Application of the continuous variation method in equimolar solutions in the presence of an auxiliary ligand

Abstract

A general expression has been derived for the “ a max” (=( C A / C M + C A )) max) obtained when the continuous variation (c.v.) method in equimolar solutions is carried out varying a metal, M, and ligand, A, continuously in the presence of a constant concentration C B , of an auxiliary ligand, B. Unlike the previous treatments found in the literature, no restriction has been imposed on the magnitude of C B employed. When M p A q and M r B s are the only two complex species formed in all the metal-ligand mixtures, the expression for “ a max” turns out to be a max ⁡ = ( q / ( p + q ) ) − ( r q Q 2 / ( p C + q C ) ( z 0 + s 2 Q 2 ) ) × ( z 0 ( 1 − r ) + s 2 Q 2 ) where Q 2 and z 0 are the equilibrium concentrations of M r B s and B respectively in the solution in which the concentration of M p A q is maximum and C = ( C M + C A ). The validity of the equation has been demonstrated by applying the c.v. method to (i) Fe(II)-1,10 phenanthroline system in the presence of the auxiliary ligands NTA, Oxalic acid and EDTA and (ii) Fe(II)-EDTA system in the presence of constant concentration of 1,10-phenanthroline. Some graphical procedures are suggested to arrive at the composition of the complexes formed with the main as well as auxiliary ligand. The utility of an auxiliary ligand forming a coloured complex of known composition with the metal, to arrive at the composition of the colourless complex formed between the metal and main ligand is also demonstrated.