Assay of Copper Sulphate


Determine the percentage purity of given sample of copper sulphate .


1. Practical pharmaceutical inorganic chemistry by Dr. Deval Rao


Apparatus: Iodine flask, Burette, conical flask

Reagents : Copper suphate, 0.1N Sodium thiosulphate solution, starch solution,  0.1 N KIO3 solution

Copper Sulphate Crystals 


This is an iodometry type of titration. It depends upon the instability of cupric iodide which is formed in the reaction between CuSO4 and potassium iodide with the liberation of free iodine. When CuSO4 was allowed to react with KI in the presence of acetic acid, cupric iodide is formed. 

2 CuSO4 + 4KI → 2CuI2 + 2K 2SO 4


The cupric iodide formed in the above reaction is unstable, so it decompose to give cuprous iodide with the liberation of free iodine.

2CuI2 → 2Cu 2I 2 + I2


The liberated iodine is titrated with 0.1N sodium thiosulphate using starch solution as indicator.

I2 + 2Na 2S 2O 3 → Na 2S 4O 6 + 2 NaI

                          Sodium tetrathionate 


Preparation of 0.1N Sodium thiosulphate

Dissolve 24.8 g of sodium thiosulphate pentahydrate in 800 ml of boiled and cooled water and mix thoroughly by shaking for approximately 15 minutes. Make up the volume to 1000 ml.

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 and titrate with sodium thiosulphte using starch solution as indicator until the blue colour is disappeared. 

N 1V 1=N 2V 2

N 2 = N 1V 1/  V 2

or refer 0.1N sodium Thiosulphate

Assay of copper sulphate

Weigh accurately about 1 gm and dissolve in 50 ml of water, add 3gm of KI, 5 ml of acetic acid and titrate the liberated iodine with 0.1N sodium thiosulphate using starch solution as indicator. Continue the  titration till a faint blue colour remains, add 2g of potassium thiocyanate stir well and continue the titration until the blue colour disappears. 

 Each ml of 0.1N sodium thiosulphate = 0.02497 gm of CuSO4

Percentage purity of the given Copper sulphate 

= VxE x AN x 100

        W x RN

V =Volume of Na2S2O3

E = Equivalent weight factor (0.02497 gm)

AN= actual normality of Na2S2O3

W = weight of sample(gm)

RN = Required Normality of Na 2S2O3

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