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Subcritical water

The term subcritical water refers to liquid water at temperatures between the atmospheric boiling point and the critical temperature (374 C) of water. Very often interest is on temperatures above 200 C. Alternative terms include pressurized hot water (PHW), hot compressed water (HCW), near-critical water (NCW) or superheated water.


Pressure is applied to keep water in liquid state. The pressure may be equal to the vapour pressure of water at a given temperature or higher. The necessary pressure range therefore is at least 16 bars at 200 C and up to 226 bars at 374 C.


When water is heated well above 100 C its dielecric constant decreases and its ionic product increases. At 200 C the dielectric constant of water is the same as that of room temperature methanol. At 297 C benzene becomes completely miscible with water. Above 200 C water may be an acid or base catalyst because its H3O+ and OH- ion concentrations are perhaps orders of magnitude higher than in ambient water.

Subcritical water is therefore a much better solvent for hydrophobic organics than ambient water. It can itself be a catalyst for reactions which normally require an added acid or base.


Within the Compressed Fluid Technologies-network the following organizations are active in pressurized hot water (PHW) research:


- University of Helsinki, Department of Analytical Chemistry,
contact: kari.hartonen (at)
- VTT Process Chemistry, contact: olli.aaltonen (at)