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Preparation and Standardization of 0.1 M Sodium Thiosulphate

Learn how to prepare and standardize the 0.1 M Sodium Thiosulphate solution using Potassium Dichromate, Potassium Iodide and Starch indicator.


•  Take about 100 ml of water in a cleaned and dried 1000 ml volumetric flask.
•  Add about 25 gm of Sodium Thiosulphate with continues stirring. 
•  Add about 0.2 gm of Sodium Carbonate with continues stirring. 
•  Add more about 700 ml of water, mix. 
•  Make up the volume 1000 ml with water. Mix solution thoroughly.
•  Keep the solution for at least one hour and than carryout the standardization.


•  Accurately weigh about 210 mg of primary standard Potassium Dichromate, previously pulverized and dried at 120°C for 4 hours.
•  Dissolve in 100 ml of distilled water in a glass-stoppered, 500 ml conical 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 distilled water, and titrate the liberated iodine with the Sodium Thiosulfate solution until the solution is yellowish green in color.
•  Add 3 ml of Starch indicator, and continue the titration until the blue color is discharged.
•  Perform a blank determination.
•  Calculate the molarity by following formula:
              K2Cr2O7 in mg 
M = --------------------------
         49.04 x Na2S2O3 in ml

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