Molybdic acid ionisation under hydrothermal conditions to 300 °C

This UV spectrophotometric study was aimed at providing precise, experimentally derived thermodynamic data for the ionisation of molybdic acid (H2MoO4) from 30 to 300 °C and at equilibrium saturated vapour pressures. The determination of the equilibrium constants and associated thermodynamic parameters were facilitated by spectrophotometric measurements using a specially designed high temperature optical Ti–Pd flow-through cell with silica glass windows.The following van’t Hoff isochore equations describe the temperature dependence of the first and second ionisation constants of molybdic acid up to 300 °C: log10K1=-96.125-0.02875·T+17.660·ln(T)log10K2=-30.082-0.02690·T+5.9366·ln(T)The resulting ionisation constants of molybdic acid demonstrate that in low sulphur containing hydrothermal fluids in the Earth’s crust, the transport of molybdenum is favoured by the HMoO4-/MoO42- species while the role of the associated H2MoO4 is of negligible importance at elevated temperatures above 200 °C.