Henry’s Law Constant

If the particular extracting technique applied to a solution depends on the volatility of the solute between air and water, a parameter to predict this behavior is needed to avoid trial and error in the laboratory. The volatilization or escaping tendency (fugacity) of solute chemical X can be estimated by determining the gaseous, G, to liquid, L, distribution ratio,KD, also called thenondimensional,ordimensionless, Henry’s law constant, H'.

The larger the magnitude of the Henry’s law constant, the greater the tendency for volatilization from the liquid solvent into the gaseous phase. According to equation (above), the Henry’s law constant can be estimated by measuring the concentration of X in the gaseous phase and in the liquid phase at equilibrium. In practice, however, the concentration is more often measured in one phase while concentration in the second phase is determined by mass balance. For dilute neutral compounds, the Henry’s law constant can be estimated from the ratio of vapor pressure, P_vp, and solubility ,S, taking the molecular weight into consideration by expressing the molar concentration:

Where P_vp is in atm and S is in mol/m³, so H is in atm.m³/mol.