ANALYTICAL EDITION
APRIL 15, 1939 (7) (8) (9) (10)
Horst, F. W., (?hem.-Ztg.,45, 572 (1921). Isaacs, M. L., J. Am. Chem. SOC.,44,1662 (1922). Jamieson, G. S., Am. J.Sci., 44, 150 (1917). Jellinek, K., and Krebs, P., 2. anorg. allgem. Chem., 130, 263
(1923). Halle. Wilhelm (11) Kausch. 0.. “Das WasserstoffsuDeroxvd.” Knapp, 1938. (12) Kingzett, C. T., J . Chem. SOC.,37, 802 (1880). (13) Knecht, E., and Hibbert, E., Be?., 38,3324 (1905). (14) Kolthoff, I. M., Pharm. Weekblad,56,949 (1919). (15) Maasa, O., and Hatcher, W. H., J . Am. Chem.8oc.,42,2559(1920).
197
(16) Machu, W., “Das Wasserstoffperoxydund die Perverbindungen:” Vienna, Julius Springer, 1937. (17) Muller, E., and Holder, G., 2, anal. Chem., 84, 410 (1931). (18) Rius, A., Trans. Am. Electrochem. SOC.,54, 347 (1928). (19) Rothmund, V., and Burgstaller, A., Monatsh., 34, 693 (1913). (20) Rupp, E., and Siebler, G., Pharm. Zentralhalle, 66, 193 (1925). (21) Schales, O., Ber., 71,447 (1938). (22) Willard, H. H., and Young, P., J . Am. Chem. Sac., 55, 3260 (1933).
Standardization of Sodium Thiosulfate by Copper, Using Perchloric Acid JOSEPH J. KOLB Lanlcenau Hospital Research Institute, Philadelphia, Pa.
I
N THE available methods for standardizing thiosulfate iodometrically against copper, the copper is dissolved by the oxidizing action of concentrated nitric acid, and various means are used subsequently to eliminate oxides of nitrogen that would interfere in the titration. Among the expedients used, all of them somewhat cumbersome, are the following: addition of sodium hypochlorite and phenol (3); addition of sulfuric acid, evaporation, then addition of hydrochloric acid and further evaporation (1); addition of bromine water and boiling until the excess is removed (8); and the use of urea and boiling (2). Such treatment is followed by the addition of ammonia in slight excess, boiling off the excess, then addition of acetic acid to adjust the acidity. The method described here consists simply of dissolving copper in concentrated boiling perchloric acid, adding an equal volume of water, boiling for 2 minutes, and diluting to volume. Aliquots are titrated with thiosulfate after addition of potassium iodide using starch and a soluble thiocyanate as described by Foote (1). Perchloric acid is a good reagent for dissolving copper since solution is fairly rapid on boiling; all the by-products are instantly and completely removed by volatilization ( B ) , while the dilute cold perchloric acid itself does not interfere in the titration (6). No chloride could be detected in solution.
Reagents Copper, 99.96 per cent pure (4).
Perchloric acid, concentrated (68 to 70 per cent), technical. Potassium iodide, iodate-free, 1 N solution. Sodium thiosulfate solution standardized against potassium iodate, the purity of which had been checked against iodine. Starch solution prepared according to Sutton ( 7 ) . Potassium thiocyanate, reagent grade.
Procedure The following procedure applies to the standardization of an approximately 0.025 N thiosulfate solution. To a 0.60- to 0.65-gram sample of copper in a 100-cc. volumetric flask add 6 to 8 cc. of concentrated erchloric acid (about 11 M ) ; heat to boiling (hood!). Boil gent& until solution is complete, continue boiling for a few minutes, then cool slightly. Add an equal volume of water and boil for 2 minutes to drive off any chlorine that may be still present. After cooling dilute to volume. To 10-cc. portions add 5 cc. of 1 N potassium iodide. Let stand for 2 minutes and titrate with sodium thiosulfate until the yellow color is nearly discharged. Add 5 cc. of starch solution
and titrate to near the end point. Add 1.5 to 2.0 grams of potassium thiocyanate (1) and titrate to the disappearance of the blue color. The end point should be checked by the addition of 0.025 N iodine, since a t the end point the titration mixture is not white but flesh-colored.
TABLE I. STANDARDIZATION OF THIOSULFAT~ Normality 0.02484 0.02488 0.02483 0.02484 0,02480 0.02490
Average Error Against KIOs
t o .00002
*0.00001 Against Cu(C104)z t o . 00003
t o . 00001 *0.00002 ~0.00001
Number of Titrations 3 3
0 3
9 3
If 10 to 12 cc. of 5.5 M perchloric acid are used, solution will be much slower. Eight cubic centimeters of acid of this concentration will not completely dissolve the copper. Variation of the acid concentration in the copper solution from 0.3 N to 0.7 N has no apparent effect on the titration value. Greater acidity than this should be avoided, since the amount of thiosulfate consumed tends to become slightly too high (about 0.30 per cent). Titrating under artificial light, the end point tended to appear too early (about 0.25 per cent). No blank correction was necessary. The mean value against copper as well as against iodate is 0.02486 += 0.00002.
Acknowledgment The copper used in this work was kindly supplied by W. M. McNabb, University of Pennsylvania. The writer wishes to thank G. Toennies of this institute for his suggestions and criticisms.
Literature Cited (1) (2) (3) (4) (5)
Foote, J. Am. Chem. SOC.,60, 1349 (1938). Hill, IND.ENQ.CHEX.,Anal. Ed., 8, 200 (1936). Kendall, J . Am. Chem. SOC.,33, 1947 (1911). McNabb, W. M., private communication. Smith, G. F., “Perchloric Acid,” Vol. I, 2nd ed., p. 12, Columbus, Ohio, G. Frederick Smith Chemical Co., 1933.
(6) Ibid., p. 15. (7) Sutton, “Volumetric Analysis,” 11th ed., p. 137, Philadelphia, P. Blakiston’s Son & Co., 1924. (8) . . Treadwell-Hall, “Quantitative Analysis,” Vol. 11, 6th ed., p. 578, New York, John-Wiley & Sons, 1924.
