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