Henry's Law

When liquid activity methods are used, the standard state fugacity for a component is the fugacity of the component as a pure liquid. This basis is not very useful for dissolved gases however, especially when they are above their critical temperature. Therefore, it is more convenient to use a standard state defined at infinite dilution. Henry's Law may be used to accomplish this. The Henry's Law approach is also useful for representing trace solutes such as organic pollutants in water.

Thermodynamically, the Henry's constant of a light gas (solute) in a solvent is defined as the infinite-dilution limit of the ratio of fugacity to mole fraction:

 Hij = limit fi / xi as xi -> 0 and xj -> 1.0

where: i designates the solute and j designates the solvent

Unless the pressure is high or there is vapor phase association, the fugacity may be replaced by the component partial pressure, yiP, where yi is the component vapor mole fraction and P is the system pressure. Thus the K-value (yi/ xi) may be expressed as:

 Ki = Hij / P

PRO/II correlates Henry's constants to the following functional form:

 ln Hij = C1 = C2/ T + C3 ln T + C4 P

where: T, P = temperature and pressure.

Coefficients may be read from the PRO/II databanks or entered directly by the user.

Refer "Henry's Law" in the PRO/II Reference Manual, Volume I, Chapter 2 for more information on this approach.