Air-water partitioning can be viewed as the determination of the solubility of a gas in water as a function of pressure, as first studied by William Henry in 1803. A plot of concentration or solubility of a chemical in water expressed as mole fraction x, versus partial pressure of the chemical in the gaseous phase P, is usually linear at low partial pressures, at least for chemicals which are not subject to significant dissociation or association in either phase. This linearity is expressed as “Henry’s Law.” The slope of the P-x line is designated H’, the Henry’s law constant (HLC) which in modern SI units has dimensions of Pa/(mol fraction). For environmental purposes, it is more convenient to use concentration units in water C

of mol/m

yielding H with dimensions of Pa m