Equilibrium studies of solutions containing Pb 2+ or Zn 2+, and cysteine, orthophosphate and a carboxylic acid

Abstract

A pH-titration study is reported, at 25·0 ± 0·1°C and μ = 0· 1 M NaClO 4, of the equilibria in solutions containing a metal ion, Pb 2+ or Zn 2+, and three ligands, cysteine, phosphate and a simple carboxylic acid, either naturally occurring citric or synthetic nitrilotriacetic (NTA). Equilibria in solutions containing the above ligands in pairs were also studied. These systems were chosen to simulate ionic equilibria associated with the uptake and transport of metal nutrients by plants. In all systems, mixed two-ligand complexes predominate, although significant amounts of quaternary complex form in solutions of Pb, cysteine, phosphate and NTA. In neutral solutions, M(cysteine)(carboxylate) complexes predominate, and both Pb 2+ and Zn 2+ are inefficient binders of phosphate. Metal phosphate interaction is stronger in the pH range 4–6. Pb 2+ complexes are particularly soluble in the presence of NTA, and the suggestion is made that any build-up of NTA in soils could promote the dispersion of Pb through living tissues.