ISSN:0003-2654    

DOI:10.1039/AN95378FX009

出版商:RSC    

年代:1953

数据来源: RSC

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No. 10 March, 1953 THE SOCIETY OF PUBLIC ANALYSTS AND OTHER ANALYTICAL CHEMISTS BULLETIN ANNUAL GENERAL MEETING, MARCH 6th, 1953 THE seventy-ninth Annual General Meeting of the Society was held at 2.45 p.m. on Friday, March 6th, 1953, in the meeting room of the Chemical Society, Burlington House, London, W.l. The Chair was taken by the President, Dr. J. R. Nicholls, C.B.E., F.R.I.C. The financial statement for 1952 was presented by the Honorary Treasurer and approved, and the Auditors for 1953 were appointed. The Report of the Council for the year ending March, 1953, was presented by the Honorary Secretary and adopted. The Scrutineers reported that the following had been elected Officers for the coming year- President-D. W. Kent- Jones, B.Sc., Ph.D., F.R.I.C. Past Presidents serving on the Council-Lewis Eynon, G.W. Monier-Williams, J. R. Nicholls and George Taylor. Vice-Presidents-A. J. Amos, T. McLachlan ‘and Eric Voelcker. Honorary Treasurer- J. H. Hamence. Honorary Secretary-K. A. Williams. Other Members of Co.unciZ-The President declared the following to have been elected Ordinary Members of Council for the ensuing two years-A. L. Bacharach, R. C. Chirnside, D. C. Garratt, H. M. N. H. Irving, Miss Mary Comer and H. W. Hodgson. C. A. Adams, N. L. Allport, B. S. Cooper, N. Heron, H. E. Monk and H. C. S. de Whalley, having been elected members of the Council in 1952, will, by the Society’s Articles of Associa- tion, remain Ordinary Members of the Council for 1953. T. W. Lovett (Chairman of the ru’orth of England Section), R.S. Watson (Chairman of the Scottish Section), A. M. Ward (Chairman of the Microchemistry Group), J. Haslam (Chairman of the Physical Methods Group) and H. 0. J. Collier (Chairman of the Biological Methods Group) will be ex-oflcio members of the Council for 1953. FORTHCOMING MEETINGS Ordinary Meeting of the Society, April lst, 1953 AN Ordinary Meeting of the Society will be held at 7 p.m. on Wednesday, April lst, 1953, in the Meeting Room of the Chemical Society, Burlington House, London, W.l. The subject of the meeting will be “The Determination of Small Amounts of Lead in Foods and Biological Materials,” and the following papers will be presented- “A Reversion Method for the Absorptiometric Determination of Traces of Lead with Dithizone,” by H. M. Irving, M.A., D.Phil., F.R.I.C., L.R.A.M., and E.J. Butler, B.A., B.Sc., D.Phil., A.R.I.C. “Preliminary Procedure for the Preparation of Biological Materials for the Micro Determination of Lead,” by K. F. Milton, BSc., Ph.D., F.R.I.C. “Sample Preparation for Determination of Lead in Foodstuffs,” by D. A. Elvidge, B.Sc., and D. C. Garratt, B.Sc., Ph.D., F.R.I.C.Meeting of the Scottish Section, April loth, 1953 A MEETING of the Scottish Section will be held at 7.15 p.m. on Friday, April loth, 1953, at the George Hotel, Edinburgh. At this meeting Dr. Christina C. Miller, F.R.S.E., F.H.-W.C., will give a lecture on ”Modern Methods of Analysis in the Training of the Student.” Meeting of the Physical Methods Group, April 14th, 1953 THE Fortieth Ordinary Meeting of the Physical Methods Group will be held at 6.30 p.m.on Tuesday, April 14th, 1953, in the Meeting Room of the Chemical Society, Burlington House, London, W.l. This meeting has been arranged by the Polarographic Discussion Panel of the Group. The following papers on “Polarography” will be presented- “The Polarographic Determination of Fluoride,” by B. J. MacNulty. “The Amperornetric Titration of Zinc,” by C. C. Washbrook and D. Pickles. third paper is being arranged. PAPERS ACCEPTED FOR PUBLICATION IN THE ANALYST THE following papers have been accepted for publication in The A7zaZyst, and are expected to appear in the near future. It is not possible to enter into any correspondence about any of them. if Large-Scale Chromatographic Separation of Sucrose - Raffinose Mixtures on Powdered Cellulose for the Determination of Raffinose in Raw Sugars,” by D.Gross and N. Albon. The possibility of separating large amounts of sucrose - raffinose mixtures on columns of powdered cellulose has been investigated. Artificial mixtures containing disproportionately small amounts of raffinose have been efficiently separated in up to 15 g of mixture per column. The liquid chromatogram technique enables complete elution of the separated sugars to be attained, with nearly quantitative recovery of the minor constituent. The method has been applied to the separation of raffinose from raw beet-sugars, 20 g of sugar being handled per run. Separation and recovery were satisfactory, and results show that agreement with raffinose analyses by the paper chromatographic method is gwcl.“Determination of Theobromine in Cocoa Residues,” by K. W. Gerritsma and Miss J. Koers. A simple, accurate method for the determination of theobromine in cocoa residues is described. The material is shaken for 5 minutes with chloroform in an ammoniacal medium ; after dehydration with anhydrous sodium sulphate, the chloroform solution is filtered and the residue and filter are washed with chloroform. The chloroform is removed from the filtrate by distillation and the residue is dissolved in water, after which silver nitrate is added and the liberated nitric acid is titrated with alkali. The results obtained by this method are compared with the values found by Wadsworth’s method, and are in agreement with them. “A Critical Examination of the Ferrocyanide Determination of Zinc,” by Mary R.Richardson and Alexander Bryson. The titration of zinc with potassium ferrocyanide has been critically examined and the end-point of the titration when internal indicators are used has been correlated with the potential drop occurring at the end of the potentiometric titration.“The Separation of Zinc and Cadmium by the Use of Activated Copper,” by Alexander Bryson and S. Lenzer Lowy. A method of separating 5 to 200-mg amounts of cadmium and zinc is presented. Metallic copper in complex cyanide solution containing tartrate as an additional complexing agent displaces cadmium but not zinc from solution. Zinc is precipitated from the filtrate as zinc sulphide by adding sodium sulphide, it is filtered, re-dissolved in acid and can be determined either volumetrically or gravimetrically.Cadmium is recovered from the copper by extraction with an alkaline potassium cyanide solution containing small amounts of hydrogen peroxide. It is precipitated as cadmium sulphide by sodium sulphide and can be determined w-ith hydroxyquinoline after filtering and dissolving in acid. “The Determination of Zinc in Lubricating Oils by Amperometric Titration. Zinc is titrated amperometrically with di-sodium ethylenediamine tetra- acetate (versene) at the dropping-mercury electrode in a strongly alkaline solution containing cyclohexylamine, or in a buffered solution containing ammonium acetate. A potential of - 1.4 volts against the saturated calomel electrode is used with a mercury drop-time of 3 to 4 seconds.,4 simple polarising unit is used. Advantages are that bulky precipitates, which might foul the electrodes, are not formed, and titration can proceed quickly because of rapid attainment of equilibrium after titrant addition. The method is applied to the determination of zinc in new and used lubricating oils, and methods of dealing with interfering elements are discussed. Determinations are made in new oils and additives containing barium and phosphorus, and in used lubricating oils containing barium, calcium, iron, copper, lead and phosphorus. Copper and iron are removed by ethereal extraction of cupferron complexes; barium and lead are removed as sulphates, and calcium is suppressed by addition of sodium fluoride. The acetate buffer method is suitable for solutions containing appreciable amounts of phosphates. An accuracy of k0.3 per cent.is attained in the titrations a t the level of 1 mg of zinc. A precision of better than 2 per cent. can be attained in the analysis of new lubricating oils and additives. Part I. Amperometric Titration of Zinc with Versene,” by D. Pickles and C. C. Washbrook. “The Determination of Methyl-Chlorophenoxyacetic Acids in MCPA Formulations,” by F. Freeman and K. Gardner. A method is described for the chromatographic separation and determina- tion of 4 : 6-dichloro, 6-chloro, 4-chloro and unchlorinated 2-methylphenoxy- acetic acids in commercial methyl-chlorophenoxyacetic acid formulations. The four acids are successively eluted with a mixture of ether and chloroform from a kieselguhr column treated with phosphate buffer, and are determined by titration with dilute sodium hydroxide solution.The effect of impurities is discussed and results are shown for synthetic samples prepared from the pure acids and various commercial preparations. ‘‘The Reaction Between Periodic Acid and Polyhydroxy Compounds with Particular Reference to the Colorimetric -Determination of Formaldehyde with Chromotropic Acid,” by S. L. Tompsett and D. C. Smith. The liberation of formaldehyde when a number of polyhydroxy and aminohydroxy compounds react with periodic acid has been studied by two procedures. The formaldehyde is determined colorirnetrically with chromotropic acid. The analytical application of this reaction is discussed and applications are made to more complex substances, human urine and blood.“Determination of Molybdenum by Ammonium Thiosulphate and Sodium Hypophosphite,” by H. N. Ray. Molybdenum is determined by precipitation as molybdenum sulphide by ammonium thiosulphate and sodium hypophosphite from acid solution, ignition and weighing as the oxide. Certain of the inconveniences of the direct hydrogen sulphide precipitation method are avoided, but several elements interfere. The method has been successfully applied to molybdenum steels and ferro-molybdenum. NOTICE Fourth International Colloquium on Spectroscopy IT has been announced that the Fourth International Colloquium on Spectroscopy, which is due to take place in Germany this year, has been arranged for September 8th to 12th, 1953, in Mi’anster, Westphalia. Anyone proposing to attend the Colloquium is asked to inform the Honorary Secretary of the Physical Methods Group, R. A. C. Isbell, Esq., Hilger k Watts Ltd., Hilger Division, 98, St. Pancras Way, London, N.W.l, before April 23rd. PRINTED BY W. HEFFER & S O N S LTD.. CAMBRIDGE, ENGLAND

ISSN:0003-2654    

DOI:10.1039/AN953780X011

出版商:RSC    

年代:1953

数据来源: RSC

ISSN:0003-2654    

DOI:10.1039/AN95378BX015

出版商:RSC    

年代:1953

数据来源: RSC

ISSN:0003-2654    

DOI:10.1039/AN95378BP027

出版商:RSC    

年代:1953

数据来源: RSC

摘要:

MARCH, 1953 Vol. 78, No. 924 THE ANALYST EDITORIAL PROCEEDINGS OF THE FIRST INTERNATIONAL CONGRESS ON ANALYTICAL CHEMISTRY, OXFORD, SEPTEMBER ~ T H - ~ T H , 1952 IN fulfilment of the promise made in the October number of The Analyst for last year that a separately bound volume containing the full proceedings, the lectures and the papers pre- sented at the Oxford Congress should be published early in 1953, we are now able to announce that a limited edition of this work has been completed on behalf of the Congress Committee and is ready for publication. In addition to the matter that has already appeared in the November and December numbers of The Analyst, this volume contains a foreword by Sir Robert Robinson, full lists of the various Committees who were responsible under the leadership of Mr.Chirnside for the organisation of the Congress and for administrating its activities in being, and a complete list of the members of Congress. The three Congress Lectures : “Research in Analytical Instrumentation,” by Ralph H. Muller; “The Value and Economic Importance of Chemical Analysis in Industry and Manufacture,” by L. H. Lampitt; and “A Contemporary Assessment of the Place of Classical Methods in Chemical Analysis,” by C. J. van Nieuwenburg, are reproduced in full. The scientific papers are grouped by subjects into nine sections : Microchemical Methods, Biological Methods, Electrical Methods, Optical Methods, Radiochemical Methods, Organic Comdexes. Presentation of Data. Adsomtion and Partition Methods, and General, which inclides an important paper on the “Agling of Crystalline Precipitates” by Professor I. M. Kolthoff. A catalogue of the Laboratory Demonstrations, with a brief description of the exhibits Details of the publication are given in a prospectus supplied with this issue. This book makes the lectures and papers presented at the Congress available to all chemists, whether or not they are members of the Society or subscribers to this journal. It is distinct from the yearly volume of The Analyst and complete with its own index, and it forms a valuable and interesting memento of the Oxford Congress-an important event in the history of analytical chemistry. is included. 133

ISSN:0003-2654    

DOI:10.1039/AN9537800133

出版商:RSC    

年代:1953

数据来源: RSC

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134 DETERMINATION OF GELATIN IK MEAT [Vol. 78 PROCEEDINGS OF THE SOCIETY OF PUBLIC ANALYSTS AND OTHER ANALYTICAL CHEMISTS DEATHS WE regret to record the deaths of Francis Clifford Dyche-Teague Robert Henry Slater. Analytical Methods Committee RECOMMENDATIONS OF THE MEAT EXTRACTS SUB-COMMITTEE Determination of Gelatin in Meat Extract and Meat Stocks Interim Report THE Analytical Methods Committee has received the following interim report from the Meat Extracts Sub-committee, and its publication has been duly authorised. In the analysis of meat extracts, stocks and allied products, the gelatin content is a valuable guide to the quality and possible admixture or adulteration of the product for both the analyst and the food technologist. Determination of gelatin nitrogen in these products presents exceptional difficulties, and the method generally recommended, which involves precipitation by alcohol, can only yield approximate results.During the Sub- Committee's examination of methods for the analysis of meat extracts and meat stocks, a method for the determination of gelatin, elaborated by Mr. G. Spall in his laboratory, gave promising results and appears to be worthy of further and wider trials. Although the Sub- Committee is still working on this method, particularly with a view to reducing the amount of impurities associated with the gelatin complex, it has been decided to publish details of ? hr method in order to stimulate interested laboratories to do further work on it. The method clcpc~-ids on the formation of a gelatin - formaldehyde complex, nearly insoluble in water, ly.evaporation of the test solution to dryness in the presence of formalin. At the time of preparing the Interim Report, the attention of the Sub-committee was drawn to a private communication by the (then) British Food Manufacturers Research Association (Food Research, July, 1929, Yol. 1, No. 4, pp. 92-93), which outlines a method essentially the same as that descrilxd kelow. The fuller investigations carried out by the Sub-committee amply confirm the results described in this early analytical note. Method-Weigh l o g of the extract into a 250-ml beaker and add 125ml of distilled water. Bring to the boil while stirring constantly and add 0 6 m l of glacial acetic acid. Allow the mixture to digest on a steam-bath for 15 to 30 minutes until the insoluble material has coagulated.Filter through a Whatman No. 4 filter-paper into a 250-ml calibrated flask. Wash well with hot distilled water and after cooling make up to 250 ml. Transfer 25 ml of this solution in a pipette to a porcelain basin (capacity about 190ml; Royal Worcester, Form 5, No. 4 is suitable). Add 0-25 ml of 40 per cent. formaldehyde solution and mix well. Concentrate the mixture to a thick consistency, add a further 0.25 ml of 40 per cent. form- aldehyde solution and thoroughly mix with a glass rod. Smear the mass over the inner surface of the basin to within 1 inch of the rim with the glass rod, then bake hard on a vigorously boiling steam-bath for 2 hours. Extract the contents of the dish twice with 100 ml of 1 per cent.formaldehyde solution at 40" C ; allow 1 hour for each extraction and maintain the temperature at 40" C during the extraction. Filter each washing through a Whatman No. 54 filter-paper and during the final extraction break up the complex and loosen it from the dish. Transfer the complex to the filter-paper and wash it with 100ml of 1 per cent. formaldehyde solution at 40" C. Determine nitrogen in the complex by the Kjeldahl method, The factor 6.55 is used to calculate gelatin from nitrogen,March, 19531 EXTRACT AND MEAT STOCKS 135 Experiments with commercial gelatin show the formaldehyde-precipitable nitrogen to be 95 to 97 per cent. of the soluble nitrogen. Table I shows the recdvery of gelatin added to meat extract as determined by five laboratories. TABLE I RECOVERY OF GELATIN ADDED TO MEAT EXTRACT Proportion of added commercial Proportion of added gelatin recovered, oh gelatin in r A I mixture, A B C D Yo 20 127 133 115 - - Means . . 126 60 116 116 111 - Means . . 114 91 101 113 114 105 115 104 94 96 102 - - 125 127 - - - 126 116 116 - - - 116 110 113 - - - 112 103 104 104 - 104 The removal of impurities adsorbed on the gelatin - formaldehyde review. E I22 127 - - - 125 120 116 - - 118 complex is still under

ISSN:0003-2654    

DOI:10.1039/AN9537800134

出版商:RSC    

年代:1953

数据来源: RSC

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March, 19531 EXTRACT AND MEAT STOCKS 135 The Determination of Carbonyl Compounds by Semicarbazide and Hydroxylamine With Special Reference to Fatty-Acid Oxidation Products BY A. J. FEUELL AND J. H. SKELLON (Presented at the meeting of the Society on Wednesday, October l s t , 1952) A volumetric method of determining aldehydes and ketones with semi- carbazide has been devised. It is suitable for carbonyl compounds that readily precipitate semicarbazones, but it is not applicable t o oxidised fatty-acids and esters. For estimating the carbonyl groups in the products of oxidation of fatty acids and esters by gaseous oxygen, a modified hydroxylamine method is described. The modified method is especially useful for coloured samples. DURING investigations into the oxidation of monoethenoid fatty acids and esters the need arose for a convenient method of determining carbonyl compounds of the keto- or ketol-acid type, which are known to be formed during these 0xidations.lJ,3~~,~ In an earlier study of an analogous problem, Marks and MorrelP concluded that the phenylhydrazine method of Maclean7 was often unreliable, but the method of Ellis8 was suitable.Ellis’s method involves the use of rather specialised apparatus, which is not always convenient when, as here, carbonyl determinations are required only occasionally and irregularly. For such occasional deter- minations a straightforward volumetric method seemed to be preferable. Volumetric methods based on hydroxylamine are in common use,9 and a selective list of references to the various modifications has been given by Maltby and Primavesi.lO Xever- theless, it seemed that there would be advantages in devising an alternative method that would permit the isolation of some derivative of the carbonyl compound simultaneously with its quantitative determination, as an aid to characterisation.No volumetric methods based on 2 :4-dinitrophenylhydrazine could be traced, although several gravimetric and colorimetric136 FEUELL AND SKELLON THE DETERMINATION OF CARBONYL [Vol. 78 procedures have been described.ll ,12 ,13 ,14 ,15 Of the other common carbonyl reagents, semi- carbazide seemed worth investigating, as volumetric methods for its estimation are a ~ a i 1 a b l e . l ~ ~ ~ ~ J8 J ~ ~ J ~ ~ ,22 One of the simplest of these methods is that of Smith and which involves direct titration with standard iodate.Smith and Wheat identified the carbonyl compounds by finding the equivalent weights of the semicarbazones. For this purpose they prepared the derivative and titrated a weighed amount of it, but did not attempt to determine the percentage of carbonyl group in compounds or unknown mixtures. USE OF SEMICARBAZIDE In the first part of our work an attempt was made to extend the method of Smith and Wheat22 to the direct determination of the carbonyl group in a given material. But an important modification was made in the titration procedure. Smith and Wheat used the iodine monochloride method,23 but it has been stated2* that Lang's iodine ~yanidemethod~*,~~126 is preferable for hydrazine and its congeners, although its use for semicarbazide has apparently not been reported.Experiment showed that semicarbazide could be titrated quantitatively and rapidly by Lang's method. DETERMINATION OF SEMICARBAZIDE- A 0-20 to 0.25-g sample of semicarbazide hydrochloride was dissolved in 25 to 30 ml of water, 20 ml of 5 N hydrochloric acid and 5 ml of 10 per cent. potassium cyanide solution were added and the solution was titrated with 0.05211 potassium iodate solution. When the solution became light brown (at about 80 per cent. of the complete titre), starch was added and titration was continued until the starch colour disappeared for at least 30 seconds. (1 ml of 0.05 M potassium iodate solution = 0.005577 g of semicarbazide hydrochloride.) As some hydrocyanic acid is evolved during the reaction, it is safer to titrate in a Biichner flask, with the side-arm connected by a flexible tube to a soiirce of gentle suction.METHOD FOR PURE CARBONYL COMPOUNDS- Reagent-Dissolve 3 g of semicarbazide hydrochloride and 3 g of sodium acetate crystals in 100 ml of water. This solution is fairly stable, but a slight loss of strength is unimportant, because a blank determination is always made. The usual titre is about 6 ml of 0.05 ill iodate per millilitre of reagent. Procedure-Dissolve 0.2 g of carbonyl compound (for an expected carbonyl content of 20 per cent. ; otherwise in proportion) in 10 ml of water or other solvent (see below, p. 137), add exactly 10ml of reagent and set aside. Filter off the precipitated semicarbazone and TABLE I 1':FFECTS OF TIME AND EXCESS OF REAGENT ON FORMATION OF SEMICAKBAZONES Salicylaldehyde Benzaldehj-de A v 7 -7 /-------------h---. Time allowed >CO found >CO found >CO found >CO found >CO found for precipitation, with E = 1.26, with E = 1-41, with E = 1.21, u-ith IZ = 1.,58), with E == 2.42, 10 22.1 22.2 25.5 2 5 6 - 20 22.2 22.2 25.4 25.5 - 30 22.1 22.4 25.4 25.6 6'4.5 60 22.1 22.4 25.5 25.7 - O/ /O 0 minutes % /O % i0 O? >CO calculated, "/b .. 22.9 26.4 SOTE--E is the molar ratio of semicarbazide hydrochloride to aldehyde. wash the flask and precipitate with two separate 5-ml portions of water, passing the rinsings through the filter and adding them to the filtrate. To the filtrates, whose combined volume is 301111, add 5 iV hydrochloric acid and potassium cyanide, and titrate as above.For the blank, omit the carbonyl compound and dispense with the filtration, but add 10ml of water before adding the acid and cyanide. Each millilitre difference between titrations is equi\-alent to 0.0014 g of ;-CO. The choice of solvent and the precipitation period are discussed below. Results-Preliminary tests were made with freshly-distilled salicylaldehyde and benz- aldehyde. The first factors studied were the length of time necessary for precipitation andMarch, 19531 COMPOUNDS BY SEMICARBAZIDE AND HYDROXYLAMINE 137 the effect of excess of reagent. These compounds were always dissolved in 50 per cent. acetic acid and, as an equal volume of aqueous reagent was added, the final acid content of the reaction mixture was about 25 per cent.; the significance of this is mentioned below.The percentage of >CO found is shown in Table I. I t is seen that with these substances useful results can be attained with a precipitation time of only 10 minutes and a 20 to 60 per cent. excess of reagent. APPLICATION TO KETOHYDROXYSTEARIC ACID- The method was next applied to a compound typical of those encountered in oxidation researches, namely, ketohydroxystearic acid. A pure specimen was prepared by King's method,l but the 9:lO and 10:9-isomers were not separated, as this was unnecessary for the purpose in hand. The purity of the specimen, found by titrating an alcoholic solution with standard alkali, was 99.2 per cent., corresponding to a carbonyl content of 8.84 per cent. (calculated, 8-91 per cent.).King1 showed that the semicarbazone of this acid could be precipitated from 60 per cent. alcohol, and these conditions were used for the determination. Procedure-Dissolve 0.4 g of the ketol acid in 16 ml of alcohol and add 10 ml of reageht. Precipitation of the semicarbazone begins within about half-an-hour and is completed overnight. Filter, rinse, and titrate the filtrate as previously described; make a blank determination simultaneously. Three separate determinations gave 8.75,9.06 and 8.87 per cent. for the carbonyl content. EFFECTS OF SOLVENT AND TEMPERATURE ON STABILITY OF SEMICARBAZIDE SOLUTIONS- Trials were made to determine the effect of temperature, as it seemed likely that it might occasionally be desirable to precipitate at other than room temperature.Erratic results under certain conditions were traced to unequal changes in titre between sample and blank, owing to decomposition of the semicarbazide. These changes were investigated systematically by setting aside blanks containing a fixed amount of the semicarbazide reagent in water or in one of three different strengths of acetic acid or alcohol for various times at three widely different temperatures. The results in Table II show the differences between initial and final titrations for pairs of similar blanks. It must be remembered that in practice decom- position will occur in both sample and blank; if these changes are equal there will be no error, but, owing to differences in concentration arising from the removal of semicarbazide in the precipitate, the amounts of decomposition will not generally be identical. The titration errors will, however, be less than the differences shown in Table 11, which can be regarded as the maximum.TABLE I1 EFFECTS OF TIME AND TEMPERATURE ON DECOMPOSITION OF SEMICARBAZIDE IN DIFFERENT SOLVENTS Titre differences after setting aside at Concen- I A 1 tration 0" C 16°C 16°C 16°C 45°C 45" C 45°C of for for for for for for for % v/v ml ml ml ml ml ml ml Solvent solvent, 8 hours, 2 hours, 4 hours, 8 hours, 2 hours, 4 hours, 8 hours, Water .. 100 0.05 0.00 0.10 0.15 0.05 0.15 0.15 Aceticacid . . 75 0.45 0.30 1.10 1-75 7-35 12.55 - 50 0.15 0.30 0.86 1.60 5-90 10.40 - 25 0.05 0.20 0.25 0.60 2-85 6.25 - Alcohol . . 75 0.50 0-55 0.50 0.55 1-15 1-25 2-15 60 0.35 0.25 0.30 0.40 0.90 0.75 1-05 25 0.05 0.10 0.16 0.20 0-40 0.30 0.40 It is clear that prolonged heating is undesirable when acetic acid is used as the solvent; but, in determinations made as above, in which the final concentration of acid in blank and sample solutions is about 25 per cent.and the proctdure is completed in 30 minutes or less, decomposition causes only a slight error. Water or dilute alcohol can be safely used for fairly long precipitation periods at room temperature, and at low temperatures any of the solvent mixtures appears to be suitable.138 FEUELL AND SKELLON THE DETERMINATION OF CARBONYL [Vol. 78 APPLICATION TO OXIDISED MATEKIALS- Monoethenoid fatty-acids and esters, after catalytic autoxidation for several hours at temperatures above 100" C, devclop reducing properties, which are apparently due to the formation of various types of carbonyl compound^.^ ,3 The application of the semicarbazide method to oxidised oils possessing marked reducing properties was disappointing, as semi- carbazones could not be precipitated; the method consequently failed.It was difficult to find a suitable solvent; the oxidised products are not soluble in 50 per cent. acetic acid, and, in view of the results shown above, a solution of any greater strength could not be used. Although soluble in 95 per cent. alcohol, the oils are precipitated as soon as the added water reduces the alcohol content to less than about 85 per cent.; hence the 60 per cent. alcohol conditions suitable for a pure ketol acid are inapplicable. Pyridine, which Hopper27 showed to be a useful solvent for preparing semicarbazones, proved unsuccessful. Moreover, its basicity caused a further complication, which would have made its use doubtful in any event ; for although additional concentrated hydrochloric acid was added before titration, a buffering effect seemed to retard the iodate reaction, and the end-point, which showed con- siderable drift, was reached slowly.Immiscible solvents such as chloroform were also investigated, but, even after shaking and setting aside for long periods, semicarbazones were not precipitated. U S E OF HYDKOXYLAMINE In view of the inapplicability of the semicarbazide method to oxidised materials it was decided to use hydroxylamine, and the recent method of Maltby and Primavesilo was selected.Careful matching of the bromophenol blue indicator at the end-point is essential in this method, but it was at once found that the procedure as published was not reliable for oxidised fatty compounds, since these were generally reddish-yellow or brown; they changed the tint of the indicator so much that it could not be properly matched against the standard. In addition, matching was found to be rather difficult in artificial light. As a result of further work, Maltby and Primavesi's method was modified in two ways, as follows- (1) By the use of two reagent solutions instead of a single one, so that interference from colour inherent in the sample is eliminated and, to a lesser extent, variations from neutrality in the sample are simultaneously compensated. Both these improve- ments can be achieved by using only about 25 per cent.(by weight) more of sample than is required for a determination by the original method. (2) By making the matching procedure equally easy in either daylight or artificial light. The method devised is as follows. METHOD- Dissolve 20 g of hydroxylamine hydrochloride in 100 ml of water, make up to 1 litre with 95 per cent. alcohol, and add 25 ml of a 0.2 per cent. alcoholic solution of bromophenol blue. Heat on a water-bath for 30 minutes to react with any carbonyl compounds in the alcohol, cool, and adjust the colour as seen in the bulk to the neutral dichroic green-red with 2 N alkali. Prepare as for reagent A but use 5 g of sodium or ammonium acetate instead of the hydroxylamine; this imparts a slight buffering action.This solution need not be heated. Adjust its colour with 2 N acid until it matches that of reagent A. The exact tint is not critical within reasonable limits, and it is immaterial whether reagent B is adjusted to match reagent A or vice veysa. The solutions keep well, but should be re-matched before they are used. Prepare 4 volumes of reagent A for each volume of reagent B. Matching-Match in paired boiling or Nessler tubes instead of in conical flasks as in the original method, using about 20 ml of each solution. Hold the tubes vertically side-by-side and view horizontally against a piece of opal glass illuminated from behind by a fairly strong light. In the small thickness of liquid viewed, the indicator colour is a distinct green in contrast to the dichroic green-red displayed by a larger bulk in a flask, and slight differences in tint are more easily perceptible.By day, reflected sunlight from walls or a strong north light are equally suitable for illuminating the opal glass screen, whilst at other times a 60, 75 or 100-watt bulb placed about a foot behind the screen is adequate. Moreover, slightly turbid solutions, which are difficult to compare, Hydroxylamine reagents-Reagent A. Reagent B. This technique is both convenient and accurate.March, 19531 COMPOUNDS BY SEMICARBAZIDE AND HYDROXYLAMINE 139 even in tubes, against ordinary backgrounds, can be matched against the opal screen, the turbidity apparently disappearing. Procedure-Dissolve the sample (corresponding to 0.002 to 0.004 gram-molecules of carbonyl compound) in 25 ml of water or alcohol, and add 20 ml of this solution to a flask containing 60ml of reagent A.Add the remaining 5ml to 15ml of reagent B in one of a pair of tubes. The colour of the indicator in both reagents is thus equally affected by dilution and inherent sample colour. Set the sample aside for 5 to 10 minutes and then titrate it with 0.2 N alkali to match reagent B, a proportionate amount of water (one-quarter of the titration volume) being added to reagent B. With a little practice it is possible to titrate the liquid in the flask to a point just on the acid side of neutrality, as seen by the colour in bulk. Pour some of the mixture into the other tube and compare the two as described. If the liquids are not matched return the contents of the tube to the flask and cautiously continue to titrate, transferring a suitable volume to the tube from time to time for exact comparison.When the solutions are matched, TABLE I11 RESULTS BY PROPOSED METHOD APPLIED TO OXIDISED MOKOETHENOID ESTERS >CO found at Sample f A \ 55O c, 85" C, 120" c, % % % Oxidation products of ethyl oleate . . .. 0.64 1.86 1.95 ,9 n-propyl oleate . . 0.78 1.91 1.88 ,7 n-butyl oleate .. 1.09 1.99 1.83 set them aside for a further 5 to 10 minutes and compare them again, as some compounds react slowly. Finally, loosely stopper the two flasks, place them on a water-bath for 20 minutes, cool and titrate if necessary to ensure final matching; this may be necessary for compounds that do not react in the cold. Water must always be added to reagent B in the same pro- portion, one-quarter of the titration volume, before final matching.A slight departure from neutrality, such as is given by many organic acid samples, causes no serious interference. Strongly acid or alkaline samples are dissolved in water or alcohol, a drop of bromophenol blue solution is added and the colour adjusted approximately to the neutral tint with alkali or acid. The volume is then made up to 25ml and the procedure carried out as described above. An improvement in technique,28 which allows more rapid manipulation, is effected by titrating the sample in a standard-joint flask having a long narrow neck equal in diameter to the tube containing reagent B. For matching, a stopper is inserted, the flask inverted, and the columns of liquid in the neck and tube are compared as usual.Results-Since hydroxylamine is well established for estimating the commoner aldehydes and ketones, trials with the modified method were mainly concerned with the effects of colour in the sample. Satisfactory results were obtained with salicylaldehyde and anisaldehyde each tinted with brown dye; mean percentages of >CO group found were, rspectively, 22.7 and 20.1 per cent. (calculated, 22.9 and 20.6 per cent.). Ketohydroxystearic acid, mixed with various amounts of brown dye in imitation of the oxidation products for which the method was ultimately to be used, was then tried. Even with highly coloured samples there were no difficulties with the modified method. The carbonyl content found in a series of runs with 0-5 to 0.6 g varied from 8.88 to 9-13 per cent. (calculated, 8.91 per cent.).An even more representative test was made by dissolving some of the ketol acid in oleic acid and again adding brown dye, thus closely simulating a possible oxidation product. The mixture contained 15-4 per cent. w/w of ketol acid (equivalent to 1-37 per cent. of >CO). Three separate determinations indicated 1.42 to 1-45 per cent. of >CO, which corresponded to 15.9 to 16.3 per cent. w/w of ketol acid. Application of the method to oxidised monoethenoid esters has enabled the influence of temperature and constitution on formation of carbonyl groups to be studied. Some typical results are indicated in Table 111, which shows the percentage of >CO found in three homologous esters autoxidised at different temperatures in presence of a uranium catalyst.Subsequent work on oxidised materials has shown that the method can be scaled down if necessary and applied to smaller samples with the use of 0.1 N alkali, a match to within140 FEUELL AND SKELLON Pol. 75 acid.28 1. 2. 3. 4. 6. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 26. 26. 27. 28. one drop still being readily attainable. Occasionally it is desirable to titrate with alcoholic alkali, as certain complex products, such as those from oxidised butyl oleate, are precipitated if appreciable amounts of aqueous alkali are added; a proportionate volume of alcohol instead of water is then added to reagent B before matching. The method has been found useful for studies of the carbonyl content of the various fractions obtained in the separation of the complex end-products of oxidised fatty-acid esters (being prepared for publication elsewhere).It has also been used to follow the steady decrease in carbonyl content occurring during the thermal catalytic autoxidation of ketohydroxystearic REFERENCES King, G., J . Chem. SOC., 1936, 1788. Skellon, J. H., Ibid., 1948, 343. Skellon, J. H., and Thruston, M. N., Ibid., 1949, 1626. Ellis, G. W., Ibid., 1950, 9. -, Biochem. J., 1950, 44, 129. Marks, S., and Xlorrell, R. S., Analyst, 1931, 56, 508. Maclean, I. S., J . Biol. Chem., 1913, 7, 611. Ellis, G. W., J . Chem. SOC., 1927, 848. Analytical Methods Committee, “The Determination of Aldehydes other than Citronellal,” A nulyst, Maltby, J. G., and Primavesi, G. R., Ibid., 1949, 74, 498. Iddles, H. A., and Jackson, C. E., Ind. Eng. Chem., Anal. Ed., 1934, 6, 454. Iddles, H. A., Low, A. W., Rosen, B. D., and Hart, R. T., Ibid., 1939, 11, 102. Houghton, R. E., Amer. J . Plzarm., 1934, 106, 62. Lappin, G. R., and Clark, L. C., Anal. Chem., 1951, 23, 541. Pool, M. F., and Klose, A. A., J . A.mer. Oil Chem. SOC., 1951, 28, 216. Hovorka, V., Coll. Trav. Chim.. Tchdcosl., 1931, 3, 285. Horlay, V., J . Pharm. Chim., 1936, 23, 199. Veibel, S., Ibid., 1936, 24, 499. Bartlett, P. D., J . Amer. Chem. SOC., 1932, 54, 285i3. Miller, C. 0.. and Furman, N. H., Ibid., 1937, 59, 161. Smith, S. G., J . Chem. SOC., 1937, 1925. Smith, G. B. L., and Wheat, T. G., Ind. Eng. Chem., Anal. Ed., 1939, 11, 200. Jamieson, G. S., “Volumetric Iodate Methods,” Chemical Catalog Co., New York, 1926. Bottger, W., and Oesper, R. E., “Newer Methods of Volumetric Chemical Analysis,’’ Chapman Mitchell, A. D., and Ward, A. M., “Modern Methods in Quantitative Chemical Analysis,” Longmans, Lang, R., 2. anorg. Chem., 1932, 122, 332. Hopper, I. V., J . Roy. Tech. Coll. Glasgow, 1929, 2, 52. Parkinson, T. L., private communication. 1934, 59, 105. & Hall Ltd., London, 1938. Green & Co., Ltd., London, 1932. ACTON TECHNICAL COLLEGE HIGH STREET, ACTON LONDON, W.3 August 22nd, 1962

ISSN:0003-2654    

DOI:10.1039/AN9537800135

出版商:RSC    

年代:1953

数据来源: RSC

摘要:

March, 19531 MONK 141 A Brief Survey of the Development of Electrographic Analysis With Special Reference to Recent British Apparatus BY P. R. MONK (Presented at the meeting of the Physical Methods Group on Tuesday, February 19th, 1952) A brief account is given of the development of electrographic analysis, from the time of its origination by Glazunov and Fritz in 1929 up to the present day. The technique is described, and details are given of recent British apparatus of two types, including several examples of their use. IN 1906, Baumannl devised a method of showing sulphide inclusions in steels by placing the polished specimen in close contact with ordinary photographic paper soaked in dilute sulphuric acid. The silver salts in the emulsion were blackened by sulphides, and not only was the presence of sulphide demonstrated, but a print showing the shape and location of the inclusions was produced.Since then, this method, and others for spot-testing metals,2 have been applied widely, but all suffer in consequence of the strongly acidic media required for dissolving the metals. These acids are not only unpleasant to handle, but often preclude the use of certain sensitive reagents. In 1929, this trouble was overcome by Glazunov3 and Fritz,4 working independently and with different aims. In place of strong acids, they used electrolytic attack with neutral salt solutions. With the sample as anode, the solution was held in porous paper, and another metal, which did not participate in the reaction, was made the cathode. Glazunov applied the technique to give metallographic prints, and Fritz worked on spot tests for identification purposes.Little further work was published until the beginning of the war, when Hermance6 gave details of the practical use of the method, both for identifying metals and for checking plated coatings, such as those on telephone contacts, for Near. He described methods for chromium, nickel, lead, tin, copper and zinc, but did not describe his apparatus in detail. Shortly afterwards, in 1941, Calamari6 developed a method for chromium in steels. With a graphite rod as counter-electrode, he applied 9 volts from six large dry cells. Then Hunter, Churchill and Mean7 made a wide survey, covering a large number of metals, and suggested the use of a block of eighteen metals as a “standard.” Their counter-electrode was aluminium.In 1943, Calamari, Hubata and Roths described a method for molybdenum in steel, which utilised the equipment they had described earlier.6 From this time onwards publications were fairly f r e q ~ e n t , ~ , ~ O , ~ ~ and in 1949, Hermance and Wadlow presented a comprehensive paper at a Symposium on Rapid Methods for the Identification of Metals before the American Society for Testing Materials.I2 But without the pioneer work of Fritz Feig1,l3P14 who developed the use of filter-paper fur spot tests, it is doubtful whether the modern electrographic method would have developed so far. His discovery that the use of filter-paper increases the sensitivity of many spot tests has been invaluable and the method includes many of the tests he has devised.MODERN ELECTROGRAPHIC METHOD- The test sample is made anode in a small cell, and the electrolyte, which soon contains some of the metal in solution, is “developed,” ie., treated with a reagent to give a characteristic colour. Instead of working in “bulk” solution, the electrolyte is absorbed in filter-paper, Whatman No. 3 filter-paper being suitable for spot-testing. This paper is placed between the sample and a counter- electrode or cathode, generally made of aluminium. Current is passed for a short time (10 to 15 seconds), and finally appropriate reagents are added. The whole test can often be completed in 1 minute or less. Their solution to the problem was simple and effective. . - The method depends mainly on electrolytic solution.142 Generally, a simple salt solution, e.g., sodium nitrate or ammonium chloride solution, which plays no part in the subsequent development, is used as electrolyte. But occasionally it is of great help to use as the electrolyte a salt that will form a colourless complex with an interfering metal.Sodium fluoride is used in this way to suppress the effect of iron in certain tests. The developers used are frequently the new organic r e a g e n t ~ , l ~ + , ~ ~ but in some tests these do not work sufficiently well under the working conditions and the older inorganic tests are applied. The sensitivity of the tests is high, oftm only a microgram or less of the metal being required. This is one of the advantages of the method, as little damage is caused to the MONK: A BRIEF SURVEY OF THE DEVELOPMENT OF [Vol.78 c---lT A-- B \ \ \ E I D i Fig. 1. Power supply unit of spot-test apparatus (laboratory model) Fig. 2. Sample holder (laboratory model) specimens beyond a slight staining. Hence the composition of finished parts can be checked without destruction. There are also several other applications : metallic inclusions can be identified readily and quickly, metallographs can be prepared, which show the distribution of alloy ingredients, and it is an excellent and sensitive method for detecting pinholes in the coatings of painted and plated parts. APPARATUS- The basic requirements are simple; a source of low-voltage direct current, and various electrodes, but for consistent working, or for “standardised” tests on coatings, more elaborate and better controlled equipment is needed.Fig. 1 shows recent apparatus designed specifically for this type of work. The “Laboratory Model” is A.C.-mains powered, as dry batteries were considered unsatisfactory for this purpose. The output can be varied continuously from 0 to 10 volts by control A, at up to 5 amperes in three ranges. Meters are fitted; the voltmeter, B, indicates the potential difference across the sample - paper - counter-electrode combination, and the range switch, C, on the ammeter, D, also sets the maximum current available in each range. This switching arrangement prevents damage to the instrument should the paper be pierced accidentally; a short circuit can be maintained indefinitely without harm. As the length of time during which current is passed is important when methods are standardised, a process timer, E, is fitted.This eliminates the human error associated withMarch, 19531 ELECTROGRAPHIC ANALYSIS 143 stop-clocks, and enables the operator to prepare the next specimen while the first is being tested. Two pilot lights are fitted, F indicating “Power on, timer motor running” and G “Output on.” The sample holder (Fig. 2) is separate, being connected to the power supply unit by a flexible cable. This is detachable and although this is normally a flat plate, it can be easily replaced by specially shaped electrodes when required. Placed centrally above this is a pressure-bar, B, which applies a known load and makes electrical contact with the sample. The load can be varied up to 25 lb by rotating the hand-wheel, C, at‘the top.The thrust is shown on the small scale, D, in the lower part of the bar. An apparatus of this kind is not always convenient or necessary, especially when spot tests only are required, as in metal stores and scrap yards, and Fig. 3 shows a special “Pocket Model” for these purposes. It does not incorporate all the controls of the “Laboratory Model,” but is nevertheless efficient for detecting a wide range of metals. The timer is variable up to 60 seconds, and is re-set automatically. The base carries the aluminium counter-electrode, A. Fig. 3 Spot-test apparatus (pocket model) The body, A, carries two standard pocket lamp cells, B, so fitted that the case is made positive. This is connected to the sample via the slotted contact ring, C, at the end.The counter-electrode is a centrally fitted aluminium plunger, D, which is spring loaded, and connects to the negative pole of the battery via a simple switch. A small circle or strip of filter-paper damped with the appropriate electrolyte is put on the sample, and the plunger is placed against it. With care being taken that the contact ring does not touch the paper, the apparatus is pushed home firmly, so that the contact ring connects to the sample. The pressure is maintained for 10 to 15 seconds, then the paper is removed and treated with the appropriate developer. PRACTICAL APPLICATION OF THE METHOD- Both models are suitable for spot-testing metals in order to check or discover their identity. For example, a stainless steel may be specified as containing molybdenum, but some doubt may have arisen as to whether in fact it does.A rapid test can be carried out as follows. Rapid procedwe-Use either 5 per cent. hydrochloric acid or 5 per cent. ammonium chloride solution as electrolyte. Pass current at about 3 volts for 10 to 15 seconds. and develop the stain by adding 1 drop of 1 per cent. ammonium thiocyanate solution. Allow the paper to soak for about 1 minute in 1 per cent. stannous chloride solution dissolved in 5 N hydrochloric acid. A brick-red stain, remaining after the disappearance of the deep red ferric thiocyanate colour confirms the presence of molybdenum. Alternatively, it may be necessary to distinguish between zinc and cadmium plating. With 10 per cent. sodium nitrate solution as electrolyte and 10 per cent.sodium sulphide solution as developer, cadmium gives a vivid yellow colour and zinc no colour. The presence of cobalt in tool and magnet steel can be shown by using 5 per cent. sodium fluoride solution as electrolyte and a saturated solution of potassium thiocyanate in acetone as developer. This is an example of the use of a complexing electrolyte ; the sodium fluoride prevents interference from iron. In all, rapid and economical methods such as these have been worked out for sixteen metals. Operation is simple. A vivid blue-green colour indicates cobalt.144 MONK [Vol. 78 Metallic inclusions in metals are best detected by means of the “Laboratory Model,” which gives a print showing not only the presence of any inclusion, but also its shape, size and position.For this work a paper coated with hardened gelatin is better than filter-paper, which is coarser grained. Both Glazunov2 and Herman~e79~2 have had success with these methods. For example, trouble may have been experienced with suspected iron inclusions in an aluminium alloy. With ammonium chloride as electrolyte and potassium ferrocyanide as developer the presence of the inclusions can be confirmed. The procedure is also useful for the detection of pinholes in painted and plated finishes. Again the “Laboratory Model’’ should be used, but reagents should be chosen to detect the underlay or base metal. For example, porosity in chromium plated over nickel can be checked with sodium fluoride as the electrolyte, and pinholes can be shown by the red nickel colour given with dimethylglyoxime or “Nioxime.” Pinholes in paint applied over iron can be detected with the ferrocyanide method mentioned above.APPLICABILITY OF THE METHOD- Electrographic technique is rapid and useful for spot-testing metals and alloys, and for detecting and delineating imperfections in both solid metals and coatings. No strongly corrosive acids are required, and damage to the sample is negligible. Apparatus has been developed both for simple spot-testing and for more detailed metallographic work. Methods are available for a wide range of metals and alloys, and the technique should be of great value to all those who use and test metals. . I thank the Directors of Baird & Tatlock (London) Ltd., for their permission to publish this paper.1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. REFERENCES Baumann, R., Me‘tallurgie, 1906, 3, 416. Evans, B. S., and Higgs, D. G., Analyst, 1945, 70, 75. Glazunov, A., Chim. et Ind., 1929, 21, 425; 1930, 23, 247; Chenz. Zentr., 1930, 11, 1104; 1932, I, Fritz, H., Z. anal. Chem.. 1929, 78, 418. Hermance, H. W., Bell Lab. Rec., 1940, 18, 269. Calamari, J. A., Ind. Eng. Chem., A n d Ed., 1941, 13, 19. Hunter, M. S., Churchill, J. R., and Mears, R. B., Metal Progr., 1942, 42, 1070; Reprinted in Calamari, J. A., Hubata, R., and Roth, P. B., Ind. Eng. Chem., Anal. Ed., 1942, 14, 535. Hughes, H. D., J . Electrodep. Tech. SOC., 1945, 66, 169. Zeeh, C. J., Metal Progr., 1947, 52, 824. Levy, M. E., Iron Age, 1949, 164, 98. A.S.T.M., Special Report No. 98, Symposium on Rapid Methods for the Identification of Metals. Feigl, F., 2. anal. Chem., 1921, 60, 1. -, “Qualitative Analysis by Spot Tests,” Cleaver-Hume Press Ltd., London, 1947. “Organic Reagents for Metals,” Hopkin and Williams Ltd., London, 1943. Yoe, J. H., and Sarver, L. A., “Organic Analytical Reagents,” John Wiley & Sons, Inc., New York, 1398; 1934, I? 3164. Metal Ind., 1943, 62, 354 and 373. 1941. 17. Mellan, I., “Organic Reagents in Inorganic Analysis,” Blakiston & Co., Philadelphia, 1941. RAIRD AND TATLOCK (LONDON) LIMITED RESEARCH AND DEVELOPMENT DIVISION 14/17 ST. CROSS STREET, HATTON GARDEN LONDON, E.C. 1 First submitted, March 4th. 1952 -4mended, September 22nd, 1952

ISSN:0003-2654    

DOI:10.1039/AN9537800141

出版商:RSC    

年代:1953

数据来源: RSC

摘要:

March, 19531 CLARK AND HALE The Identification of Alloys and Stainless Steels by Electrographic Methods BY G. C. CLARK AXD E. E. HALE (Presented at the meeting of the Physical Methods Group o n Tuesday, February 19th, 1952) A method is described for the identification of certain stainless steels and other alloys used in the construction of chemical plant. An electro- graphic method, which obviates drilling or cutting, is used for testing the sample. The constituents shown on the electrogram are identified by a series of spot tests applied systematically to classify the type of material from which the plant is made. The method is simple, the apparatus is portable and can be used on any site. COPPER and nickel alloys and stainless steels are becoming increasingly important in the construction of chemical plant.The presence of certain “stabilising elements” in the stainless steels has a considerable effect on their resistance to corrosion. If the construction of new plant is to be considered, it is possible to test known specimens before a decision is made as to their suitability. However, when it is necessary to check the performance of existing plant or to purchase second-hand plant or job lots of sections or plate the question of identity arises. As records are frequently unreliable or even unobtainable, recourse has to be had to some analytical procedure that will supply the information. A fully equipped metallurgical laboratory would be able, eventually, to identify a specimen, which would have to be obtained by drilling or cutting, but this method could obviously not be used for existing plant or plant offered for sale.The time that elapses before the full result becomes known may be too long in a second-hand plant sale. A simple method of identifying plant and plant materials is set out here, which although it has only been applied to a limited range of alloys, is amenable to elaboration. The sample is taken, by means of an electrogram, on filter-paper moistened with a neutral electrolyte.1,2 An electrolytic solution of the test material is produced by connecting the positive pole of a source of continuous current to the test material and by completing the circuit through the moist paper to an electrode connected to the negative pole. The ions are quickly driven into the paper, where they can be identified by a series of spot t e s t ~ .~ , * , ~ METHOD APPARATUS- A source of continuous current is required, which can be two or three dry cells in series. A cycle-lamp battery will give 3 volts or the slightly larger rectangular battery will give 4& volts and provide the electrogram in a shorter time. Filter-paper pads should be made by folding strips of Postlip 633A, Whatman No. 1, or other similar grade of filter-paper, 1 inch wide by 5 inches long, six times to make pads approximately 1 inch long by 2 inch wide. Two electrodes of copper or aluminium are also needed. The electrodes can be con- veniently made from 16-gauge copper or aluminium sheet about 2 inches square. One piece is bent at a right angle, about 2 inch from one edge, to form a finger grip. Single leads of flexible wire about 1 yard long are attached to each electrode. When testing a piece of plant a third electrode is useful.This can be a piece of brass rod about 2 inch in diameter or the central carbon rod from an exhausted dry battery. REAGENTS- dropping pipettes for the spot tests. A N solution of potassium chloride and bottles of reagents provided with capillary[Vol. 78 PROCEDURE- To prepare a suitable site for the test on the plant or specimen, remove adherent dirt, scale or paint by scraping an area of about 1 square inch. This site can be on the rim outside or inside of a vessel, as is most convenient. The inside can be used if a vessel is constructed from mild steel clad with an alloy. Avoid highly polished surfaces, as mirror polishes will be dulled by the electrolysis.Connect the electrodes to the battery, with the bent electrode to the negative pole. Moisten a pad of the filter-paper with 4 or 5 drops of N potassium chloride solution, avoiding any excess, and place it on the cleaned area of the specimen. Hold the pad firmly in place by exerting firm pressure with the bent, that is, negative, electrode. Hold the positive electrode on the specimen, adjacent to, but not touching, the moistened paper pad. This completes the circuit and generally produces a suitable electrogram after about 1 minute. To identify the metals add drops of spot-test reagents from a capillary dropper to portions of the electrogram. It is important that no excesses of reagents be used or the paper will become flooded.Unfold the paper pad and cut off the portion of the strip that has been in contact with the specimen. Cut this strip, which is usually coloured, into six narrow strips. Apply the tests in the following way, but modify the scheme as necessary to suit any specific requirements. 1. Hold one of the strips in ammonia vapour- A bright blue colour indicates copper. A paler inauvish colour indicates nickel. A brown or dirty-green colour indicates iron. A greyish colour indicates iron plus copper or nickel. 146 CLARK AND HALE: THE IDENTIFICATION OF ALLOYS AND Copper is confirmed by spotting the strip with sodium di-ethyldithiocarbamate; a brown stain develops. Nickel is confirmed by spotting the strip with dimethylglyoxime solution, which gives a red colour.Iron is confirmed by spotting a fresh strip, which has not been held in ammonia vapour, with potassium ferrocyanide ; a blue colour develops. Nickel in the presence of iron is confirmed by adding a drop of tartaric acid solution to a fresh strip and then adding a drop of dimethylglyoxime solution. On holding the strip in ammonia vapour a red colour develops. Copper in the presence of iron is confirmed by the same procedure except that sodium di-ethyldithiocarbamate is used in the place of dimethylglyoxime. 2. Apply a spot of potassium thiocyanate solution to another strip. A red-brown colour indicates iron. Apply stannous chloride to the red-brown stain. If this colour is discharged and a carmine colour develops, molybdenum is present. 3. Apply stannous chloride to a fresh strip and then add a spot of chroniotropic acid solution.The minimum amount of stannous chloride should be used as it reduces the sensitivity of the test. As most chromotropicacid solutions are brown, it is advisable to place some on a piece of filter-paper and to compare this with the strip of the electrogram used for the testing for titanium. 4. Apply a drop of diphenylcarbazide solution to the remaining strip. A violet colour indicates chromium. Iron, nickel, chromium and molybdenum indicate a stainless steel stabilised with molybdenum. Iron, nickel, chromium and titanium indicate a stainless steel stabilised with titanium. Nickel and copper indicate Monel or a similar alloy. Iron, nickel and chromium indicate Inconel or an ordinary stainless steel.I t will be noted that one electrolyte, potassium chloride, has been used for all these tests. This is not necessarily the best solution for all specimens and if the method is to be used for regular routine work, solutions of other salts may give better results with certain elements. As an example, with certain of the stainless steels a better test for chromium is given when potassium nitrate is used as the electrolyte, and for copper alloys sodium sulphate solution gives slightly better results. So far the test has only been used qualitatively, and the following suggestion is offered and may lead, if not to a strictly quantitative test, at least to a comparative test that might A reddish-brown colour indicates titanium.March, 19531 STAINLESS STEELS BY ELECTROGRAPHIC METHODS 147 be of value in distinguishing between the austenitic 18 - 8 stainless steels and the low chromium - nickel steels that are also used for chemical plant.I t is quite possible that paper chromatography would separate inorganic elementss 9’ 98,9 from the electrogram. The resulting chromatogram when developed with appropriate reagents would give coloured areas proportional to the quantity of the elements present and so the amount of the elements present in the original specimen could be assessed. A certain amount of success along these lines has been achieved, but further work is needed before the method can be established. The authors express their thanks to the Directors of Howards and Sons, Ltd., of Ilford, Essex, for their permission to publish the results of this work, which was carried out in the Works Analytical Department.1. 2. 3. 4. 5. 6. 7. 8. 9. REFEREKCES Glazunov, A., Chim. et Ind., 1929, 21, 425; Chem. Zentr., 1930, 11, 1104, and 1932, I, 1398. Fritz, H., 2. anal. Chenz., 1929, 78, 418. Feigl, F., “Qualitative Analysis by Spot Tests,” Cleaver-Hume Press LM., London, 1947. “Organic Reagents for Metals,” Hopkin & Williams, Ltd., London, 1943. “The B.D.H. Book of Organic Reagents for Analytical Use,” The British Drug Houses, LM., Arden, T. V., Burstall, F. H., Davies, G. R., Lewis, J. A., and Linstead, R. P., Nature, 1948, Burstall, F. H., Davies, G. R., Linstead, R. P., and Wells, R. A., Ibid., 1949, 163, 64. Tolley, G., Mfg. Chenz., 1949, 20, 215. Poole, Dorset, 1948. 162, 691., , , J . Chem. SOC., 1950, 516. ---- WORKS ANALYTICAL LABORATORY UPHALL WORKS HOWARDS AND SONS, LTD. ILFORD, ESSEX March 4th, 1952 DISCUSSION ON THE FOREGOING TWO PAPERS MR. A. A. SMALES asked if either author had yet been able to identify stainless steels containing up to 1 per cent. of niobium or tungsten and whether it was possible to say with certainty whether a steel contained more than a specification minimum, when this minimum was about 0.5 per cent. of the elements titanium, niobium and tungsten. He also asked if the electrographic technique had proved of value in the identification of minerals. MR. MONK said that neither he nor Mr. Hale could offer a suitable chemical spot test for niobium. He had conducted experiments with tungsten, for which the Rhodamine B test might be suitable, without success.In reply to Mr. Smales’ second question, he said that, provided suitable tests were available, comparison under controlled conditions with standard samples should give a reasonable estimate of the amounts present. MR. HALE said that i t was doubtful whether an assessment of the quantity of alloying metal could be made by the method described in their paper. If a rigorously standardised procedure were adopted, i.e., if the time of electrolysis, pressure of electrode, current density, thickness of paper, and so on, were made the same, then by comparing electrograms from standard sampks of known composition with the electrograms of the test sample a semi-quantitative result could be obtained. To Mr. Smales’ third question he said that the electrographic identification of minerals described by R.Jirkovsky (Chem. Listy, 1931, 25, 254; Chem. Abstr., 1931, 25, 5640) was one of the early applications of electrographic analysis. The mineral must be a conductor of electricity. M R . D. G. HIGGS said that he had been much impressed by what he had seen a t the meeting. His own experience of electrographic analysis had been fairly limited; he had abandoned the procedure early in the forties, as i t was too restricted for his purposes, viz., spot testing. I t was interesting to hear that Mr. Hale had also been unable to find a suitable spot-test reaction for niobium in stainless steel, it being one of the three elements that had completely outwitted him in his own researches. With regard to the other metals used as stabilisers in the 18 - 8 type of steel, he had reliable tests for titanium, tungsten and molybdenum, but not for tantalum, which was often used as a substitute for niobium.He asked Mr. Hale whether the technique he had described could detect as little as 0.1 to 0.3 per cent. of titanium easily, since if molybdenum was also present the amount of titanium needed to convert all the carbon to carbide could be considerably reduced below the amount already indicated, namely, 1 per cent.148 CLARK AND HALE [Vol. 78 Mr. Higgs drew Mr. Smales’ attention to a paper, which included good multicolour electrographs of various metallic minerals (Gutzeit, G., American Institute of Mining and Metallurgical Engineering, Tech- nical Publication No.1457, March, 1942). He had always maintained that it was impossible to interpret spot-tests quantitatively, there being so many variables that could not readily be controlled. The apparatus shown by Mr. Monk might go a long way towards achieving this end; would the author say something about his experience in this connection. MR. MONK agreed that quantitative interpretation of the results should be made with caution, but by rigidly controlling conditions, as with the “laboratory model,” results should be reasonably accurate. Comparison should be made with known samples of similar type. He said, on the second point, that he had had little personal experience, as he had been concerned mainly with the development of the apparatus. Mr. Higgs might like t o see an account of the work of Glazunov and his associates (Chem.Zentr., 1941, 11, 2565; Korrusion u. Metallschutz, 1946, 16, 341). MR. HALE said that the electrographic method should detect as little as 0.1 to 0.3 per cent. of titanium, if care was used in applying the spot-tests. The chromotropic acid solution should be freshly prepared, and the minimum quantity of stannous chloride solution, preferably diluted, used to suppress the iron colour. MR. H. STANT asked why pressure was varied between electrodes and why aluminium was made cathodic. He asked if it was possible to detect iron, copper, manganese and cobalt in pure nickel and whether it was possible to distinguish between residual elements, low alloy elements and highly alloyed materials. MR. MONK said that pressure was made variable so that the optimum conditions might be chosen and allowance could be made for different sample areas. In anodic dissolution (the opposite of electroplating) the sample was the anode and, hence, the aluminium, which acted only as the second electrode to complete the circuit, was the cathode. Other metals could be used, platinum was ideal, but aluminium was generally suitable. Iron, copper and cobalt should be fairly easy and manganese somewhat more difficult, but all tests depended on the actual proportions present, which might be below the sensitivity limit. I t should be remembered that the total sample taken was only a few micrograms. In reply to the last question, he said that distinction depended on amounts present and on the sensitivities of the tests for the metal concerned. In general, highly alloyed materials would give a strong colour, low alloy elements a weak but definite colour, and residual elements weak or indefinite results.

ISSN:0003-2654    

DOI:10.1039/AN9537800145

出版商:RSC    

年代:1953

数据来源: RSC

摘要:

March, 19531 BISHOP 149 Replacement of Standard Cell and Salt Bridge by Indicator Electrodes and the Use of Non-Aqueous Solutions in Potentiometry Part 11. Iodoinetry and Iodimetry in Aqueous Solution BY E. BISHOP* The use of reference indicator electrodes in the titration solution to replace the standard reference cell and salt bridge in potentiometric analysis has been extended to electron transfer reactions, and indicator electrodes responsive to ion combination reactions have been applied to redox reactions involving the iodine - iodide system. The glass reference indicator electrode, in solutions not too strongly acid, has been found admirable in all three types of procedure and gives unexceptionable results. The silver reference indicator electrode is eminently suited to procedures involving the titration of reducing agents with standard iodine, and to titration of iodine, either standard or liberated by an oxidant from an excess of iodide, with standard reductants.In procedures involving direct titration of an iodide with an oxidant, its behaviour is not that of a constant potential electrode, but depends on the normal potential of the oxidant. The behaviour of the antimony reference indicator electrode is complicated by its existence in alternative valency states ; however, the electrode gives excellent results in all three types of procedure. I t is shown that accuracy with reference indicator electrodes equals that of the accepted technique, with a stand- ard calomel half-cell and salt bridge, and that qualitatively (equilibration speed and sensitivity of measurement with low impedance potentiometers) some advantage, in addition to simplicity, over the classical method is offered. THE titration cell used in potentiometric analysis consists of an indicator electrode, whose potential varies with the concentration of a reacting ion, in combination with a reference electrode, whose potential remains constant during the determination.Usually the second electrode is a standard half-cell, such as the saturated calomel electrode, electrically connected by means of a salt bridge to the titration solution containing the indicator electrode. In titration work, the reference electrode should furnish a steady potential, unaffected by the nature of the reaction being studied, and one to which the varying potential of the indicator electrode can be referred.An indicator electrode in a solution stabilised or buffered with respect to the indicated ion will furnish the required steady potential, and there is no reason why such a “reference-indicator” electrode should not be placed in the titration solution itself, provided there is no interaction between the system of ions maintaining the reference indicator potential and the reagents whose reaction is being followed by the “reaction indicator” electrode. Bishop1 applied this principle in the use of pH indicating reference electrodes in precipitation reactions, when glass and antimony gave excellent results in argentometric determinations of halides and thiocyanate in aqueous media containing free nitric acid, and also in non-aqueous amphiprotic media stabilised with ammonium salts.The poising of the solution with respect to the reference indicator ion was not critical. The application of a redox reference indicator electrode to these reactions did not give a constant reference potential, but gave accurate results of diminished sensitivity in unpoised solutions ; in redox systems poising gave rise to unexplained anomalies. EXPERIMENTAL This paper begins an examination of the application of “ion combination reference indicator” electrodes to electron transfer reactions. Certain electrodes function in more than one way; for instance, antimony, arsenic, hydrogen and quinhydrone are used as pH indicating electrodes, but depend upon or are affected by oxidation reactions, and mercury * Present address: Washington Singer Laboratories, University College of the South-West, Exeter.150 BISHOP: REPLACEMENT OF STANDARD CELL AND [Vol.78 as a pHg indicating electrode is influenced by oxidation; this makes them inapplicable to electron transfer reactions as reference indicator electrodes, unless the effect can be overcome and a valency state fixed and retair,ed. An ideal reference electrode should neither influence (electrically) or contaminate (chemically) the titration solution, nor be influenced or conta- minated by the titration solution. Within the pH limits set by the acid and alkali errors, the glass electrode fulfils this condition admirably, and it was from this realisation and its application in cerate oxidimetry some years ago, that the reference indicator electrode principle1 originated and the present work developed. Previous applications of the glass electrode to titration work have been made by Lykken and Tuemmler2 and have been embodied in standard analytical methods in the petroleum i n d ~ s t r y .~ Bates4 has used the same principle to eliminate liquid junctions in the precise determination of ionisation constants by titration, for which the potential of the reference electrode had to be both accurately known and perfectly steady, by a combination of a silver - silver chloride electrode in solutions of uniform ionic strength with a hydrogen reaction indicator electrode. A great number of useful procedures are based on the iodide -iodine redox system6 and these can be classified into three groups as follows- (1) Direct titration of iodide by an oxidising agent to give free iodine. (2) Direct titration of reducing agents by means of standard iodine solutions. (3) Titration of free iodine, either as a standard solution, or liberated from an excess of iodide by an oxidant, by means of a standard reductant, such as thiosulphate or arsenite.For the second and third of these groups the end-point is normally indicated by starch, which is both convenient and accurate. Reactions of the first group require electrical methods for locating the end-point, or an indirect back-titration method, inherently of lower accuracy, and it is with this group that the principal advantage of the potentiometric approach lies. I n the first group, loss of iodine by volatilisation, instead of being a source of error carefully to be avoided, is often advantageous.Furthermore, it is often possible to work in an acidity range in which direct or induced air oxidation of iodide is minimised. A possible disadvantage lies in oxidation of iodine to iodate; the acidity and salt concentration of the solution are insufficient to support formation of I' ions. This can often be overcome by controlling acidity and removing the liberated iodine by a solvent and, although it may reduce the size of the potential break at the end-point, it does not necessarily cause errors, since consumption of oxidant by this reaction does not occur until the end-point has been passed. Although less popular for reactions in groups 2 and 3, the end-point curves for the potentiometric method are particularly steep, and the enhanced precision of end-point location thereby offered, coupled with the ease and speed of operation, compensate for the additional complication in technique.Errors due to volatilisation of iodine can be overcome in suitable apparatus, and the methods ROW proposed eliminate the disadvantage of increased internal electrode resistance and sluggish potentiometer response on the low-potential side of the end-point, which may give rise to uncertainties of 50 mV or more with standard low resistance potentiometers and the calomel cell and salt bridge. To provide for this second possibility, titrations of iodine, standard or liberated, by thiosulphate, arsenite or antimonite are preferred to titrations of reductants by standard iodine delivered from the burette.Ion-combination indicator electrodes include pH electrodes (glass, antimony, arsenic, and so on), precipitation electrodes (silver), complexation electrodes (silver) and association electrodes (mercury). Provided there is no chemical interaction between reference indicator and reaction indicator systems, any of these electrodes can theoretically be used for reference purposes in electron transfer reactions. Glass, antimony and silver electrodes have beeit successfully applied to various titrations involving iodine. In all reactions, the glass electrode acts perfectly as a constant potential generator, and curves obtained with the glass reference indicator electrode are completely superimpsable, by movement of the potential ordinate, on the standard curves obtained with a saturated calomel half-cell and potassium sulphate salt bridge.Trouble may arise if the platinum reaction indicator electrode runs negative to glass, as the unscreened glass electrode is apt to give spurious potentials when the pH meter connections are reversed. Solutions with acid concentrations above normal can be titrated with the glass electrode, but this is not considered to be good practice, since most electrodes are subject not only to the acid error-March, 19531 SALT BRIDGE BY INDICATOR ELECTRODES 161 migration of H,O' ions-in this range, but also to short-term vagaries and to drifts from which recovery is slow. The behaviour of the silver reference indicator electrode varies with conditions.In group 1 reactions, its potential tends to follow that of the platinum reaction indicator electrode with a difference of some 670 mV (silver negative), so that it is polarised or acts after the 800 > E 600 *;: ai E 400 .: ld E 500 300 400 5. 5 t YI -. 7 -I In 3 .- 3 600 dicator electr dicator electrod 21 22 0.1 M cerate added, ml 24 Fig. 1. A, end-point curves for titration of iodide with permanganate; B, end-point curves for titration of iodide with cerate Reactions of group 1. fashion of an inert electrode. With powerful oxidants such as cerate and permanganate, the potential forms a peak exactly at the end-point (Fig. 1) and then returns towards its former value. This indication is reproducible and accurate, but the height attained by the peak may vary from 40 to 160 mV.A semblance of a normal form of end-point has been observed with iodate, but no indication a t all occurs with ferricyanide or chloramine-T. In reactions of groups 2 and 3, when an excess of iodide ions is present, the silver - silver iodide electrode is presumably set up (very quickly), and the silver reference indicator electrode then functions excellently as a constant potential generator, giving normal curves of the usual sharpness that are superimposable on the curves obtained by the standard salt bridge calomel cell method. Since the electrode circuit is all metal, its ohmic resistance is low and the potentiometer response is highly sensitive, except in reduced solutions, when the resistance rises and the response becomes sluggish.However, the uncertainty of measurement with the Tinsley potentiometer remains within 2 to 3 mV, which is much superior to the response with the salt bridge and calomel cell. No treatment such as scraping or activation is required between titrations, and the potentials are reprodiicible. The behaviour of the antimony reference indicator electrode is complicated because i t can exist in two forms: Sb/Sb"' (or Sb/Sb,O,) and Sb/Sb""' (or Sb/Sb,O,), with other possible forms of Sb"'/Sb""' and Sb/Sb,O,. If preserved in one state or other, it acts as a constant potential generator and a stable reference indicator electrode ; but in electron transfer reactions it is liable to change from one state to the other during the reaction. If converted to the higher valency form by oxidants, it does not retain this state on exposure152 BISHOP: REPLACEMENT OF STANDARD CELL AND [Vol.78 to air, but reverts, presumably by reaction between Sb and Sb""', to the lower valency form. The consumption of oxidant or reductant that must occur during conversion from one form to the other is small, being less than 0.01 ml of 0.1 N reagent for a fairly large electrode surface. In titration of iodine solutions by reductants no error is caused, as oxidant consumed by conversion to quinquivalency is repaid on reduction to tervalency. On moving from ter- to quinquivalency, the reference potential should rise, i.e. , the difference potential, platinum minus antimony, should fall as platinum is positive to antimony, whilst the converse 850 950 400 550 > < E E > I , E E 300 450 250 400 500 > x .- E +I c E C d .- +I h 400 350 450 E $ 800 900 3 .- m - .350 3 3 < 700 800 300 22 23 24 0.1 M ferricyanide added, mi 26 Fig. 2. A, end-point curves for titration of iodide with iodate; B, end-point curves for titration of iodide with ferricyanide should hold during the reverse motion. Initial equilibration of the antimony elect rode depends upon the titration and the previous treatment of the electrode. The scraped electrode reaches equilibrium more quickly, since the appropriate valency state is deposited immediately, but it is unnecessary to scrape or clean the electrode between titrations as it reaches equilibrium with time enough to record the correct end-point even in the fastest titration.As the normal state is the tervalent, equilibrium is immediately reached in reducing solutions, and the electrode gives a steady reference potential in the titration of bivalent tin, thiosulphate and tervalent arsenic and antimony with standard iodine solution. After the end-point is passed, the electrode is fairly slowly changed to the quinquivalent condition and the platinum- antimony potential falls off (Fig. 3). In oxidising solutions, the quinquipositive form is produced so that the electrode potential rises to equilibrium in the titration of iodine by thiosulphate , arsenious or antimonous solutions, steadies when the end-point is approached, falls correctly a t the end-point and becomes the steady reference terpositive value when the end-point is passed.In group 1 reactions, the electrode is brought up to the quinquivalent state and preserved therein, when it acts as a constant potential electrode. An anomalous effect in which the reference potential falls, causing a magnified potential change over the end-point , has occasionally been observed, particularly with chloramine-T. This is being further investigated. Again the negligible ohmic resistance of the electrode circuit gives a greatly increased sensitivity of potentiometer response over the calomel cell - salt bridge system, which reduces the uncertainty of measurement in reduced solutions to within 2 mV. As in precipitation reactions1 the reference indicator electrodes all show an increase in speed of equilibration over the calomel cell - salt bridge system.Reactions of group 1 are inherently slower than those of groups 2 and 3, and require 4 to 6 minutes for the potentials to reach equilibrium at the end-point with the calomel cell and salt bridge; the reference indicator electrodes require about half this time. With the platinum - glass electrode system measured on a pH meter, no change in sensitivity, i.e., cell chain resistance, is noticeable, Reactions of group 1.March, 19631 SALT BRIDGE BY INDICATOR ELECTRODES 153 as it is swamped by the high input impedance of the meter. With platinum - calomel, platinum - silver and platinum - antimony electrode systems metered on a low impedance potentiometer, the metering sensitivity suffers a sharp drop exactly a t the end-point in titration of iodine by reductants on passing from high to low potentials, and remains insensitive in the presence of excess of reductant.A sudden increase takes place in reverse titrations. In terms of galvanometer deflection with a Tinsley 50-ohm per volt potentiometer and standard I-metre taut-suspension mirror galvanometer at full sensitivity, the change is of the order +I00 440 600 500 - T - T l 500 -I .- -_ ... -x--x-x-Standard calomel cell and s a l t bridge - -Glass - reference indicator electrode -Antimony reference indicator - electrode -+-+-+-Silver reference indicator electrode I I_____ __ . -300 0 200 25 26 27 A 23 24 25 B 100 24 22 0-1 M iodine added, ml Fig. 3. Reactions of group 2. A, end-point curves for titration of tin" in tartrate buffer with iodine; B, end-point curves for titration of arsenite in acetate bufTer with iodine of 0-05 to 50 mV per mm with calomel cell and salt bridge, 0.01 to 3 mV per mm with thc silver reference indicator electrode and 0.01 to 2niV per nun with the antimony reference indicator electrode.Since this applies to tin, antimony and arsenic titrations as well as to thiosulphate, irreversibility of the electrode is not a likely explanation. A sudden high resistance appears to be generated in the cell chain; it is the more marked the higher the ohmic resistance of the cell chain. Indeed, this sudden change in resistance can be used to locate the end-point accurately, but is so sharp and sudden that little preliminary warning is given. A similar, but less marked, increase in resistance occurs with the silver and calomel and to a lesser degree with the antimony electrodes in group 1 reactions at, and after, the end-point.In group 2 or 3 reactions involving arsenic or antimony, it is necessary to buffer the solution at a fairly high pH. Buffering is also necessary when the glass electrode is used for reference indicator purposes at a pH value of more than 2. The potential jumps are less There is a recovery in sensitivity well past the end-point.[Vol. 78 in acetate than in bicarbonate buffer (Fig. 4) as the normal potentials of the SbO,”’/SbO,”‘ and AsO,” ’/AsO,‘” redox systems increase with decreasing pH. Thiosulphate cannot be used safely in bicarbonate buffer as some oxidation to sulphate takes place, but in acetate 154 BISHOP: REPLACEMENT OF STANDARD CELL AND 450 300 -50 400 450 A B 0.05 M antimonyl tartrate added. ml A, end-point curves for titration of iodine in acetate buffer with antimonyl tartrate; B, end-point curves for titration of iodine in bicarbonate buffer with antimonyl tartrate buffer, free acetic acid, dilute sulphuric and hydrochloric acids, tartrate buffer and tartrate - bicarbonate medium results are excellent.Selenium, cuprous , chromic, zinc , manganous and cerous ions, either free or complexed (e.g., as tartrate) are without influence; this allows determination of such materials as selenite, copper, hydrogen peroxide, sulphides, per- manganate , cerate , dichromate , iodate , bromate, ferricyanide and vanadate by back-titration methods with arsenite or thiosulphate. Where such reactions, e.g., selenite or dichromate , require acid concentrations higher than are acceptable to the glass electrode, the solution after liberation of iodine is treated with an excess of sodium acetate, or nearly neutralised with sodium hydroxide in the presence of tartaric acid; precipitation of potassium hydrogen tartrate, if potassium ion should be present in the system, does not interfere, although the abrading action of the stirred precipitate on the silver electrode causes momentary kicks in the platinum - silver potential. The use of carbon tetrachloride to remove free iodine in reactions of group 1 is advan- tageous with oxidants of lower normal potential, such as ferricyanide, iodate and chloramine-T, and leads to smoother curves; but, to be effective, the solvent must be thoroughly dispersed in the solution.The iodine solution SO formed is liable to cause surface corrosion and staining of the antimony electrode, which does not affect its action; but it seems desirable to clean the electrode after such an experiment. Fig. 4. Reactions of group 3. METHODS APPARATUS- The apparatus has previously been discussed.l A Marconi mains-operated pH meter was used for measurements with Marconi glass electrodes, and a Tinsley 50-ohm per volt precision vernier potentiometer for other electrodes. Platinum and silver wire and thin cast antimony rod, all glass sheathed, formed the metal electrodes and a saturated calomel half-cell with a salt bridge of saturated potassium sulphate was used as the standard reference electrode, Several electrodes were studied simultaneously by multiple potentiometers.A magnetic stirrer driven by an induction motor was found to be more satisfactory than the original arrangement1 and eliminated commutator interference and erratic speed variation. No interference from the magnetic fields, either on the electrodes or on the valve potentiometer, whose high input impedance renders it susceptible to interference pick-up, was observed. Magnetic rotors sealed in glass were used, as polythene is attacked by iodine. Times wereMarch, 19531 SALT BRIDGE BY INDICATOR ELECTRODES 155 taken by stop-watch, and equilibration was regarded as complete when the potential did not alter in 1 minute. REAGENTS- when possible, calibrated glassware being used. and various standard volumetric procedures.PROCEDURES AND RESULTS- results are discussed under each heading. REACTIONS OF GROUP 1: DIRECT TITRATION OF IODIDES- Permautganate-Dilute an aliquot of standard 0.1 M iodide solution to 120 ml, add 15 ml of 2 N sulphuric acid and titrate with standard ~errnanganate.~,~ The accuracy of Purified reagents were used throughout, and solutions were prepared by direct weighing Solutions were further checked by gravimetric Since the procedures are well known, only brief experimental details are given, and the Titrant Per man gan a te Cerate . . Iodate . . .. Ferricyanide . , TITRATION OF Amount taken 5.01 4-88 4-90 25.26 25.19t 24-97 50.00 49.66 49-20 5.01 5.22 4-90 22-88t 24-75 25.26 49.66 49.20 43.35 9.90 9.85t 19.80 20.2 1 22-88 49.20 49.66 4-88 5-01 4.90 24-75t 25.18 22.88 50-00 49-66 TABLE I IODIDES WITH Found by calomel cell and salt bridge 5.01 4.88 4.90 25-26 25.19 24.96 49.98 49-66 49.18 5.01 5.22 4.9 1 22-88 24.73 25-26 49.66 49.18 42.34 9.90 9-84 19.80 20.20 22.87 49.18 49-65 4.87 5-00 4-88 24.76 25-18 22.86 50.01 49.64 OXIDANTS* Found by reference indicator electrodes I A - Glass 5.01 4.88 4.90 26.26 25.19 24.96 49.99 49-67 49.19 5.01 5-22 4-91 22.88 24.74 25-26 49.66 49.19 42.35 9.90 9.85 19-80 20.21 22.87 49.19 49.66 4-87 5-00 4-88 24-76 25.18 22.86 50.02 49-63 Silver 5-01 4.89 4.90 25.26 25.19 24.96 49.99 49-67 49.19 5.02 5.22 4.92 22.88 24.75 25-27 49.68 49.19 42-35 9.95 - - I - - - - - - - - - I Antimony 5-00 4-88 4-90 25.26 25.19 24-95 49.98 49.66 49.18 5-00 5.22 4.89 22-88 24-74 25.26 49.66 49.19 42.36 - 9.85 19-80 20-19 22.86 49.18 49.67 - - - 24.76 25.19 22-86 50.01 - * Results expressed in milli-equivalents.f End-point curves of these titrations are shown in Figs. 1 and 2. the method has been extolled8 and proposed for the standardisation of permanganate solutions, Attainment of equilibrium near the end-point has been found to take rather longer than previously indicateds; about 4 minutes with calomel cell and salt bridge and 2 to 3 minutes with glass, silver and antimony reference indicator electrodes. At and after the end-point, each increment of permanganate causes a rise in potential; the potential reaches a maximum15G BISKOP : REPLACEMENT OF STANDARD CELL AND [Vol. 78 after 3 to 4 minutes, then as iodate forms the potential falls, reverting to its original value in a further 10 minutes.This has been found to continue long after the correct end-point has been passed if complete equilibrium is awaited after each drop. The results by the reference indicator methods are in excellent agreement with theory (Table I), and the curves with glass and antimony superimpose well on the standard calomel electrode curves. The silver curve is in the forni of a peak (Fig. 1). Cerate-Dilute an aliquot of standard iodide solution with water to 125 ml and titrate with standard 0.1 M sulphato-cerate solutiong in N sulphuric acid solution.10 Equilibrium TABLE I1 TITRATION OF REDUCTANTS WITH STANDARD IODINE SOLUTION* Amount Titration of taken Arsenite . . 0 . .. 5-23 6.23 4-98 24.17t 25.00 24.90 49-70 49-20 49.30 Antimony1 tartrate Thiosulphate .. 5.00 6-00 4.!)2 25.13 25.1 3 24.3;; 48.61 49-00 49.00 25.75 25-75 24.38 49.50 49.50 48-73 Stannous tin . . .. 5.63 5.63 25-04? 25.04 Found by calomel cell and salt bridge 5.23 5.23 4-98 24.17 25-00 24-86 49.68 49.2 1 49.2 1 5.00 5-0 1 4.92 25.12 25.1 1 24.35 48.59 49.01 49-01 25.76 25.76 24-38 49.47 49.45 48.73 5.6 1. 5 6 2 25.04 25.03 Found by reference indicator electrodes r Glass Silver Antimony 5.23 5.23 5.23 5.23 (5.23 5.24 4.98 4-98 4.98 24.17 24.17 24.17 25.00 25.00 25-00 21-89 24-88 24-90 49.68 49-68 49-68 49-20 49.2 1 49.2 1 49.21 49.21 49.22 A \ 5.00 5-02 4.92 25.13 25-12 25-35 48.60 49.00 49-01 6-00 5.01 4.92 25.12 26-12 25.36 48.60 49.00 49-01 5-00 5.01 4.93 25.12 25.12 25.34 48.61 49.0 1 49.00 25-75 26-76 25-78 25.75 26-76 25.76 24.38 24.38 24.38 49.48 49-48 49.48 49.47 -19.45 49.49 43-73 48.73 48.73 5.61 5.61 5.61 5.63 5.62 5.63 25-04 25.04 25.04 25.03 25.03 25.03 Q Results expressed in milli-equivalen ts.7 End-point curves of these titrations are shown in Fig. 3. is attained in 5 minutes at the end-point with calomel, and in 2 minutes with glass, antimony and silver electrodes. Owing to the increase in acidity of the solution during titration, the curves of the glass and antimony reference indicator electrode systems. diverge slightly from the standard calomel curve, but the end-points are excellent (Fig. 1) and the results accurate (Table I). The silver reference indicator electrode again gives a peak at the end-point. Iodate-Dilute an aliquot of standard 0.1 M iodide solution to 100 ml, add 10 ml of 2 N sulphuric acid and 10 ml of carbon tetrachloride, and titrate with 0.02 M iodate solution.llJ3 Equilibration speed was about the same as for permanganate.The potential break is much smaller than with permanganate or cerate. The curves with glass and antimony are excellent (Fig. 2) and the results satisfactory ('Fable I). The silver electrode is unsatisfactory. Chloramine-T-Titrations with chloramine-T will form the subject of a separate communication. Ferricyanide-To an aliquot of 0.1 M iodide solution add 75 ml of a solution 0-2 M in acetic acid and 0.2 M in sodium acetate (later referred to as 0.2 M acetate buffer), 30 ml of 0.2 M zinc sulphate solution and 10 ml of carbon tetrachloride; dilute to 160 ml and titrate with 0.1 M ferricyanide s01ution.l~March, 19531 SALT BRIDGE BY INDICATOR ELECTRODES 167 The electrodes can be cleaned by brief immersion in N sodium hydroxide solution and subsequent washing with water, acid and water. Equilibration was fairly rapid.The end- point is retarded in the absence of acetate buffer and organic solvent, and absorption of carbon dioxide by the unbuffered solution affects the glass and antimony electrode potentials. Silver gives a uniform potential without inflection, The curves are smooth (Fig. 2) and the results good (Table I). REACTIONS OF GROUP 2: DIRECT TITRATION OF REDUCTANTS WITH STANDARD IODINE SOLUTIOK- One acid concentration only has been investigated for each reductant. Add 75 ml of 0.2 M acetate buffer solution to an aliquot of 0-1 N antimony1 tartrate, arsenious or thiosulphate solution, dilute to 125ml with water, and titrate with standard TABLE I11 TXTRATION OF STANDARD IODINE WITH REDUCTANTS* Titrant Medium Antimony1 tartrate .. IJicarbonste buffer Acetate buffer Arsenite . . . . Bicarbonate buffer Acetate buffer Thiosulphate . . . . Acetate buffer Decinormal acid Amount taken 5.00 4.45 4-45 24.75.1 24.70 25.69 25.69 25.69 24-70t 48-66 45.99 45.99 25.69 24-70 24-70 48-66 48.66 5.00 5.00 4.45 25.69 26-69 24.75 4.45 4-45 5.00 24.56 24-56 25.69 24.50 24.50 25.69 45.99 45-99 Found by calomel cell and salt bridge 5.0 1 4.45 4-45 24-76 34.72 25.68 25-68 25-67 24.70 48-65 45.99 46-00 28-70 24.70 24.7 1 48-67 48-64 5.00 5.00 4.44 25-68 25.67 24-76 4-45 4.45 5.01 24-56 24.55 25.69 24.50 24.50 25.68 45.97 45-96 Found by reference indicator electrodes r -I A Glass 501 4.45 4.45 24-76 24.72 25-68 25.68 25.67 24.70 48.65 46.00 55-98 25.69 24-70 24.70 48-67 48-64 5-00 5.00 4-44 25.68 25.68 24-76 4.45 4.45 5.00 24.56 24.55 25.69 24-60 24.50 25.68 45-98 45.97 Silver 5.01 4-45 4.45 24.76 24.72 25-68 25.68 26-67 24.70 48.66 46.00 45.99 25-69 24.70 24.70 48-67 48-64 5.00 5-00 4.44 25.68 25.67 24.76 4-45 4.45 5.00 24.56 24.55 25.69 34-50 24-50 25.68 45.97 45.97 Antimony 5-01 4.45 4-45 24-76 24.72 25.68 26-67 25.67 24.70 48.64 45.98 45.99 26.69 24-70 24-7 1 48.67 48.65 5.00 6.00 4-44 25.68 25.68 34-76 4.45 4.46 5-00 24-56 24-55 25.70 24.50 24.50 25.68 45.98 45.96 * Results expressed in milli-equivalents.t End-point curves of these titrations are shown in Fig. 4.0.1 M iodine in 4 per cent. potassium iodide solution. For stannous tin (prepared in 2 N hydrochloric acid), add 5 g of tartaric acid, sufficient 5 N sodium hydroxide solution to neutralise the free acid and 5 g of sodium bicarbonate; dilute to 126 ml and titrate. The antimony, arsenic and thiosulphate curves are much alike, so only one example is shown (Fig. 3), but the lower normal potential of the stannous - stannic electrode leads to a much larger potential jump (Fig. 3). Equilibration was rapid with all electrodes. Electrodes on the low impedance potentiometer show a high resistance, which vanishes sharply at the end-point. With the antimony reference indicator electrode, conversion to quinquivalency158 BISHOP: REPLACEMENT OF STANDARD CELL AND [Vol.78 occurs in excess of iodine, causing the difference potential to decrease, so that the curve falls off after passing the end-point (Fig. 3). Results are shown in Table 11. REACTIONS OF GROUP 3 : TITRATIONS OF IODINE WITH REDUCTANTS- (i) Bicarbonate bufler-Dissolve 5 g of sodium bicarbonate in water, add an aliquot of standard 0.1 M iodine solution, dilute to 125 to 150 ml and titrate quickly with 0.1 N reductant (0.05 M arsenite or antimonyl tartrate, 0.1 M thiosulphate). (ii) Acetate bzt$er-Add an aliquot of standard iodine to 75 ml of 0.2 M acetate buffer, dilute to 125 to 150m1, and titrate quickly with standard reductant. (iii) Decinormal acid-Dilute 7.5 ml of 2 N sulphuric acid with water, add an aliquot of standard iodine, dilute to 125 to 150 ml and titrate quickly with standard thiosulphate.With arsenite and antimonyl tartrate, the reaction is incomplete in acid solution, and with thiosulphate partial oxidation to sulphate causes low results in bicarbonate medium. With antimony and arsenic the potential break at the end-point is less in acetate buffer TABLE IV TITRATION OF LIBERATED IODINE WITH THIOSULPHATE" Substance determined Cerate . . .. .. Iodate . . .. . . Dichromate . . .. .. Selenite . . .. .. Permanganate . . .. Ferricyanide . . .. Copper .. .. .. Anioun t taken 25.22 27.44 24.65 25.00 12.47 22.10 38.72 Found by calomel cell and salt bridge 25.22 27.44 24.66 25.01 12.48 22.08 38.72 Found by reference indicator electrodes A I \ Glass Silver Antimony 25.22 25.22 25.22 27.44 27.44 27.45 24.66 24-66 24.66 25.02 25.02 25.02 12-48 12.47 12.48 22.08 22.07 22-09 38.72 38.72 38.725 * Results, expressed in milli-equivalents, are means of three replications.than in bicarbonate buffer, as predicted, since the normal potential of antimonyl - antimonic and arsenious- arsenic electrodes rises with decreasing pH, but this is offset by a rather greater sharpness of end-point . The thiosulphate end-point shows little variation in deci- normal acid, acetate buffer, 0.5 N acetic acid and tartrate buffer. As usual, the potentiometer sensitivity with a 50-ohm per volt Tinsley potentiometer is low on the reduced side, but excellent in an excess of iodine, the transition being smooth and sharp. Equilibration was rapid with all electrodes. Glass and silver reference indicator electrodes give normal smooth curves, and antimony, being converted to the quinquivalent form before the end-point, does not attain complete equilibrium, bnt joins the other curves at the end-point, giving the usual sharp drop.Since the curves are all much alike, one example only is given (Fig. 4) ; this shows the difference between bicarbonate and acetate buffers for antimony. The results are excellent (Table 111). Many procedures are based on the liberation of iodine from an excess of iodide and titration of the excess with arsenite or thiosulphate. Some of these reactions, e.g., dichromate and selenite, require a high acid concentration for the initial stage, others, such as copper, permanganate, cerate and so on, will react completely at a pH of 1 or higher.In order to protect the glass electrode from excessive acidities, the liberation of iodine can be allowed to proceed in strong acid, and then the excess of acid is neutralised with alkali or removed with sodium acetate; if necessary in the presence of a complexing agent such as tartaric acid to prevent hydrolysis of metal ions with the formation of a precipitate that may adsorb iodine. Some csxmples are given below. Yema~zgnizate, cerate, iodate-Work in a volume of 125 ml that is 0.1 N in sulphuric acid, and titrate the liberated iodine immediately. Ferric-yanidc-Work in a volume of 125 ml, containing 30 ml of 0.2 M zinc sulphate and 75 ml of 0.2 M acetate buffer, and titrate the liberated iodine immediately. Dichronzate-Work in 2 N hydrochloric acid, set aside for 3 minutes, add 5 g of tartaric acid, dilute to 150 ml, add the calculated amount of 5 N sodium hydroxide to neutralise the mineral acid and half the tartaric acid, and titrate the liberated iodine immediately.Selenite-Work in 2 N hydrochloric acid, set aside for 2 minutes, add 8 g of sodium acetate, dilute to 150 ml and titrate the liberated iodine immediately. The solution mustMarch, 19531 SALT BRIDGE BY INDICATOR ELECTRODES 159 be vigorously stirred near the end-point to desorb iodine from the flocculated precipitate of selenium. Copper-Treat the sample in a volume of 125 ml containing 6 ml of glacial acetic acid with 5 g of potassium iodide in a little water and titrate immediately. The curves are all satisfactory and similar to the normal thiosulphate - iodine curves, which shows that there is no interference from the reaction products, i.e., no seleniding or copper plating of the silver or antimony; the results are excellent (Table IV). Individual graphs are not shown, as they are similar to those in Fig. 4. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. REFERENCES Bishop, E., Proceedings of the International Congress on Analytical Chemistry, Oxford, September, Lykken, L., and Tuemmler, F. D., I n d . Eng. Chem., Anal. Ed., 1942, 14, 67. David, V. W., and Daniels, P. H., private communication. Bates, R. G., Siegel, G. R., and Acree, S. F., J . Res Nut. BUY. Stand., 1943, 30, 347. Lundell, G. E. F., and Hoffman, J. I., “Outlines of Methods of Chemical Analysis.” John Wiley Hendrixson, W. S., J . Amcr. Chem. SOC., 1921, 43, 14. Kolthoff, I. M., Rec. Trav. Chirn., 1921, 40, 532. Kolthoff, I. M., and Furman, N. H., “Potentiometric Titrations,” John Wiley & Sons h e . , Smith, G. F., “Cerate Oxidimetry,” G. F. Smith Chemical Co., Columbus, 1942, p. 13. Willard, H. H., and Young, P., J . Amer. Cheun. Soc., 1928, 50, 1368. Kolthoff, I. M., Rec. Trav. Chinz., 1920, 39, 212. Hendrixson, W. S., .J. Amer. Chem. SOC., 1921, 43, 861. Miiller, E., 2. nnorg. Chem., 1924, 135, 265. 1952; Analyst, 1952, 77, 672. & Sons Inc., New Yorlr; Chapman & Hall Ltd., London, 1938, pp. 159-162. New York; Chapman & Hall Ltd., London, 1931, p. 259. UNIVERSITY OF DURHAM KING’S COLLEGE NEWCASTLE-ON-TYNE June 27th, 1952

ISSN:0003-2654    

DOI:10.1039/AN9537800149

出版商:RSC    

年代:1953

数据来源: RSC