Titrimetric Determination of Cu Solution

In weakly acidic solution, Cu reacts with iodides according to  

2 Cu +4T-).2CuI+l2 The liberated iodine is titrated against standardised sodium thiosulphate solution 

At a high pH, the reaction is slow and at a low pH, T is likely to be oxidised in air. A moderate excess of iodide ensures complete reaction. 

The precipitated Cul and the adsorption of iodine by the Cul make the end-point difficult to detect. This is mitigated by adding KSCN just before the end-point Pipette 25.0 cm of -0.1 M Cu solution into a conical flask and add 10 cm of 10% KI solution. Titrate the liberated iodine with a standardised 0.05 M sodium thiosulphate solution until the iodine colour fades to a straw colour. Then add 2 cm of freshly prepared starch solution indicator and continue titration until the blue colour just fades. Then add 10 cm of 10% thiocyanate solution when the blue colour becomes more intense. Complete the titration quickly until the blue colour does not reappear within a minutes. The flesh-coloured precipitate Cul remains at the end-point. 

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