Spectrophotometric measurements of metal complex formation at high temperatures: the stability of Mn(II) chloride species

The equilibrium formation constants of manganese(II) chloride complexes have been determined spectrophotometrically in the temperature range from 25°C to 300°C at saturated water vapour pressure at chloride concentrations from 0.022 to 0.80 mol kg−1. Mn2+ and MnCl+ were the only species identified at temperatures from 25°C to 100°C. From 150°C to 300°C, the uncharged MnCl20 complex was identified and it becomes the predominant species in concentrated chloride solutions at T≥300°C. It is of interest to note that both MnCl+ and MnCl20 increase considerably in stability with increasing temperature. From 25°C to 300°C, logβ1 increases from 0.85 (±0.08) to 4.21(±0.12) and from 150°C to 300°C, logβ2 changes from 0.88 (±0.10) to 4.63 (±0.15). With increasing temperature from 25°C to 300°C, the formation of MnCl+ is characterised by increasingly endothermic enthalpies and large positive entropies, indicative of greater electrostatic interaction. Chloride complexes will play an important role in the transport and deposition chemistry of manganese by hydrothermal fluids migrating through the earthʹs crust.