Reliability in standardization of sodium thiosulfate with potassium dichromate

• Standardization of sodium thiosulfate with potassium dichromate was discussed.
• Accuracy of the standardization method of sodium thiosulfate was evaluated.
• Sodium thiosulfate was assayed through two different paths based on coulometry.
• An appropriate iodine liberation condition in the standardization was clarified.
• This study enables analysts in the field to have accurate results.

Sodium thiosulfate is often standardized with potassium dichromate. In the standardization, iodine (triiodide) liberated by potassium dichromate in an acidic potassium iodide solution is titrated with a sodium thiosulfate solution. The iodine liberation process significantly affects the titration results. In the present study, the accuracy of the liberation process was examined by assaying potassium dichromate through two different paths: assaying directly by coulometric titration with electrogenerated Fe(II), and assaying by gravimetric titration through the iodine liberation reaction with a sodium thiosulfate solution of which concentration was standardized by coulometric titration with electrogenerated iodine. The accuracy of the standardization of a sodium thiosulfate solution by potassium dichromate was discussed from the apparent assays of potassium dichromate under different measurement conditions.