Sugar complexes with metal(2+) ions: thermodynamic parameters of associations of Ca(2+), Mg(2+) and Zn(2+) with galactaric acid.

A solution study on the complex ability of galactaric acid (GalaH(2)) for complexation with Ca(2+), Mg(2+) and Zn(2+) 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 DeltaH degrees and DeltaS degrees values are also determined. The formation of the complex species is an endothermic process and DeltaH degrees and DeltaS degrees values suggest a chelate coordination mode of galactaric acid involving carboxylic oxygen and alpha-hydroxylic group. The prevailing species at acidic or neutral pH is [MGala] (M=Ca(2+), Mg(2+), Zn(2+)) 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.