Sugar complexes with metal2+ ions: thermodynamic parameters of associations of Ca2+, Mg2+ and Zn2+ with galactaric acid Original Research Article

A solution study on the complex ability of galactaric acid (GalaH2) for complexation with Ca2+, Mg2+ and Zn2+ ions is reported. The stability constants of the complex species are determined by means of potentiometric measurements. From the dependence of stability constants on temperature, the ΔH° and ΔS° values are also determined. The formation of the complex species is an endothermic process and ΔH° and ΔS° values suggest a chelate coordination mode of galactaric acid involving carboxylic oxygen and α-hydroxylic group. The prevailing species at acidic or neutral pH is [MGala] (M=Ca2+, Mg2+, Zn2+) which is also isolated in the solid state and characterised by means of IR spectroscopy. On increasing pH, the [MGalaH−1]− species is also formed, where the coordinated OH group undergoes deprotonation.