 on the edge plane.4 It is interesting to compare this result with that obtained by an independent method. A simple model of graphite is to associate the active sites with an edge-plane An investigation of the X-ray line profiles by using the 110 and 004 diffraction lines gives values of 1, and lc, the dimensions of the crystallites parallel and perpendicular to the layers, respectively. These can be converted to an edge-plane surface of 46 m2g-l and a basal- plane surface of 54 m2 g-l.Neither the X-ray diffraction photographs nor the nitrogen BET surface are sensitive to thermal treatment of the surface. Although the mass spectro- meter method can detect both small and large changes in the active surface area it is not yet possible to put this on an absolute basis as the coverage also varies with the initial tem- perature and pressure of chemisorption. Saturation coverage, or total active surface, has been taken arbitrarily for Graphon as that occurring at 300°C and 500 mtorr. The theoretical concepts are not understood and at the moment saturation coverage for a carbon can only be determined by amassing sufficient experimental results around these temperatures and pressures.The final problem concerning the interpretation of the Acheson thermal analysis results is one that is common to a number of carbons and graphites, including poly(viny1idene chloride) char. The chemisorption or depletion of oxygen follows a graph similar to Fig. 1 in that, when it is plotted as the log of the pressure veysus time, there is a linear oxidation region in accordance with equation (1) (Fig. 2). The desorption results from the same chemi- sorptions, however, give linear (Elovich) log t$o v e y s w pressure graphs.* This presents ( t o 1February, 19711 IN THERMAL DESORPTION AND OXIDATION OF CARBON 113 the mechanistic problem that the theory of chemisorption assumes a constant activation energy while the desorption shows a variation of activation energy with coverage.There is some evidence that this variation is not even a simple linear function as has been suggested for Graphons and poly (vinylidene chloride) char.lO To obtain such graphs of activation energy against coverage, the isothermal pressure data are used to fit an nth degree Chebebysevs polynominal equation to give continuous pressure versus time kinetic curves. Suitable curves were found by using variable values of n on a computer programme. Usually n was in the range 6 to 12. The same pro- gramme is used to provide the rate data, which are plotted as log rate vemm accumulative desorption. A small extrapolation of the often linear curves permits the application of the Arrhenius equation at one value of coverage (8). The use of a number of temperature intervals then makes the plotting of a graph of activation energy versus coverage possible.At the moment the temperature intervals of 100 "C are not entirely satisfactory for this calculation and a linear temperature rise may offer a more complete solution. An already difficult problem is complicated by the fact that both the carbon monoxide and carbon dioxide desorptions independently follow the Elovich equation. For Acheson graphite the activation energy versus coverage functions are not identical; the carbon dioxide function is linear while the carbon monoxide function is clearly non-linear.8 NUCLEAR-KEACTOR GRAPHITE- The problem with thermal analysis data on this material is that the gas evolution from a surface of 0.6 m2 g-1 is so small that it is on the limit of the most sensitive vacuum micro- balances.For the mass spectrometer method this means a conversion to high vacuum baking techniques to reduce the residual gas pressure to less than 1 ptorr. Table I1 shows the cumulative desorption from a nuclear graphite in pg g-1. The number of significant figures is as yet uncertain and it will require many more results to estimate an error for the smallest TABLE 11 CUMULATIVE THERMAL DESORPTION FROM ORIGINAL NUCLEAR GRAPHITE Quantities per gram of samplelpg r A \ Carbon Carbon Hydro- Temperature range/"C Hydrogen monoxide dioxide Water Nitrogen carbons 0 to 200 0.04 0.57 5-66 1.08 1.58 0.31 0 to 400 0.10 6.75 14-98 2.45 5.33 1-12 0 to 600 0-55 8.25 48.69 2-45 7-62 2.01 0 to 800 1-27 14-74 54.19 2.45 8.09 2-01 0 to 900 1-78 20.12 56.34 2-45 8.66 2.01 gas evolutions.Three general points are of interest. The proportion of active surface is high, possibly as much as 30 per cent. of the total surface. This means that these graphites are of low reactivity because of the reduced specific surface and its limited accessibility rather than of the perfection of the crystalline graphite. This is confirmed by an X-ray analysis, which gives an apparent edgeplane surface of the order of 50 m2 g-1. It follows that most of this must be completely inaccessible to the gas phase. The graphs of oxygen chemisorption at low temperatures are no longer logarithmic, but can be plotted as linear pressure vemus dtime. The reactivity has apparently moved out of the region of control by surface chemical reactivity. The most likely rate-controlling step is a diffusion mechanism.ll ,12 POLY (VINYLIDENE CHLORIDE) CHAR- By analysis methods similar to those already described, the authors have been able to establish some relationship between the mechanisms of adsorption and desorption for this material.13 The very large surface of 1 000 m2 g-l makes detailed calculations possible.The transient formation of a surface oxygen complex during a chemisorption has been shown to obey Elovich's adsorption equation. The thermal desorption products carbon monoxide and carbon dioxide have been found individually to obey the Elovich equation, and a linear graph of increasing activation energy of desorption of carbon monoxide versus decreasing coverage (0) of the active sites was obtained.The log oxygen pressure veysuus time graphs still retain the linear oxidation region but equation (1) has to be modified to account for the formation of a surface complex conforming to Elovich's equation.114 AUSTIN et al. : USE OF PARTIAL-PRESSURE MASS SPECTROMETRY [Analyst, Vol. 06 The data above are all in the region of extensive coverage, ie., that corresponding to 300°C and 200 mtorr pressure. A large amount of data concerning the carbon monoxide - carbon dioxide ratio has been collected generally for the carbon - oxygen system and more resulted from this particular investigation, the mass-spectrometer method facilitating its collection. Even so, the r61e of the different carbon monoxide and carbon dioxide evolutions, both in oxidation and desorption, is still not clearly understood in relation to carbon - oxygen kinetics .For oxidation it has been suggestedl* that the carbon monoxide - carbon dioxide ratio follows an exponential law .. ' * (2) carbon monoxide/carbon dioxide = 103s4 e-12 400/BT .. I I 1565 rntorr 30 L I t' 176 rntorr L 20 E 2 ? 10 a 9 8 7 6 5 4 3 --. 2 I l l 0 40 80 120 160 200 240 280 320 360 Ti rne/m i n u tes Fig. 3. Graphs of carbon dioxide evolution during the oxidation of poly(viny1idene chloride) char at 300°C I I I I 0, 100 200 300. Time/minutes Fig. 5. Graphs of carbon dioxide evolution during the oxidation of poly(viny1idene chloride) char at 172 mtorr initial oxygen pressure 50 I I 40 i- 30 t 1565 mtorr A 91 mtorr I - 51 mtorr I Time/m inutes Fig.4. Graphs of carbon monoxide evolution during the oxidation of poly(viny1idene chloride) char at 300°C 0 50 100 150 200 250 300 350 Ti me/m inu tes Fig. 6. Graphs of carbon monoxide and total oxygen evolution during the oxidation of poly- (vinylidene chloride) char at 172 mtorr oxygen pressureFebruary, 19711 IN THERMAL DESORPTION AND OXIDATION OF CARBOK 115 At the low pressures used in the present and similar mass-spectrometel.4 experiments the secondary reaction C + CO, -+ 2CO can be reduced, such that it is of minor importance. In this event the meaning of the carbon - carbon dioxide ratio as an equilibrium constant is not clear. It may be related to the distribution of carbon monoxide and carbon dioxide active sites. Equation (2) can only be a general trend, as an inspection of Figs.3 and 4 shows that the axygen pressure has only a small effect on the carbon monoxide evolution but alters the carbon dioxide evolution markedly. These figures show the carbon dioxide and carbon monoxide evolution during chemisorption at 300 "C for various pressures. The reason for the log pressure v e m u time graph is that the chemisorption at 176, 91 and 51 mtorr all gave linear log pressure versus time graphs for oxidation. Equation (1) can be used to strictly define two regions; complex formation plus oxidation, followed by oxidation at constant 0. A mass conservation of oxygen must then apply for oxygen depletion and carbon monoxide and dioxide evolution. The simplest solution to this condition would be a linear logarithmic evolution law but Figs.3 and 4 show that this could only apply approximately and after excessive time. The same difficulty occurs with graphs plotted at different temperatures as shown in Figs. 5 and 6. These are at a pressure of 172 & 4mtorr. For completeness, a graph of total oxygen (3 + co2) is included in Fig. 6. The above result indicates some of the difficulties in the current theories for the carbon-oxygen reaction. It also shows that gravimetric work depending on carbon burn-off will give a different solution from that derived from oxygen depletion results. The authors think that this may be due to the com- plexity of the mathematics of concurrent gas depletion and evolution equations in a closed volume, and that this problem requires further clarification. CONCLUSION co Because current oxidation theories are not entirely satisfactory there is room for specu- lation with regard to this problem.The formation of the complex has been shown to follow the Elovich15 adsorption equation and Fig. 7 shows an attempt to describe the concurrent carbon monoxide evolution in a similar way. The initial period may well be in agreement with Elovich's findings. The second linear region and others at greater time are rather specu- lative as it is very easy to construct apparent linear portions on a curve of decreasing slope. 1 2 3 4 5 10 15 Pressure/mtorr Fig. 7. Elovich log time vemws pressure graphs for carbon monoxide evolution during the oxidation of poly(viny1idene chloride) char a t 172 mtorr initial oxygen pressure 1 10 100 1000 Ti me/minutes Fig.8. Log pressure versus log time graphs for chemisorption a t 350°C and 172 mtorr initial oxygen pressure (poly- (vinylidene chloride) char)116 AUSTIN, BROWN, DOLLIMORE, FREEDMAN AND HARRISON The curves in Figs. 4 to 6 can be made linear by plotting log pressure versus log time. This is shown in Fig. 8 for the 350°C system kinetics, the slopes of which are approximately 0.5, indicating that p cc t’. This was the criterion associated with diffusion for the nuclear graphite. It is difficult, however, to see for poly(vinylidene chloride) char how complex formation and its subsequent thermal desorption conform to the Elovich equation while the carbon monoxide and carbon dioxide evolutions during oxidation are diffusion controlled. The use of the constant volume pressure fall method has been criticised in the past.It has been suggested that the kinetics are a function of the system. The correct analysis, however, should account for this and such systems have led to many advances in the under- standing of heterogeneous catalysis.16 Chemisorption at an initial oxygen pressure of 1 565 mtorr, as shown in Fig. 3, is an interesting example. In this case the pressure is so high that oxygen depletion is too small to be measured. However, the gradual decrease in the rate of carbon monoxide and carbon dioxide evolution shows that saturation is being achieved and that, despite the abundance of oxygen present, the oxidation reaction is decaying rapidly. The decay rate is greater than the usual logarithmic decay associated with what is often called “Gaedes equation’’ for pumping speed in a constant volume, where the gas is removed at a constant volume rate.This is analogous to Langmuir adsorption models where L- 9 is P dt c onst ant. The intention of this paper is to illustrate the data that can be obtained on a complex reaction system by the use of the partial-pressure mass-spectrometer method. The fact that the carbon - oxygen system is an as yet unresolved problem adds further point to the investigation. B. H. Harrison acknowledges the financial support of a research studentship from the Gas Council and J. Dollimore acknowledges the opportunity and invitation to present this paper to the Thermal Analysis Group of the Society for Analytical Chemistry on November 13th, 1969.As this was also the occasion of Dr. D. Dollimore’s becoming the Chairman of the Thermal Analysis Group, J. Dollimore also acknowledges the interest shown and advice given by his brother on this subject over many years. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. REFERENCES Jacobs, P. W., and Tariq Kureishy, A. R., “Fourth International Symposium on the Reactivity Broadbent, D., Dollimore, D., and Dollimore, J., J . Chew. SOC., A, 1966, 1491. Brown, J. G., Dollimore, J., Freedman, C. M., and Harrison, €3. H., “Eighth Conference on Vacuum Walker, P. L., jun., Laine, R. N., and Vastola, F. J., “Proceedings of the Fifth Conference on Car- Lussow, R. O., Vastola, F. J., and Walker, P. L., jun., Carbon, 1967, 5, 591. Harker, H., Horsley, J. S., and Parkin, A., J . Nucl. Muter., 1968, 28, 202. Good, R. J., Ginifalco, L. A., and Kraus, G., J . Phys. Chew., 1958, 62, 1418. Dollimore, J., Freedman, C. M., and Harrison, B. H., “Third Conference on Industrial Carbon and Graphite,” The Society of Chemical Industry, London, in the press. Tucker, B. G., and Mulcahy, M. F. R., Trans. Faraday SOC., 1969, 65, 274. Dollimore, J., Freedman, C. M., Harrison, B. H. and Quinn, D. F., Carbon, 1971. Barrer, R. M., and Brook, D. W., Trans. Faraday Soc., 1953, 49, 1049. Sevenster, P. G.. Fuel, 1959, 38, 403. Dollimore, J., Austin, F., Freedman, C. M., and Harrison, B. H., Carbon, 1971. Arthur, J. R., Trans. Faraday SOC., 1951, 47, 164. Elovich, S. Yu., and Zhabrova, G. M., Zh. Fiz. Khim., 1939, 13, 1761. Low, M. J. D., Chem. Rev., 1960, 60, 267. of Solids,” Elsevier, Amsterdam, 1960, p. 352. Microbalance Techniques,” Wakefield, Mass., U.S.A., 1969. bon,” Volume 11, Pergamon Press, Oxford and London, 1963, p. 211. Received March 19th, 1970 Accepted August 26th, 1970

ISSN:0003-2654    

DOI:10.1039/AN9719600110

出版商:RSC    

年代:1971

数据来源: RSC

摘要:

Analyst, February, 1971, Vol. 96, pp. 117-122 117 The Determination of Fluorine in Rock Materials by y-Activation and Radiochemical Separation BY J. S. HISLOP, A. G. PRATCHETT AND D. R. WILLIAMS (Analytical Sciences Division, Atomic Energy Research Establishment, Harwell, Berks.) A technique is described for the determination of fluorine in rock materials involving irradiation in a source of high energy y-photons to induce the 1:F (y,n) 18F reaction. Fluorine-18 is then separated from the radioactive matnx by distillation and its activity measured either in the distillate or as precipitated calcium fluoride and compared with that of irradiated calcium fluoride standards. The technique has been applied to the analysis of standard rock materials G1 (638 p.p.m.), W1 (221 p.p.m.), T1 (476 p.p.m.) and to Apollo 11 lunar fines (76 p.p.m.).Precautions are taken to eliminate interferences. The results obtained for the standard rocks are in good agree- ment with those of conventional methods but disagree with other activation results. The limit of detection of the method is 0.002 pg. SEVERAL methods have been developed for the determination of fluorine in rocks. These may be conveniently classified into chemical and activation techniques. Chemical methods, particularly those involving the spectrophotometric measurement of the coloured complex of fluorine with zirconium - Eriochrome cyanine R1s2 can be extremely sensitive but are susceptible to contamination errors unless rigorous precautions are taken. Activation methods on the other hand are much less susceptible to contamination errors and are independent of reagent blanks.Radiometric methods for the determination of fluorine have been reviewed by F~reman.~ Jeffery and Bakes4 have proposed a non-destructive technique involving fast-neutron activation in which the 19F (n,cc) 16N reaction is used, but because of the high limit of detection, viz., 0.5 per cent. of calcium fluoride, this technique is only applicable to fluorine ores and concentrates. Thermal-neutron activation to give fluorine-20 has also been used by Mapper at A.E.R.E., Hanvell, but the short half-life (11-2 s) of this nuclide and interference from the 28Si (n,p) 28A1 reaction restricts its application. On the other hand, activation by y-photons permits the determination of fluorine to be made via the 19F (7,n) l8F reaction.Fluorine-18 is a pure p+-emitter and its non-destructive determination in a rock matrix, except in a few exceptional cases, for example, in the absence of titanium-45 (/?+, half-life 3.1 hours), is likely to be di&cult. This technique, by using a Betatron, has recently been reported by Kosta and Siunecko5 for the non-destructive analysis for fluorine in a variety of other matrices. Car- penter6 claims that this technique is applicable to samples of minerals from marine and terrestrial sediments. Fluorine-18 produced by the 19F (n,2n) 18F reaction has been used to determine fluorine non-destructively in fluoro-organic materials.' The half-life of fluorine-18 (110 minutes) is sufficiently long to enable it to be separated chemically from the irradiated matrix.Wilkniss, Skinner and Cheek have used this technique following y-activation for the determination of fluorine in synthetic rain water8 by distillation of fluorine-18, and in sea waterg by adsorption of fluorine-18 on calcium sulphate. The distillation technique has also been reported by Reed10 and Reed and Jovanovicll for the determination of fluorine in meteorites and rock material. This technique has, however, produced significantly higher results for the analysis of standard rock materials than those reported by more conventional methods. The work reported in this paper is based on radio- chemical separation of fluorine-18 from y-irradiated rock matrices by using a modification of Willard and Winter's distillation technique, and efforts have been made to eliminate several possible sources of error in the technique reported by Reed,lo particularly with regard to interferences and standards. 0 SAC; Crown Copyright Reserved.118 INTERFERENCES- Fluorine-18 is not produced solely from fluorine and it is necessary to consider alternative reactions by which it can be formed.Several reactions may occur during y-activation, mainly 20Ne (y,d) and (y,np), 21Ne (y,t) and (y,nd), 22Ne (y,tn) and 23Na (r,na), together with a considerable number involving charged-particle activation, which include l60 (a,d) and (3He,p) and 1 8 0 (p,n). Because of the extremely low concentration of neon present in rock material, interference from this element is likely to be small. Similarly the flux of charged particles during y-irradiation is also likely to be low.Nevertheless, complete dismissal of charged- particle interference may not be justified. Wilknissl2 has investigated the production of low levels of fluorine-18 activity from y-irradiation of pure water and has shown that it may be produced by the l80 (p,n) 18F reaction, the protons resulting from the l60 (y,p) lSN reaction. As rock matrices are complex this or similar reactions may occur, but their effect would be expected to be reduced the lower the energy of y-radiation used. Similarly fluorine-18 may be produced by both the 19F (y,n) 18F and 19F (n,2n) 18F reactions, and thus the possibility of interference exists should a higher fast-neutron flux be present in the sample than in the standard resulting from the presence of an element with high y,n cross-section in the former but not in the latter.The relative specific activities of the two reactions under the experimental conditions used, however, suggest that this interference will be small. The most significant source of fluorine-18 interference in rock materials is that from 23Na (y,ncc), bearing in mind the high sodium concentration in many rocks. There are two methods of overcoming this interference : (a) irradiation can be carried out with y-radiation with maximum energy below 20.9 MeV, the threshold for the 23Na (y,na) reaction, and above 10.4 MeV, the threshold for the 19F (y,n) reaction; (b) alternatively, irradiation can be per- formed at an energy higher than the threshold energy for the 23Na (y,ncc) reaction, but with the inclusion of sodium standards from which a correction can be made for sodium inter- ference when the sodium concentration of the sample is known.With the Harwell 45-MeV electron linear accelerator it is extremely difficult to operate reliably at an energy as low as 20 MeV for long periods of time, consequently a conveniently low energy (23 to 25 MeV) was used and the sodium correction procedure adopted. Certain samples, particularly those with high sodium content, were irradiated by using the 17-MeV electron accelerator (Linac) at Harwell’s Wantage Research Laboratory, which eliminated sodium interference in these analyses. HISLOP et al. : DETERMINATION OF FLUORINE IN ROCK [Analyst, Vol. 96 EXPERIMENTAL IRRADIATION- About 50 to 100 mg of material were packaged in an aluminium sample container in double aluminium foil cups, 6 mm in diameter, which were sandwiched between two standards of Optran pure grade calcium fluoride and two standards of AnalaR sodium sulphate.The total volume occupied by the standards and sample was a right cylinder 6 mm in diameter and 0.5 cm long. The samples were irradiated for 30 minutes id the brehmsstrahlung produced by bombarding an air-cooled &inch thick tungsten target with electrons of maximum energy of about 23 to 25 MeV by using the Harwell Linac, or electrons of maximum energy 17 MeV by using the Wantage Linac. The Harwell irradiation facility has been described e1~ewhere.l~ The Wantage Linac, manufactured by Vickers, is a double-section, single klystron machine with energy variable between 5 and 17 MeV.Unlike that of the Harwell Linac the y-irradia- tion facility is not a permanent feature of the machine and the tungsten converter and sample container were placed in the dead-ahead electron beam position prior to each irradiation. To ensure homogeneous distribution of flux across the diameter of the samples the sample container was rotated about the axis through the centres of the samples during irradiation. Beam currents, measured on the tungsten converter, of 5 to 10 pA were obtained with the Harwell Linac and 50 to 60pA with the Wantage Linac. Samples were cooled by air in both instances. Specific activities of fluorine-18 were comparable with the two machines, the lower energy of the Wantage machine, and hence lower activation cross-section, being compensated for by the higher beam currents available.February, 19711 MATERIALS BY Y-ACTIVATION AND RADIOCHEMICAL SEPARATJON 119 CHEMICAL SEPARATION- After irradiation the sample was weighed into a platinum basin, 50 mg of Optran calcium fluoride carrier were added and mixed thoroughly with the sample, and the mixture fused for 2 minutes with 0.7 g of sodium hydroxide.The melt was cooled and extracted with 14 ml of distilled water. The solution was neutralised with 3 ml of 35 per cent. perchloric acid and transferred with 13 ml of water to a single-necked 50-ml distillation flask containing 25 ml of 35 per cent. perchloric acid, 1 ml of silver perchlorate (prepared as in reference 14) and some glass beads.Distillation of fluorine as fluorosilicic acid (H2SiF6) was then carried out by using a modification of Kubota's method.15 This technique, which involves the distillation of the fluorosilicic acid in an apparatus containing a heated (145' C ) outer jacket, in which the dilute acid is used as the source of steam, permits high, reproducible recoveries of fluorine in a small volume of distillate to be achieved. Perchloric acid was used in preference to sulphuric or phosphoric acids to simplify yield determinations. A 25-ml volume of distillate was collected in about 1 hour, which was either counted directly in a 50 mm diameter polycarbonate container or the fluoride precipitated as calcium fluoride. Yield determination was carried out in one of two ways: (i) if fluorine was counted in the 25 ml of distillate the yield was determined when the activity had decayed, by using the method of Popov and Knudson,16 which involved the precipitation of fluorine with excess of lanthanum nitrate and determination of the excess of lanthanum with cupferron ; and (ii) initially, problems were encountered in obtaining a reliable method of precipitating fluorine in a form that could be used directly for counting and for yield determination. That eventually used consisted of co-precipitation of calcium fluoride with calcium carbonate by a method similar to that used by Berze1ius.l' The pH of the 25 ml of distillate was adjusted to 8 by addition of 2 N sodium carbonate and 1 ml added in excess.To the heated solution were then added 10 ml of 0.5 N calcium chloride and, on further heating for 2 to 3 minutes, calcium fluoride - calcium carbonate was precipitated.The precipitate was cooled, centrifuged and washed with 10ml of hot water. It was then transferred to a platinum basin with ethanol and ignited in an oven at 700" C for 2 minutes. The residue was crushed and calcium carbonate decomposed with 10 ml of 10 per cent. acetic acid. After evaporating the solution to dryness the residue was re-heated on a hot-plate for 15 minutes. The residue was transferred to a filter tube with 10 ml of water and washed with a further 20 ml of water. The filter-paper and contents were then ignited at dull red heat and the residue transferred to a weighed counting tray. The total time required for distillation and preparation of precipitated calcium fluoride sources was about 3 hours.Experiments with radioactive calcium fluoride showed that yields from distillation were in excess of 85 per cent. and, for distillation PLUS precipitation, in excess of 70 per cent. A similar tracer experiment resulted in no significant loss of fluorine-18 during fusion. A further tracer experiment also showed that yields obtained by the lanthanum fluoride - cupferron technique were in excellent agreement with those for the radiochemical technique. SOURCE PREPARATION- Calcium fluoride standards used when the fluorine-18 from the samples was counted as 25 ml of distillate were dissolved in boric acid - nitric acid solution, a suitable dilution being made up in 25 ml with water and counted in similar geometry to the sample.When samples were counted as precipitated calcium fluoride sources the standards consisted of weighed aliquots (about 250 mg) of a solution of fluorine-18 prepared from the calcium fluoride stan- dards evaporated to dryness on a counting tray with an infrared lamp. To provide maximum P+-annihilation, and to obtain a reproducible source geometry, both fluorine standards and the sample, precipitated as calcium fluoride, were counted on aluminium trays sandwiched between two discs of copper sheet (100 mg cm-2). Amounts of about 30 mg of sodium sulphate irradia- ted to monitor 23Na (y,na) 18F interference were either diluted and counted in 25 ml of water or counted directly on a counting tray sandwiched between copper sheets as described.DETECTION OF ACTIVITY- All distillates or samples precipitated as calcium fluoride were examined with a Laben 512-channel 3 x 3-inch NaI (Tl) y-ray spectrometer to determine whether activities other than p+-annihilation radiation were present; in no instance was this found to be the case. The activity of all samples was then followed for at least four half-lives of fluorine-18 by120 HISLOP et al. : DETERMINATION OF FLUORINE IN ROCK [Analyst, Vol. 96 using a 100-channel 3 x 3-inch NaI (Tl) y-ray spectrometer incorporating a multi-position sample changer. The area under the 0-51-MeV annihilation full energy peak was then calcu- lated by Covell’s method,ls and decay curves obtained to verify the purity of the calcium fluoride sources. The peak area values were also analysed by a least squares fitting technique, which enabled accurate determinations of fluorine-18 activity at the end of irradiation, and hence the fluorine concentration of the samples, to be made.In addition, the errors resulting from counting statistics of the samples and standards were calculated. SENSITIVITY- Specific activities of fluorine-18 (at end of irradiation) obtained from irradiation of calcium fluoride standards were of the order of 10 to 100 counts s-l pg-l of fluorine with the irradiation and counting conditions already described. The background count on the detector used was about 5 counts s-l in the region of 0.5 MeV. Assuming that samples can be counted for a 2-hour period, 2 hours after irradiation the detection limit for fluorine (giving a fluorine count equivalent to 30 of the background) is 0.02 to 0.002 pg under the present conditions.RESULTS The calculated fluorine concentrations together with the calculated counting errors for individual analyses of three standard rocks and Apollo 11 lunar fines are given in Table I. G1, a granite, and W1, a diabase, are samples of standard rock material issued by the U.S. Geological Survey. T1 is a tonalite issued by the Geological Survey of Tanganyika. A sum- mary of the results calculated for each material is included. TABLE I FLUORINE CONCENTRATIONS OF ROCK MATERIALS Irradiation, Calculated interference Corrected fluorine concentration, p.p.m. MeV Sample from sodium, per cent.* (with counting errors) 23 to 25 w 1 13.7 214 f 9 t 23 to 25 W1 16.1 227 f 9 t 23 to 25 w 1 3 233 & 3t 23 to 25 G1 9.5 651 f 141.17 G1 - 620 f 7$ 17 G1 - 617 f 14: 17 G1 - 648 f lo$ 17 G1 - 655 f 13: 17 T1 - 496 f 18: 17 T1 - 488 f 9$ 17 T1 - 474 f 8$ 17 w 1 - 227 f 2$ 17 w 1 - 203 3 5f 17 T1 - 477 f 5: 23 to 25 Lunar fines 1.1 77 f I t 23 to 25 Lunar fines Nil 75 f 2: Summavy- Mean chlorine Number of Coefficient of variation, Sample concentration 8 determinations per cent. G1 638 18 5 3 w 1 22 1 12 5 6 T1 484 10 4 2 Apollo 11 fines 76 - 2 - * Based on fluorine-18 activity induced in sodium sulphate and sodium concentration obtained 7 Counted as 25 ml of distillate. : Counted as calcium fluoride. from literature. DISCUSSION Certain aspects of this work require further comment. The results obtained at higher irradiation energies and corrected for sodium interference are not significantly different from those obtained at lower energy in the absence of inter- ference, and confirm the validity of the interference correction.This lack of dependence on irradiation energy also indicates that charged-particle reaction interferences are not significant.February, 197 11 MATERIALS BY 7-ACTIVATION AND RADIOCHEMICAL SEPARATION TABLE I1 SUMMARY OF LITERATURE RESULTS FOR DETERMINATION OF FLUORINE IN STANDARD ROCKS24 9" 121 Author Huang and Johns2 . . Evans and Sergeant1 . . Shapirole. . .. .. Ingamells20 . . .. Jeffery21 . . .. .. Jeffery81 . . .. .. Shima22 . . .. .. Peek and Smithzs . . Reed and Jovanovicll . . Reedlo . . .. .. Present work G1 705 f 92 (3) 622 -f 8 (5) 700 720 (3) 629 (4) 627 (4) 720 & 70 600 (3) 831 & 66 935 f 509 1153 f 90 1080 f 116 1138 f 52 1023 1050 638 f 18(5) W l T1 208 f 5 (4) 228 f 8 (6) 200 - 390 f 10 (2) 455 & 5 (2) 305 (2) - 290 (4) - 290 (4) - 190 f 30 - 220 (3) - 279 f 28 - 329 426 f 35 - 552 f 24 - - - 221 f 12(5) 484 f lO(4) Techniques Spectrophotometric Spectrophotometric Spectrophotometric Chemical Photometric Titrimetric Colorimetric Spectrophotometric y-Activation y -Activation y- Activation Numbers in brackets indicate number of determinations.The precision of each determination calculated on the basis of counting statistics alone is significantly less than the standard deviation calculated from several replicate analyses, which is in agreement with our experience of y-activation analysis and may largely be attri- buted to uncertainties in the technique used for the calculation of the relative y-photon flux in the irradiation position of the sample compared with that of the standards.Because of this uncertainty, the accuracy of individual analyses is estimated to be not better than & 10 per cent. A summary of the values recorded in the literature for the fluorine concentrations of standard rocks is given in Table 11. Several features can be observed: the present results are in general agreement with most of the published values, and are in particularly good agreement with those of Evans and Sergeant1 and Peek and Smith'3; and there are significant differences between the results of the present work and those of Reedlo and Reed and Jovano- vicll who also used a y-activation technique.Reed and Jovanovicll have commented on the differences between the two sets of results that they reported for G1 and W1 and attribute them to the fact that two different aliquots were used. This heterogeneous sampling may also be the reason why their results are higher than those of other workers. The fact that the results of the present work are in general agreement with those obtained with a range of other techniques, in which, presumably, a range of aliquots was used, indicates that y-activation does not necessarily produce high results for fluorine concentration in rock materials. We are indebted to the staff of the Harwell Linac, and to Mr. R. W. Marriott of the Wantage Linac, for their assistance and co-operation during the irradiation of samples.1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. REFERENCES Evans, W. H., and Sergeant, G. A., Analyst, 1967, 92, 690. Huang, W. H., and Johns, W. D., Geochim. Cosmochim. Acta, 1967, 31, 597. Foreman, J. K., Analyst, 1969, 94, 425. Jeffery, P. G., and Bakes, J. M., Ibid., 1967, 92, 151. Kosta, L., and Siunecko, J . , Analyt. Chem., 1970, 42, 831. Carpenter, R., Diss. Abstr. (B), 1969, 29, 2796. England, E. A. M., Hornsby, J. B., Jones, W. T., and Terrey, D. R., Analytica Chim. Acta, 1968, Wilkniss, P. E., Skinner, I(. J., and Cheek, C. H., Radiochim. Acta, 1968, 10, 76. Wilkniss, P. E., Ibid., 1969, 11, 138. Reed, G. W., Geochim. Cosmochim. Acta, 1964, 28, 1729. Reed, G. W., and Jovanovic, S., Earth Plan. Sci. Lett., 1969, 6, 316. Wilkniss, P. E., I n t . J. Appl. Radiat. Isotopes, 1967, 18, 809. Hislop, J. S., and Wood, D. A., U.K. Atomic Energy Authority Research Reflort, A.E.R.E. R.6165, 40, 365. H.M. Stationery Office, London, 1969.122 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. HISLOP, PRATCHETT AND WILLIAMS “Standard Methods of Analysis,” United Steel Co. Ltd.. Fifth Edition, Percy Lund, Humphries Kubota, H., Microchem. J., 1967, 12, 525. Popov, A. I., and Knudson, G. E., Analyt. Chem., 1954,26, 892. Kodama, K., “Methods of Quantitative Inorganic Analysis,” First Edition, Interscience Publishers, Covell, D. F., Analyt. Chem., 1959, 31, 1785. Shapiro, L., Prof. Pap. U.S. Geol. Surv., 575-D, 1967, 233. Ingamells, C. O., Talanta, 1962, 9, 607. Jeffery, P. G., Geochim. Cosmochim. Acta, 1962, 26, 1355. Shima. M., Sci. Pap. Inst. Phys. Chem. Res., Tokyo, 1963, 57, 150. Peek, L. C., and Smith, V. C., Talanta, 1964, 11, 1343. Fleischer, M., Geochim. Cosmochim. Acta, 1966, 29, 1263. -, Ibid., 1969, 33, 65. & Co. Ltd., London, 1961, p. 191. a division of John Wiley & Sons Inc., New York and London, 1963, p. 448. Received August loth, 1970 Accepted September 24th, 1970

ISSN:0003-2654    

DOI:10.1039/AN9719600117

出版商:RSC    

年代:1971

数据来源: RSC

摘要:

Analyst, February, 1971, Vol. 96, $$. 123-126 123 Pyridine-2,3-diol as Metal Indicator in the Chelatometric Determination of Iron(II1) with EDTA BY D. P. GOEL* AND R. P. SINGH (Department of Chemistry, University of Delhi, Delhi-7, India) Pyridine-2,3-diol can be satisfactorily used as an indicator in the chelato- metric determination of iron(II1) over the pH range of 1 to 4, the end-point bei-ng sharp and distinct. The indicator is effective in the presence of common bivalent metal ions. Interferences from some quadrivalent, tervalent and bivalent metal ions have been prevented by using masking agents, but oxalate and thiocyanate seriously interfere. Borate, tartrate and citrate do not interfere. However, the presence of a large excess of acetate ions must be avoided. AMONG the i n d i c a t o r ~ ~ ~ ~ ~ ~ ~ ~ that have been used with varying degrees of success for the titrimetric determination of iron(II1) are tiron, salicylic acid, thiosalicylic acid, pyrocatechol violet, kojic acid, thiocyanate ions and triphenylmethane dyes.Recently 2- and 6-hydroxy- m-toluic acids? 5-hydroxy-m-toluic acid,g 1-hydroxy-2-naphthoic acid7 and 3-hydroxy- 2-naphthoic acid* have also been recommended for the purpose. During the course of investigations on the spectrophotometric determination of iron,9 it was observed that the intense colour of the complex of iron with pyridine-2,3-diol (2,3-di- hydroxypyridine) is discharged by the addition of EDTA, which indicated that pyridine- 2,3-diol might be capable of use as a metal indicator in the complexometric determination of iron.The present investigations were carried out to ascertain its usefulness for this purpose. EXPERIMENTAL REAGENTS AND APPARATUS- Standard irout(l11) chloride solution-Prepare a 0.05 M standard solution of iron(II1) chloride by dissolving freshly precipitated iron(II1) hydroxide in AnalaR concentrated hydrochloric acid and diluting to 1 litre with doubly distilled water. The iron solution was standardised gravimetrically. Titrant-Prepare a 0.05 M stock solution of analytical-reagent grade EDTA, disodium salt, by dissolving it in doubly distilled water. Buffer solutions-Prepare 1.0 M stock solutions of hydrochloric acid and anhydrous sodium acetate and use for preparing buffers as required. These chemicals were of AnalaR grade. Pyridine-2,S-dioZ indicator-A 0.5 per cent.solution of pyridine-2,3-diol (Aldrich Chemi- cals Co., U.S.A.) in ethanol. All other solutions were prepared with analytical-reagent grade chemicals by using doubly distilled water. A Metrohm pH meter, E-350, was used for pH measurements. CONDITIONS FOR CHELATOMETRIC TITRATION- The colour of the iron(II1) - pyridine-2,3-diol complex varies from sky blue to red, depending on the degree of acidity, and is sky blue up to a pH of 2.0, above which it changes to purple - red. Thus if titration is carried out at pH below 2.0, the colour change is from blue to colourless (yellowish with large amounts of iron). The colour change at the end-point between pH 2.0 and 4.5 is from purple - red to colourless or yellow, depending on the con- centration of iron. at Present address: St.Stephen’s College, Delhi-7, India. 0 SAC and the authors.124 GOEL AND SINGH: PYRIDINE-2,3-DIOL AS METAL INDICATOR I N THE [ArtabSi!, VOl. 96 EFFECT OF pH- Titrations of iron(II1) against EDTA were carried out at various pH values. The metal and indicator concentrations were kept constant and the pH was adjusted with sodium acetate - hydrochloric acid buffers of different pH. The results are shown in Table I. TABLE I Amount of iron(II1) taken, 5.585 mg ( ~ 2 . 0 ml of 0.05 M EDTA) ; 5 to 10 drops of 0.5 per cent. indicator solution TITRATION OF IRON(II1) AT DIFFERENT pH VALUES pH . . . . . . . . 0.7 1.0 1.5 2.0 2-4 2.8 3.2 3.5 4.0 4.3 4.6 Volume of 0.05 M EDTA required/ml . . . . 2-08 2.01 2.00 2.00 2.00 2.00 2-00 2.00 2.01 2-03 2.06 I t is evident that satisfactory results are obtained in the pH range of 1.0 to 4.0. Below pH 1.0 the amount of EDTA required is more than that indicated by theory. Higher results are also obtained above pH 4.0 and appear to be caused by the complexing behaviour of acetate ions.In the presence of larger amounts of acetate ions even higher results were obtained. However, the use of acetate cannot be avoided as its use is necessary to prevent the precipitation of iron(II1) hydroxide. CONCENTRATION OF IRON- Titrations with different amounts of iron were carried out at pH 2.0, with acetate buffer, at which pH the colour intensity is appreciable and most of the bivalent metals do not interfere. Moreover, the effect of acetate ions is negligible.Results of these titrations showed that when iron is present in high concentrations (higher than 70 mg per 100 ml), detection of the end-point becomes difficult as the iron(II1) - EDTA complex imparts a more or less pronounced yellow colour to the solutions. Up to about 70 mg of iron(II1) in a total volume of about 100 ml could be fairly accurately determined. With larger amounts of iron (up to 150 mg) the end-point could still be detected to within 0.05 to 0-10ml of standard 0.05 M EDTA solution, and in such instances is indicated by the disappearance of the last trace of greenish shade, the solution becoming yellow. AMOUNT OF INDICATOR- The titrimetric determination of iron(II1) was carried out by adding different volumes of 0.5 per cent. indicator solution, and it was found that 5 to 10 drops were sufficient at all pH values between 1-0 and 4.0.Increased amounts of indicator were required in the presence of larger amounts of complexing anions such as tartrate or citrate. EFFECT OF TEMPERATURE- It was observed that the titrations could be performed within a fairly wide range of temperatures (10 to 60 "C). At temperatures below 25 "C the titration must be carried out slowly as the formation of the iron(II1) - EDTA complex is slow. At room temperature (30°C) the titration can be carried out fairly accurately without much delay (not more than 1 minute). Above 6OoC, the iron(II1) - pyridine-2,3-diol complex dissociated to an appreciable extent and the end-point was reached a little sooner, thus giving low results. RECOMMENDED PROCEDURE- To an acidic solution containing 3.0 to 70.0mg of iron, add 5 to 10 drops of 0.5 per cent.solution of indicator. Adjust the pH to about 2.0, with pH indicator paper, by adding dilute ammonia solution, then add 10ml of buffer solution of pH 2.0. Dilute the solution with doubly distilled water to nearly 100ml and add, from a microburette, standard 0.02 to 0 . 0 5 ~ EDTA solution until the blue colour completely disappears. The colour change is from blue to yellow with larger concentrations of iron(II1). Results obtained by using the procedure outlined above for some of the titrations are recorded in Table 11, which shows that the method can be used to determine amounts of iron(II1) ranging from 3.3 to 70 mg, with an error not exceeding +Om5 per cent.February, 197 13 CHELATOMETRIC DETERMINATION OF IRON (111) WITH EDTA TABLE I1 TITRATION OF IRON WITH EDTA 1.0 ml of 0.05 M EDTA = 2.793 mg of iron; pH 2.0 125 Iron(II1) taken/mg 0.14 0.28 0.56 1.12 3-38 7.82 15.64 31.28 54.74 78-20 Iron (111) found/mg 0.14 0.279 0.558 1.11 3.36 7.84 15.67 31.36 54-90 78.50 Error, per cent.0.0 -0.35 - 0.36 -0-9:) + 0-30 + 0.25 +0.19 + 0.25 + 0.29 + 0-39 TNTERFERENCES- Interferences caused by various cations and anions in the determination of iron have been investigated in the pH range of 1-5 to 2.0. The anions oxalate and thiocyanate interfere seriously. No interference is caused by SO,2-, Br-, I- (at pH values above 2G), borate, tartrate and citrate when present in 100-fold excess. The tolerated amounts of the anions POq3-, S,032-, F- and 10,- are given in Table 111.The cations vanadium(V), bismuth(II1) and nickel( 11) interfere seriously even in the presence of the usual masking agents. Most of the common bivalent metals, viz., zinc(II), magnesium (11) , cadmium (11) , manganese (11) , barium (11) , strontium (11) , cadmium( 11) , beryllium(I1) and U0,2+ do not interfere at the pH used for the titrations, even when present in considerable excess. Interferences caused by some quadrivalent, tervalent and bivalent metal ions have been prevented by using masking agents (Table 111). A solution of iron(II1) perchlorate was used with lead(II), silver(1) and thallium(1) ions. TABLE I11 EFFECT OF FOREIGN IONS Amount of iron(III), 5-58 mg (35 ml of 0.02 M EDTA) Foreign ion added Copper(II), pH 3.0 Aluminium(II1) .. Antimony(II1) . . Zirconium( IV), pH 3-0 Thorium(1V) . . Cobalt(I1) . . .. Lead( 11) .. Amount of foreign ion/mg .. 25.0 .. 10.0 .. 50.0 .. 50.0 .. 25.0 .. 20.0 .. 60.0 Titanium(IV), pH 3.0 . . 20.0 Molybdenum(V1) . . .. 100.0 Tungsten(V1) . . .. 100.0 PO,s-, pH > 2.5 .. .. 100.0 103-, pH 2-5 . . .. 50.0 S,O,+, pH 3-5 . . .. 50.0 F-, pH 3.0 . . .. .. 50.0 Volume of 0.02 M EDTA 4-98 5.0 1 5.02 - 4.98 Sodium citrate 4.97 Sodium potassium tartrate 4-97 Sodium potassium tartrate 4-98 Sodium citrate 4-97 Sodium potassium tartrate 5.01 Sodium potassium tartrate 5.01 Sodium potassium tartrate 4.99 - 4.99 - 4.98 - 4.98 - solution used/ml Masking agent Thiourea in presence of NaF and NaF and NaF DISCUSSION Of the indicators made use of in the chelatometric determination of iron with EDTA,1,2,3,4 salicylic acid, thiosalicylic acid and tiron have been widely used, including the determination of iron(II1) in ores, soils and cements.Unfortunately, with these indicators titanium(IV), zirconium(IV), thorium(IV), antimony(II1) and bismuth(II1) interfere. The pH range for accurate determination is 2.0 to 3.0. Results with thiocyanate are dependent on the concentration of the indicator, and the pH must also be controlled strictly in the range of 2-0 to 2.4. To obtain good results the use of 50 per cent. acetone medium has been recommended by some workers.126 GOEL AND SINGH Kojic acid is used within the narrow pH range 2-0 to 3.0. The advantage with this indicator is that it can be used successfully within the wide temperature range of 40 t o 100 “C.The ions copper(II), nickel(II), mercury(II), bismuth(III), thorium(IV), POP3-, F-, oxalate and tartrate interfere in the determination. Pyrocatechol violet has also been used successfully, but thorium(1V) and aluminium(II1) interfere. Triphenylmethane dyes can be used successfully within a narrow pH range. The pH range for 2-, 5- and 6-hydroxy-nz-toluic acids is narrow in comparison with that for pyridine- 2,3-diol and the same is true for the hydroxynaphthoic acids. Compared with the above indicators for use in the chelatometric determination of iron(II1) with EDTA, pyridine-2,3-diol is superior as it can be used over a wider pH range (1.0 to 4.0) and has a sharp and distinct end-point. The value of this indicator has been enhanced by using tartrate, citrate and fluoride as masking agents for the cations that normally interfere in the determination of iron with EDTA when using other indicators. We thank Professor R. P. Mitra, Head of the Department of Chemistry, for providing research facilities, and the Ministry of Education, Government of India, for the award of a research fellowship to one of us (D.P.G.). 1. 2. 3. 4. 5. 6. 7. 8. 9. REFERENCES Schwarzenbach, G., “Complexometric Titrations,” Methuen and Co. Ltd., London; Interscience Flaschka, H. A., “EDTA Titrations,” Pergamon Press Ltd., Oxford, 1964. Welcher, F. J., “The Analytical Uses of Ethylenediamine Tetraacetic Acid,” D. Van Nostrand Ptibil, R., Talanta, 1965, 12, 925. Pande, C. S., and Srivastava, T. S., 2. analyt. Chem., 1960, 175, 29. Sen, A. B., and Chauhan, V. B. S., Indian J . Appl. Chem., 1962, 25, 127. Pande, C. S., and Srivastava, T. S., 2. analyt. Chem., 1960, 172, 356. Katyal, M., Goel, D. P., and Singh, R. P., Talanta, 1965, 15, 711. Publishers Inc., New York, 1960. Co. Inc., Princeton, New Jersey, New York, 1961. , , Ibid., 1962, 25, 130. -- Received October 16th, 1969 Accepted August 26th, 1970

ISSN:0003-2654    

DOI:10.1039/AN9719600123

出版商:RSC    

年代:1971

数据来源: RSC

摘要:

Analyst, February, 1971, Vol. 96, pp. 127-129 127 Spectrophotometric Determination of Vanadium with N-Benzo yl- o -tolylh ydroxylamine BY A. K. MAJUMDAR AND S. K. BHOWAL (Department of Inorganic and Analytical Chemistry, Jadavpur University, Calcutta-32, India) The method prescribed by Majumdar and Das for the spectrophotometric determination of vanadium(V) with N-benzoyl-o-tolylhydroxylamine has been re-examined. The validity of the method originally reported is supported by the further results presented. MA JUMDAR and Das introduced N-benzoyl-o-tolylhydroxylamine as a specific reagent for the spectrophotometric determination of vanadium1 ,2 in the quinquivalent state and recom- mended the use of sodium fluoride to prevent interference by titanium. In their method they prescribed the extraction of vanadium(V) from 4 to 8 N hydrochloric acid solution with an ethanol-free chloroform solution of the reagent and measurement of the optical density of the extract at 510 nm.Jeffery and Kerr, while using this reagent for the determination of vanadium in rocks and mineral^,^ preferred the use of carbon tetrachloride to chloroform as the extractant because of the possible presence of ethanol in the latter solvent. Moreover, during the preparation of calibration graphs they observed that somewhat lower optical densities were obtained unless the standard vanadate solution was re-oxidised prior to reaction with the reagent. They found, however, that the vanadium complex in chloroform solution gave calibration graphs similar to those obtained with solutions in carbon tetrachloride, isobutyl methyl ketone and toluene.These observations led the present authors to re-evaluate the method of Majumdar and Das. Thus ethanol-free chloroform was found to be a more satisfactory extractant for the vanadium - benzoyl-o-tolylhydroxylamine complex than carbon tetrachloride. Both the reagent and its vanadium complex are highly soluble in chloroform. While only two extrac- tions with chlorofonn are sufficient to extract the vanadium complex completely, at least four extractions with carbon tetrachloride are needed for its complete removal. Moreover, the vanadium complex has a higher optical density in ethanol-free chloroform than in carbon tetrachloride, the spectrophotometric sensitivities (log I,/I = 0.001) being 0.010 8 and 0.012 3 pg cm-2 of vanadium, respectively.The optical densities remain the same whether or not the aliquots of standard vanadate solution are oxidised before treatment with the reagent solution. However, in the procedure followed by Jeffery and Kerr for the determination of vana- dium in silicate minerals, oxidation of vanadium was obviously necessary because after the decomposition of the mineral vanadium was present in a lower state of oxidation. For the preparation of the calibration graph from a standard metavanadate solution, re-oxidation of the vanadium seems to be unnecessary. The presence of a very small proportion of vanadium(1V) should not affect the accuracy of the result because the present study has shown that when a solution of vanadium(1V) in 4 to 8 N hydrochloric acid is maintained in contact with a solution of the reagent in chloroform for some time and the mixture shaken, the chloroform layer slowly assumes the red - violet colour characteristic of the vanadium(V) complex with the same maximum absorption region at 510nm.Moreover, the solid complex isolated from a 4 to 8 N hydrochloric acid solution of vana- dium(1V) (A. K. Majumdar and B. C. Bhattacharyya, unpublished work) has been found to be identical with that isolated when the vanadium is in the quinquivalent state.4 The compounds obtained with both vanadium(1V) and (V) melt with decomposition at 133" C 0 SAC and the authors.128 [Analyst, Vol. 96 and are diamagnetic. Thus it is reasonable to assume that as vanadium(V) in the presence of the reagent forms a more stable complex, the reduction potential of the vanadium(1V) - vanadium(V) system increases so that vanadium(1V) becomes rapidly oxidised by air to vanadium(V) to furnish the complex.Two measuring instruments of different sensitivities have been used in this investigation for comparison of optical densities. Two procedures, one being that suggested by Majumdar and Das and the other in which permanganate was used as oxidant, with carbon tetrachloride or chloroform as the extractant, are described. MA JUMDAR AND BHOWAL : SPECTROPHOTOMETRIC DETERMINATION METHOD APPARATUS, REAGENTS AND SOLUTIONS- Two spectrophotometers, a Unicam SP600 and a Hilger Uvispek, were used for optical density measurements. A standard solution of vanadium was prepared, as described by Majumdar and Das,l by dissolving ammonium metavanadate in water rendered aminoniacal. The chloroform used for extraction was freed of ethanol by washing it successively with dilute sulphuric acid followed by dilute ammonia solution and water, then dried over fused calcium chloride and distilled.A 0.5 per cent. solution of N-benzoyl-o-tolylhydroxylamine in carbon tetrachloride or purified ethanol-free chloroform was used. All chemicals used were of analytical-reagent grade. PROCEDURE FOR EXTRACTION AFTER RE-OXIDATION- Acidify an aliquot of the standard vanadate solution (4 ml containing 200 pg of vanadium) in a 50-ml separating funnel with 4 or 5 drops of dilute sulphuric acid. Add dropwise dilute potassium permanganate solution until the pink colour of the solution persists for more than 5 minutes.Add 2 ml of 0-05 M sulphamic acid solution followed by 10 ml of 8 M hydrochloric acid, then add 4 ml of a 0.5 per cent. solution of the reagent in purified chloroform or carbon tetrachloride. After the addition of another 4-ml portion of chloroform or carbon tetra- chloride, shake the mixture for 1 to 2 minutes and transfer the organic layer to a 25-ml calibrated flask. Extract the aqueous layer once with a 5-ml portion of chloroform or three times with 5-ml portions of carbon tetrachloride. Make the volume of the combined extracts up to 25 ml with the respective solvents and measure the extinctions at 510 nm against the pure solvents as reference (see Table I). TABLE I VARIATION OF OPTICAL DENSITY WITH SOLVENT Optical density Optical density Solvent (Unicam SPSOO) (Hilger Uvispek) Chloroform .. . . . . . . 0.750 0.800 Chlorof o m . . . . . . . . 0.745 0.797 0.706 Carbon tetrachloride 0.645 Carbon tetrachloride . . . . . . 0.645 0.710 . . . . . . PROCEDURE FOR EXTRACTION WITHOUT RE-OXIDATION- The extraction procedure followed was that suggested by Majumdar and Das, the same amount of vanadium(V) being used as that given for the above procedure. The results obtained are shown below. Optical density Optical density Solvent (Unicam SPSOO) (Hilger Uvispek) Chlorof o m . . . . . . . . 0-745 0.797 Carbon tetrachloride . . . . . . 0.650 0.710 The molar extinction coefficients of the vanadium complex calculated for ethanol-free chloroform and carbon tetrachloride solutions were 4 776 and 4 202, respectively, with the Unicam SP600, and 5 095 and 4 496, respectively, with the Hilger Uvispek.February, 19711 OF VANADIUM WITH N-BENZOYL-O-TOLYLHYDROXYLAMINE 129 PREPARATION OF THE VANADIUM(IV) COMPLEX- To a calculated amount of vanadyl dichloride (VOC1,) solution diluted to 30m1, add 401nl of concentrated hydrochloric acid. Pass sulphur dioxide gas through the solution to ensure that the vanadium is present in the quadrivalent state, and boil off the excess of sulphur dioxide. Add 0.01 mole of the reagent in 140 ml of acetone, when the violet complex almost immediately begins to separate out.After cooling the solution for about 3 hours, filter off the complex, wash and dry it in a vacuum desiccator. The complex melts, with decomposition, at 133 "C and is diamagnetic. REFERENCES 1. 2. 3. 4. Majumdar, A. K., and Das, G., Artalytica Chirn. A&, 1964, 31, 147. -,- , Ibid., 1966, 36, 454. Jeffery, P. G., and Kerr, G. O., Analyst, 1967, 92, 763. Majumdar, A. K., Bhattacharyya, B. C., and Das, G., J. Indian Chenz. Soc., 1968, 45, 964. Received October 28th. 1969 Accepted May 21st, 1970

ISSN:0003-2654    

DOI:10.1039/AN9719600127

出版商:RSC    

年代:1971

数据来源: RSC