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where Dv is diffusivity (in square centimeters per second), T is absolute temperature (Kelvin), |A is viscosity of solution (in centipoise), VA is molar volume of solute as liquid (25.6

cm3 mol~1 for oxygen [358]), i)jb is the association parameter for solvent (2.26 for water) (358), and MB is the molecular weight of solvent.

For the diffusion of oxygen in water at 20°C:

Substituting these values into the Wilke-Chang equation above gives a diffusivity of oxygen in water at 20°C of 1.972 X 10"5 cm2 s~\ which is approximately 10,000 times lower than in air at the same temperature.

OXYGEN DIFFUSION IN SOILS Ideally, the soil gas diffusion coefficient is measured by laboratory or field methods, but in practice this is likely to be too time-consuming and expensive, so it is simpler to use empirical models that relate the gas tortuosity factor t? to specific bulk soil properties that can be readily measured (139).

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