Equilibria of complex formation between bivalent metal ions and 3,3'-dimethyl-5,5'-bis[(N,N-bis(carboxymethyl)amino)methyl]-phenolphthalein

Interactions of 3,3'-dimethyl-5,5'-bis[N,N-bis(carboxymethyl)amino)methyl]phenolphthalein complexon (OCP) with some of the divalent metal ions (viz., Mn(II), Co(II), Cu(II), Zn(II), Pb(II), and Cd(II)) have been studied by potentiometry in aqueous medium at ionic strength (I) = 0.1 mol dm(-3) NaClO4 and t = (30 +/- 0.1) degrees C. These metal ions form 1:1 and 2:1 (metal:OCP) chelates with OCP. A study over a wide pH suggests the formation of various stable complex species (viz., MH(2)L(2-), MHL(3-), and ML(4-) dominant species occurred for a 1:1 metal:ligand ratio and MH(2)L(2-) and M(2)L(2-) species occurred for a 2:1 metal:ligand ratio). The equilibrium constants along with the percentage of metal ions distributed among various complex species formed are given. The proton-ligand dissociation constants for OCP have also been evaluated.