Preparation and standardization of 0.1 M Sodium Thiosulphate BP Volumetric Solution


Accurately weigh and dissolve 25 g of Sodium thiosulphate and 0.2 g of Sodium carbonate in sufficient carbon dioxide free water to produce 1000 mL.


To 10 mL of 0.033 M Potassium Bromate add 40 mL of water, 10 mL of Potassium Iodide solution and 5 mL of Hydrochloric acid R1. Titrate with the Sodium Thiosulphate solution using 1 mL of Starch solution, added towards the end of the titration, as indicator.

1 mL of 0.1 M Sodium Thiosulphate is equivalent to 2.783 mg of KBrO3 or 0.5 mL of 0.033 M Potassium Bromate.


Molarity (M) = [0.1 x Weight of KBrO3 (in g) x 10]/[ Titration reading (in mL) x 1000 x 0.002783]

Limit: 0.1 M ± 5 % (0.0950 M – 0.1050 M)


a. 0.033 M Potassium Bromate:

Dissolve 5.5670 g of Potassium Bromate in sufficient water to produce 1000 mL.

b. Potassium Iodide solution:

A 16.6 w/v solution of Potassium Iodide.

c. Hydrochloric acid R1:

Dilute 70.0 g of Hydrochloric acid to 100 mL with water.

d. Starch solution:

Triturate 1.0 g of soluble starch with 5 mL of water and whilst stirring pour the mixture into 100 mL of boiling water containing 10 mg of Mercuric Iodide.

Carry out the test for sensitivity each time the reagent is used.

Test for sensitivity: To a mixture of 1 mL of the Starch solution and 20 mL of water, add about 50 mg of Potassium Iodide and 0.05 mL of Iodine solution R1. The solution is blue.

e. Iodine solution R1:

To 10 mL of 0.05 M iodine add 0.6 g of Potassium Iodide and dilute to 100 mL with water. Prepare immediately before use.

f. 0.05 M iodine:

Dissolve 12.7 g of Iodine and 20 g of Potassium Iodide in water and dilute to 1000 mL with the same solvent.


Store in glass stoppered bottles.

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