Phase transitions in hydrothermal K2HPO4 solutions

A 0.1 M potassium phosphate (K2HPO4) solution was reacted in a flow-through cell pressurized to 22 MPa. Reduced light transmission through the cell windows was observed at a setpoint temperature ≥400 °C, along with a decrease in effluent conductivity, but with no effect on flow. These observations suggest solution separation at ∼360 °C, with accumulation of a salt-concentrated liquid in the cell body and transition of a dilute liquid to a supercritical fluid at temperature >374 °C. High-pressure differential scanning calorimetry experiments confirm an onset temperature of 354 °C with an endothermic transition at 377 °C and 22 MPa. For apparent density, ρ = 150–500 kg/m3, the average transition temperature for 0.1 M solutions, 375 ± 5 °C, is slightly elevated relative to that of water at 371 ± 4 °C. Highest deviation for 1.0 M solutions, 365 ± 15 °C, is attributed to increased K2HPO4 hydrolysis and polymerization reactions.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► K2HPO4 solutions are reacted at high temperature and pressure. ► Formation of salt-concentrated liquid and supercritical fluid occurs at T ≥ 400 °C. ► Transition temperature varies with K2HPO4 concentration and apparent density. ► Hydrolysis and polymerization reactions increase with T and salt concentration.