Stability constants of polynuclear mercaptoacetate complexes of nickel and zinc

Abstract

In mercaptoacetic acid solutions containing zinc ions the major complex species are ZnL, ZnL₂^2–, ZnL₃^4–, Zn₂L₃^2–, and Zn₃L₄^2–. Stability constants for these species have been calculated from pH titration data by the non-linear least-squares method using an electronic computer. They are at 20° and l= 0·1 (NaClO₄), log β₁₁= 7·80, log β₂₁= 14·96, log β₃₁= 17·80, log β₃₂= 25·20, and log β₄₃= 36·47. The composition of the binuclear zinc complex was confirmed from direct measurements of free metal ion concentrations using a zinc amalgam electrode. The pH titration data for mixtures of nickel ions and mercaptoacetic acid could be best fitted by assuming the formation of the complexes NiL₂^2–, NiL₃^4–, Ni₂L₃^2–, Ni₃L₄^2–, and Ni₄L₆^4–. Computed stability constants were log β₂₁= 13·01, log β₃₁= 14·99, log β₃₂= 22·68, log β₄₃= 33·27, and log β₆₄= 49·85.