1144
ANALYTICAL CHEMISTRY
( 5 ) Butler, C. L., and Cretcher, L. H.,
(6) (7) (8) (9) (10)
(11) (12) (13) (14)
J. Am. Chem. SOC.,51, 1519 (1929). Conrad, C. M . , Ibid., 53, 1999 (1931). Dickson, 9.D., Otterson, H., and Link, K. P., Ibid., 52, 775 (1930). Dore, W. H., Ibid., 48,232 (1926). Ehrlich, F., and Schubert, F., Ber., 62, 1974 (1929). Freudenberg, K., Gudjons, H., and Dumpert, G., Ibid., 74, 245 (1941). Kapp, E., J . B i d . Chem., 134, 143 (1940). Lefevre, K. U.,andTollens, B., Ber., 40, 4513 (1907). McCready, R. M., Swenson, H. A., and Maclay, W. D., TND. ENG.CHEM.,hx.4~. ED.,18, 290 (1946). McKinnis, R. B., J. Am. Chem. SOC.,50, 1911 (1928).
(15) Nanji, D. R., Paton, F. J., and Ling, A. R., J . SOC. Chem. I n d . , 44, 253T (1925). (16) National Formulary, 8th ed., p. 374, 1946. (17) Phillips, M . , Goss, M. J., and Browne, C. A, J. Assoc. O f i c . A&. Chemists, 16, 289 (1933). (18) Shorey, E. C., and Martin, J. B., J. Am. Chem. Soc., 52, 4907 (1930). (19) 1'0s~.W.. and Pfirschke. J.. Ber.. 70B. 631 (1937). (20j Whistler,' R., Martin, A . , and Harris, M:, J . Research Natl. Bur. Standards, 24, 13 (1940). (21) Wurz, O., and Swoboda, Papier-Fabr., 38, 299 (1940).
RECEIVED December 26, 1947. Published with the permission of t h e direct o r of t h e North Dakota Agricultural Experiment Station.
Direct Determination of Chromate Ion with Standard Arsenite and Diphenylamine as Indicator ZOLTAN SZABO AND LADISLAUS CSANYI University of Szeged, Institute for General and Physical Chemistry, Szeged, Hungary
analysis it is known that arsenious acid reduces Iever,thequalitative chromate ion to chromic ion. Jellinek and Kuhn ( I ) , hoawere unable to work out a quantitative method on this S
basis. An indirect determination of the chromate ion with arsenite has been suggested by Spitalsky ( 3 )and by Kolthoff and Sandell ( 2 ) . Zintl and Zaimis ( 4 ) described a direct potentiometric titration. The authors' aim was to work out on the basis of this reaction a direct and accurate method with a reversibly working indicator. As the standard oxidation potentials of the systems Choy-,' Cr+++ and As04---/AsOa--are +1.30 and +0.58 volt, respectively, the potential a t the equivalent point is about $0.94 volt. Among the most used oxidation-reduction indicators the oxidation potential of diphenylamine-+O.i6 volt in 2 N sulfuric acid-corresponds best to this value. When diphenylamine is added to a chromate solution in the presence of sulfuric acid, the color changes initially from dirty yellow to brownish red and after addition of arsenious acid it varies from red to violet-red, deep blue, or violet. This final color does not change on addition of arsenious acid even in excess, so that this reducing process cannot immediately be indicated by diphenylamine. The violet-blue color of the diphenylamine-chromate mixture changes instantly to green, if sulfurous acid or ferrous sulfate is added. However, no color change is caused by arsenite, even if a small amount of ferrous sulfate is employed as a transfer agent, although the reduction of chromate has been completed. The color change of the diphenylamine that takes place on the action of arsenite is reversible, if a trace of potassium iodide is present The probable explanation is that the iodide ion catalyzes the reduction of the blue colored diphenylbenzidine violet to colorless diphenylbenzidine. This conclusion is suggested by the fact that the necessary amount of potassium iodide is proportional to the added diphenylamine. In itself the reaction between arsenite and chromate proceeds too slo~$lyto base a direct measurement upon this process. Manganese ion has proved to be the most satisfactory catalyst for the reaction. The catalytic effect of the manganese ion has been discussed by Zintl and Zaimis. Fortunately the catalyst exhibits its maximal effect in the same concentration range of sulfuric acid that is necessary for the reduction of a chromatearsenite system. On the other hand, the potential of the equivalent point is independent of the concentration rates of the system. Thus the determination of chromate can be made in a wide range. Between 1.0 and 0.01 gram every chromate quantity can be estimated. Owing to the different aquo complexes of the chromic ion the
color of the solution can differ a t the end point of the titration. The end point, however, is easy to observe. The formation of aquo complexes is impaired by the presence of neutral salts and by cooling. The presence of neutral salts causes the reappearance of the blue color after the titration. Therefore it is expedient to fix the color of aquo complexes through standardization of the temperature. The method was iodometrically controlled. SOLUTIONS
Decinormal Arsenite. The pure, dry substance (4.948 grams) is dissolved in a solution of 5 grams of sodium hydroxide, which is prepared in a small amount of water, and, if necessary, boiled. The solution is transferred into a 1-liter flask, diluted with water, and acidified by 10 ml. of concentrated sulfuric acid, and then the flask is filled up to the mark with water. Thus the acidified arsenite solution is compared with alkaline arsenite, very constant in strength, especially after removal of catalyzing impuritiesiron, sulfur dioxide, etc. Catalyst Solution. I t is expedient t c employ the catalyzing ions in a common solution. \Then 0.021 gram of potassium iodide and 0.005 gram of manganese sulfate are dissolved in 250 ml. of water, 5 ml. of this solution contain the quantity of catalvst necessary for one determination. Indicator Solution. One gram of diphenylamine is dissolved in 100 ml. of concentrated sulfuric acid. One drop of this solution indicates the titration end point with great sharpness. PROCEDURE
The chromate solution sample is acidified with 5 to 6 ml. of concentrated sulfuric acid and distilled water is added to make 50
Table I. Determination of Chromate K2Cr20: Taken
KzCrz0: Found
Gram
Gram
Gram
470
0,7088 0.7088 0.7088 0.7088 0,7088 0,4359 0,4359 0.4339 0,4359 0.1856 0.1866 0.1856 0.1856 0,11818 0.11818 0.11818 0.11818 0,05534 0.03534 0.01073 0.01073
0.7091 0,7088 0,7092 0.7088 0.7085 0,4363 0,4359 0.4366 0,4359 0.1856 0,1835 0,1854 0.1855 0.11813 0,11820 0.11813 0,11818 0.05540 0.05529 0.01072 0.01074
+ O . 0003 0.0000 4-0.0004 0.0000 - 0.0003 + O . 0004 0 0000 - 0.0003 0.0000 - 0.0003 -0.0001 - 0.0002 0.0001 - 0.00005 +o. 00002 - 0.00005 + O0.0000 , 00006
0.04 0.0 0.05 0.0 0.04 0.09 0.0 0.07 0.0 0.03 0.05 0.1 0.05 0.04 0.01 0.04 0.0 0.1 0.09 0.1 0.1
Difference
-
- 0.00005 -0.00001 +0.00001
V O L U M E 21, NO. 9, S E P T E M B E R 1 9 4 9
1145
Table 11. Determination of Chromium in Steel Steel Taken
Grams 1.0457 1.0478 0.5393 0.5293 0.3814 1.4848 0.4326 0,2982 0.2738 0.2339
Chromium Found Gram
0.01126 0.00408 0,00293 0.00257 0.00230
Chromium Found
% 0.81 0.77 0.79 0.79 0.68 0.75 0.941 0.983 0.940 0.985
to 60 ml. To the cooled sample are added 5 ml. of catalyst solution and 1 drop of indicator solution. The arsenious acid ib allowed t o run into the chromate solution from a buret just until the blue color changes to light green. If the blue color reappears within 5 minutes, one more drop of arsenite is added.. The indicator error can be estimated experimentally. This error originates from the oxidation of diphenylamine to diphenylbenzidine through chromate, which is the only compound to shon color later and is reformed again after disappearance of the blue. This correction amounts to +0.05 ml. of a 0.1 N arsenite solution. Table I shows that the proposed method gives satisfactory results. Mercury salts and thiocyanate ion disturb the determination. The former slacken the development of the diphenylamine color. and the rhodanide gives with the indicator a dirty green precipitate. I n the determination of chromium in steel according to this method care must be taken to remove the excess of iron, which
would disturb the reaction with diphenylamine. This can be achieved easily with alkaline or ammonium fluoride. To mask I gram of ferric sulfate, 0.5 gram of fluoride is enough. A 0.3- to 1.0-gram sample of steel is taken in an Erlenmeyer flask, and 30 to 35 ml. of water and then 5 to 6 ml. of concentrated sulfuric acid are added. The liquid is boiled over a small flame until complete solution. After evolution of hydrogen has stopped, potassium permanganate solution is added dropwise to oxidize the chromic ion to the chromate stage, which is shown by the dark brown color of the precipitated manganese dioxide. The excess of permanganate is decomposed by boiling. After cooling to room tem erature the solution is filtered until the filtrate is clear. The &ter is washed with 20 ml. of hot water, 0.5 or 1 gram of ure fluoride salt is then added, and after 1 minute 5 ml. of catafyst solution, 1 drop of indicator, and 0.05 N arsenious acid are run int,o the solution until the color changes to light green. The arsenite solution is standardized against potassium dichromate in the presence of the amount of fluoride salt indicated above. If a part of the chromium in steel is present as carbide, this does not dissolve in sulfuric acid. I n this case 1 to 2 ml. of concentrated hydrochloric acid are added to the sulfuric acid and the solution is evaporated to dryness to expel the hydrochloric acid. Otherwise the procedure is the same as above. LITERATURE CITED
(1) Jellinek and Kuhn, Z.anorg. u . allgem. Chem., 138,81(1924). (2) Kolthoff and Sandell, IND.ENQ.CEEM.,ANAL. ED.,2, 140
(1930). (3) Spitalsky, 2.anorg. a2Zga. Chem., 69, 179 (1911). (4) Zintl and Zaimis, Z.angew. Chem., 41,543 (1928). REPFIVED
June 14, 1948.
Analysis of Barytes SILVE KALLMA",
Ledoux & Co., New York, N. Y .
'HE evaluation of barytes iiivolves the determination of barium sulfate, usually after removal of acid-soluble barium salt?. Frequently, silica, iron, alumina, strontium, calcium, and magnesium must also be determined. For the determination of barium sulfate, the acid-insoluble residue (or the original sample, if a separate determination of the soluble barium salts is preferred) is fused with sodium and/or potassium carbonate. The melt is evtracted with hot water to dissolve out the alkali sulfate. According to the method of the Yew Jersey Zinc Company (4), the barium carbonate, together with the other insoluble carbonates, is dissolved in hot dilute hydrochloric acid. If the sample is free from or low in strontium, the barium is then directly precipitated by the very SIOK addition of a hot ammonium sulfate solution. This method requires prior information as the amount of strontium present, but such information frequently can be obtained only by actual analysis of the sample. Tests recorded in Table I indicate that strontium is largely coprecipitated with the barium sulfate, no matter whether the reagent is added slowly or all a t once. Calcium is also coprecipitated, although its interference is less serious than that caused by strontium. For larger amounts of strontium (41,the insoluble carbonates are dissolved in nitric acid and neutralized with ammonium hydroxide, and barium is preripitated as the chromate in a buffered acetic acid-acetate solution. The proper conditions for this precipitation were recently described by Beyer and Rieman (1). After reprecipitation, the barium chromate is dissolved in hydrochloric acid and hydrogen peroxide and the barium is finally precipitated as the sulfate. The filtrate from the barium chromate is not well suited for the determination of strontium, as no prior separation from calcium and other elements r a s carried out. Strontium and other im-
purities of the sample must therefore be detrrrniued in separate portions. Kallmann ( 2 ) recently suggested a 20% solution of hydrogen chloride in n-butyl alcohol for the separation of barium and strontium from calcium, and a 4 to 1 mixture of 11.0 .V hydrochloric acid and n-butyl alcohol for the separation of barium from strontium. This method is intended for mixtures of the chlorides of the three alkaline earths not exceeding 1 gram, and containing not more than 500 mg. of combined barium and strontium chlorides and not more than 250 mg. of strontium chloride. 8 s barytes essentially consists of barium sulfate with only minor quantities of calcium and strontium, some difficulty would be encountered in applying this method in its original form to the analysis of barytes. Hence, certain changes in the manipulations have been worked out which simplify considerably the analysis of this mineral.
Table I.
Precipitation of Barium Sulfate in Presence of Strontium and Calcium
(BariuIn sulfate precipitated from hot weakly hydrochloric acid solution of harium, strontium, a n d calcium b y slow addition of hot ammonium sulfate solution) BaSOi Found BaUz BrCh CaClz Equivalent Taken Taken Taken to BaClz Error Gram Gram Gram Gram Gram
0.9000 0,9000 0.9000 0.9000 0.9000 0.9000 0,9000 0,9000 0.9000
0.9002 0.8997 0.9026
O.Qb52
0.0104 0.0104 0.0208 0.0416
..
..
0,9056
o.oi20 0.0240
0,9064 0,9112 0.9198 0.9015 0.9045
+o, 0002 - 0.0003 +O. 0026 +0.0056 +0.0064
+0.0112 f0.0198 0,0015
+ +0.0048
,
