To determine the strength of an unknown solution of CuSO₄ with the help of Sodium thiosulphate (hypo) solution and determine the amount of Cu in the copper ore.

Introduction

It is redox iodometric titration (double titration). Analytical reactions using iodine as the oxidizing agent are called iodimetric titration while procedures using iodide ion as the reducing agent are called iodometric titration. When a measured quantity of CuSO₄ is treated with KI, the Cu2+ present in the CuSO₄ oxidizes the corresponding amount of KI to iodine. The liberated iodine is estimated by titrating against hypo solution, using starch as an indicator. Known CuSO₄ solution is used to standardize hypo solution

The reactions involved are as follows:


1. 2CuSO₄ + 4KI -----------> Cu₂I₂ + 2K₂SO₄ +I₂
   
   
2. I₂ + 2Na₂S₂O₃ -------------> 2NaI + Na₂S₄O₆
   
   
3. I₂ + Starch -------------> Starch-iodo complex

Iodine is slightly soluble in water but it soluble in solutions containing iodide ion. Iodine forms the tri iodide complex with iodide. Excess potassium iodide is added to the reaction mixture to increase the solubility of iodine and to decrease its volatility.

I₂ + KI ------------->KI₃