3. Preparation & standardization of Sodium Thiosulphate Solution- By Ms. Lipsa Samal
AIM:To determine the normality of given sample of sodium thiosulphate.
REQUIREMENTS:
Apparatus : Burette, Volumetric flask, Pipette,
Reagents : 0.1 N sodium thiosulphate, Potassium iodate, Potassium iodide.
PRINCIPLE:
The principle of standardization of sodium thiosulphate is based on redox iodometric titration
with potassium iodate (primary standard). Potassium iodate is a strong oxidizing agent, it is
treated with excess potassium iodide in acidic media which liberates iodine which is back
titrated with sodium thioslphate. Uniformity of reactions between iodine and sodium
thiosulphate forms basis for utilizing the standard solution of iodine in the analysis of sodium
thiosulphate. Sodium thiosulphate is usedin the analysis of iodine. In the analysis of iodine
starch solution is used as indicator.
KIO3+ 5KI + 3H2SO4→ 3K2SO4+3I2+3H2O I2
3I2+2Na2S2O3→ 2NaI + Na2S4O6
PROCEDURE:
Preparation of 0.1N Sodium thiosulphate
Dissolve 24.8g of sodium thiosulphate pentahydrate(Na2S2O3.5H2O) in 800 ml of freshly
boiled and cooled water and mix thoroughly by shaking for approximately 15 minutes. Make
up the volume to 1000 ml.
Preparation of 0.1N Potassium Iodate
Weigh accurately about 356 mg of KIO3 and dissolve in 100 ml distilled water
Preparation of Starch indicator
Take 1 gm of soluble starch and triturate with 5 ml of water and add it to 100 ml of Boiling
water containing 10 mg of Mercuric iodide with continous stirring
Standardisation of 0.1N sodium thiosulphate
Take 10 ml of Potassium Iodate solution .Add 2 gm of Potassium Iodide and 5 ml of dilute
H2SO4,keep it in dark for 10 minutes, add 2 to 3 drops of starch indicator andtitrate with
sodium thiosulphte using starch solution as indicator until the blue colour is disappeared.
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