0.1 n Sodium thiosulfate Preparation and Standardization

0.1 n Sodium thiosulfate Preparation and Standardization

Preparation of 0.1 N Sodium thiosulfate Solution

• Take 25 g of sodium thiosulfate pentahydrate (Na2S2O3·5H2O) in 500 mL of freshly boiled and cooled water
• Add 0.11 g of sodium carbonate (Na2CO3).
• Dilute to 1 L with freshly boiled and cooled water, and let stand for 24 h.
• Store the solution in a tightly closed glass bottle.

Standardization of  0.1 N Sodium thiosulfate

• Take 1 g of primary standard potassium dichromate (K2Cr2O7), transfer to a platinum dish, and dry at 120°C for 4 h.
• Cool in a desiccator.
• Weigh accurately 0.21g of the dried K2Cr2O7,
• Transfer to a 500-mL glass-stoppered conical flask.
• Add 100 mL of water, swirl to dissolve, remove the stopper,
• Quickly add 3 g of potassium iodide (KI), 2 g of sodium bicarbonate (NaHCO3), and 5 mL of hydrochloric acid (HCl).
• Stopper the flask quickly, swirl to ensure mixing, and let stand in the dark for 10 min.
• Rinse the stopper and inner walls of the flask with water and titrate with the Na2S2O3 solution until the solution is yellowish green.
• Add 2 mL of starch solution (10 g/L), and continue the titration to the disappearance of the blue color.
Initial point Color

End point color

Calculation

Normality of Na2S2O3 solution

N = W/0.04904 * V

where:
N  = normality of the Na2S2O3 solution,
W = grams of K2Cr2O7 used, and
V = millilitres of Na2S2O3 solution required for titration of
the solution.

Ex-
0.2105 grams of K2Cr2O7 used, and
43.6 of Na2S2O3 solution.

N = 0.2105/0.04904*43.6
= 0.09845 N

Stability

Re-standardize weekly

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