ISSN:0003-2654    

DOI:10.1039/AN95075FX001

出版商:RSC    

年代:1950

数据来源: RSC

摘要:

2 FURNESS : THE DETERMINATION OF CHROMIUM IN CHROMITE [Vof. 75 The Determination of Chromium in Chromite* Part I. Volumetric Determination of Chromate with Special Reference to the Effects of Vanadate and Arsenate BY W. FURNESSt SyNopsIs-Various forms of the titrimetric determination of chromate by reduction with ferrous ammonium sulphate solution are considered with reference to their use as the final step in the determination of chromium in chromite. In the reduction of pure chromate with the ferrous solution and back-titration with permanganate solution, the use of various redox indicators (disulphine blue, barium diphenylamine sulphonate, N-phenyl anthranilic acid or ferroin), or no indicator at all, gave equally accurate results (mean error f0.03 per cent.). Vanadic acid, but not arsenic acid, interferes.This interference can be avoided by use of nitroferroin as indicator, with higher acidity in the solution. In the direct titration of chromate with ferrous ammonium sulphate solution (standardised against permanganate and this against sodium oxalate or arsenious oxide) positive errors, e.g., 0-3 per cent. were obtained. Satisfactory reproducibility may be obtained by reducing the chromate with excess of ferrous ammonium sulphate solution and titrating back with dichromate solution. To ensure accuracy in this procedure potassium dichromate must be taken as primary standard, and the conditions specified for titration in the assay must be reproduced in the standardisation. Vanadium, however, interferes. * During 1944 the Chrome Ore, Magnesite and Wolfram Control of the Ministry of Supply had occasion to review methods for determining the chromium content of chrome ores in common use at that time in this country.A small committee representing various interested Government Departments was formed by the Controller of Chemical Research and Development, Ministry of Supply, to explore the possibilities of co-ordinating the various methods of analysis in use. A memorandum was circulated to seven Government Departments and to fifty-three metallurgical firms, or firms of analysts, asking for their views, and as a result many descriptions of current methods (some of which showed considerable divergencies) and helpful suggestions were received. As there was clearly a certain amount of doubt about the simplest and most reliable procedure a t various stages, it was decided to undertake the investigation now reported.This and the following papers describe the contributions made by repre- sentatives (F. J. B., W. F.) of the Chemical Inspectorate of the Ministry of Supply, and by a representative (P. J. H.) of the Department of the Government Chemist. t Present address-Brotherton and Co., Ltd., Central Research Department, Kirkstall Lane, Leeds 5. These contributions are gratefully acknowledged.Jan., 19501 PART I. VOLUMETRIC DETERMINATION OF CHROMATE 3 INTRODUCTfoN-Methods proposed for quantitatively converting the chromium content of the mineral chromite into soluble chromate fall into two main groups- (i) Solzltion methods: dissolution of the sample in 70 per cent. perchloric acid, the same acid being used at the boiling-point as oxidant1; or dissolution and oxidation by means of a mixture of sulphuric and perchloric acids2; or dissolution in a mixture of sulphuric and phosphoric acids, perchloric acid being used as principal oxidant but followed by ~ermanganate.~ (ii) Fmion methods : fusion with alkaline oxidising fluxes based on sodium peroxide4v6s6 and extraction of the melt, followed by an auxiliary oxidation, e.g., with silver nitrate and persulphate in acid solution.4~5 Methods of the first group provide the only practicable means whereby chromite minerals can be dissolved completely in aqueous solvents.However, the use of perchloric acid as oxidant for chromium received a set-back in 1931 when Lundell, Hoffman and Bright7 pointed out that oxidation by that reagent alone was rarely more than 99.5 per cent.effective. This was later attributed to partial reduction of chromic acid by traces of hydrogen peroxide resulting from decomposition of the perchloric acid, but improvements in procedure since recommended by Smith and other^^>^+^ have largely off set this drawback. Nevertheless, methods depending on perchloric acid as oxidant are not commonly employed in this country for the assay of chrome ores. Methods of the second group are generally employed when only the chromium content of the ore is required. The best known procedures are those of Hillebrand and Lundell" and of Cunningham and M~Neill.~ In the former method chromium is separated from iron by filtration of the aqueous extract of the melt and the insoluble matter is returned for a second fusion; in the latter method a single fusion only is made and the melt extracted with dilute acid, so that chromium and iron are brought into solution together.Both authorities recommend an auxiliary process of oxidation to ensure complete conversion of chromium to the sexavalent condition, after which an excess of ferrous ion is added and the excess determined by titration with permanganate. The present investigations relate primarily to methods of the second group, and the results are being reported in three parts. In this, the first part, difficulties of the titration procedure are discussed with special reference to the choice of primary standards, satisfactory indicators, and the development of a procedure applicable in presence of vanadate and arsenate.There follows in Part I1 a study of methods for the determination of chromium in the melts obtained by fusion of mixtures of potassium dichromate with certain metallic oxides in alkaline fluxes containing sodium peroxide. Finally, in Part 111, a selected procedure for the analysis of chromite is described and tested by reference to the U.S. National Bureau of Standards Chrome Refractory No. 103. VOLUMETRIC PRIMARY STANDARDS-The following standard reagents, maintained at 18' to 20" C. throughout the investigation, were used. Potassium dichromate-Willard and Gibson1 observed that potassium dichromate of C.P. grade may contain excess of chromic acid. However, in the present investigation the chromate content of a specimen of AnalaR potassium dichromate was not significantly changed on re-crystallising twice from water.A solution standardised as follows was therefore accepted as one of the primary standards. The solution contained approximately 0.50 g. of re-crystallised potassium dichromate per 50ml. The actual weight of potassium dichromate delivered by a 50-ml. pipette at 20" C. was determined by emptying the pipette into a tared platinum dish, evaporating, and heating at 150" C. to constant weight. Three checks (0.5046 g., 0.5047 g., 0.5044 g., mean 0.5046 g., for example) showed what variation might be expected between successive deliveries from that pipette. Potassiztm permafiganate-The decinormal solution was standardised on alternate days, but no significant variation was detected during three weeks..Two primary standards were used, viz., dried AnalaR sodium oxalate (procedure of Fowler and Bright9), and dried AnalaR arsenious oxide (procedure of Metzler, Myers and Swiftlo). Ferrous ammonium sul$hate-The approximately decinormal solution of this salt in diluted sulphuric acid (1 in 10) was standardised afresh a t the time of use by the appropriate methods given below.4 FURNESS : THE DETERMINATIOS' OF CHROMIUM IN CHROMITE DETERMINATION OF CHROMATE BY REDUCTION WITH EXCESS OF FERROUS ION FOLLOWED BY BACK-TITRATION WITH PERMANGANATE [Vol. 75 In the absence of vanadate the chief indiciltors already available for this titration are: (a) the colour due to excess of permanganate; (b) disulphine blue, 0.1 per cent.aqueous solution; (c) barium diphenylamine sulphonate, 0.3 per cent. aqueous solution ; (d) N-phenyl anthranilic acid, 0.1 per cent. in 0-1 per cent. sodium carbonate solution; (e) 1 : 10-phenanthroline ferrous sulphate (Ferroin) solution, 0.025 N . The following tests were made with each indicator to simulate the conditions that would obtain in an actual analysis of chromite- 0.5046 g. of potassium dichromate, delivered by pipette into 500 ml. of 2 N sulphuric acid, was reduced with 100 ml. of approximately 0.106 N ferrous ion. Using each indicator in turn, and with 25 ml. of phosphoric acid in cases (a), (b) and (c), the excess of ferrous ion was titrated with standard decinormal permanganate. Concurrently, 100 ml. of the ferrous ion solution in 500 ml.of 2 N sulphuric acid were titrated with the decinormal permanganate, using the respective indicators and with phosphoric acid as before. Assuming that indicator blanks cancel, the results of duplicated titrations were calculated and are set out in Table I. TABLE I 0.5046 g. of potassium dichromate taken in each case Indicator a a b b c c d d e e Permanganate equivalent ml. 0.10103 N 104.90 - 3.05 = 101.85 104.90 - 3.00 = 101.90 104.75 - 2-87 = 101.88 104.75 - 2.95 = 101.80 105.08 - 3.28 = 101.80 105.05 - 3.26 = 101.79 104.63 - 2.74 = 101.89 104.65 - 2.78 = 101.87 104.65 - 2.78 = 101.87 104.65 - 2.80 = 101.85 Potassium dichromate found, g. 0.5046 0.5048 0.5047 0.5043 0.5043 0.5043 0.5048 0.5047 0-5047 0.5046 Error €5 0~0000 + 0.0002 + 0~0001 - 0*0003 - 0*0003 - 0*0003 + 0~0002 + 0~0001 + 0~0001 0~0000 The end-points with barium diphenylamine sulphonate (c) and ferroin (e) were the most easily distinguishable; With N-phenyl anthranilic acid (d) and disulphine blue (b) the end-points were less sharp.Without any added indicator ( a ) , the troublesome procedure of comparing the deep blue-green colour of the solution in the region of the end-point with that of another solution containing an equivalent amount of chromic sulphate had to be adopted. In each case the accuracy of the results is seen to be ver'y nearly as good as the precision with which the pipette delivered the standard dichromate solution. In other similar experiments 5 ml. of 0.100 N potassium arsenate were introduced before the addition of excess of ferrous ion.No interference ensued in the determination of dichroma te. In the presence of vanadate, however, interference was encountered. Vanadate is quanti- tatively reduced by ferrous ion to the vanadyl state and re-oxidation with decinormal permanganate is rather slow, especially towards the end of the reaction, if it is carried out in 2 N sulphuric acid at 20" C. Willard and Youngll showed that the oxidation of vanadyl ion proceeds more rapidly at elevated temperatures (40" to 50" C.), especially in solutions of higher pH such as may be obtained by buffering with sodium acetate. Oxidation is also accelerated in presence of phosphoric acid. However, no simple modification of procedure along these lines alone afforded results for chromate comparable in accuracy with those reported in Table I.Barium diphenylamine sulphonate and N-phenyl anthranilic acid are not applicable, of course, because in presence of vanadate both indicators assume their oxidised forms. Nor is disulphine blue satisfactory, fosr in the final stages of the back-titration there is a pronounced lag due to a slow reaction between the oxidised form of the indicator and vanadyl ion. To facilitate observation of the equivalence-point in such circumstances, Willard and Young (Zoc. cit.) used ferroin, but mentioned that this indicator suffers decomposition at temperatures above 50" C. All specimens of ferroin used by the present author showedJan., 19501 PART I. VOLUMETRIC DETERMINATION OF CHROMATE 5 distinct signs of decomposition a t 40" C .and observation of the colour change was consequently impaired. On the other hand, at temperatures below 40" C. the action of the indicator is sluggish, for in the conditions just preceding the equivalence-point, the oxidised form of the indicator is able to oxidise vanadyl ion only very slowly. The practicability of substituting 5-nitro-1 : 10-phenanthroline ferrous sulphate (Nitro- ferroin), an indicator of higher redox potential, for the determination of dichromate in the presence of vanadate has therefore been investigated. When introducing this indicator in 1934, Hammett, Walden and Edmonds12 observed that in M sulphuric acid solution it did not appear to be oxidised in presence of vanadic acid, but in titrations of ferrous ion with ceric ion it did not give a sharp end-point. This indicator was later used by Smith and Get213 in cerate oxidimetry for titrations of oxalate and arsenite, and has been further described by Smith and Ri~hter.1~916 Now the formal oxidation - reduction potentials of the systems MnO,'/Mn", VO,"'/VO", nitroferroin/nitroferroin in sulphuric acid solution depend on the concentration of this acid.The relative values of these formal potentials, and that of the ferric/ferrous system in sulphuric acid solutions up to 9 N concentration, are shown approximately in Fig. 1. Data for the indicator and for the vanadate/vanadyl system are taken from Smith and Richter.l*~~~ I .7 1.4 -J a 1.1 W 5 1.0 0.9 0.9 W a 0.7 I 2 3 4 5 6 7 8 9 NORMALITY OF SULPHURK AClO SOLUTION Fig. 1 From these data it may be expected that in a direct titration of ferrous ion with permanganate the precision and accuracy of the end-point as indicated by nitroferroin should improve as the normality of the sulphuric acid is raised.On the other hand, whilst nitro- ferroin is easier to oxidise at the higher acid concentrations, oxidation of vanadyl ion becomes increasingly more difficult until in 8 N or 9 N sulphuric acid the formal oxidation - reduction potentials of the two systems are approximately equal. In other words, when ferrous ion alone is being titrated with permanganate one may expect to observe the sharpest indicator reaction at the higher acidities, but if vanadyl ion has to be completely oxidised before nitro- ferroin changes appreciably to its oxidised form, lower acidities should be more favourable. The possibility of successfully titrating ferrous and vanadyl ions together, using nitroferroin as indicator, therefore rests on a satisfactory compromise between the two opposing factors.In order to arrive at such a compromise two series of titrations were carried out at 20" C. In each series 25.00 ml. of approximately 0-1 N ferrous ion in 300 ml. of sulphuric acid solution, which varied in concentration throughout the series from 1 N to 6 N , were titrated with standard decinormal permanganate, 0.1 ml. of 0.025 N nitroferroin being added as indicator. In one series, however, 1-00 ml. of 0.100 N ammonium vanadate solution was added; the other series contained no vanadate. Table I1 summarises the results.6 Normality of sulphuric acid FURNESS THE DETERMINATION OF CHROMIUM IN CHROMITE [Vol.75 TABLE :I1 Titration of 0.1 N ferrous ion in absence of vanadate (Note 1) ml. End-point 23.5-24-0 very indistinct 23.7-23.9 indistinct 23.85 fairly sharp 23.83 sharp 23.82 extremely sharp 23.83 extremely sharp Titration of 0.1 N ferrous ion in presence of 1.00 ml. 0-100 N vanadate r 1 ml. End-point 23.5-24.5 very indistinct 23.9 indistinct 23.84 fairly sharp 23.84 distinct (note 2) 23.82 distinct (note 3) 23-80 distinct (note 4) A 300 NOTE 1-Using ferroin as indicator in a control titration, 25.00 ml. of the ferrous ion solution in ml. of 2 N sulphuric acid required 23.82 ml. of the same permanganate solution. NOTE %In approaching the end-point the last 0.5 ml. of permanganate solution required 5 minutes NOTE 3-In approaching the end-point the last 0.5 ml.of permanganate solution required 8 minutes NOTE &In approaching the end-point the last 0.6 ml. of permanganate solution required 10 minutes to react. to react. to react. In absence of vanadium the sensitivity of the colour change (orange to pale grey-blue) in sulphuric acid of concentration 5 N or 6 N is at least as good as that of any other indicator so far investigated. On the other hand the oxidation of vanadyl ion is inconveniently slow in sulphuric acid of concentration greater than 4 N , but a good end-point, which coincides very closely with the true equivalence point, can be reached readily in 3 N or 4 N sulphuric acid. In most analyses of chromite samples, the amount of vanadium encountered will generally be less than the equivalent of 1 ml. of decinormali vanadate solution, and for such purposes it is suggested that nitroferroin will be most useful in a sulphuric acid solution of concentration 4 N .Further experiments, using nitroferroin as indicator, were made along the same lines as those which gave the results reported in Table I, except that in some instances ammonium vanadate was added to the potassium dichromate before reduction with excess of ferrous ion. The precise procedure was as follows- (i) Standardisation of ferrous ammonium sulphate sol&ion-l00 ml. of the solution, which was approximately 0.12 N , were transferred by pipette to a 1000-ml. flask, 400 ml. of 5 N sulphuric acid were added (so that the concentration of .sulphuric acid would finally be approximately 4 N ) and the solution was titrated with the standard decinormal permanganate until within about 5 ml. of the equivalence- point.Then 0.2 ml. of 0.025 N nitroferroin indicator was added and the titration continued to the end-point. (ii) Determination of potassizlm dichromate-0.5009 g. of potassium dichromate in solution was transferred by 50-ml. pipette to a 1000-ml. flask. Various volumes (from 0 to 2.0 ml. in different experiments) of 0.100 N ammonium vanadate were intro- duced, the solution was diluted to 1010 ml. with water and then 400 ml. of 5 N sulphuric acid were added. 100ml. of the ferrous ammonium sulphate solution were added, followed by 0.2 ml. of 0.025 N nitroferroin indicator, and the excess of ferrous ion was titrated with the standard decinormal permanganate solution.TABLE 111 0.5009 g. of potassium dichromate taken in each case 0.100 N vanadate added ml. nil nil 0-5 0.5 1.0 1.0 2.0 2.0 Permanganate equivalent ml. of 0.10120 N KMnO 118.50 - 17.56 = 100.94 118.50 - 17.60 = 100.90 118.50 - 17.58 = 100.92 118.60 - 17.58 = 100.92 118.46 - 17.51 =z 100.95 118.46 - 17.48 = 100.98 118.46 - 17.48 = 100.98 118.46 - 17.50 = 100.96 Potassium dichromate found 0.5009 0.5007 0.5008 0.5008 0.5009 0.501 1 0.50 1 1 0.5010 Error g. 0~0000 - 0.0002 - 0~0001 - 0~0001 0~0000 + 0.0002 + 0.0002 + 0*0001Jan., 19501 PART I. VOLUMETRIC DETERMINATION OF CHROMATE 7 The results are recorded in Table 111. In no case was there any undue delay in the final stages of the permanganate titration, and the colour change of the indicator, although superimposed on the deep blue-green colour of the chromic sulphate, was clearly observed and found to be very stable.The introduction of 5 ml. of 0.100 N potassium arsenate caused no interference. DETERMINATION OF CHROMATE BY DIRECT TITRATION WITH STANDARD FERROUS AMMONIUM SULPHATE SOLUTION Using an electrometric method of titration, Eppley and Vosburgh16 showed that under certain conditions potassium dichromate appeared to oxidise more than its equivalent of ferrous ion. Titrating 25 ml. of 0.1 N dichromate in 250.ml. of 2 M hydrochloric acid solution with 0.1 N ferrous sulphate standardised against permanganate, an excess of ferrous ion above the theoretical amounting to from 0.3 to 0-5 per cent. was required. Similarly, in the reverse titration of standard ferrous sulphate solution with dichromate, a 0.10000 N dichromate solution appeared to have a normality of 0.10037. Willard and Gibson, have also reported inconsistencies in electrometric titrations of dichromate with ferrous ion as the acidity and concentration of the dichromate solution were varied.The following experiments were therefore undertaken to decide whether inaccuracies might also arise in the direct titration of dichromate with ferrous ion standardised against permanganate, using the indicators (a) barium diphenylamine sulphonate, (b) N-phenyl anthranilic acid, (c) ferroin. In view of the results obtained the procedures are described in detail. (i) Standardisation of ferrous ammonium sulphate solution-50 ml. of decinormal permanganate, standardised by the two methods mentioned on p.3, were added to 100 ml. of 2 N sulphuric acid. The ferrous ammonium sulphate was immediately titrated into the acidified permanganate solution, and, when near the equivalence- point, in one series 10ml. of phosphoric acid (sp.gr. 1.75) and 2 drops of barium diphenylamine sulphonate indicator were added, and in the other series 2 drops of ferroin indicator alone were added. Titration was then continued to the end-point. The two series of titrations gave the values 0.10664 and 0.10662, the mean, 0.10663, being accepted for the normality of the ferrous ion solution. (ii) Determination of potassium dichromate-0.5046 g. of potassium dichromate was transferred by 50-ml. pipette ( c j .p. 3) to a flask containing 400 ml. of 2 N sulphuric acid. The dichromate was titrated with the freshly standardised ferrous ion soluti6n until within a few ml. of the equivalence-point. The appropriate indicator was added, together with 25 ml. of phosphoric acid in the case of indicator (a), and the titration continued to the end-point. The end-point was distinct with indicators (a) and (b), but ferroin was less satisfactory in this procedure. Single titrations only were made with each indicator, and the results, calculated on the assumption that indicator blanks in the standardisation cancel those in the determination, are recorded in Table IV opposite indicators a,, b,, c,. (iii) Repetition of titrations-On the next day the ferrous ion solution was re-standardised, on this occasion by titration with the standard decinormal permanganate, using ferroin and correcting for the indicator blank.The determination of dichromate was repeated with appropriate corrections for indicator blanks, and the figures shown in Table IV opposite indicators a,, b,, c, were obtained. The standardisation of ferrous ion solution and titrations of dichromate were again repeated. Now each indicator was prepared in the “just reduced” form, i.e., in separate small beakers the amount of indicator required was first oxidised with dichromate in dilute sulphuric acid solution, then just converted to its fully reduced form with 0-01 N ferrous sulphate before being added to the titration flask. The results are shown opposite a3, b,, c3 in Table IV.All the results shown in Table IV are high, the mean error being +O.OOl6g. Hardwick,,’ working independently, also observed under similar conditions a discrepancy of +O.OOlSg. in the titration of 06g. quantities of potassium dichromate with ferrous ammonium sulphate solution standardised against permanganate. It is considered that these errors are too large to be attributed entirely to manipulative or calibration errors.8 FURNESS: THE DETERMINATION OF CHROMIUM IN CHROMITE cvoi. 75 Amounting approximately to 0.3 per cent., they are of the same sign and magnitude as the error recorded by Eppley and Vosburgh. TABLE IV 0.5046 g. of potassium dichromate taken in each case Potassium Indicator Ferrous ion titration dichromate found 96.80 ml. 0-10663 N 97.03 ml.0.10663 N 96.94 ml. 0.10663 N 98-03 ml. 0.10529 N 98.00 ml. 0.10529 N 97-98 ml. 0.10529 N 97-95 ml. 0.10521 N 98.03 ml. 0.10521 N 98.08 ml. 0.10521 N 0.5061 0.5073 0.5069 0-5061 0.5060 0.5059 0.5053 0,5059 0.5060 Error g. + 0.0016 + 00027 + 0.0023 + 0.0015 + 0.0014 + 0.0013 + 0.0007 + 0.0013 + 0.0014 For the purposes of chrome ore analysis it i s possible that the error could be eliminated by standardising the ferrous ion against pure potassium dichromate. Such a procedure would not, however, be entirely free from objection, and until fresh evidence can be produced misgivings must arise in connection with this method for determining chromium. The presence of arsenate does not, apparently, influence the result obtained for chromate, but in the presence of vanadate the method is not applicable.DETERMINATION OF CHROMATE BY REDUCTION WITH EXCESS OF FERROUS ION FOLLOWED BY In applying this procedure to the analysis of chromite the purest available potassium dichromate is conveniently accepted as the primary standard. The ferrous ammonium sulphate solution is accordingly standardised by titration with a primary standard potassium dichromate solution. However, in view of the observations of Eppley and Vosburghls and in accordance with the suggestions of Willard arid Gibson,l it is desirable to arrange that the end-point in this titration shall be reached under conditions approximating closely to those existing in the solution derived from the chromite sample. Thus, besides using equal amounts of the same indicator, the volume of ferrous ammonium sulphate taken in the standardisation must be equal to that required in the assay, and the conditions of dilution, acid concentration and temperature must be specified for both.The final conditions have been specified by Hardwick in Part I1 of this investigation, but taking, for example, 0.5046 g. of potassium dichromate in 400 ml. of 2 N sulphuric acid and 25 ml. of phosphoric acid (spgr. 1.75), adding 100 ml. of approximately 0.105 N ferrous ammonium sulphate, then titrating back the excess with standard decinormal dichromate at 20" C., the relative mean deviation of this method for determining chromate does not exceed 0.04 per cent. when barium diphenylamine sulphonate is used as indicator. The colour change of N-phenyl anthranilic acid is less satisfactory, whilst the oxidation - reduction potentials of ferroin and nitroferroin are too high for use in this titration.Arsenate does not interfere in this procedure. Vanadium, however, remains quantita- tively in the quadrivalent state at the end-poin t indicated either by barium diphenylamine sulphonate or N-phenyl anthranilic acid, and the interference of vanadate could not be prevented by any variation of this procedure. BACK-TITRATION WITH DICHROMATE Thanks are expressed to Mr. S. H. Bales, Chemical Inspectorate, Ministry of Supply, for encouragement and advice during the prosecution of this work, and to Dr. P. J. Hardwick for a sample of 5-nitro-1 : 10-phenanthroline. Acknowledgment is also made to the Chief Scientist, Ministry of Supply, and to the Government Chemist for permission to publish this work. REFERENCES 1. Willard, H. H., and Gibson, R. C., Ind. Eng. Chem., Anal. Ed., 1931, 3, 88. 2. Smith, G. F., McVickers, L. D., and Sullivan, V. R., J . SOC. Chem. Ind., 1935, 54, 3 6 9 ~ . 3. Smith, G. F., and Getz, C. A., Ind. Eng. Chem., Anal. Ed., 1937, 9, 518. 4. Hillebrand, W. F., and Lundell, G. E. F., Ajbplied Inorganic Analysis, 1929, p. 408. 6. Cunningham, T. R., and McNeill, T. R., Ind. Eng. Chem., Anal. Ed., 1929, 1, 70.Jan., 19501 PART I. VOLUMETRIC DETERMINATION OF CHROMATE 9 6. 7. a. 9. 10. 11. 12. 13. 14. 15. 16. 17. Theobald, L. S., Analyst, 1942, 67, 287. Lundell, G. E. F., Hoffman, J. I., and Bright, H. A., Chemical Analysis of Iron and SteeE, 1931, Smith, G. F., and Getz, C . A., Ind. Eng. Chem., Anal. Ed., 1937, 9, 378. Fowler, R. M., and Bright, H. A., J . Research Nut. Bur. Standards, 1935, 15, 493. Metzler, D. E., Myers, R. J., and Swift, E. H., Ind. Eng. Chem., Anal. Ed., 1944, 16, 625. Willard, H. H., and Young, P., Ibid., 1934, 6, 48. Hammett, L. P., Walden, G. H., and Edmonds, S. M., J . Amer. Chem. SOC., 1934, 56, 1092. Smith, G. F., and Getz., C. A., Ind. Eng. Chem., Anal. Ed., 1938, 10, 304. Smith, G. F., and Richter, F. P., Ibid., 1944, 16, 580. perties and Applications to AnaZysis. Eppley, M., and Vosburgh, W. C . , J . Amer. Chem. SOC., 1922, 44, 2148. Hardwick, P. J. , Private communication. p. 298. -- , , Phenanthroline and Substituted Phenanthroline Indicators. Their Preparation, Pro- Issued by the G. F. Smith Chemical Co., 1944. CHEMICAL INSPECTORATE MINISTRY OF SUPPLY March, 1949

ISSN:0003-2654    

DOI:10.1039/AN950750002b

出版商:RSC    

年代:1950

数据来源: RSC

ISSN:0003-2654    

DOI:10.1039/AN95075BX003

出版商:RSC    

年代:1950

数据来源: RSC

ISSN:0003-2654    

DOI:10.1039/AN95075BP005

出版商:RSC    

年代:1950

数据来源: RSC

摘要:

Jan., 19501 The PART I. VOLUMETRIC DETERMINATION OF CHROMATE Determination of Chromium in Chromite 9 Part 11. Determination of Chromium in a Synthetic Sample of Known Composition BY P. J. HARDWICK SYNOPSIS-Four methods in common use for the determination of chromium in chromite were tested on a synthetic mixture of potassium dichromate and “mixed oxides” equivalent in composition to the mineral. All the methods involved fusion with sodium peroxide and removal of most of the excess of peroxide by boiling with water, and the chromate formed was titrated by adding excess of ferrous ammonium sulphate solution and titrating back with dichromate solution. Two of the methods gave satisfactory results (within 1 part in 2000 of the chromium present) after allowance was made for the small amount of vanadium present; they were selected for trial on a standard sample of chrome ore.The other two were more laborious and required amending for accurate work. FOLLOWING the study of titration methods for the determination of chromate described in Part I,l an examination has been made of four methods in common use in this country for determining chromium in chromite. In all four methods the ore is decomposed and chromium oxidised to chromate by fusion with sodium peroxide, usually in a nickel crucible. Most of the excess of peroxide is then destroyed by boiling with water. In three of the four methods the interference of residual peroxide is eliminated by an auxiliary oxidation in acid solution with ammonium persulphate and silver nitrate2s3 (Method 1) or potassium permanganate in small or large excess4~6 (Methods 2 and 3, respectively). In the fourth method the alkaline solution of peroxide is boiled for a longer time.The interference of manganese as pennan- ganate or manganese dioxide is eliminated in all cases by boiling with hydrochloric acid (for small amounts of manganese) or by filtration. In the present investigation the chromium as chromate was determined by addition of excess of ferrous ammonium sulphate solution and back-titration with standard potassium dichromate solution, using barium diphenvlamine sulphonate as indicator in presence of phosphoric acid. This titration method was chosen for its convenience and for the sharpness of the end-point. Also, provided the ferrous ammonium sulphate solution is standardised against potassium dichromate under the same conditions, particularly with regard to acidity, volume of solution and concentration of dichromate,6,7 the procedure is capable of yielding exact results.It is necessary, however, to correct for vanadium. The main object of the investigation was to test the accuracy of these methods, working with a known amount of chromium. For this purpose a synthetic sample was used containing in typical proportions all the principal constituents of chromite, all as oxides or carbonates except chromium, which was present as potassium dichromate. Unnecessary wastage of a standard sample of chromite was thereby avoided. Moreover, it was an advantage to start with all the chromium in the form of a soluble salt, corresponding with complete decomposition of the ore.10 HARDWICK THE DETERMINATION OF CHROMIUM I N CHROMITE [Vol.75 The results obtained showed that two of the methods (Methods 1 and 2) were suitable for accurate work. The other two methods required amending. EXPERIMENTAL PREPARATION OF A SYNTHETIC SAMPLE OF KNOWN COMPOSITION- The composition of the mineral chromite varies within wide limits, but as commonly purchased in this country it contains on average about 48 per cent. of chromic oxide. The remainder consists mainly of oxides of iron, aluminium, magnesium and silicon, and may include small amounts of calcium, barium, manganese, titanium, vanadium and nickel. For the purpose of this investigation a synthetic representative material (subsequently referred to as “mixed oxides”) was prepared consisting of an intimate mixture of ignited oxides and carbonates in the following proportion by weight: Fe,O, 16; A1,0, 15.5; MgO 14; SiO, 4; CaCO, 1; MnO, 0.5; BaCO,, TiO,, V20, and Ni203, each 0.25.Addition of an accurately known weight of K,Cr,O, to a weighed portion of the “mixed oxides” in the ratio 1-79 to 1 provided a sample of known composition containing all the principal con- stituents of an average chromite in typical proportions with a little K,O in addition. FUSION AND EXTRACTION PROCEDURE- The sample of “mixed oxides” and added potassium dichromate (total weight about 0.7g.) was fused with 5g. of sodium peroxides at dull red heat for 5 minutes in a nickel crucible of about 40-ml. capacity. On cooling, the crucible and lid were transferred to a 600-ml.beaker fitted with a cover glass, 100 ml. of water were added and the solution was boiled for 10 minutes to extract the chromate and decompose most of the excess peroxide. The crucible and lid were then rinsed with about 50ml. of water and removed. In preliminary experiments the melt was maintained at dull red heat for 15 minutes, but attack of the nickel by the peroxide was severe and crucibles of wall-thickness about 0.7 mm. could not be safely used for more than 2 or 3 fusions. Parallel experiments in which a mixture of sodium peroxide and sodium hydroxide (Theobald’s mixtureg) was used as flux showed reduced attack of the nickel and the crucible served for 6 fusions. In subsequent work with sodium peroxide, however, it was found that the time of fusion could be shortened to 5 minutes for complete decomposition of the sample.Under these conditions about 0.4 g. of nickel was dissolved during a fusion and the average life of a crucible was 5 or 6 fusions. Examination of the crucibles spectrographically showed that the nickel contained only a trace of chromium (less than 0.01 per cent.) and approximately 0-3 per cent. of manganese. No vanadium was detected. TREATMENT OF THE EXTRACT BY DIFFERENT METHODS- Method 1-Auxiliary oxidation with amnzonium persdphate and d v e r nitrate2 J-The extract was acidified with 120 ml. of diluted sulphuric acid (1 + 3), warmed to dissolve the precipitate and transferred to a 1000-ml. conical flask. After dilution with water to 500 ml. to decrease the sulphuric acid concentration to about 2 N , the auxiliary oxidation was carried out in the usual manner2 and the small amounts of permanganate and manganese dioxide present were destroyed by boiling with dilute hydrochloric acid.After cooling, a measured volume (100 ml.) of standard ferrous ammonium sulphate solution was added from a pipette and the small excess titrated back with 0.1 N potassium dichromate solution after addition of 25 ml. of phosphoric acid (sp.gr. 1-75) and 5 drops of a 0.3 per cent. solution of barium diphenylamine sulphonate in water as indicator. A t the end-point the indicator was sensitive to 0.02 ml. of 0.1 N potassium dichromate solution added to a total volume of about 600 ml. of solution. Small departures from standard practice2 were the higher initial concentration of hydro- chloric acid used (20 ml.of diluted hydrochloric acid (1 + 3) per 500 ml. of solution) and the longer time of boiling given (15 minutes) to destroy persistent traces of manganese dioxide and remove chlorine. Separate experiments in which known amounts of potassium dichromate were boiled with hydrochloric acid under similar conditions showed that no reduction of dichromate occurred. Method 2-Auxiliary oxidation with Permanganate in small excess-The extract was acidified and diluted to 500 ml. as in Method 1, and 1 to 2 ml. of approximately 0.1 N potassium perrnanganate were added to the hot solution to give a small excess, shown by theJan., 19501 PART 11. DETERMINATION OF CHROMIUM IN A SYNTHETIC SAMPLE 11 deepening amber colour.After boiling for 5 minutes, the excess of permanganate was destroyed by boiling with hydrochloric acid and the chromate titrated as in Method 1. Method 3-Auxiliary oxidation with Permanganate in large excess (VignaZ Method4+- The extract was acidified and diluted to 500 ml. as in Method 1. It was then heated to boiling and a 5 per cent. solution of potassium permanganate was slowly added until a brown precipitate appeared. Approximately 5 ml. of a 10 per cent. solution of manganous sulphate were added and the solution was boiled for 5 minutes, cooled and filtered by suction through a closely packed pad of asbestos (previously boiled with dilute nitric acid) or through a sintered glass crucible (Jena 3 G4). The precipitate of hydrated manganese dioxide was washed with hot water and the filtrate and washings were diluted to 500 ml.and titrated as in Method 1. Method 4-AZkaZine~ltrations-The extract was boiled for a further 5 minutes to destroy most of the remaining peroxide and the precipitate of hydroxides allowed to settle. The solution was then filtered through a hardened paper (Whatman No. 52, 12.5 cm.) a t room temperature to avoid reduction of the chromate, or through sintered glass, and the precipitate washed 8 to 10 times with 30-ml. quantities of water until the washings were colourless. The combined filtrate and washings were acidified with 120ml. of diluted sulphuric acid (1 + 3), cooled to room temperature and titrated as in Method 1. RESULTS The results obtained for the chromium content of samples containing a known weight of potassium dichromate added to 0.26g. of “mixed oxides” are summarised in Table I.The weight of potassium dichromate taken was checked by titration against standard ferrous ammonium sulphate solution of an equal amount of dichromate weighed out at the same time as the test sample. In Methods 1 and 2 the chromic oxide found was corrected for a small consistent “blank” with the “mixed oxides” amounting to 0-0004 g. of chromic oxide, which was chemically equivalent to the known amount of vanadium present (0.00125 g. V,05). TABLE I DETERMINATION OF CHROMIUM IN A SYNTHETIC SAMPLE BY DIFFERENT METHODS Method Test No. 2 3 4 1 1 2 3 4 5 6 7 8 1 2 3 4 1 2 1 2 3 4 Potassium dichromate added g. 0.4643 0-4643 0.4643 0.4643 0.4644 0.4644 0.4643 0.4643 0.4642 0-4642 0.4641 0.4641 0.4644 0.4644 0.4642 0.4642 0.4642 0.4642 Equivalent wt.of chromic oxide g. 0.2399 0-2399 0.2399 0-2399 0.2400 0-2400 0-2399 0-2399 0.2399 0-2399 0-2398 0-2398 0-2400 0-2400 0.2399 0.2399 0.2399 0-2399 Chromic oxide found Error g- 0.2398 0-2398 0.2399 0.2399 0.2399 0-2399 0,2399 0.2400 0.2399 0.2398 0.2399 0.2399 0.2436 0,2429 0.2394 0-2330 0-2353 0-2351 R- - 0~0001 - 0*0001 o*oooo o*oooo - 0~0001 - 0~0001 0~0000 + 0~0001 o*oooo - 0~0001 + 0~0001 + 0-0001 + 0-0036 + 0.0029 - 0.0005 - 0.0069 - 0.0046 - 0.0048 The results, calculated as chromic oxide, show satisfactory agreement with the weights taken in all the tests of Methods 1 and 2. The maximum error is about 1 in 2000 which is admissible in most analytical work.The results obtained by Method 3 are 1 to 2 per cent. too high. “Blanks” with the “mixed oxides” by this method were also high and variable. Separate experiments showed that the error probably arose through peptisation of the hydrated manganese dioxide precipitate; it was reduced by washing the precipitate with a 1 per cent. solution of sulphuric acid instead of water and was completely eliminated10 when a wash solution containing 5 per cent. of potash alum and 1 per cent. of sulphuric acid was used. Amended in this way the method gave satisfactory results but was more laborious than Methods 1 and 2.12 BRYANT AND HARDWICK: THE DETERMINATION OF CHROMIUM IN CHROMITE [VOl. 75 The results obtained by Method 4 are low. The error was found to be due mainly to retention of chromium by the bulky insoluble residue, consisting largely of nickel hydroxide.Separate experiments1O in which the sample was fused with Theobald’s mixture instead of sodium peroxide, to reduce the amount of nickel dissolved from the crucible, showed that the residue still retained a small amount of chromium (about 0.0005 g. of Cr,O,) even after repeated washing with hot water. By fusing the ignited residue with the flux, however, as in the method described by Hillebrand and Lundel12 using sodium peroxide , the chromium was almost completely recovered. The amended method, involving two fusions and extractions followed by an auxiliary oxidation with ammonium persulphate and silver nitrate in acid solution, and removal of traces of permanganate by boiling with hydrochloric acid, gave results comparable in accuracy with those obtained by Methods 1 and 2 but was laborious. Methods 1 and 2 were therefore considered the simplest and most reliable of the methods tested and were selected for further trial with a standard sample of chrome ore.ll I am indebted to the Government Chemist and to the Chief Scientist, Ministry of Supply, for permission to publish this paper. 1. 2. 3. 4. 6 . 6. 7. 8. 9. 10. 11. REFERENCES Furness, W., Analyst, 1960, 75, 2. Hillebrand, W. F., and Lundell, G. E. F., Applied Inorganic Analysis, 1929, pp. 408-415. Cunningham, T. R., and McNeill, T. R., Ind. Eng. Chem., Anal. Ed., 1929, 1, 70. Vignal, H., Bull. SOC. Chim., 1886, 45, 171. Lundell, G. E. F., Hoffman, J. I., and Bright, H. A., Chemical Analysis of Iron and Steel, 1931, Eppley, M., and Vosburgh, W. C., J . Amer. Cl’zem. SOC., 1922, 44, 2148. Willard, H. H., and Gibson, R. C., Ind. Eng. Chew., Anal. Ed., 1931, 3, 88. Mellor, J. W., and Thompson, H. V., Quantitative Inorganic Analysis, 1938, pp. 527-532. Theobald, L. S., Analyst, 1942, 67, 287. Furness, W., private communication. Bryant, F. J., and Hardwick, P. J., AnaZyst, 1950, 75, 12. p. 298. DEPARTMENT OF THE GOVERNMENT CHEMIST GOVERNMENT LABORATORY, W.C.2 28th September, 1949

ISSN:0003-2654    

DOI:10.1039/AN9507500009

出版商:RSC    

年代:1950

数据来源: RSC

摘要:

12 BRYANT AND HARDWICK: THE DETERMINATION OF CHROMIUM IN CHROMITE [VOl. 75 The Determination of Chromium in Chromite Part 111. Determination of Chromium in a Standard Sample of U.S. National Bureau of Standards Chrome Refractory No. 103 by Selected Methods BY F. J. BRYANT AND P. J. HARDWICK SYNoPsIs-The two selected methods, both involving fusion with sodium peroxide, were simple, rapid and capable of giving reproducible results. In the standard sample dried to constant weight a t 10.5' to 110OC. they indicated chromium equivalent to 36.96 and 37-03 per cent. of Cr,O, respec- tively. The higher results by the second method were attributed to incomplete elimination of chlorine from the decomposition of traces of manganese dioxide on boiling with dilute hydrochloric acid before the chromate titration.When the boiling was prolonged, the second method gave consistent values of 36-97 per cent. Allowing for the presence of 0.08 per cent. of vanadium in the sample the average results by the two methods were 36.92 and 36.93 compared with the certificate figure of 36.97 per cent. of Cr,O,. h T R o D u c T I o N - o f the four methods previously tested (Part 111) using a synthetic sample containing a known amount of chromium, two (Methods 1 and 2) gave satisfactory results after correction for the small amount of vanadium present and seemed suitable for routine or umpire determinations of chromium in chromite. This Part describes independent analyses of chrome refractory No. 103 of the U.S. National Bureau of Standards by these two methods.In both methods, any vanadium present reacts similarly to chromium, being reduced from Vv to VIV on the addition of ferrous ammonium sulphate and not re-oxidised in the back-titrationJail., 19501 PART 111. DETERMINATION OF CHROMIUM IN A STANDARD SAMPLE 13 with potassium dichromate. On the other hand, titration with potassium permanganate causes the re-oxidation of vanadium to Vv but the reaction is slow, and even under the optimum conditions described in Part I2 there is some sacrifice in the sharpness of the end- point. From information available it seemed that vanadium was unlikely to be present in more than small amounts in most ores. In view of the convenience of the dichromate titration and the satisfactory results obtained with the synthetic sample, it was decided to follow the procedure described in Part I1 and apply a correction for vanadium determined separately, if necessary, by a colorimetric method.EXPERIMENTAL The test sample was a finely ground standard sample of U.S. National Bureau of Standards Chrome Refractory No. 103, previously well mixed (sieving tests showed that all but a very small fraction of the sample passed a 200-mesh B.S. sieve, of aperture 0-0030 inch). Weighed portions of the sample were dried a t 105" to 110°C. to constant weight before test. The certificate value of the percentage of chromium in the sample was unknown to the analysts during the work. The volumetric techniques used by the two independent analysts varied slightly, and are described below. (1) In one case (F. J.B.) the strength of the ferrous ammonium sulphate solution was so adjusted that when 100ml. were added to the chromate solution the excess was of the order of 1 to 5 ml., which was determined by adding the standard dichromate solution from a 10-ml. burette. The standard dichromate solution was accordingly made slightly weaker than the ferrous ammonium sulphate solution and the standardisation performed by taking 100 ml. of the iron solution, adding 100 ml. of the dichromate solution and completing the titration from the 10-ml. burette. The actual concentrations selected, after initial experiments had shown the ore to contain approximately 25 per cent. of chromium, were 0.074 N dichromate and 0.075 N ferrous ammonium sulphate in 5 per cent. sulphuric acid. Potassium dichromate was taken as the primary standard and one calibrated 100-ml.pipette and one 10-ml. burette were used throughout. (2) In the other case (P. J. H.) the graduated apparatus was limited to a 50-ml. pipette and a 50-ml. burette, both previously re-calibrated. Ferrous ammonium sulphate -solution in 5 per cent. by volume sulphuric acid was freshly standardised against 0.1 N potassium dichromate solution both before and after each group of titrations. To minimise burette drainage errors, the ferrous ammonium sulphate solution was made slightly stronger than the dichromate solution and the main bulk of the titrant was added from the pipette. The titration was then ' completed by the addition of dichromate solution from the burette. Usually, on re-standardising the ferrous ammonium sulphate reagent after a period of about an hour, titrations agreed to within 0.02 ml. in a titration of about 50 ml.of 0.1 N dichromate. All reagents were of AnalaR quality and the potassium dichromate was re-crystallised and dried to constant weight a t 105" to 110" C. Spectrographic examination of the crucibles used for sodium peroxide fusions showed that the nickel contained negligible amounts of chromium and vanadium and approximately 0.3 per cent. of manganese. A "blank" with the reagents and a control determination with a known amount of potassium dichromate (added as standard dichromate solution to the alkaline extract of the fused peroxide) were carried out with each group of test determinations. METHODS AND RESULTS METHOD AUXILIARY OXIDATION WITH AMMONIUM PERSULPHATE- This method, which had given satisfactory results in previous work with the synthetic sample, seemed suitable for routine or umpire determinations of chromium in chromite.It is described in detail here for convenience of reference. An accurately weighed amount, approximately 0.5 g., of the well mixed, finely gtuund sample dried to constant weight a t 105" to 110°C. was transferred to a nickel crucible of about 40-ml. capacity and was thoroughly mixed with 5g. of sodium peroxide. A thin layer of peroxide was sprinkled over the surface of the mixture, the crucible was covered with a nickel lid and heated gently over the small flame of a: Bunsen burner for about 5 minutes until the contents were molten. Heating was then continued at dull redness for a further14 BRYANT AND HARDWICK: THE DETERMINATION OF CHROMIUM IN CHROMITE [Vol.75 5 minutes with occasional careful swirling of the contents of the crucible to ensure thorough mixing. On cooling, the crucible and lid were transferred to a 600-ml. beaker having a cover glass, 100 ml. of water were added and the solution was boiled for 10 minutes to decompose most of the peroxide. The crucible and lid were then thoroughly rinsed and removed, the alkaline extract was acidified with 120 ml. of diluted sulphuric acid (1 + 3), warmed to dissolve the precipitate, and transferred to a 1000-ml. conical flask. (In the first four tests the slightly turbid solution was filtered at this stage and the small residue ignited and examined for unattacked ore.The residue was found to consist mainly of manganese dioxide with a trace of chromium (0.0001 g. of Cr,O,) probably not completely washed from the filter paper). Water was added to increase the volume to 500 ml. and followed by 5 ml. of nitric acid (sp.gr. 1-42), 25 ml. of 1 per cent. silver nitrate solution and 5 g. of ammonium persulphate. After addition of one OT two silica chips to prevent bumping, the solution was heated to boiling and boiled vigorously for 15 minutes to destroy excess of persulphate. The solution was cooled slightly, 20 ml. of diluted hydrochloric acid (1 + 3) were added to destroy permanganate and vigorous boiling was continued for a further 16 minutes to expel chlorine. On cooling, a measured volume (100 ml.) of standard ferrous ammonium sulphate solution was added from a pipette and the excess titrated back with potassium dichromate solution after addition of 25 ml.of phosphoric acid (sp.gr. 1-75) and 5 drops of a 0.3 per cent. solution of barium diphenylamine sulphonate in water as indicator. The results obtained by both analysts are set out in detail in Table I. Tests 1 to 12 were carried out a t one laboratory (P. J. H.) and 13 to 20 at the other (F. J. B.). In each group of tests the “blank” was less than 0.00005 g. of Cr,O, (= 0.02 ml. of 0.1 N K,Cr,O,). All weights of Cr203, in this and subsequent tables, are expressed to the nearest 0.05 mg. TABLE I DETERMINATION OF CHROMIUM IN A STANDA:RD SAMPLE OF CHROME ORE BY METHOD 1 Test No. 1 2 3 4 5 6 7 8 Control 9 10 11 12 13 14 15 16 17 18 19 20 Control Control Control Control Control Wt.of air-dried sample g. 0.5238 0.5229 0.5238 0.5213 0.5220 0.5243 0.5232 0.5275 0.5140 0.5102 0.6146 0.5104 Loss on drying at 105” to lloo c. g. 0.0026 0.0026 0.0026 0-0026 0-0025 0-0025 0.0029 0.0029 0.0023 0-0023 0.0025 0.0025 Moisture 0-50 0.50 0.50 0.50 0-48 0.48 0.55 0.55 0.45 0.45 0.49 0-49 % Wft. of dried r-A-------, Total wt. Wt. of Cr,O, found sample taken 0.5205 0-5197 0. 5 20 2 0.5178 0.112730 0.5190 0.5180 0.11 2730 0.5203 0.5246 0. I2730 0.5117 0.5079 0.12740 0.5 12 1 0.5079 0- 112740 0*5000 0*5000 0.5000 0.5000 0*5000 0*5000 0.5000 0-Ei000 0. I. 875 9. in solution g. 0-19200 0.19170 0.1 9260 0-19110 0.12725 0.191 65 0.191 10 0.12735 0.19280 0.1 9390 0.12745 0-18935 0.1 8770 0.12740 0.18910 0.1 8765 0.1 2740 in of Cr,O, residue found g* g.0.00010 0.19210 0.00010 0.19180 0.00010 0-19270 0~00010 0*19120 0.00010 0.12735 0.19165 0.19110 0.12735 0.19280 0.1 9390 0.12745 0.18935 0-18770 0.1 2740 0.18910 0.1 8765 0.12 740 0*18480 0.18500 0.18470 0.1 8520 0.1 8475 0.18460 0.18460 0.18485 0.18760 cr*o, in dried sample yo found 36.91 36.91 37-04 36-93 36-93 36.90 37.06 36.96 37.00 36.96 36.93 36-95 36-96 37.00 36.94 37.04 36.95 36.92 36.92 36.97 Mean .. 36.96 The difference of 0.16 per cent. between t.he extreme values found for the percentage Cr20, in the dried sample was greater than expected in view of the consistent results obtained with the synthetic sample by this method and the sharp titration end-point, which was sensitive to within 0.02 ml. of 0.1 N potassium dichromate in a total titration volume ofJan., 19501 PART 111.DETERMINATION OF CHROMIUM I N A STANDARD SAMPLE 15 about 75 ml. in tests 1 to 12 and 95 ml. in tests 13 to 20. The control determinations, on the other hand, showed an excellent recovery of chromium in line with the precision of the titrations. The slight excess of chromium found in one instance (Control 7, 8) was probably due to traces of chlorine remaining after decomposition of permanganate (derived from the nickel 1 :rucible) with hydrochloric acid although the solution was boiled for 15 minutes before itration and the "blank" was satisfactory. Th: variation in the percentage of Cr,O, found in the test samples was probably not due to differences in moisture content as all samples were dried overnight a t the same temperature (105" to 110" C.) to constant weight.The percentage moisture found in the sample (0.5 &- 0.05 per cent.) varied slightly in tests made on different days. The average percentage of Cr,O, in the dried sample, corrected for the presence of 0.08 per cent. of vanadium" is 36.92 per cent., which is slightly lower than the certificate value of 36.97 per cent. of Cr,O, for chrome refractory No. 103. METHOD 2-hJXILIARY OXIDATION WITH PERMANGANATE I N SMALL EXCESS- Satisfactory results were obtained by this method in previous work (Part 11) with the synthetic sample. With the advantages of being somewhat shorter than Method 1 and requiring less expensive reagents, the method was considered more suitable for routine work. The procedure of fusion with sodium peroxide, subsequent boiling with water and acidi- fication with 120ml.of diluted sulphuric acid (1 + 3) was the same as in Method 1. After addition of 5 ml. of nitric acid (sp.gr. 1.42) and warming to dissolve the precipitate, the almost clear solution was transferred to a 1000-ml. conical flask and diluted to 500 ml. with water. 1 to 2 nil. of 0.1 N potassium permanganate solution were added to give an excess as shown by the deepening amber colour, and the solution was then boiled for 5 minutes. After cooling slightly, 20 ml. of diluted hydrochloric acid (1 + 3) were added and boiling continued for 15 minutes to destroy permanganate or manganese dioxide and remove traces of chlorine. Addition of one or two silica chips helped to maintain even boiling. On cooling, a measured volume (100 ml.) of standard ferrous ammonium sulphate solution was added from a pipette and the excess was titrated back with 0-1 N dichromate solution as described in Method 1.The results obtained are shown in Table 11. TABLE I1 DETERMINATION OF CHROMIUM IN A STANDARD SAMPLE OF CHROME ORE BY METHOD 2 Test No. 21 22 Blank Control 23 24 Blank Control 25 26 Blank Control Wt. of air-dried sample g. 0.5251 0.5229 0-5094 0.5138 0-5 163 0.5086 Loss on drying at 105" to 110" C. 6. 0.0029 0.0026 0.0025 0.0026 0*0025 0.0025 Moisture 0.65 0*60 % 0-49 0.49 0.48 0.49 Wt. of dried sample taken g. 0.5222 0.5203 0.1 2 730 0.5069 0.5113 0.12740 0.5138 0.5061 0.12740 Wt. of found g- 0.1 9335 0.19265 nil 0-12745 0.18780 0.1 8945 0.00030 0.12775 0.19020 0.18735 nil 0.12755 Mean Cr2OfJ Cr20, in dned sample yo found 37.02 37.03 37.05 37.05 37.01 37.02 37-03 The results obtained by this method are more consistent than those obtained by Method 1 and the maximum difference of 0.04 per cent.of Cr,O, is of the order expected from the precision of the titration. The mean percentage of Cr,O, in the sample, however, is 0.07 higher than the average of the twenty determinations by Method 1. Allowing for 0.08 per cent. of vanadium in the sample, the corrected figure of 36.99 per cent. of' Cr,O, is in close agreement with the certificate value of 36.97 per cent. A slight excess of chromium was found in control determinations by this method and in one instance (Blank 23,24) a small "blank" was recorded. These discrepancies were probably due to traces of chlorine remaining in the solution after boiling with dilute hydrochloric acid to destroy permanganate and suggested that the percentage of Cr,O, found in the test samples * 1 ml.of 0.1 N K,Cr,O, = 0.0051 g. of V = 0.00253 g. of Cr,O,.16 BRYANT AND HARDWICK: THE DETERMINATION OF CHROMIUM IN CHROMITE [VOl. 75 was slightly greater than the true value. The presence of chlorine in the final solution after 15 minutes’ boiling may have been due to the slow reaction of hydrochloric acid with the traces of manganese dioxide remaining after treatment of the boiled alkaline solution with dilute sulphuric acid. In Method 1, oxidaiion with ammonium persulphate and silver nitrate in the presence of nitric acid converted all the manganese to permanganate, giving a clear solution, especially noticeable with the “blank.” No difficulty was then experienced in removing traces of chlorine by boiling for 115 minutes with dilute hydrochloric acid in blank or control determinations except in one instance (Control 7, 8).In the present method it appeared necessary to boil for at least 15 minutes after complete reduction of permanganate and the disappearance of the last traces of turbidity due to manganese dioxide. This was confirmed by the results of the following series of tests (Table 111) in which, after addition of dilute hydrochloric acid, boiling was continued for 5 to 10 minutes until the solutions were quite clear and then for a further 15 minutes to expel traces of chlorine. Tests 27 to 30 were carried out at one laboratory (P.J. H.) and 31 to 34 at the other (F. J. B.). It is seen that satisfactory results were obtained in the control determinations, having regard to the limits of experimental error set by the sensitivity of the titration end-point (0.02 ml. of 0.1 N K,Cr,O, = 0.00005 g. of Cr,O,). Consistent results were obtained for the percentage of Cr,O, in the test sample but, as expected, the average was slightly lower than that previously found. After correcting for 0.08 per cent. of vanadium, the average percentage of Cr,O, was 36.93, which agrees with 36.92 per cent. found by Method 1 and compares reasonably well with the certificate value of 36.97 per cent. DETERMINATION Test No. 27 28 Blank Control 29 30 Blank Control 31 32 33 34 Blank Control Wt. of air-dried sample g. 0.5031 0.5022 0.5054 0.504 1 TABLE I11 OF CHROMIUM IN A STANDARD SAMPLE OF CHROME ORE BY MODIFIED METHOD 2 Loss on drying at 106” to 110” C. g. 0.0024 0.0023 0.0024 0.0023 Wt. of dried sample Moisture taken 0.48 0.5007 0.46 0.4999 %I g. 0.12730 0.48 0-5030 0.46 0-5018 0.12730 0.5000 0-5000 0.5000 0.5000 0.1875 Wt. of found g. 0.18500 0.18485 0.00005 0.12740 0.18585 0.18555 nil 0.12735 0-18475 0.18510 0- 18495 0.18485 0~00020 0.1 8750 Mean . . cr*os cr*o, in dried sample yo found 36.95 36.98 36-95 36-98 36.95 37.02 36-99 36.97 36.97 Acknowledgment is made by one of qs (F. J. B.) to Mr. E. Booth for experimental This paper is published by permission of the Chief Scientist, Ministry of Supply, and the assistance. Government Chemist. REFERENCES 1. 2. Hardwick, P. J., Analyst, 1950, 75, 9. Furness, W., Ibid., 1950, 75, 2. CHEMIST INSPECTORATE MINISTRY OF SUPPLY DEPARTMENT OF GOVERNMENT CHEMIST CLEMENT’S INN PASSAGE, LONDON, W.C.2 September, 1949

ISSN:0003-2654    

DOI:10.1039/AN9507500012

出版商:RSC    

年代:1950

数据来源: RSC

摘要:

Jan., 19501 GENTRY AND SHERRINGTON 17 Extraction and Photometric Estimation of Some Metals with 8-Hydroxyquinoline BY C. H. R. GENTRY AND L. G. SHERRINGTON SYNoPsIs-The pH conditions for the complete extraction of certain metals from aqueous solutions by means of a 1 per cent. solution of 8-hydroxy- quinoline in chloroform are reported. Metals investigated include aluminium, copper, ferric iron, manganese, molybdenum, nickel and stannic tin. For the photometric estimation of traces of stannic tin, extraction with 8-hydroxy- quinoline is put forward as a valuable addition to known methods. As a further example of the application of the results, brief mention is made of the separation of iron, nickel, cobalt, copper and manganese from very pure molybdenum trioxide. It is shown that the extraction of 8-hydroxy- quinolinates of the heavy metals is a useful method of purifying solutions of many reagents used in trace analysis. THE solubility in chloroform of the 8-hydroxyquinolinates of many metals can be of analytical importance in three respects : (a) as a means of separation of trace impurities prior to their determination by other methods, (b) as providing a medium for photometric or fluorimetric analysis, since the chloroform solutions are strongly coloured and some show a marked fluorescence, and (c) for the purification of reagents by extraction of the metallic impurities.The first systematic publication on the extraction of metals with a chloroform solution of 8-hydroxyquinoline, by Moeller,l described the use of a 0.01 M reagent solution with four extractions for each determination.Under these conditions the pH ranges for complete extraction were very narrow for some of the metals. By using a 1 per cent. (about 0.07 M ) reagent solution, the extraction of aluminium was made effectively complete in one operation over a wide pH range.2 The same conditions have been adopted by Westwood and Maye9 for the determination of cerium in cast iron. Lacroix: who investigated the properties of the oxinates of aluminium, gallium and indium, used a 0.1 M solution of reagent (about 1.45 per cent.). The work of this author is of special interest for the valuable theoretical treatment given. In the present work the 1 per cent. reagent concentration has been retained and the other conditions of extraction also follow those previously used for the extraction of aluminium, with the exception of a shorter time of shaking.The primary object of the investigation has been the establishment of the pH range for complete extraction of the several metals. As is to be expected, the results differ widely from those found by Moeller. Of special significance in the present work is the new method of photometric analysis offered by the extraction of stannic tin. In view of the paucity of good methods €or the estimation of traces of tin, special attention has been given to this finding. EXPERIMENTAL For each element a series of experiments was made at various pH values, extracting in each test 50 ml. of the aqueous solution, containing a fixed amount (50 or 200 pg.) of the element, with 10ml. of a 1 per cent.solution of 8-hydroxyquinoline in chloroform. After shaking for 1 minute, the chloroform layer was run off into a small flask containing about 1 g. of anhydrous sodium sulphate. A blank determination was made, omitting the element under investigation. The acid buffer solutions were made from sodium acetate solution to which either hydrochloric or acetic acid was added, and the alkaline buffer solutions were prepared from mixtures of sodium hydroxide and bicarbonate. In the determination of some of the metals 10 ml. of 10 per cent. sodium potassium tartrate was included in the buffer solution. In order to keep the blank value at a low level it was found necessary to purify the solutions of sodium acetate, sodium bicarbonate and sodium potassium tartrate by the method to be described later.The absorption of the chloroform solution was measured on the Spekker absorptiometer, using the mercury vapour lamp and a 1-cm. cell. The filter combination used was Ilford 601 AnalaR reagents were used where available.18 GENTRY AND SHERRINGTON : EXTRACTION AND PHOTOMETRIC [voi. 75 with Chance No. 8 (O.V.l), excepting for iron, for which Wratten 50 and Chance 0.B.2 were used. The choice of filters was in accordance with the transmission curves reported by Moeller. In the determinations, the pH of the aqueous solution was measured with the glass electrode immediately after the separation of the chloroform layer. (a) ALUMINIUM- The pH conditions for the extraction of aluminium in the absence of tartrate, which interferes, have been described previously.2 For the sake of completeness the graphical representation of the results is included in Fig.1. Replacement of the Wratten No. 2 filter, previously used, by the Ilford 601 filter, together with the Chance No. 8 filter gave a linear calibration curve. The deviation from linearity previously reported was due to the transmission of some red radiation by the filter pair then used. (b) COPPER- The results obtained with copper in the presence of tartrate are shown in Fig. 1. It can be seen that complete extraction of copper occurs in the pH range 2.8 to 14. Throughout this work the term “complete extraction” is used where at least 98 per cent. of the metal present has been extracted in one operation, as shown either by a second extraction or by comparison of the drum reading with results at other pH values.(c) IRON- Ferric iron in the presence of tartrate can be completily extracted in the pH range 2.5 to 12.5 (Fig. 1). With a double or triple extraction the iron could be completely extracted from even more acid solution, say at pH 2, a fact of value in the separation of iron from other heavy metals. (a) MANGANESE- On attempting the extraction of bivalent manganese in the presence of tartrate, it was found that, although extraction occurred in the alkaline range, the Spekker readings showed small but significant variations a t slightly differing pH values. I t was thought that these variations might be due to partial atmospheric oxidation of the manganese. Two series of experiments were therefore made, the first in presence of 5 ml.of 5 per cent. sodium sulphite solutions and the second in presence of 1 mg. of potassium ferricyanide. Under the latter oxidising conditions, complete extraction and constancy of drum readings were found in the pH range 7.2 to about 12.5. These conditions are therefore recommended for the photometric determination of manganese; but for the separation of manganese, the ferricyanide may be omitted . RESULTS (e) MOLYBDENUM- The colour of the molybdenum hydroxyquinolinate extract was not so intense as for the other metals, and it was necessary to use 200pg. of molybdenum in each experiment. Tartrate interfered with the extraction, but in its absence molybdenum could be completely extracted in the pH range 1.6 to 5.6.(f) NICKEL- range 4-5 to 9.5 (Fig. 1). The complete extraction of nickel in the presence of tartrate was possible in the pH (g) TIN- It was found that stannic tin could be ex.tracted from an acid solution, pH 2-5 to 5.5, to give a yellow chloroform layer. The transmission curve of the chloroform solution is shown in Fig. 2, from which the broad minimum at 3850 A. can be seen. This decided the choice of filters: Chance No. 8 and Ilford 601 transmitting the mercury 4078 and 4047 A. lines. By the treatment of a series of solutions containing different amounts of tin, all at pH 3.5, a calibration curve was obtained. This was linear over the range examined, i.e., up to 280pg., corresponding to a drum difference reading of 0.70. A 2-minute shaking period was used, and the extraction was made promptly after adjusting the pH of the solution,Jan., 19501 ESTIMATION OF SOME METALS WITH 8-HYDROXYQUINOLINE 19 as otherwise somewhat lower results were obtained.The presence of tartrate seriously interfered with the extraction of tin, but in the presence of 0.4g. of ammonium oxalate 95 per cent. of the 2OOpg. of tin present could be extracted in the pH range 5 to 6. MOLYBDENUM (Tart rate Absent) A .J I\ v - - 100 I RON (Tart r a t e Pr e sent) 0 W 'OO"-".- TIN (Tartrate Absent) 01 I00 z 0 F o V c x W SIOO $ 0 I00 0 I00 \ rq.0 1.5 I f 2i3 COPPER (Tartrate Present) LUMlNlUM Absent) V'I - - * - - a - - I$--, 12.5 ', MANGANESE \ \ (Tartrate and Ferri- \ \ cyanide Present) I 1 I 5 0 1 2 3 4 5 " 6 7 8 9 1011 12 I 3 1 4 P" Fig.1. Effect of pH on extraction 0 = No extraction x = Part extraction 0 = Complete extraction (h) OTHER METALS- Attempts to obtain conditions for the complete extraction of bismuth in one operation were unsuccessful, although partial extraction was possible in the pH range 5 to 12. Even in the absence of all tartrate and chloride, and using a bismuth sulphate solution and acetate buffers, the absorptiometer readings showed marked changes with pH.20 100' 90 80 70 $ 6 0 50 40 30 - z a t- 20 i$ 10. GENTRY AND SHERRINGTON : EXTRACTION AND PHOTOMETRIC [Vol. 75 0 l ~ ~ ' ' ~ " ' ~ " " ' ~ 350 360 370 380 390 400 40 420430440 450460 470 480490500 WAVELENGTH, mp Fig. 2. Transmission curve for tin 8-hydroxyquinolinate Cobalt, like manganese, gave peculiar results, thought to be due to oxidation by air in alkaline solution.Complete extraction of cobalt was possible in a pH range of about 5-7 to 96, but the drum readings showed variations larger than the expected experimental errors. Attempts to obtain constant readings under either oxidising or reducing conditions, as used for manganese, were not successful. In view of the analytical association of arsenic, antimony and tin, the extraction of the first two elements was investigated under the conditions used for tin. Arsenic did not iriterfere but antimony could be extracted from acetate solutions to some extent over the range for the complete extraction of tin. Tartrate partially suppressed the extraction of antimony, whilst the presence of oxalate prevented extraction from solutions with a pH greater than 5.0.DISCUSSION As was to be expected, with the present conditions, the pH ranges for complete extraction differ markedly from those reported by Moe1ler.l Where possible, tartrate has been added to the aqueous solution, to prevent precipitation of the hydrous oxides, which would otherwise occur. The presence of these hydrous oxides would considerably increase the extraction time necessary for the complete formation of the 8-hydroxyquinolinates. Molybdenum, tin and aluminium, however, cannot be extracted completely in the presence of tartrate. I t is nevertheless possible to remove all but a negligible amount of aluminium in the presence of tartrate from a solution at pH 9.5 by using a triple extraction. Whether tartrate is added in a particular application will depend on the con- ditions required.An example of the application of this method as a means of separating trace impurities is the determination of iron, nickel, cobalt, copper and manganese in very pure molybdenum trioxide. By double extraction with a chloroform solution of 8-hydroxyquinoline at pH 9 in the presence of tartrate the impurities from 5 g. of sample are separated from the molybdenum and concentrated into 10 ml. of chloi-oform solution. I t is then a comparatively simple matter to determine the impurities by standard photometric methods on a micro- scale. I t is possible by this means to determine the above-mentioned impurities in amounts down to 0.00002 per cent. in the molybdenum trioxide. For the direct photometric determination of most of the elements investigated, chloroform extraction of the 8-hydroxyquinolinate has no advantages over other available methods, particularly in view of its non-selective nature.An exception to this is the determination of aluminium by the masking of interfering elements, a procedure described in a previous paper. A further exception is the photometric determination of tin, which is not easily determined in trace amounts by known methods. The application of the 8-hydroxyquinoline method is dependent on the ready separation of tin from other elements by distillation of the chloride or bromide. An accurate photometric estimation of tin in the distillate can then be made by the procedure previously described. Conditions of distillation must beJan., 19501 ESTIMATION OF SOME METALS WITH 8-HYDROXYQUINOLINE 21 carefully controlled to prevent the interference of antimony, but otherwise the method is specific among common elements.This new method for the determination of tin has been applied to samples of tungsten and tantalum compounds with a considerable saving in time over the methods previously used. A third general application of the extraction of the 8-hydroxyquinolinates is found in the removal of heavy-metal impurities from analytical reagents. This method is generally applicable to soluble salts of ammonium, the alkali metals or the alkaline earths, provided that the pH of the solution falls within the desired range, preferably from 4-5 to 9.5, as can be seen by inspection of Fig. 1.The procedure adopted is to shake the solution of the salt with a 1 per cent. solution of 8-hydroxyquinoline in chloroform, run off the chloroform layer, and repeat the process until the organic layer is colourless. Excess of 8-hydroxyquinoline can be removed from the aqueous solution by several extractions with chloroform. Finally, the chloroform dissolved in the solution can be removed by bubbling air through the solution. Experiments have shown that this procedure will reduce the heavy metal content of solutions to a very low level and the method is very suitable for the preparation of chemicals used in trace analysis. CONCLUSIONS The work reported in this paper was originally undertaken as part of an investigation of analytical methods for the refractory metals, but the results are felt to be of more general interest. As a means of separation of trace impurities and as a method of reagent purification, chloroform extraction of 8-hydroxyquinolinates should have many applications. For the estimation of traces of tin, the direct photometric method using 8-hydroxyquinoline would seem to have some advantages over other methods and it should repay study. We wish to thank Mr. J. A. M. van Moll and the Directors of Philips Electrical Limited, for permission to publish this paper. REFERENCES 1. 2. 3. 4. Lacroix, S., Anal. Chim. Ada, 1947, 1, 260. PHILIPS ELECTRICAL LTD. Moeller, T., I n d . Eng. Chem., Anal. Ed., 1943, 15, 346. Gentry, C. H. R., and Sherrington, L. G., Analyst, 1946, 71, 432. Westwood, W., and Mayer, A., Ibid., 1948, 73, 275. MATERIAL RESEARCH LABORATORIES MITCHAM JUNCTION, SURREY May, 1949

ISSN:0003-2654    

DOI:10.1039/AN9507500017

出版商:RSC    

年代:1950

数据来源: RSC

摘要:

Jan., 19501 ESTIMATION OF SOME METALS WITH 8-HYDROXYQUINOLINE 21 A Method for the Dry Assay of Sulphides and Oxides of Lead and Bismuth BY J. A. SMYTHE SYNOPSIS-A rapid test tube method of dry assay of lead in galena is based on the observation that if a solution of galena in molten caustic potash is heated with stannous sulphide the lead is quantitatively reduced to metal and may be obtained as a button. The method is applicable also to the oxides of lead. Bismuth in its oxide or sulphide may be similarly assayed. The action of the stannous chloride is discussed: heated alone with malten caustic potash it is entirely oxidised to the stannic state with evolution of hydrogen. PELLETS of caustic potash melt at 170" C. If galena is present, it begins to dissolve-at once in the molten alkali and there is formed, with mineral of high grade, a clear solution, coloured more or less red according to whether the galena contains much or little iron.The solution contains lead oxide and potassium sulphide and addition of water to it reverses the reaction, the black sulphide being precipitated.22 When caustic potash and stannous sulphide are slowly heated together the alkali, as soon as it is melted, attacks the stannous sulphide and hydrogen is liberated, increasing in quantity as the temperature rises and producing at 180" to 190" C. vigorous effervescence. The melt is clear and yields a clear solution with water. On acidification, hydrated stannic oxide, with perhaps some stannic sulphide, is precipitated. No stannous compound is formed in this reaction.In presence of galena the reaction between the alkali and stannous sulphide is modified, in so far as the yield of hydrogen is greatly reduced ; at the same time metallic lead is separated quantitatively. This reaction is suggested here as the basis of a method for the assay of galena and it has also been found capable of extension to the oxides of lead and to the sulphide and oxide of bismuth. SMYTHE: A METHOD FOR THE DRY ASSAY OF SULPHIDES [Vol. 75 METHOD- The operation is carried out in a test tube and the composition of the charge found most suitable, having regard to accuracy of result, ease of manipulation and economy of time is: PbS 0-35 g., SnS 0.70 g. and KOH 4.0 g. The galena, crushed to pass a 60- or 80-mesh sieve, is shot into the test tube down a chute of glazed paper and the potash pellets are added and melted in a small flame, a roll of filter paper being placed in the tube to absorb moisture. When the galena has dissolved, or nearly so, the paper is removed and the charge of stannous sulphide added in 5 or 6 portions through a small funnel; the heating and swirling of the tube are so regulated that the froth, caused by the escape of hydrogen, does not rise more than about an inch up the tube, and particles of stannous sulphide, borne upwards on the froth, subside into the liquid.The lead soon separates as a spongy mass and the heating is increased gently until the melting-point of lead (327" C.) is reached. The metal then appears as a brilliant bead in a translucent liquid and the reaction is complete. After solidification the melt is dissolved in water, the muddy liquid decanted and the bead cleaned with a little dilute hydrochloric acid, dried on filter paper and weighed.The whole operation can be done in 5 to 10 minutes. If the tube should spfing a small leak after the lead has separated, but before complete melting, the sponge can be extracted, pressed between filter papers and heated with a small amount of potash, and the metal recovered without loss. It may be mentioned that it seems necessary, in order to get accurate results, that the galena should be dissolved, or nearly so, in the potash before the addition of the reducing agent. Early experiments in which the two were ground finely together gave results almost invariably 1 or 2 per cent.low. The method has been tested on three samples of galena: (a) a selected cleavage cube without visible impurity, (b) a high-grade concentrate from Sipton, Northumberland, and (c) a low-grade concentrate from Halkyn, Flintshire, containing much dirty gangue and blende. The dry assay by the ordinary crucible method was done on (b) and (c), and the determination of lead by wet methods on all three. The results, as percentages of lead, are given below. Those by the new method are from three consecutive trials and the means of these. New method, 76 Crucible Wet -------_7 Sample assay assay Mean - 85-0 85.1, 85.3, 84.6 85.0 82.8 83.0, 83.3, 82.3 82.9 79.8 57.5 61.3 61.2, 61.2, 60.9 61.1 Yo % (4 (4 (4 Some experiments have been made on the effect of adding to the charge sulphides, commonly associated with galena, that are easily reducible and yield metals soluble in lead at a relatively low temperature.These were added in such quantity as would give, on complete reduction and absorption by the lead, an alloy containing 3 per cent. of the metal. The effect of addition of orpiment was not reflected in the quantitative results and the bead was malleable and contained little more than a trace of arsenic. With stibnite somewhat more antimony was found in the bead, but its weight was not seriously affected. It is probable that these sulphides form sulpho-salts not easily reduced. With bismuth sulphide, however, there was complete reduction to bismuth, which was taken up by the lead. Applicatiort to oxides of lead-Peroxide of lead is "balled up" by the potash and appears to be but slightly soluble in it, but this does not affect its reducibility. The two other oxides are fairly soluble.The method used is the same as for galena, but it is advantageous toJan., 19501 AND OXIDES OF LEAD AND increase the quantity of stannous sulphide to 1 g. and mental results, given below as percentage of lead, show BISMUTH 23 that of potash to 5g. The experi- good agreement with those from the wet assay. PbO .. Pb,O, . . PbO, . . Application to sulfihide rather more easily than with of the metal (268” C.). The are given below. Bi,S, . . Bi,O, . . Wet assay % .. .. 89-8 . . .. 89.4 .. .. 86.6 and oxide of bismuth-The New method % 90.0, 90.1 89.6, 89.8 85.3, 85-0 reaction with these comDounds goes the compoinds of lead, being facilitated by the low melting-p&nt charge is as for galena.Some results, as percentages of bismuth, Wet assay New method % % .. .. 77.5 17.7, 17.6, 7 7 4 .. . . 89.5 89.0, 89.2, 89-2 The experimental results cited above attest the value of this reaction as a quantitative one. The obvious advantages lie in the economy of material and fuel and the great saving in time. Duplicate determinations of galena, for example, can be completed in 30 to 40 minutes, including the weighings, and the requisite skill for rapid and successful operation is easily acquired. Perhaps the only drawback with galena is that the silver cannot be determined in the bead. Metallic tin brings about the same reaction as stannous sulphide, but it cannot be used in its place, first because of slow reaction, and secondly because some tin is dissolved by the precipitated lead and protected thereby from reaction with the potash.A bead obtained in this way was found to contain 20 per cent. of tin. NOTE ON STANNOUS SULPHIDE- The material bought as stannous sulphide and used in these experiments is uniform, black, lustrous and well crystallised; it contains but little impurity. It is, however, not pure stannous sulphide, but a mixture, obviously a solid solution, of stannous and stannic sulphides. The analytical figures are: Sn (total) 74.4, S 24.5 (98.9), Sn (stannous) 57.3 per cent. From these the proximate composition is calculated as: SnS 73.5 and SnS, 26.5 per cent., or perhaps more accurately as: SnS 51.6 and Sn,S, 48-4 per cent.For the sake of simplicity the trade name stannous sulphide* has been used in the foregoing account, but it is desirable in what follows to distinguish it from pure stannous sulphide and it may be conveniently, if loosely, termed “tin sulphide.” Experiments on the liberation of hydrogen from tin sulphide and potash are informative and throw some light on the reactions involved in the method of assay. The volume of hydrogen equivalent to 1 g. of tin is calculated as 188 ml. at N.T.P. One gram of tin sulphide contains 0673 g. of stannous tin and should therefore yield 107.7 ml. of hydrogen. The volume actually observed was 107-5 ml., in excellent agreement with this. In presence of added PbS (the two ground together finely) the volume of hydrogen was greatly reduced, being 64.7 ml.from a 2 : 1 mixture and 19 ml. from a 1 : 1 mixture. If the mixing were of the intimacy inherent in a solid solution, then further diminution in the yield of hydrogen would be expected. A suitable solid solution for testing this was made by heating together lead and tin sulphide. The preparation contained a small amount of Pb - Sn alloy and its proximate composition was: PbS 48.9, SnS 45.6, Pb 4.3 and Sn 1.2 per cent.; total Pb 46.7 per cent., total Sn 37.1 per cent. This material with molten potash gave 23.5ml. of hydrogen per g. and 46.1 per cent. of Pb. Calculating from the hydrogen value, the amount of SnS destroyed by the alkali and hence the residue available for reduction can be found. This is equivalent to 47.1 per cent.of PbS, and would thus yield 40.8 per cent. of Pb. Adding the free lead to this gives the total reduced lead as 45.1 per cent., which is identical with the observed value. When the same solid solution was ground with its own weight of PbS and melted with alkali, not a trace of hydrogen was evolved and the yield of lead was 69.0 per cent. The whole of the tin was there employed in the reduction, the lead equivalent of which is 64.8. Adding to this the free lead gives the total yield as 69.1 per cent., a figure almost identical * This reagent was supplied by The British Drug Houses Limited, who will guarantee supplies to contain not less stannous sulphide than that used in these experiments.24 WILLMOTT AND RAYMOND: THE DETERMINATION OF SMALL [Vol. 75 with the observed value.These results have been substantiated by similar tests with another solid solution, richer in tin and poorer in lead, the details of which may be omitted. It is evident that the reducing action of stannous sulphide in these reactions is not brought about by hydrogen, but that the active agent is stannous oxide. This, in molten alkali, is oxidised by water; but when oxides of lead and bismuth are present, these act directly as oxidising agents. There is a further possibility that in some cases the stannous oxide suffers self-oxidation and reduction, yielding stannic oxide and tin and that the latter plays the part of the reducing agent. With tin sulphide, the presumed constituent of the sesqui-sulphide is resolved into stannous and stannic sulphides and the latter, being converted directly into stannic oxide, has no influence on the reduction process.NOTE ON BEHAVIOUR OF MOLYBDENUM SULPHIDE- Since the work described above was carried out, I have examined the reaction between MoS, and caustic potash. The molybdenum sulphide was prepared from molybdenite and contained 56.4 per cent. of molybdenum, equivalent to 94 per cent. of MoS,. ’ This yielded when melted with potash 127 ml. of hydrogen per gram, that is 135 mi. for the pure sulphide. This is close to the calculated yield of 139 ml., based on the assumption that MoS, is con- verted into MOO, which is then oxidised by water, as is SnO. When MoS, is fed into the melt of PbS and KOH, lead separates as a sponge in a sludge greatly thickened with molybdenum compounds. It seems impossible to melt this lead owing to the excessive frothing that takes place towards the end of the reaction. The sponge can, however, be recovered and melted into a bead with a little fresh caustic potash, but the yield is low, of the order 80 per cent. The reaction is therefore useless for quantitative work. It may be mentioned that FeS, does not yield hydrogen with caustic potash, nor is it capable of reducing PbS in the alkali melt. KING’S COLLEGE NEWCASTLE-ON-TYNE Febrtsary, 1949

ISSN:0003-2654    

DOI:10.1039/AN9507500021

出版商:RSC    

年代:1950

数据来源: RSC

摘要:

24 WILLMOTT AND RAYMOND: THE DETERMINATION OF SMALL [Vol. 75 The Determination of Small Quantities of Copper in Lead and Lead Alloys BY P. L. WILLMOTT AND F. J. RAYMOND SYNoPsIs-The method is based on the formation of the copper complex with sodium diethyldithiocarbamate in strongly ammoniacal solution, its extraction therefrom by ether and photometric reading of the ether extract. A 6-g. sample is dissolved in nitric acid and evaporated and, after addition of sulphuric acid, the major part of the tin, antimony and lead is filtered off. Interference by nickel and cobalt are avoided by addition of dimethylglyoxime to the neutralised filtrate and filtration. For complex formation the filtrate is treated with ammonia, ammonium citrate and the reagent. The ether extract is read in a Spekker absorptiometer.Bismuth interferes, producing a similar but much feebler colour. A correction for it is made by carrying out a similar determination in which the formation of the copper complex is prevented by prior addition of cyanide. Copper contents up to 0.01 per cent. are determinable to within 0.00005 per cent. By suitable choice of sample weights and dilutions the method may be adapted to copper contents up to 10 per cent. DURING the past year there has been an increasing demand for chemical lead and also lead pipe made to B.S. 1085. In order to meet the heavy demand, it was essential that the works refining processes were not delayed, and that results of chemical tests on the partially refined metal were reported as quickly as possible.For determining small quantities of copper there were two methods available. The method given by the British Standards Institution is a very lengthy process, and not suitable for routine works control. An alternative method is to electrolyse the copper from solution, so that it may be redissolved in a small volume and determined colorimetrically with ammonia.Jan., 19501 QUANTITIES OF COPPER IN LEAD AND LEAD ALLOYS 26 This method is also very lengthy, since it requires taking large quantities of the sample because of the rather insensitive colour reaction. Among the most sensitive reagents for copper is sodium diethyldithiocarbamate. A brown precipitate of the very slightly soluble copper carbamate is formed when an aqueous solution of sodium diethyldithiocarbamate is added to a very slightly acid, neutral or alkaline (ammoniacal) solution of a cupric salt.In very dilute solutions a colloidal suspension is formed which is suitable for colour comparison. Gum arabic, gum acacia, gum tragacanth or gelatin may be used as protective colloid to prevent the coagulation of the precipitate. The copper carbamate complex is soluble in a number of organic solvents, such as amyl alcohol, amyl acetate, bromo-benzene, chloroform or ethyl ether. The resulting brown solution may be used for the colorimetric determination of copper, either by comparison with standards similarly prepared or by measuring its light absorption. With many of the heavy metals sodium diethyldithiocarbamate forms precipitates, most of which are soluble to a greater or less extent in the organic soltents named.Zinc, cadmium, mercuric mercury, silver, lead and tin salts give white precipitates, whilst ferric iron yields a brown-black precipitate in neutral or acid solutions. However, in alkaline citrate solutions there is no reaction if the pH is maintained above 9. Bismuth yields a similar colour to that given by copper, although the colour produced is only about one-thirtieth to one-fiftieth as intense. Of the metals that interfere with the copper determination, nickel, cobalt and bismuth are the worst offenders. Zinc, cadmium, mercuric mercury, silver, lead and tin salts cause no interference after extraction because their solutions have 100 per cent. transmittancy. Iron is not likely to be found in large quantities in lead.The interference from nickeland cobalt is avoided by adding 1 ml. of 0-5 per cent. dimethyl glyoxime solution to thealkaline solution. The cobalt remains in the aqueous layer as an orange complex, which is not extracted with ether. Copper diethyl- dithiocarbamate is destroyed by dilute solutions of potassium cyanide, whilst the bismuth compound is not affected. Strong solutions of potassium cyanide, however, make the extrac- tion of the bismuth complex difficult, and may cause bismuth to revert to the aqueous phase if an ethereal solution of the complex is shaken with very concentrated potassium cyanide solution. Investigation by the authors has shown that less potassium cyanide is required to prevent the formation of the copper complex than is required to destroy the complex once it has been formed.Solutions of copper diethyldithiocarbamate in ether are very stable and undergo no change in intensity over a period of many months if kept in stoppered bottles to prevent evaporation of the ether. The procedure described is very accurate and detects as little as 1 pg. of copper. The total time taken in a determination is about 20 minutes, made up as follows: preparation of the sample, 1 rnin.; weighing the sample, 1 min.; solution of sample, 4 rnin.; precipitation and cooling, 4 min. ; filtering, 5 min. ; extraction and comparing colour, 5 min. In order to check the accuracy of the method, a solution of pure lead nitrate was prepared and different amounts of pure copper nitrate were added. The following table shows the results obtained.The nickel is precipitated and removed by centrifuging or filtering. The interference from bismuth is best overcome by a difference method. Copper added, Copper found, % on amount of lead present yo on amount of lead present 0.00005 0~00010 0.00015 0.00050 0.00 100 0.00350 0.00440 0.0 1000 0.03000 0.00005 0~00010 0-000 15 0-00050 0-00095 0-00355 0-00440 0-01000 0-03050 The synthetic samples contained lead nitrate equivalent to 5 g. of lead. A further series of tests was performed on synthetic samples containing different amounts of bismuth, and in every test the result found was within 0.00005 per cent. of the true figure when calculated on the amount of lead present.26 WILLMOTT AND RAYMOND: THE DETERMINATION OF SMALL METHOD SOLUTIONS REQUIRED- (1) Sulphuric acid, 50 per cent. v/v.[vol. 75 (2) Ammonium citrate solution. (3) Potassium cyanide solution in water, 0.5 g. per 100 ml. (4) Sodium diethyldithiocarbamate solution in water, 0.5 g. per 100 ml. Dissolve 250g. of citric acid in 250ml. of water. Cool and add 250ml. of aqueous ammonia, sp.gr. 04380. PROCEDURE- Weigh two separate 5-g. portions of the sample and place each in a 300-ml. tall-form beaker. Dissolve egch in 15 ml. of water and 7 ml. of nitric acid, sp.gr. 1.42. The sample will dissolve rapidly if in the form of thin rollings. When it has dissolved, boil down to small volume and then take up in the minimum quantity of water. This treatment will eliminate the bulk of the tin and antimony from solution. Add 10 ml. of 50 per cent.sulphuric acid and cool thoroughly. Filter off the lead sulphate through a fairly fine paper; a Whatman No. 530 is recommended. Wash well with cold water until free from acid. (If the presence of nickel or cobalt be suspected, add also 1 ml. of arnmoniacal 0-5 per cent. dimethylglyoxime solution. Allow to stand 5 minutes and then filter through a Whatman No. 530 paper. In absence of nickel and cobalt this treatment can be omitted.) To one of the two test solutions add a 10-ml. excess of aqueous ammonia (sp.gr. 0.880), 10 ml. of ammonium citrate solution (Reagent 2) and 10 ml. of sodium diethyldithiocarbamate solution (Reagent 4). Rinse out the beaker with 25 ml. of ether and add the rinsings to the solution in the funnel. Shake vigorously under running water for about 1 minute and allow the two layers to separate.(The lower, aqueous, layer should be colourless, and should remain so when a few ml. of the sodium diet hyldit hiocarbamat e solution are added.) Collect the coloured ethereal layer in a 100-ml. graduated measuring flask. Rinse out the funnel with ether saturated with reagents and add the rinsings to the main solution in the flask. Shake to mix the extract and determine the transmittancy as a Drum Reading 1, on the Spekker absorptiometer, with a water-to-ether setting of 1.00. For copper contents below 0.003 per cent. (calculated on sample) use an Ilford violet filter No. 601 and the large 4-cm. cells. For copper contents between 0.002 and 0.010 per cent. use the I-cm. cells and the same filter. The 1-cm. cells with an Ilford blue-green filter No.603 should be used for copper contents between 0.01 and 0.03 per cent. The Drum Reading 1 is a measure of copper plus bismuth, and in order to obtain the true copper figure a difference method is employed. To the other test, which has been left standing after the filtrate has been neutralised with ammonia as described above, add 1 ml. of 0.5 per cent. potassium cyanide (Reagent 3) for each 0.01 per cent. or part of 0-01 per cent. of copper plus bismuth found from the first uncorrected test. Add a 10-ml. excess of aqueous ammonia (sp.gr. 0*880), 10 ml. of the ammonium citrate solution and 10 ml. of the sodium diethyldithiocarbamate and continue as before. This is due to bismuth alone. The drum difference due to copper is equal to Drum Reading 1 minus Drum Reading 2.Standard graphs are prepared from pure standard copper nitrate solution and drum differences plotted against per cent. of copper (on 5 g. sample). Neutralise the filtrate with ammonia, using litmus as indicator. Cool and transfer to a 200-ml. separating funnel. Dilute to the 100-ml. mark with ether saturated with reagents. Determine the transmittancy as Drum Reading 2. ABBREVIATED METHOD- For routine testing the test may be modified. Weigh only one sample of 5 g. and proceed Determine the trans- Pour the ethereal solution back into the separating funnel Shake Allow the two layers to separate and discard the aqueous layer. Determine The difference in drum reading as in the method given above to determine copper plus bismuth. mittancy as Drum Reading 1.and add about 30 ml. of water and 5 ml. of 0.5 per cent. potassium cyanide solution. for 1 minute. the transmittancy of the ethereal layer as Drum Reading 2. is that due to copper, and the percentage may be read off the appropriate graph.Jan., 19501 NOTES- When the solution is neutralised with ammonia a precipitate may form. This is due to antimony and is caused by insufficient boiling down. The precipitate should be filtered off through a Whatman No. 541 paper. The quantities of the ammonia, citrate and carbamate prescribed should be adhered to. Excess of sodium diethyldithiocarbamate causes a white precipitate to form which does not dissolve in the ether. It does not effect the test except by making separation of the two layers difficult. If very small quantities of copper are present, an initial weight of 10 g. should be taken and the extract made up to only 25 ml. In this case, if the graphs used are based on 5-g. samples, the result must be divided by 8. High copper contents can be determined by the above procedure if smaller weights of sample are taken. For a 10 per cent. copper-lead master alloy it is recommended that 0.5 g. be taken and dissolved, the lead precipitated and filtered off, and the filtrate made up to 500 ml. and 10 ml. (equivalent to 0.01 g. of sample) taken for the determination. QUANTITIES OF COPPER IN LEAD AND LEAD ALLOYS 27 The authors thank the management of the Millwall Branch of the Associated Lead Manufacturers, in whose laboratories this work was carried out, for permission to publish this paper. 136 SIBLEY GROVE MANOR PARK LONDON, E.12 March, 1949

ISSN:0003-2654    

DOI:10.1039/AN9507500024

出版商:RSC    

年代:1950

数据来源: RSC

摘要:

Jan., 19501 QUANTITIES OF COPPER IN LEAD AND LEAD ALLOYS 27 Gravimetric Determination of Copper, Iron, Aluminium and Titanium with N-Benzoylphenylhydroxylamine BY SUDHIR CHANDRA SHOME* SYNoPsIs-N-benzoylphenylhydroxylamine, which can be easily prepared, and preserved indefinitely, has been used successfully for the gravimetric determination of copper, iron, aluminium and titanium. Copper, iron and aluminium can be determined by weighing the precipitate directly and the presence of appreciable amounts of other metals like beryllium, cobalt, cadmium, manganese, nickel, uranium (sexivalent) and zinc does not interfere with the procedure. The precipitation of iron is interfered with by the presence of aluminium and chromium. I t is possible to determine copper in presence of phosphoric acid, but the separation of iron and aluminium from phosphoric acid is not possible.In the determination of titanium with benzoylphen ylhydroxylamine, the precipitate should be ignited to oxide before weighing. Titanium has been separated from aluminium but iron and phosphoric acid interfere with the precipitation of the metal. IN an attempt to improve upon certain defects of cupferron, the ammonium salt of N-nitroso- phenylhydroxylamine, as an analytical reagent, a series of allied organic compounds were examined by the auth0r.l It was observed that N-benzoylphenylhydroxylamine (C,H,-N-OH) \ CO--C,H, unlike cupferron (free acid) (CGH5-N-ONHJ \ possessed the following useful properties : (i) the compound was stable towards heat, light and air and could be preserved indefinitely, (ii) it was soluble in hot water, (iii) the precipitates * Present address, Corrosion Section, Department of Metallurgy, Cambridge University.28 SUDHIR CHANDRA SHOME GRAVIMETRIC DETERMINATION OF COPPER, [Vd.75 formed with metal ions became granular on heating and (iv) the precipitates were not con- taminated with the organic reagent when formed in hot solution and therefore were directly weighable. The object of the present investigation was to study the use of this new organic reagent in the determination of copper, iron, aluminium and titanium. EXPERIMENTAL PREPARATION AND PROPERTIES OF BENZOYLPHENYLHYDROXYLAMINE- Benzoylphenylhydroxylamine was prepared by a slight modification of the method of Bamberger.2 Phenylhydroxylamine (30 g.) was dissolved in warm water (1200 ml.) and the solution was filtered.The filtrate was cooled, a little sodium hydrogen carbonate was added and benzoyl chloride (45 g.) was then added drop by drop while the phenylhydroxylamine solution was stirred vigorously. About 30g. of sodium hydrogen carbonate were added in small quantities at a time to keep the mixture faintly alkaline. The stirring was continued for 90 minutes and the resulting solid (a mixture of monobenzoyl- and dibenzoyl-phenyl- hydroxylamine) was filtered and washed with water. The solid was then triturated with 10 per cent. sodium hydrogen carbonate solution in a porcelain mortar for half an hour, filtered and washed with water. By this treatment the entrapped drops of benzoyl chloride were removed.The monobenzoyl derivative was separated from the dibenzoyl derivative by treating the white mixture with aqueous ammonia (sp.gr. 0-SS), in which only the mono- benzoyl compound dissolved. The solution was filtered and the filtrate was added to a slight excess of dilute sulphuric acid cooled with ice and salt. The monobenzoylphenyl- hydroxylamine that separated out was then filtered and further purified by recrystallisation from alcohol. ' Benzoylphenylhydroxylamine is very slightly soluble in cold water but soluble in hot water to the extent of about 0.5 per cent.; it is soluble in alcohol, benzene, ether, acetic acid and aqueous ammonia solution. The compound is stable towards heat, light and air. I t is very weakly acidic in character and no ammonium salt is formed when gaseous ammonia is passed through its solution in ether.Its melting-point is 121" to 122" C. REACTIONS WITH THE METAL IONS- Bamberger2 reported that benzoylphenylhydroxylamine gave a green precipitate with copper, and a red coloration with iron salt solutions. He did not study the reactions of the compound with the other metal ions. In the present investigation it was observed that benzoylphenylhydroxylamine formed precipitates with the following ions in weakly acidic or weakly alkaline medium: Pd", Pt"", Zr"", Pb', Hg", Hg', Ag', UO,", Ce"", Mn", Cr'**, Fe'", Fe", Cu", Al"', Ti"", Sn"", Sn", Th"", Ni", Co", Zn", Cd", VO,', WO,',' and MOO,". Copper, iron and aluminium ions were precipitated separately from hot aqueous solution by adding an alcoholic solution of the organic reagent in slight excess.The precipitates were thoroughly washed with hot water and dried at 110" C. Analysis of these pure com- plexes indicated that their compositions were as follows : CU(C,H~~O~N)~, Fe(C,,HloO,N), and Al(Cl,Hlo02N),. The copper, iron and aluminium complexes were green, red and white and their melting- (also decomposition-) points were 198" to 199" C., 187" to 188" C. and 238" to 239" C. respectively. The complexes were decomposed by moderately concentrated mineral acids. The yellow titanium complex was prepared from the cold solution and purified from excess of reagent by crystallisation from alcohol. Analysis of the titanium complex showed that it was not of any definite composition. The titanium complex was decomposed in presence of considerable amounts of mineral acids.All these metallic com- plexes were soluble in organic solvents. Copper and iron complexes were slightly soluble in 50 per cent. alcohol, whilst those of aluminium and titanium were more soluble. REAGENTS USED IN THE DETERMINATION OF THE METALS- (1) Metals to be determined-Standard solutions of copper sulphate, ferric alum, potash alum and titanium sulphate were prepared separately by the usual methods. A standard solution of copper chloride was used when copper was to be determined in presence of lead and mercury. (2) Foreign ions-In the study of the effect of different ions on the precipitation of the metals to be determined, alkali salts were used for the solutions of the anions, and chlorides, nitrates and sulphates were used for the solutions of the cations.Jan., 19501 IRON, ALUMINI WM AND TITANIUM WITH N-BENZOYLPBENYLHYDROXYLAMINE 29 (3) BenzoyZphenyZhydroxyZarnine soZzltiout-An alcoholic solution of the organic reagent was used for the precipitation of the metal ions.PH Adjustment-The pH of the solutions was usually adjusted by adding requisite quantities of 10 per cent. solution of sodium acetate and 1.08 N sulphuric acid. Glacial acetic acid was used instead of sulphuric acid when copper was determined in presence of lead and mercury. In the determination of titanium, the standard solution was first neutralised with 6 N ammonia solution and then acidified with the necessary amounts of concentrated hydrochloric acid (d. 1.15). A11 the chemicals used were of A.K.quality. DETERMINATION OF COPPER, IRON AND ALUMINIUM PROCEDURE- Take a known quantity of copper, iron or aluminium solution containing about 0.025 g. of copper, 0-015 g. of iron or 0.01 g. of aluminium and add 5 ml. of 1.08 N sulphuric acid solution (in the determination of iron in ferric alum, the addition of this acid is not necessary when the ferric alum solution contains the same amount of sulphuric acid). Dilute the solution with distilled water to about 400 ml. and heat to boiling. Dissolve benzoylphenylhydroxyl- amine (one and three-quarter times the theoretical quantity) in alcohol (15 to 20ml.), warm the solution and add to the hot solution of the metal. Add 10 ml. of 10 per cent. sodium acetate solution to raisk the pH of the solution to about 4-0.Occasionally stir the precipitate formed and heat it on the boiling water-bath for 1 to 2 hours (I hour for copper precipitate and 2 hours for iron and aluminium precipitates). Filter the precipitate on a No. 4 sintered glass crucible and keep the filtrate for subsequent pH measurement by means of a glass electrode. Wash the precipitate thoroughly with hot water and dry it at 110" C. to constant weight. Calculate the metal content on the basis that the precipitate contains 13.03 per cent. of copper, 8.064 per cent. of iron or 4.064 per cent. of aluminium. Notes-(@) Iron and aluminium precipitates tend to form hard lumps when precipitated above 70°C. and hence the precipitation is carried out at about 65°C. There should be good stirring when the flocculent precipitate is changed to the granular forh by heating on the water-bath.(b) During the precipitation of the metals, the benzoylphenylhydroxylamine solution is not allowed to fall on the sides of the beaker, otherwise the organic reagent would be deposited on the sides owing to the evaporation of alcohol and necessitate the use of a large volume of wash-water. (c) In the precipitation of aluminium, the alcohol in the final volume of solution should be not more than 5 per cent., because the aluminium complex is moderately soluble in alcohol. (d) The aluminium complex is very slightly soluble in hot water at 90" C. but is insoluble in boiling water containing a small amount of reagent. The aluminium precipitate is washed with warm water at about 45" C. RESULTS- The results of determinations of copper, iron and aluminium are shown in Table I.They indicate that these metals can be determined by weighing their complexes directly. EFFECT OF pH ON THE PRECIPITATION OF COPPER, IRON AND ALUMINIUM- The precipitation of copper, iron and aluminium was quantitative between the pH values of 3.6 to 6.0, 3.0 to 5.5 and 3.6 to 6.4 respectively. The metals were not precipitated completely when the pH of the solution was lower than these ranges and at higher pH values slightly high results were obtained. EFFECT OF FOREIGN IONS ON THE PRECIPITATION OF COPPER, IRON AND ALUMINIUM- (a) The presence of anions-Copper was determined in presence of phosphoric, arsenic and arsenious acids by means of benzoylphenylhydroxylamine. A small amount (5 ml.of 10 per cent. solution) of Rochelle salt was added to the copper solution in ordel: to prevent precipitation of copper phosphate, copper arsenate or copper arsenite when the pH of the solution was raised to about 4-6 by adding sodium acetate solution. The results are given in Table 11.30 SUDHIR CHANDRA SHOME : GRAVIMETRIC DETERMINATION OF COPPER, [Vol. 75 Attempts to separate iron and aluminium from phosphoric acid with the help of benzoyl- phenylh ydroxylamine were not successful. Copper, iron and aluminium were determined separately in presence of large amounts of tartrate ion. Benzoylphenylhydroxylamine gave precipitates with vanadate, molybdate and tungstate ions in acid medium and therefore copper, iron and aluminium could not be separated from these ions.TABLE I DETERMINATION OF COPPER, IRON AND ALUMINIUM WITH BENZOYLPHENYLHYDROXYLAMINE pH of the solution == 3.9 to 4.0 Metal taken g* 0.03156 (CU) 0.02777 97 0.02651 )) 0.02525 97 0.01728 (Fe) 0.01656 79 0.01690 35 0.01440 97 0-01021 (Al) 0.00899 ) 9 0*00817 9> 0.00817 7 9 Wt. of ppt. g. 0.2426 0.2136 0.2037 0.1935 0.2148 0.2045 0.1964 0-1780 0.2506 0.2220 0.2014 0.1999 Metal foun g. 0.03 161 0.02783 0.02655 0,02522 0-01732 0.01 649 0.01584 0.01435 0.0 101 8 0.00902 0.008 18 0.00812 Id Error g- + 0.00005 + 0.00006 + 0.00004 + 0.00004 - 0.00003 - 0.00007 - 0.00006 -0*00005 - 0.00003 + 0.00003 + 0~00001 - 0.00005 (b) TIze 9resence of catiom-Preliminary experiments revealed that cobalt, cadmium, lead, mercuryn, manganese, nickel, uraniumv1, and zinc did not form any precipitate with benzoylphenylhydroxylamine at pH 4.0.Copper, iron and aluminium were, therefore, TABLE I1 pH of the solution = 4-5 to 4.7 g. g. g. g. €5 0.02525 0.16 (P20s) 0.1930 0.02515 - 0*00010 79 0.02 n 0.1938 0.02525 0~00000 97 0.10 (Ago6) 0.1942 + 0-00005 ?9 0.10 (AqO,) 0.1943 0.02531 + 0.00006 DETERMINATION OF COPPER IN PRESENCE OF PHOSPHORIC, ARSENIC AND ARSENIOUS ACIDS Copper taken Acid added wt. of ppt. Copper found Error 0.02530 determinable in presence of the above-mentioned ions. Results are recorded in Table 111, and show that copper, iron and aluminium can be separated from many other different metal ions. Benzoylphenylhydroxylamine precipitated tin, titanium and zirconium ions in acid solution. Copper, iron and aluminium could not be determined in presence of each other or in presence of any of the three above-mentioned ions by means of this organic reagent.The precipitation of iron with benzoylphenylhydroxylamine was interfered with by the presence of chromic ion. DETERMINATION OF TITANIUM PROCEDURE- Take a known quantity of the titanium sulphate solution containing about 0.1 g. of TiO, and add distilled water to make the volume about 400ml. Neutralise the solution with ammonia solution and add 5 ml. of concentrated hydrochloric acid. Precipitate titanium by adding slowly a 10 per cent. alcoholic solution of benzoylphenylhydroxylamine (about double the theoretical quantity) to the clear solution of the metal, with constant stirring. Allow the precipitate to stand for 45 minutes with occasional stirring, filter and wash with dilute hydrochloric acid containing the organic reagent (to prepare the wash-solution, add 10ml.of benzoylphenylhydroxylamine solution to 1 litre of warm distilled water, cool and mix with 3 ml. of concentrated hydrochloric acid). Ignite the precipitate carefully in a platinum crucible to constant weight and weigh as titanium dioxide. Notes-(a) A gummy substance is formed when titanium is precipitated from a warm solution and hence the titanium solution is kept below 25" C. for precipitation.Jan., 19.501 IRON, ALUMINIUM AND TITANIUM WITH N-BENZOYLPHENYLHYDHOXYLAMINE 31 TABLE I11 DETERMINATION OF COPPER, IRON AND ALUMINIUM IN PRESENCE OF pH of the solution = 3-9 to 4.1 FOREIGN METAL IONS Metal taken g. 0.02967 (CU) 0.02525 99 99 99 99 99 99 tt 99 97 0.03156 97 99 9) 79 99 0.01440 (Fe) 99 99 99 9Y 99 99 97 99 0.00817 (Al) 99 99 93 9s Y Y t? 99 77 99 79 Foreign metal added €5 0.08 (Pb) ” (Hg) 0.01 (Be) 0.12 (CO) 0.16 (Cd) 0.08 (Zn) 0.06 (Mn) 0-12 (Ni) 0.16 (U) 0.14 (CO) 0.17 (Ni) 0.28 (Mn) 0.24 (U) 0.12 (Zn) 0.02 (Be) 39 (Ni) 3% (Mn) $9 (Zn) 99 (Co) ” (U) Wt.of ppt. g. 0.2280 0.2271 0.1939 0.1930 0.1939 0.1944 0.2427 0.2434 0.2421 0.1791 0.1784 0.1 783 0.177% 0,1793 0.2008 0.2000 0.1999 0.20 18 0*2000 0.2004 Metal found g. 0-02970 0.02959 0.02526 0.02515 0.02526 0.02533 0-03161 0.0317 1 0.03154 0.01444 0.01438 0.0 143 7 0.01434 0.01445 0*00816 0.00813 0.00812 0.00820 0.00813 0.008 14 Error €5 + 0*00003 - 0.00008 + 0~00001 - 0*00010 + 0~00001 + 0-00008 + 0*00005 + 0.000 16 - 0~00002 + 0.00004 - 0~00002 - 0.00003 - 0.00006 + 0.00006 - 0~00001 - 0.90004 - 0.00005 - 0.00004 - 0.00003 + 0*00003 (b) The titanium precipitate is moderately soluble in alcohol and therefore the alcohol in the final solution should not be more than 5 per cent.RESULTS- It was found that this determination was possible when concentrated hydrochloric acid up to 20 ml. was added; the precipitation of titanium was incomplete if more acid was used. The results of the determination of titanium are shown in Table IV. TABLE IV DETERMINATION OF TITANIUM WITH BENZOYLPHENYLHYDROXYLAMINE Titanium dioxide Titanium dioxide taken found Error g* g. g. 0.1082 0.1081 - 0*0001 0-0773 0.077 1 - 0~0002 0.0742 0.0744 + 0.0002 0.0618 0.0616 - 0.0002 0.0464 0.0460 - 0.0004 EFFECT OF FOREIGN IONS- Table V.was determined in presence of large amounts of tartrate ion. from iron or phosphate ion was not possible. Results of determinations of titanium in presence of aluminium ion are recorded in The metal The separation of titanium It is seen that titanium can be determined in presence of aluminium. TABLE V DETERMINATION OF TITANIUM IN THE PRESENCE OF ALUMINIUM Titanium dioxide taken g. 0.1082 0-1082 0.0773 0.0433 0.0402 Aluminium sesquioxide added g- 0.10 0.05 0.10 0-05 0.10 Titanium dioxide found Error g - €5 0.1084 + 0~0002 0.1079 -0.0003 0.0770 - 0.0003 0-0432 - 0.000 1 0.0400 - 0.000232 BLAKE : APPLICATION OF RADIO-FREQUENCIES TO CONDUCTIMETRIC [VOl. 75 DISCUSSION The essential difference between benzoylphenylhydroxylamine and cupferron is that there is a benzoyl group in the molecule of the former instead of the nitroso group of the latter.Owing to this difference benzoylphenylhydroxylamine has some properties (e.g. , stability and solubility in water) better than those of the free acid of cupferron. In the determination of copper, iron and aluminium with this new organic reagent, the precipitate can be weighed directly, which is a distinct advantage over the cupferron method in which the precipitate should be ignited to oxide before weighing. Small quantities of the metals can be determined more accurately with benzoylphenylhydroxylamine since the weight of the metal complex produced from a given amount of metal is nine to thirteen times greater than the weight of the corresponding oxide. Some of th? metal ions such as lead and mercury, which interfere with the determination of copper by the cupferron method, have no influence when benzoylphenylhydroxylamine is employed. The results obtained by this new method are accurate. Moreover the organic reagent can be prepared easily and preserved indefinitely. The acidity of benzoylphenylhydroxylamine is less than that of nitrosophenylhydroxyl- amine (free acid of cupferron) and therefore, its complexes with the metal ions are more easily decomposed by mineral acids. Owing to this defect, iron cannot be precipitated in strong acid solution and the separation of the metal from aluminium, chromium or phosphoric acid, is not possible. In such circumstances and in the separation of titanium from phosphoric acid, the use of cupferron is preferred. The author wishes to express his gratitude to Sir J. C. Ghosh, Kt., D.Sc., F.N.I., Director, Indian Institute of Science, for the opportunities afforded to him for carrying out this investiga- tion and for advice and encouragement. His thanks are also due to Dr. S. C. Bhattacharyya for his valuable suggestions and help. REFERENCES 1. 2. Shome, S. C., Current Sci., 1944, 13, 257. Bamberger, E., Ber., 1919, 52, 1116. DEPARTMENT OF CHEMISTRY BANGALORE, INDIA INDIAN INSTITUTE OF SCIENCE Februavy, 1949

ISSN:0003-2654    

DOI:10.1039/AN9507500027

出版商:RSC    

年代:1950

数据来源: RSC