INDUSTRIAL AND EKGINEERING CHEMISTRY
200
TABLE I. PRECISION ATTAINABLE Potassium Iodide MI.
Ceric Sulfate
M1. Solution I, 0.1074 M 18.67 18.69 18.70 18.69 18.70 Av. 18.69 Av. deviation 0.8 part per 1000
10 I O 0
Solution 11, 0.09982 M 10.00
20.10 20.10 40.20 10.04
20.00 5.00
Av. deviation 0.1 part per 1000
TABLE11. EFFECTOF VARYINQ CONCENTRATION OF ACIDAND VOLUMEOF SOLUTION (10.00 ml. of potas ‘um iodide and 25 ml. of acetone in each titration. sulfate required, 18.69 ml.) Volume a t Start 9 M H&Od Ceric Sulfate Used
%kit
.M2.
MI.
.M1.
100 100 100 100 145 100 200
1 5 15 20 30 30 20
18.75 18.70 18.68 18.61 18.60 18.43 18.6Y
TABLE111. EFFECTOF BROMIDE (10.00 ml. of potassium iodide and 25 ml. of acetone in each titration. Ceric sulfate required, 18.69 ml.) 0.1 .MKRr Volume a t Start 9 .M H P S O ~ Ceric Sulfate Used
‘Ml
.
MI.
MZ.
M1.
VOL. 8, NO. 3
low results and a fleeting end point. The data in Table I1 show the effect of varying these conditions. EFFECTOF BROMIDE.Measured volumes of 0.1 M potassium bromide were added to the iodide and the titration was performed as described above. I n general, more ceric sulfate is required than for iodide alone. The larger the ratio of bromide to iodide, the greater is the excess of ceric sulfate necessary to reach an end point lasting 1 minute. This interference can be almost entirely eliminated b y diluting the solution sufficiently while keeping the acid concentration approximately constant. Within the limits investigated it was possible to titrate the iodide with an accuracy of 3 to 4 parts per thousand when the bromide-iodide ratio was 5 to 1. The results of several of these titrations are given in Table 111. EFFECTOF CHLORIDE.Moderate amounts of neutral chloride do not interfere. Titrations in the presence of 0.5, 1.0, and 5.0 grams of sodium chloride required 20.10, 20.16, and 20.23 ml. of ceric sulfate, compared to 20.10 ml. for iodide alone. I n the titration with 5.0 grams of chloride present, precipitation of the salt occurred. The excess ceric sulfate required with the larger amounts of chloride may be due to the formation of hydrochloric acid, since titrations of iodide alone in which hydrochloric acid was substituted for the sulfuric acid used excess ceric sulfate.
Summary I n the presence of acetone and sulfuric acid iodides may be titrated quantitatively with ceric sulfate to a visual end point, using o-phenanthroline ferrous ion as indicator. The effect of bromides and chlorides on this titration has been determined.
Literature Cited (1) Andrew,
a 35
ml. of acetone.
Kith the acetone, which is proportional to the concentration of acid and of acetone (4). Too little acid is undesirable, since it leads to slightly high results and the titration is timeconsuming, while too high a concentration of acid leads to
LYLE 0. HILL Central Y. M,C. A. College, Chicago, Ill.
T
HE method of standardizing sodium thiosulfate against
copper and of determining copper in samples which are soluble in nitric acid as outlined b y Gooch and Heath (1) requires long evaporations. Kendall (8) has pointed out that these evaporations eliminate nitrous acid. The use of urea for the elimination of nitrous acid was worked out independently by the author. Afterwards, in a careful study of the literature it was found that Koelsch (3) suggested a procedure essentially the same as that given below and that Pozzi-Escot (4) also suggested the use of urea, employing, however, a much longer procedure. It is felt that this simple method has been overlooked and should be called to the attention of the analytical chemist. Results with this method agree with those of Gooch and Heath (1) within one part per thousand.
L. W., J.Am. Chem. Soc., 25, 756 (1903).
(2) Berg, R., 2. anal. Chem., 69, 369 (1926). (3) Ibid., 69, 1 (1926). (4) Dawson, H. M., J . Chem. SOC.,1927, 458, (5) Hahn, F. L., and Wolf, H., Chern.-Ztg., 50, 674 (1926); Hahn, F. L., and Weiler, G., Z. anal. Chem., 69, 417 (1926). (6) Lang, R . , 2. anorg. a2Zgem. Chem., 122, 332 (1922); 142, 229 (1925); 144, 75 (1925). (7) Swift, E. H., J . Am. Chem. SOC.,52, 894 (1930). (8) Walden, G. H., Hammett, L. P., and Chapman, R. P., Ibid., 55, 2649 (1933). (9) Willard, H. H., and Young, P.,Ibid., 50,1368 (1928). RECEIVED February 13, 1936.
Procedure Weigh a sample of pure copper, 0.2 to 0.3 gram. Dissolve the sample in 2 t o 5 ml. of concentrated nitric acid. Add 0.5 ram of urea and heat to boiling. Cool, adjust the acidity by afding 6 N ammonium hydroxide until a white precipitate is formed, dissolve the precipitate with 6 N acetic acid, and add 5 ml. in excess. Add 3 grams of potassium iodide, allow to stand 2 minutes, and titrate the liberated iodine with sodium thiosulfate solution. Starch is used as an indicator ( 2 ml. of l per cent solution) and should be added about 1 ml. before the end point is reached. Variations in concentration of KO3- and NH4+ have no effect on the final precision.
Literature Cited (1) (2) (3) (4)
Gooch and Heath, 2. anorg. Chem., 55, 199 (1907). Kendall, J. Am. Chem. Soc., 33, 1947 (1911). Koelsch, Chem.-Ztg., 37, 753 (1913). Poaai-Escot, E., Ann. chim. anal., 18, 219 (1913).
RECEIVED December 23, 1935.
