Standardization of Sodium Thiosulphate Solution (Na2S2O3) by Iodometry

 

Theory

Sodium thiosulphate solution (Na₂S₂O₃) is titrated against potassium dichromate (K₂Cr₂O₇) in presence of HCl and KI. K₂Cr₂O₇ oxidizes the iodide ion in acidic medium to equivalent amount of iodine. The iodine formed in the reaction oxidizes Na₂S₂O₃ giving sodium tetrathionate ion and the end point is detected by starch solution

Apparatus

Burette, Flask, weight balance,

Chemicals

Sodium Thiosulphate, Potassium dichromate, Potassium iodide, HCl, Starch solution

Principle

Iodometric Titration (A type of redox reaction)

Reaction Equation

K₂Cr₂O₇     +    6KI  +   14HCl        -------->    3I₂   +   2CrCl₃

2Na₂S₂O₃    +    I₂         ----------->      Na₂S₄O₆     +  2NaI                 

Indicator

Starch solution

Endpoint

Colour change from dark blue to bottle green

Procedure

Preparation of sodium thiosulphate solution:

Dissolve 12.5g of sodium thiosulphate in water and make the volume up to 500ml. Keep the solution aside and filter to remove any cloudiness if appears.

Preparation of starch solution:

Add one gram of starch to few ml of water, prepare slurry and add gradually to 100ml of boiling water till a translucent solution is obtained

Standardization:

Dissolve 0.125g of accurately weighed potassium dichromate in 25ml of water present in a 250 ml flask. Add 10 ml of HCl and 2g of KI, close with stopper, shake well and keep in dark for 15 minutes. Add 100ml of water to the above mixture and titrate with sodium thiosulphate using starch as the indicator.

Calculations:

Result

 

Discussion

From the above experiment it was evident that sodium thiosulphate can be effectively standardize by using potassium dichromate and iodide with HCl and starch as the visual indicator, after performing the calculations, strength of the prepared sodium thiosulphate solution was found to be ……….. N.

 

Why did we used normality instead of molarity in this experiment?

How is normality different from molarity?