Appendix Sodium Thiosulfate

Identification Cautiously dissolve 250 mg of sample in 5 mL of glacial acetic acid, and add a few drops of a saturated solution of potassium iodide. Iodine is liberated. Add 20 mL of water and sufficient sodium thiosulfate TS to remove the iodine color. The resulting solution gives positive tests for Calcium, Appendix IIIA. 

Chloride Determine as directed in the Chloride Limit Test under Chloride and Sulfate Limit Tests, Appendix IIIB. Dissolve 1 g of sample in 100 mL of water. Any turbidity produced by a 10-mL portion of this solution does not exceed that shown in a control containing 70 p,g of chloride (Cl) ion. Ferric Iron Dissolve about 5 g of sample, accurately weighed, in a mixture of 100 mL of water and 10 mL of hydrochloric acid in a 250-mL glass-stoppered flask, add 3 g of potassium iodide, shake well, and allow to stand in the dark for 5 min. Titrate any liberated iodine with 0.1IV sodium thiosulfate, using starch TS as the indicator. Each milliliter of 0.1 N sodium thiosulfate is equivalent to 5.585 mg of ferric iron. 

Peroxide Value Determine as directed in Method II under Peroxide Value, Appendix VII however, after adding the saturated solution of potassium iodide, mix for 1 min, add 200 mL of water, and titrate with 0.05 N sodium thiosulfate, adding starch TS as the endpoint is approached, and continue the titration until the blue starch color has just disappeared. Perform a blank determination (see General Provisions), and make any necessary correction. Proceed with the calculation shown in the appendix. 

B. A 1 20 aqueous solution gives positive tests for Sodium and for Thiosulfate, Appendix IIIA. 

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