January 15, 1935
ANALYTICAL EDITION
(8) Medri, Gazz. chim. ital., 36, I , 373 (1906); Chem. Zentr., 1906, 11, 459. (9) Petersen, 2.anorg. Chem., 5 , l (1893). (10) Purgotti, Ann. ist. super. agrar. Portici, [31, 3, 47 (1929). (11) Raschig, "Schwefel- und Stickstoffstudien," P. 188, Leiprig, Verlag Chemie, 1924. (12) Roberto and Roncali, Ind. chim., 6, 178 (1904) ; Chem Zentr., 1904, 11, 616.
57
(13) Sabanejeff, 2. anorg. Chem., 20, 21 (1899). (14) Stelling, Svensk Kem. Tid., 45, 3 (1933). RECEIVED October 19,1934. Baaed upon a part of the thesie presented to the Faculty of the Graduate School of Cornell University by A. G. Houpt in partial fulfilment of the requirements for the degree of Master of Chemistry. The work of Doctor Sherk has been supported through the personal generosity of Haymo V. Pfister of the Pfister Chemical Company.
Standardization of Potassium Dichromate
vv
HOBART H. WILLARDAND PHILENA YOUNG, University of Michigan, Ann Arbor, Mich. HEN p o t a s s i u n i dichromate is used as a
The oxidaiion value of solid potassium dichromate or of solutions of the reagent may be determined very accurately against the primary standard, arsenious oxide, by treatment of arsenious acid in a sulfuric acid solution with less than its equivalent of dichromate, and titration of the excess reducing agent, in the presence of osmium tetroxide as catalyst and o-phenanthroline ferrous complex as indicator, either with ceric sulfate or with potassium permanganate. The titration may also be made potentiometrically with potassium bromate in a hydrochloric acid solution. S i x samples of reagent quality and chemically pure potassium dichromate made by different Jirms showed practically 100 per cent purity.
volumetric oxidizing agent, its o x i d a t i o n v a l u e is checked most satisfactorily in one of two ways. The c. P. salt may be recrystallized a number of times from water and the final product after drying used as a primary standard, or solutions of the approximate normality desired may be standardized, often by a procedure similar t o that followed in an actual analysis. There are objections to both of these methods. Suitable substances of definitely known content against which to standardize a dichromate solution are not always available, and e i t h e r p r o c e d u r e f o r preparing - - a standard solution may be time-consuming. Precise and more direct methods for testing the purity of a supply of potassium dichromate or for the standardization of solutions, preferably against primary reduction standards, are needed. There has been considerable doubt whether the salt as purchased is sufficiently pure for a primary standard. The present study shows that such standardizations may be made accurately and rapidly &gainst arsenious oxide.
EXPERIMENTAL METHODS AND RESULTS REAGENTS AND SOLUTIONS. Bureau of Standards arsenious oxide was dissolved in sodium hydroxide, and sufficient sulfuric acid added to react with the latter, followed by 10 grams of sodium bicarbonate. The potassium dichromate was reagent quality material as received. The ceric sulfate solutions were from large supplies made by dissolving ceric ammonium sulfate in 0.5 M sulfuric acid. They had been standardized against Bureau of Standards sodium oxalate (4). A 0.025 M solution of o-phenanthroline ferrous complex (CJf8N2.H20)aFe,was prepared by dissolving the correct amount of o-Dhenanthroline in a 0.025 M aaueous solution of ferrous sulfate'. The 0.01 M solution of the catalyst was made by dissolving 1 gram of osmium tetroxide (sometimes called perosmic acid) in 400 cc. of 0.1 N sulfuric acid.
the reaction between arsenious acid and potassium dichromate in t h e p r e s e n c e of osmium tetroxide, which appears to be a catalyst for arsenic and similar materials, would be sufficiently rapid for a direct titration. A procedure for comparing potassium dichromate against such a primary standard is of importance because the present methods of checking the normality of dichromate solutions are too indirect-such as t i t r a t i o n with ferrous sulfate, which has been checked against permanganate standardized a g a i n s t B u r e a u of S t a n d a r d s s o d i u m oxalate. This particular method, which is widely used, has been shown to be undesirable as the titration with ferrous sulfate is influenced by acidity, volume of solution, and concentration of dichromate (1, 3). In the titration of arsenious acid with potassium dichromate in a sulfuric acid solution containing two or three drops of 0.01 M osmium tetroxide as catalyst and two drops of 0.025 M o-phenanthroline ferrous complex as indicator, fairly concordant results could be obtained at 50" C. with solutions containing from 40 to 60 cc. of 10 N sulfuric acid per 100 cc. of solution a t the beginning of a titration. However, as much as 0.10 cc. of 0.1 N dichromate in excess was ordinarily used, an indication that the reaction was too slow for a visual end point. TABLEI.
POTENTIOMETRIC TITRATION OF ARSENIOUS ACID WITH POTASSIUM DICHROMATE (Osmium tetroxide
0.01 M
Os04
10 N &SO4
Drops
CC.
4 4 4 4
30 40 50 60
0 9 12
40
40 40
a6
catalyst) NORMALITY OF KzCraOr 0.1003 0.1002 0.1004 0.1004 0.00985 0.1006 0.1005
The potentiometric method was tried because it permitted the use of a higher temperature with the possibility thereby REACTION BETWEEN ARSENIOUSOXIDE AND POTASSIUM of increasing the velocity of the reaction. Twenty-cubic DICHROMATE. It has been stated recently by Gleu (2) that centimeter portions of 0.1009 N sodium arsenite were diluted the titration of arsenious acid either with ceric sulfate or with water and the volume of 10 N sulfuric acid specified with potassium permanganate, using o-phenanthroline ferrous in Table I to 100 cc. A few drops of 0.01 M osmium tetroxide complex as oxidation-reduction indicator, is satisfactory mere added and the arsenic was titrated potentiometrically a t room temperature in a sulfuric acid solution, provided a t 70" to 80" C. with 0.1004 N potassium dichromate. that a very small amount of osmium tetroxide is present as The results obtained are shown in Table I. The reaction is catalyst. His data are inadequate, however, to show the quantitative, but the titration much too slow to be of any precision to be expected. The question arises as t o whether practical value. With no catalyst too much of the dichro-
INDUSTRIAL AND ENGINEERING CHEMISTRY
58
Vol. 7, No. 1
TABLE11. TITRATION OF EXCE~S ARSENITE WITH CERICSULFATE 0.09940 N NaiAsOa Cc
.
(Osmium tetroxide as catalyst and o-phenanthraline ferrous oomplex as indicator) VOL.AT BEQINNINQ 0.09976 N OF TITRA0.08 N NORMALITY OF KzCn07 TION Ce(S04)n KzCrzO7
5 N HZ904
co.
Cc
.
25 25 25 25 25 25 25 10 50 50 50 25a 25 b 25 b
10
0
10 30 50 10 10 40 10 15 15 40 0 0 10
25 25 25 25C 25d 25*
10 10 10 10 10 10
20 20 20 15 10 20 5 45 40 40 20 20 20 0.09853 N 25 25 25 20 20 20 0.06036 N 20 50 0.02564 N
,
cc.
CC.
100 100
REMARKS
6 tandardizations
100 100 100 100 400 100 150 150 400 100 100 100
6.12 6.12 6.11 12.28 18.50 6.12 6.14 5.93 12.18 12.18 6.11 6.12 6.12
100 100 100 100 100 100
0.80 0.37 0.34 6.43 6.43 6.46
End point not as sharp
Soln. stood 5 min. after adding KzCrzOz
End point not as sharp End point not as sharp End point not as sharp 0.09682 0.09821 0.09853 0.09836 0.09836 0.09826
S o h . stood 5 min. after adding KaCrtOI Soln. stood 10 min. after adding K&rrO? Reaction a Iittle slow Reaction a little slow Had to titrate a t 50' C.
10 0.05015 100 6.06 15 10 100 5.82 0.05026 30 0.027N 0.02510N 23 30 25 10 0 100 23:31] 10 10 16 100 4.57 0.02535 10 40 50 100 8.53 0.02563 a 10 cc. of 70 per cent HClOi present. b 5 cc of " 0 8 , ap gr. 142,present. the green liquid. When c , d , e io, 15,and 25 be. of 2 M KC1 added before KsCmO7. Four drops of comes more pale. At the Os04 used.
mate solution was used, doubtless because of the slowness of the reaction, and even with 9 or 12 drops of the osmium tetroxide the reaction seemed only slightly more rapid. Since the reaction between arsenious acid and potassium dichromate is too slow to permit of direct titration, a procedure involving treatment of the former in sulfuric acid solution with less than its equivalent of dichromate, and back-titration of the excess arsenious acid with ceric sulfate in the presence of osmium tetroxide as catalyst and o-phenanthroline ferrous complex as indicator was investigated. In the experiments listed in Table 11, the measured volume of sodium arsenite was treated with sulfuric acid and sufficient water so that after the addition of the dichromate the volume of the solution was that recorded. Three drops of 0.01 M osmium tetroxide and two drops of 0.025 M o-phenanthroline ferrous complex were added, and the titration was made with ceric sulfate. Four different dichromate solutions were used. In the third column is recorded the normality of each of these solutions on the basis of 100 per cent purity and in the last column the experimental values obtained. This method of standardizing dichromate solutions is quantitative over a wide range in experimental conditions. If the excess of arsenious acid is very small a few minutes should be allowed for the reaction to be complete. It is immaterial whether the indicator is added before the dichromate or just before the titration with ceric sulfate, but the osmium tetroxide must not be added before the dichromate, The data in Table I1 indicate also the possible concentration of chloride ion for satisfactory results. To check this method more closely, four sets of experiments
were made to obtain the purity of the potassium dichromate in terms of its oxidation value. In one series, samples of arsenious oxide, of slightly more than 1 gram, were treated in a 600-cc. beaker with a little water and 1 gram of sodium hydroxide, and the mixture was warmed until solution x-as complete. Water and 50 cc. of 5 N sulfuric acid were added. Then 1 gram or slightly less of potassium dichromate, dried at 100' to 110" C. for 4 hours, was added, the solution stirred, diluted to 400 cc., and allowed t o stand for 5 minutes. Three drops each of 0.025 M o-phenanthroline ferrous complex and of 0.01 M osmium tetroxide were added and the arsenious acid was titrated with 0.025 N ceric sulfate. Because of the dark green color of the chromic salt, the volume of the solution should be 400 to 500 CC. If the beaker is placed on a piece of white paper, the operator in looking down through the solution will observe a rose tint in
S o h . stood 5 min. after adding KzCrtOr Soln. stood 5 min. after adding KzCrrOi Standardizations Soln. stood 5 min. after adding KzCn07 S o h stood 5 min. after adding KzCrtOi
near the end point, this rose color beend point it disappears and the solution changes t o a clear green in color. In the experiments listed in Table 111, the excess of arsenite was such as to require as little as 8.40 cc. of the 0.025 N ceric sulfate in one case, and as much as 43.33 cc. in another case. In the second series, the procedure was the same as in the first, except that 0.044 N potassium permanganate was used to titrate the excess of arsenious acid. In the third series, the samples of arsenious oxide were placed in 400-cc. beakers and dissolved in the usual way, the solution was treated with 10 cc. of hydrochloric acid (sp. gr. l.lS), and diluted with water to 100 cc. Potassium dichromate samples of a little less than 1 gram were added, the solution was stirred and allowed t o stand a few minutes before potentiometric titration with 0.025 N potassium bromate. The strength of the bromate solution was checked potentiometrically against 0.025 N sodium arsenite in a hydrochloric acid solution. In the fourth series, the procedure was the same as in the first, except that the dichromate had been fused in an electric furnace, and after being broken up into small crystals had been dried for 4 hours at 100" t o 110" C. TABLE 111. OXIDATIOK VALUEOF REAGENT QUALITY POTASSIUM DICHROMATE AGENTFOR TITRATINQ EXCESH Ha.4~08 0.025 N Ce(SO4)z.in HzS04 s o h . 0.044N KMnOd in HzSOa soln. 0.025 N KBrOa in HCl soln. 0.025 N Ce(S041z in HzS04 s o h . (fused KzCrzOi used)
OXIDATION VALUE OF KnCrzO 7 100.01 10001 100.00 100.01 100.05'100'02' 99.99'100 00 100 03' 100'05'100.01'1OO:Ol 99:99: 99:98:100.00: 99.99
Tests made on samples of potassium dichromate from a number of chemical houses are listed in Table IV. The samples were fused, and then broken up and dried for 4 hours at 100' to 110" C. Dilute ceric sulfate was used in determining the excess of arsenious acid. TABLEIV.
OXIDATIOSVALUE OF SAMPLESOF POTASSIUM DICHROMATE AFTER FUSION
MATERIAL Baker Analyzed Baker and Adamsor,, reagent Coleman and Bell, c. P. Mallinckrodt, A. R. Merck c. P. Manuiacturer not stated, reagent
OXIDATION VALUE
LITERATURE CITED (1) Eppley a n d Vosburgh, J . Am. Chem. SOC.,44, 2148 (1922). (2) Gleu, 2. anal. Chem., 95, 305 (1933). (3) Willard a n d Gibson, I N D . ENQ.CHEX.,Anal. Ed., 3,88 (1931). (4) Willard a n d Young, J . Am. Chem. SOC.,55, 3260 (1933).
RECEIVZDOctober 23, 1934. The authors are indebted t o the Faculty Researoh Fund of the University of Michigan for the support of this work.
