APPLICATION of CERIC SULFATE to the DETERMINATION of ARSENIC in PARIS GREEN and LEAD ARSENATE J. P MEHLIG Oregon State
R
AND
K. R. JOHNSON
College. Corvallis. Oregon
ECENT investigations ( 6 , 1 3 , 1 4 )have shown that ceric sulfate is a very satisfactory standard oxidizing agent in a wide range of analytical procedures. It has a number of decided advantages over the more commonly known and used potassium dichromate and permanganate. In particular, a number of workers (4, 9, 11, 13) have indicated that it quantitatively oxidizes arsenic in hydrochloric acid solution from the trivalent to the pentavalent condition. In the present work a study has been made of the application of ceric sulfate to the determination of arsenic in Paris green and lead arsenate, and the results have been compared with those obtained by titration with standard iodine solution. The former official method of the Association of Official Agricultural
Chemists (2, 5, l o ) , in which the arsenic is reduced with potassium iodide before titration with standard iodine, cannot be used with ceric sulfate because the latter oxidizes potassium iodide to free iodine. However, the official distillation method ( I ) , wherein the arsenic, reduced to the trivalent condition by hydrazine sulfate, is distilled as arsenic trichloride before titration, does not involve this difficulty. PREPARATION OF SOLUTIONS
Ceric ammonium sulfate was used rather than ceric sulfate because it is more easily dissolved. It was made up as a 0.1 N solution in a 0.05 molar sulfuric acid solution as directed by Smith ( 6 ) . Occasionally some grades of the salt may require additional acid for solu-
tion. The solution was standardized against arsenic trioxide of known purity which was distilled according to the official method (1). The stability of ceric sulfate solutions has been pointed out by F u m a n (3) and by Willard and Young (12). Standardizations carried out a t intervals over relatively long periods of time confirmed their observations that the solution was stable for forty weeks. Hydrazine sulfate reagent (1) was prepared by dissolving 20 g. of hydrazine sulfate and 20 g. of sodium
Dirtillolion Mclhod
Sample
KI Reduction Melhod A d a by iodine.
l o 0 8
Didillnlion Mdhcthod An01
ay rcric rulfole,
No.
%
%
iodms,
%
Difference be. lwcm ceric rulfolc and iodine dirtillotion melhadr,
%
most satisfactory time of distillation is about three hours. Shorter periods gave high results. The distillate was made up accurately to 1000 ml. a t room temperature. To a 200-ml. aliquot of this solution in a 500-ml. Erlenmeyer flask 25 ml. of concentrated hydrochloric acid was added and the mixture was heated to 50'. Then 5 to 7 ml. of iodine monochloride and one drop of ortho-phenanthroline ferrous complex indicator were added and the solution was titrated with standard ceric ammonium sulfate. The color change is from pink through orange to practically colorless. A one-minute endpoint was taken. It was found advisable to run a blank titration and make any necessary correction therefor. For comparison another aliquot of the distillate, after neutralization withsodium bicarbonate, was titrated with an iodine solution that had been standardized against distilled arsenic. The results of the determination of arsenic in eleven samples of Paris green and five samples of lead arsenate are shown, respectively, in Tables 1 and 2, which also include the figures obtained by titration with standard iodine according to both of the Association of Official Agricultural Chemists methods (1, 2, 5, 10). DISCUSSION
bromide in 1000 ml. of 1:4 hydrochloric acid. The equivalent amount of potassium bromide was found to work equally as well. Iodine monochloride, the catalyst (9) for the reaction between ceric sulfate and arsenic trichloride, was prepared as follows (7). Ten g. of potassium iodide and 6.74 g. of potassium iodate were dissolved in 90 ml. of water, 90 ml. of concentrated hydrochloric acid were added, and the solution stirred well. To the orangecolored solution 5 ml. of chlorofom were added and, after stimng, any red color in the chloroform was removed by adding drop by drop 0.005 molar potassium iodate solution, finally balancing with a drop of 0.01 molar potassium iodide solution. The final color of the chloroform must be very faint. Ten ml. of this solution were diluted to 1000 ml. The indicator, ortho-phenanthroline ferrous complex (8) was obtained, already prepared as a 0.025 molar solution, from the G . F. Smith Chemical Company. It may also be made by dissolving 6.95 g. of crystalline ferrous sulfate in 1000 ml. of water, adding 14.85 g. of ortho-phenanthroline monohydrate, and stirring until solution is complete. One drop of this solution serves for a single titration in a volume of 100 = 150 ml.
the official iodometric method. Results may be duplicated on the same sample with a precision of about t0.05 per cent. The standard ceric solution is very stable and thus has a decided advantage over iodine solution, which must be restandardized very frequently. Since the indicator, ortho-phenanthroline ferrous complex, is not subject to decomposition as is starch solution, it may be kept indefinitely. In addition the endpoint is more easily detected.
PROCEDURE
SUMMARY
As much as possible of a 1.5-g. sample of Paris green or dried lead arsenate was transferred with a spatula from the counterpoise watch glass into a 250-ml. distilling flask and the remainder was carefully washed into the flask with 50 ml. of hydrazine sulfate reagent. The solution was then distilled with concentrated hydrochloric acid according to the official method ( I ) of the Association of Official Agricultural Chemists. The
Ceric ammonium sulfate has been shown to be a satisfactory standard oxidizing agent when used instead of iodine in the determination of arsenic in Paris green and lead arsenate by the official distillation metho'd of the Association of Official Agricultural Chemists. The advantages of the cerometric method over the iodometric method have been indicated.
Titration with ceric sulfate is capable of giving results for arsenic in Paris green and lead arsenate which are within *0.10 per cent. of the values obtained by TABLE 2 RBSYLTSOBT&-D
.OR
KI Redunion Method Somole No.
As.06 by
iodine, %
L&ADAXSENAT=BY PA& CBBOUBTRIC MBTBOD Dirlillalion Mdhad AsrOa by card sulforc, %
Di$tiNolion Method
ASSO, by
iodinr, %
Diffwcncc b r lwrrn crric rullole and iodine dislillalion mdhodr.
%
AC~WLEDGMENT
The writers wish to express their sincere appreciation to L. E. Stevenson, now with the Weyerhauser Corn-
pany, Longview, Washington, for the valuable preliminary work he did on cerometric-arsenic titrations while a student at Oregon State College.
LITERATURE CITED
(1) "Methods of analysis," Association of Official Agricultural Chemists, Washingon, D. C., 4th ed., 1935,p. 43. (2) Ibd., p. 49. N.H..J. Am. Chcm. Soc... 50.~755, (1928). (3) FURMAN. ~ -, (4j ibid., 54; 4235 (1932). (5) M m , E. G., "Quantitative analysis," McGraw-Hill Book Co., Inc., New York City, 1932,p. 262. (6) SMITH, G. F., "Ceric sulfate," G.Frederick Smith Chemical Co.. Columbus. Ohio. 1935. . D. 52. (7) Ibid., p. 53. ~~
.
~
(8) SMITH.G. F., "Ortho-phenanthroline," G. Frederick Smith Chemical Co.,Columbus, Ohio, 1935,p. 6. (9) S w m , E. H.AND C. H. GREGORY, J . Am. C h n . Soc., 52, 901 (1930). ~-...,. Chem., Bull. 107,1907,p. 25. (10) U.~S;BU~. (11) WILLARD. H. H.AND P. YOUNG,J. Am. Chem. So ., 50, 1372 (1928). (12) Ibid., 51, 149 (1929). (13) Ibid.. 55.3260 11933). (14j YO&& P., J: CHEMEDUC., 11,466 (1934),
