Sodium Thiosulfate

Sodium Thiosulfate
Chemical FormulaNa2S2O3·5H2O
Formula Weight248.19
Equivalent Weight248.19 (Molar = Normal)
CAS No.10102-17-7

Sodium Thiosulfate solutions are almost exclusively used to standardize Iodine solutions or as back-titrants in titrations using Iodine. Solutions of Sodium Thiosulfate are most commonly standardized with Potassium Dichromate or Potassium Iodate solutions, which generate Iodine from Iodide. Starch indicator is typically used. The reaction with Iodine is:

I3- + 2 S2O32- → 3 I- + S4O62- (Dithionite ion)

Titrations with standard Sodium Thiosulfate solution are best conducted in the pH 5 - 9 range. Since this is not always possible, the titrations should be conducted rapidly and with adequate stirring to insure rapid reaction of the Thiosulfate, thus minimizing any acid accelerated decomposition. This will also minimize the acid-catalyzed oxidation of Iodide (usually present in a titration involving Thiosulfate) to Iodate, which could lead to decreased accuracy.

Sodium Thiosulfate solutions are subject to bacterial decomposition and chemical deterioration. Sodium Thiosulfate solutions decompose in acid solution resulting in the evolution of Sulfur Dioxide gas and precipitation of elemental Sulfur. Contact with Carbon Dioxide in the air can produce the acidity leading to this decomposition, which is faster in more dilute Thiosulfate solutions. RICCA CHEMICAL COMPANY uses aged stock solutions of Sodium Thiosulfate to prepare our standardized Thiosulfate titrants. This allows time for residual impurities, present in even the ACS Reagent Grade material, to react with the Thiosulfate so that a steady concentration can be reached. Preservatives are also added to prevent bacterial growth, control pH, and increase shelf life.

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