Preparation and standardization of 0.02 N Sodium Thiosulfate USP Volumetric Solution
1. PREPARATION :
Dissolve about 5.2 g Sodium Thiosulfate and 40 mg of Sodium Carbonate in 1000 mL of recently boiled and cooled water.
2. STANDARDISATION :
Accurately weigh about 42 mg of primary standard Potassium Dichromate, previously pulverized and dried at 120°C for 4 hours (or dried according to the instructions on its label), and dissolve in 100 mL of water in a glass stoppered, 500 mL flask. Swirl to dissolve the solid, remove the stopper, and quickly add 3 g of Potassium Iodide, 2 g of Sodium Bicarbonate, and 5 mL of Hydrochloric acid. Insert the stopper gently in the flask, swirl to mix, and allow to stand in the dark for exactly 10 minutes. Rinse the stopper and the inner walls of the flask with water, and titrate the librated Iodine with the Sodium Thiosulfate solution until the solution is yellowish green in colour. Add 3 mL Starch TS, and continue the titration until the blue colour is discharged. Perform a blank determination and make necessary correction.
Note : Refer Bottle label or CoA of Primary standard for drying condition.
Normality (N) = [Weight of Potassium Dichromate (K2Cr2O7) in mg]/[ 49.04 x (Titration reading – Blank reading) [in mL]]
Limit : 0.02 N ± 5 % (0.0190 N – 0.0210 N)
3. PREPARATION OF INDICATORS / REAGENTS :
a. Starch TS :
Mix 1 g of soluble starch with 10 mg of red Mercuric Iodide and sufficient cold water to make a thin paste. Add 200 mL of boiling water, and boil for 1 minute with continuous stirring. Cool, and use only the clear solution.
Note : Commercially available, stabilized starch indicator solution may be used.
4. STORAGE CONDITION :
Store in glass-stoppered, amber-coloured bottles.