This paper is the background for a new international formulation for the ion product of water substance (May 1980) issued by the International Association for the Properties of Steam. The ion product of water (Kw) is represented hy an equation, based on density and two 'lluH-1rJ'ltir. fllndion~ of reciprocal absolute temperature. for use from 0 to 1000°C and 1 to 10,000 bars pressure. The equation is believed to describe within ±0.01 units of log K: (where K: equals Kj(mol kg-I)2) many of the measurements at saturated vapor pressure up to 200°C, and to within ±0.02 units up to the critical temperature (374°C). It also describes within the experimental uncertainty the several sets of measurements at high pressures and should provide values within ±0.05 and 0.30 units at low and high temperatures, respectively.

Water is not restricted to moderate temperatures and low pressures, but can exist up to very high temperatures, far above its critical point at 647 K. In this supercritical regime, water can be gradually compressed from gas-like to liquid-like densities. The resulting dense supercritical states have extraordinary properties which can be tuned by temperature and pressure, and form the basis for innovative technologies. This Review covers the current knowledge of the major properties of supercritical water and its solutions with nonpolar, polar, and ionic compounds, and of the underlying molecular processes.

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The method of the Pendant Drop or Standing Bubble is applied to measure interfacial tensions between water and nonpolar fluids to high temperatures and pressures. The high pressure cell with two sapphire windows and the auxiliary equipment with several feed autoclaves is described. The shapes and sizes (about 2 mm) of drops and bubbles are recorded with microscope and video camera. A digital image processing procedure was developed which permits fast, objective and precise determination of the contour parameters.The six gases helium, neon, argon, nitrogen, methane, and propane have been investigated to 473 K (with nitrogen to 573 K) and (in part) to 2800 bar. Gas densities came close to liquid density values. For comparison, water plus liquid n-hexane, n-decane, and toluene was investigated to 473 K and 3000 bar. For these liquid hydrocarbons, the interfacial tension y always increases with pressure. At 373 K for water-n-hexane y is 41.8 mN/m at 100 bar and 47.3 mN/m at 2600 bar, respectively. In the water-gas systems y decreases with pressure and passes through a flat minimum around 1000 bar. For water-nitrogen at 373 K y = 52.5,46.5 and 48.3 mN/m at 200, 1400 and 2800 bar. Only with water-helium y increases continuously with pressure.

By use of a specially designed cell with a sapphire window the absorption of the OD-stretching vibration of HDO in H20 has been measured at frequencies from 2200 to 2900 cm-1 and at temperatures and pressures from 30 to 400°C and from 50 to 4000 bars respectively. At the supercritical temperature of 400°C and pressures below 200 bars (corresponding to a density of water of 0.1 g/cm3) the rotational structure of the vibration band of free water molecules is observed. At higher density only one intensive absorption maximum with simple shape is observed at all temperatures. This is considered as support for the continuum model for water in the liquid and in the dense supercritical state.

A high pressure autoclave equipped with sapphire windows is described. With water‐nitrogen mixtures of thirteen different concentrations (x) determinations were made of liquid‐gas phase equilibria conditions along “isopleths” between 523 and 673 K and 20 to 270 MPa. Molar volumes of the mixtures were measured at the three‐dimensional (PTx) phase equilibria surface and in the supercritical homogeneous region at 673 K. The critical curve, an envelope of the isopleths, begins at the critical point of water (647 K), has a temperature minimum (639 K) at about 75 MPa and proceeds to 250 MPa at 659 K. Phase equilibria and critical curve data are given. Values for the Henry‐constant to 647 K for mixtures dilute in nitrogen and for the nitrogen solubility from 300 to 600 K and from 10 to 200 MPa are presented. Excess volume, VE, values have been calculated for 673 K from 30 to 250 MPa. All VE values are positive. The maximum is 57 cm3 mol−1 at 70 mol per cent of H2O at 30 MPa and about 2 cm3 mol−1 at 40 mol per cent H2O and 250 MPa. Excess Gibbs energy‐values and activity coefficients are presented.

Critical Phenomena / Fluid Mixtures 1 Gases / High Pressure / Liquids 1 ThermodynamicsA high pressure autoclave with sapphire windows, auxiliary equipment and means and precautions needed for experiments with high pressure, high temperature oxygen are described. With water-oxygen mixtures of different mole fractions, x, determinations were made of liquid-gas phase equilibria conditions along "isopleths" between 500 and 660 K to 250 MPa. Molar volumes of the mixtures were measured at the three-dimensional ( P T x ) phase equilibria surface and in the supercritical region at 673 K. The critical curve, an envelope for the isopleths, begins at the critical point of water (647 K), has a temperature minimum (640 K) at about 75 MPa and proceeds to 250 MPa at 663 K. Phase equilibria and critical curve data are given. The H 2 0 -0 2 critical PT curve is very close to the critical curve recently (10) determined for H20-N2. Values for the Henry-constant from 300 K to 647 K for mixtures dilute in oxygen are presented. The Henry constant at room temperature has only about half the value of the Henry constant for nitrogen in water. At the critical temperature of water (647 K), however, both constants do not differ by more than the uncertainty of the determinations. The excess volume was calculated at 673 K from 30 to 250 MPa. All values are positive. The excess Gibbs energy and activity coefficients are presented. One isopleth for H'O-air with x(H,O) = 0.80 was measured and molar volume values for this composition at 673 K between 33 and 280 MPa are given.

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