Question

Why ammonia is used in cu estimation by colorimetry?

Answer 1

HNO3 is an oxidising agent when it reacts with copper it forms copper nitrate. For colorimetric analysis the solution must be colourised, when copper reacts with HNO3 if forms green coloured Cu(NO3)2. When this copper nitrate is diluted in water gives bright blue colour, this blue colour intensity is taken on account for copper ion concentration .

Reactions:

With conc. acid :

Cu(s) + 4HNO3(aq) ——> Cu(NO3)2(aq) + 2NO2(g) + 2H2O(l)

With dilute acid :

3Cu(s) + 8HNO3(aq) ——> 3Cu(NO3)2(aq) + 2NO(g) + 4H2O(l)