Oct., 19581 BLUNDY 555 The Determination of Chromium by a Solven t-extraction Met hod BY P. D. BLUNDY (Chemical Engineering Division, A.E.R.E., Harwell, nr. Didcot, BerRs.) A method is described for the determination of small amounts of chromium in the presence of similar amounts of iron and nickel in solutions 0.04 M in uranium and 0.005 M in copper. The chromium is oxidised to chromate with ammonium hexanitratocerate in hot acid solution. The chromate is ex- tracted with isobutyl methyl ketone from a solution M with respect t o hydro- chloric acid at a temperature of < 10" C. The chromate is then extracted from the isobutyl methyl ketone by two successive water washes and is determined colorimetrically xith diphenylcarbazide. In the presence of 100 pg each of nickel and iron, chromium was determined in the range 10 t o 100 pg with a mean recovery of 100.3 per cent.and a standard deviation of 10.8 per cent. A METHOD was required for determining 1 to 100 pg of chromium in solutions 0.04 M in uranium, 0.005 M in copper and containing 1 to 100 pg each of iron and nickel. Chromium, iron and nickel arise from corrosion of a stainless-steel containing vessel at high temperature and pressure. The colorimetric determination of chromium with diphenylcarbazide after oxidation to chromate is the most sensitive method known. The critical stage in this procedure is the oxidation of chromium to chromate. Several methods have been described for this oxidation and they are summarised in Table I. Some were tried in this laboratory, but, at the low concentrations of chromium involved, complete oxidation was evidently not achieved, the results being neither reproducible nor in agreement with those for pure dichromate solutions.TABLE I METHODS FOR THE OXIDATION OF TERVALENT CHROMIUM Oxidant Method for destroying Medium excess of oxidant Results Sulphuric acid Boiling Not reproducible Sulphuric acid and a Boiling Not reproducible Orthophosphoric acid Boiling Thorium phosphate trace of silver nitrate precipitated azide or hydrochloric acid hydrochloric acid are evolved Acid solution Decomposition with Not attempted Potassium bromate Perchloric acid - Evaporation until fumes Not attempted Ceric sulphate or ammonium Hot acid solution Addition of sodium Complete oxidation Ammonium persulphate Potassium permanganate Sulphuric acid, 0.5 N Addition of sodium Sot reproducible hexanitratocerate azide or sodium and good reproduci- Willard and Young1 describe a volumetric method for chromium in which ceric sulphate in acid solution at a temperature of 100" C is used as oxidant.Excess of ceric ions may be destroyed with sodium azide or sodium nitrite, the former being preferred. The literature contains conflicting reports about the elements that interfere in the chromate - diphenylcarbazide reaction. A variety of solvent systems are suggested for preparing diphenylcarbazide reagent solution. Urone2 studied this problem and found that acetone and ethyl acetate give the most stable reagent solutions. Bryan and Dean3 describe the use of isobutyl methyl ketone (hexone) as a selective solvent for chromate from M hydrochloric acid before a flame-photometric determination of chromium.The selective action of hexone for chromate is fully described by Weinhardt and H i ~ s o n . ~ nitrite bility556 BLUNDY: THE DETERMINATION OF CHROMIUM BY [Vol. 83 It was decided, therefore, to utilise the hexone extraction of chromium, after oxidation to chromate by ceric solutions, for the separation from interfering elements before a colorimetric determination with diphenylcarbazide. EXPERIMENTAL CERIC OXIDATION- Aliquots of chromium potassium sulphate solution containing 10 to 100 pg of chromium111 were placed in 40-0-ml centrifuge tubes. Two millilitres of 4 N sulphuric acid and 2.0 ml of approximately 0.02 N ammonium hexanitrat,ocerate were added.These solutions were diluted to about 15.0ml and the tubes were immersed in boiling water for 25 minutes. They were then removed and cooled to <lo" C. Sodium azide solution (2.0 per cent.) was added dropwise with swirling to destroy the excess of ceric ions. The solutions were then transferred to 100-ml calibrated flasks containing 3.0 ml of 4 N sulphuric acid and diluted to approximately 90 ml. Two millilitres of a 1.0 per cent. solution of diphenylcarbazide in acetone were added to each flask and the solutions were made up to the mark. After they had been set aside for 5 minutes, the optical density of each solution was measured in 2-cm cells with a Spekker absorptiometer, Ilford No. 605 filters being used, The procedure was then repeated with aliquots of potassium dichromate solution over the range 10 to 1OOpg of chromium.The results agreed with those for the chromium potassium sulphate solutions and also with those found for potassium dichromate solutions in which the ceric oxidation had been omitted. SOLVENT EXTRACTION- Aliquots of potassium dichromate solution containing between 10 and 100 pg of chromiumv1 were taken and the ceric oxidation procedure was repeated as far as the cooling stage, and then 8.0 ml of 4 M hydrochloric acid were added while the solutions were cooling. Each solution was then transferred to a 100-ml graduated separating funnel and was diluted to 32 ml with water, which made each solution M with respect to hydrochloric acid. Twenty millilitres of hexone saturated with M hydrochloric acid were then added to each.After they had been shaken for 1 minute, the layers were allowed to separate and the aqueous layers were run off and discarded. The hexone layers were then washed twice with 20.0-ml portions of water and the washings were run into 100-ml calibrated flasks containing 5.0 ml of 4 N sulphuric acid. The diphenylcarbazide colours were developed and the optical densities were measured in the same way as before. The procedure was repeated with a solution of chromium potassium sulphate; the results were in good agreement with those for the potassium dichromate solution. About 97 per cent. of the chromium is recovered by the extraction, but the loss can be compensated for by incorporating the extraction in the calibration procedure. The mean temperature of the solutions during oxidation was 93" C and before extraction it was <lo" C.EFFECT OF INTERFERING IONS- Cations-Suitable amounts of elements that might interfere in the determination were added, singly and together, to solutions containing 50 pg of chrorniumv1. Colours were developed with diphenylcarbazide and the optical densities were measured with a Spekker absorptiometer. It was found that, when the solutions contained copper or iron, the colour faded rapidly, but, in the other solutions, the colour was stable for at least 2 hours. The results in Table I1 show that uranium, thorium and nickel do not interfere, but iron and copper cause low results. Identical results were obtained from solutions of tervalent chromium, which were oxidised to the sexavalent state by ceric oxidation, the excess of ceric ions being removed with sodium azide.Table I1 also shows results obtained when ceric oxidation was followed by solvent extraction. Interference from iron and copper was greatly reduced, and a twenty-fold excess of these elements produced individual errors of 2 per cent. or less. The combined interference of solution A was apparently slightly greater, but much less than without the solvent-extraction stage. As the chromium to be determined . in these solutions arose from the corrosion of stainless steel, the ratio of iron to chromium was unlikely to exceed 20. It was only necessary to take a 1.0-ml aliquot for the deter- mination of 10 pg of chromium, and hence the copper present was not likely to exceed 0.3 mg.To test the accuracy and precision of the method, forty-four replicate determinations were carried out at 10 to 90-pg levels of chromium in the presence of 11.4 mg of uranium, 0.305 mgOct., 19581 A SOLVENT-EXTRACTION METHOD 667 of copper, 0.106 mg of nickel and 0.105 mg of iron. The mean recovery was 100.3 per cent. and the standard deviation was & 0.8 per cent. TABLE I1 Each solution contained 50 pg of chromium EFFECTS OF INTERFERING ELEMENTS AND OF SOLVENT EXTRACTION Colour develo$ed from chromiumv1 solution without solvent extraction- Amount of uranium added, mg 0.0 5.0 10.0 20.0 50.0 Amount of chromium found, pg 50.0 50.0 49.8 50.0 49.8 Amount of thorium added, mg 0-0 100 200 300 400 Amount of chromium found, pg 50.0 50.1 50.0 50.2 50.0 Amount of nickel added, mg 0.0 0.20 0053 1.06 5.28 Amount of chromium found, pg 50.0 50.0 49.9 49.8 60.0 Amount of copper added, mg 0.0 0,193 0-386 0,579 0.772 0.963 2.0 5.0 Amount of chromium found, pg 50.0 46.3 45.9 45.7 45.7 45.4 43.2 42.6 Amount of iron added, mg 0.0 0.255 0.510 0.765 1.02 1-25 2.60 6.25 Amount of chromium found, pg 50.0 47.7 46.5 45.7 45.7 43.5 43.5 44-0 Amount of chromium found, pg 50.0 44.0 42.7 40.7 Amount of copper added, mg 0.0 0.2 0.4 0.6 0.8 1.0 1.93 4.8 Amount of chromium found, pg 50.0 49.7 49.7 49.7 49.4 49.2 48.9 47.0 Amount of iron added, mg 0.0 0.255 0,510 0.765 1.02 1.25 2.50 6.25 Amount of chromium found, pg 50.0 49-7 49.8 49.2 49.0 48.2 47.2 46.5 Amount of solution A added, ml* 0.0 0.5 1.0 2.0 Amount of chromium found, pg 50.0 49.5 49.1 49.8 Amount of solution A added, ml* 0.0 0-5 1.0 2.0 Colour developed after ceric oxidation and solvent extraction of chromium111 solution- * Solution A contained, per millilitre, 11.4 mg of uranium, 0.305 mg of copper, 0,1056 mg of nickel Anions-Oxidations were carried out in the presence of 400 mg of nitrate ion and trace and 0.1251 mg of iron.' amounts of chloride, perchlorate and fluoride ; no interference was observed. REAGENTS- Ammonium hexanitratocerate, 0.02 N-Dissolve 10,965 g of analytical-reagent grade am- monium ceric nitrate, (NH,),Ce(NO,),, in water. Make the solution N with respect to sulphuric acid when diluted to 1 litre. Sulphuric acid, 4 N. Hydrochloric acid, 4 M. Hexone saturated with M hydrochloric acid-Shake 500 ml of isobutyl methyl ketone with 500 ml of M hydrochloric acid, and allow the layers to separate. Run off the aqueous layer and pass the solvent layer through a Whatman No.1 filter-paper into a clean dry bottle. Diphenylcarbazide solution, 1 per cent. w/v-Dissolve 0.5 g of diphenylcarbazide in acetone and dilute to 50.0 ml with acetone. Standard chromium potassium sulphate solutiort, 1.00 mg per ml-Dissolve 4.8031 g of analytical-reagent grade chromium potassium sulphate, CrK(SO4)J2H,O, in water, add a few millilitres of 4 N sulphuric acid and dilute to 1 litre. For calibration purposes, dilute 10.0ml of this solution to 1 litre. PROCEDURE- Place an aliquot containing 10 to 90 pg of chromium in a 40-ml centrifuge tube. Neutralise any excess of free acid with ammonium hydroxide, and then make slightly acid with sulphuric acid.If ferrous iron is known to be present, oxidise it to the ferric state with 0.02 N am- monium hexanitratocerate. Add 2.0 ml of 4 N sulphuric acid and 2.0 ml of 0.02 N ammonium hexanitratocerate, dilute to about 15 ml, and immerse in boiling water for 25 minutes. Remove, and cool to <lo" C. Add 8.0 ml of 4 M hydrochloric acid while the solution is cooling. Transfer to a 100-ml calibrated separating funnel, and dilute to 32 ml with water. Add 20.0 ml of hexone saturated with M hydrochloric acid. Shake for 1 minute, allow the layers to separate, and then run off and discard the aqueous layer. Add 20.0 ml of water to the separating funnel, and shake for 1 minute. When the layers have separated, run the METHOD' Prepare this solution freshly each day.558 BLC'KDY AXD SIMPSON: THE DETERMIXATION OF [Vol.83 aqueous layer into a 100-ml calibrated flask containing 5.0 ml of 4 A' sulphuric ticid. Repeat the washing with a further 20.0 ml of water, and add it to the previous washings. Dilute to about 90 ml, and add 2-0 ml of 1 per cent. diphenylcarbazide solution. Dilute to the mark and set aside for 5 minutes. Measure the optical density against water in 2-cm cells with a Spekker absorptiometer, llford No. 605 filters and a mercury-vapour lamp being used. Prepare a blank solution in the same manner and measure its optical density against water. Prepare a calibration graph from aliquots of chromium potassium sulphate solution in the range 10 t o 9Opg of chromium, to which interfering elements in the concentrations expected in the samples have been added. CONCL~SIOSS Compared with other published methods, the proposed method has been found tobe rapid and reliable. The interference from iron is not likcly to exceed 2 per cent. in the worst conditions envisaged. The method has been extended t o solutions obtained from the erosion and corrosion of stainless steel by thoria slurries. 1 thank Mr. W. H. Hardwick and Mr. K. Todd for helpful discussions during this work, and hlr. M. P. Sirnpson, who carried out some of the earlier experimental work. REFERENCES 1. 2. 3. 4. Willard, H. H., and Young, P., J. Amer. Chena. SOC., 1929, 51, 139. Crone, Y. F., Anal. Chem., 1955, 27, 1354. Bryan, 11, A, and Dean, J. X., Ibid., 1957, 29, 1289. Weinhardt, A. E., and Hixson, A. N., Ind. EnR. Clietn., 1951, 43, 1676. Received Februmy 26fh, I958