Determination of amount /dm3 of Cu+2 in CuSO4.5H2O using Na2S2O3 and KI by iodimetry

 

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Theory

In this experiment the amount of Cu⁺² in CuSO₄.5H₂O was determined with the help of iodometric titration. This is an oxidation and reduction reaction, as well as “Iodometric reaction”. Titration involving with iodine or dealing with iodine liberated in chemical reaction is called Iodimetric and Iodometric titration respectively. This reaction is iodometric because iodine is got from KI. The main purpose is to determine the amount of Cu⁺² ions in copper sulphate (CuSO₄.5H₂O ) for this we first determined the molarity (moles/dm3) of CuSO₄.5H₂O and then performed the mathematical calculations to obtain the end result.   

Apparatus

Burette, Burette stand, beaker, stirrer, weight balance, measuring cylinder

Chemicals

1g KI, 0.1 M Na₂S₂O₃, Sample solution (CuSO₄.5H₂O)

Principle

Redox reaction

Reaction equation

2 CuSO₄ + 4KI   →  Cu₂I₂  +  2K₂SO₄  +  I₂

I₂  +  I^-   →  I₃^-

Starch  + I₃^-  →  Starch-I₃ (dark blue)

2S₂O₃^-2  +   I₂   →    S₄O₆^2-  +  2I^-

Indicator

Starch solution

End point

Colourless

Procedure

1.    Make of 0.1 M Na₂S₂O₃ solution by dissolving 1.24g of Na₂S₂O₃ in distilled water in a 50ml beaker and then make the volume of the solution up to the mark.

2.    Prepare sample solution by taking small amount of CuSO₄ and dissolve in distilled water make solution up to 20 ml

3.    In titration flask take 10ml of sample solution and add 1g of KI in it, then dilute it with 10ml of distilled water

4.    Titrate this solution with Na₂S₂O₃ taken in burette until the colour of the solution became pale yellow

5.    Add 2ml of 5% starch solution in the above solution the colour will change into blue

6.    Again, titrate until colourless solution is obtained

Starch solution:

Make a paste of 5g of soluble starch with a little water and pour the paste with constant stirring into 100 ml of boiling water and boil for 1 minute. Allow the solution to cool and add 2-3g of KI. Keep the solution in a stoppered bottle.

Calculations:

Molarity of CuSO₄.5H₂O = M₂ = ………M  (suppose its ‘A’ moles)

 

Cu⁺²  :  CuSO₄.5H₂O

1         :         1

According to the above mole ratio 1 mole of CuSO₄.5H₂O contains 1 mole of Cu⁺² ions which means

A moles of CuSO₄.5H₂O = A mole of Cu+2

Result

Amount of Cu⁺² in CuSO₄.5H₂O is ………moles/dm3

Discussion:

In the titration of iodine, starch must not be added until just before the end point is reached. Apart from the fact that the fading of iodine colour is a good indication of the approach at the end point. If the starch solution is added when the iodine concentration is high, some iodine may remain adsorbed even at the end point.