Question

Knowing the formation constant of a metal M²⁺ with ligand EDTA is K_fMY = 6.3 x 10²¹, calculate the equilibrium concentration of free M²⁺, [M²⁺], in a solution of 0.0125M of MY^2-  at pH 3.0 and pH 8.0, in the presence of 0.005M NH₃. The four formation constants of M²⁺ with ammonia are 6.1 x 10², 2.9 x 10², 1.15 x 10²and 99 respectively.

Answer 1

EDTA is, by far, the most widely used chelator in analytical chemistry. By direct titration or through an indirect sequence of reactions, virtually every element of the periodic table can be measured with EDTA. EDTA is a hexaprotic system, designated H₂Y²⁺. The first four pK values apply to carboxyl protons, and the last two are for the ammonium protons. The neutral acid is tetraprotic, with the formula H₂Y.

The fraction of EDTA in each of its protonated forms is plotted below.

we can define a fraction of dissociation, α, of EDTA as