Question

what is the purpose of adding kcn to the water sample for the titration with EDTA

Answer 1

A complexometric titration uses the formation of a coloured complex to indicate the endpoint.

EDTA, often be written as H4Y, is a common ligand in complexometric titrations.

It has four carboxyl groups and two amine groups that can act as electron pair donors (Lewis bases).

EDTA is often used as the disodium salt, Na2H2Y.

It reacts with many metal ions to form a complex:

M^2+ + H2Y^2-  $\rightleftharpoons$ MY^2- + 2H^+

Carrying out the reaction in a basic buffer solution removes the

H^+ as it is formed.

This moves the position of equilibrium to the right and favours formation of the complex (Le Châtelier's Principle).

Also, for EDTA, pKa4 = 10.26.

Thus, if the solution is buffered to about pH 10.3, most of the EDTA will exist as Y^4- ions.

The metal ions will not have to remove the hydrogen ions from H2Y^2-.

They can react directly according to the equation

M^2+ + Y^4- ----> MY^2-

Above pH 10.3, most metal ions react quantitatively with EDTA.

Note: KCN can be used to remove the remaining two protons in the common EDTA (H4Y) form, H2Y^2- to form Y^4- and effectively bind it to the metals that are used in the EDTA titrations.