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CORRESPONDENCE
Stability of Weak Sodium Thiosulfate Solutions Sir: The Reagent Laboratory a t Ohio State University is uniquely situated to determine and redetermine the factor of volumetric solutions over long periods of time. This laboratory, which prepares various reagents and solutions, including many different volumetric solutions, for routine, research, and teaching laboratories, sometimes keeps solutions in stock for several years. When such solutions are restandardized after an extended period, and these records compiled, interesting data are accumulated. In some cases, the data contradict long accepted information. A case in point is weak sodium thiosulfate. Orders for 0.02, 0.025, or 0.01 N are usually filled by accurately diluting standard 0.1 N solution. Usually more is prepared than is ordered, and the remainder placed in stock for use should another order come within a short period of time. We have already established that 0.1 N sodium thiosulfate is stable for long periods of time by rechecking the factor on such solutions. This was reported by this author in the Journal of Chemical Education for July 1951 in the article, "Keeping Properties of Certain Volumetric Solutions." On June 10, 1974, we mixed six one-liter bottles of accumulated 0.025 N sodium thiosulfate by filtering them into one vessel. Upon titration us. standard potassium iodate, this mixture, composed of solutions which had been prepared in February 1964, September 1964, August 1966, August 1970, June 1972. and November 1972, had a factor of 0.02487 N. On July 1, 1974, this same procedure was followed with seven 0.01 N solutions which had been prepared June 1966, ,July 1966, November 1966, April 1968, February 1970, Feb-
ruary 1971, and April 1974. The factor obtained after titration was 0.00996 N . One of the precautions that should be taken in the preparation of sodium thiosulfate solutions is that the water should be boiled or recently distilled to ensure it being as near sterile as possible. Another precaution is to add 0.02% NaZC03. Without these precautions, the microorganisms (thiobacteria) which cause the slow dicomposition of the solution, are not inhibited. This decomposition is mentioned by Kolthoff and Sandell ( I ) and again by Willard and Furman (2). In our laboratory, we used demineralized double-distilled water which has a purity of approximately 1,250,000 ohms in specific resistance and 0.80 micro-ohms in specific conductance. Thus it is evident from the data presented here that weak solutions of sodium thicsulfate, when properly prepared, are stable much longer than is usually indicated in the literature. Barbee William Durham The Reagent Laboratory Ohio State University Columbus, Ohio RECEIVEDfor review July 8, 1974. Accepted August 19, 1974. (1) I. M. Kolthoff and E. B. Sandell, "Textbook of Quantitative Analysis," Macmillan. New York, N.Y., 1969, p 590, (2) H. H. Willard and N. H. Furman, "Elementary Quantitative Analysis," 3rd ed., D.Van Nostrand, New York, N.Y., 1940,p 266.
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