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 degrees C to 300 degrees C at saturated water vapour pressure at chloride concentrations from 0.022 to 0.80 mol k(-1). Mn2+ and MnCl+ were the only species identified at temperatures from 25 degrees C to 100 degrees C. From 150 degrees C to 300 degrees C, the uncharged MnCl20 complex was identified and it becomes the predominant species in concentrated chloride solutions at T greater than or equal to 300 degrees C. It is of interest to note that both MnCl+ and MnCl20 increase considerably in stability with increasing temperature. From 25 degrees C to 300 degrees C, log beta(1) increases from 0.85 (+/- 0.08) to 4.21(+/- 0.12) and from 150 degrees C to 300 degrees C, log beta(2) changes from 0.88 (+/- 0.10) to 4.63 (+/- 0.15). With increasing temperature from 25 degrees C to 3000C, the formation of MnCI+ 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 earth's crust. (C) 2000 Elsevier Science B.V. All rights reserved.