Air-Water Equilibrium Conditions

A first approach in this respect is described by the distribution coefficient KA for a more or less volatile compound, A, between the gas phase and the water phase  

Equation (4.1) expresses that the ratio of the concentrations of A in the gas phase and the water phase, respectively, is a constant at equilibrium. This constant is temperature dependent but is independent of the quantity of A as long as dilute solutions are dealt with. 

The concept of mole fraction of a component used in Equation (4.1) is a convenient measure of concentration when dealing with trace quantities and dilute solutions, often experienced in environmental systems. This is especially the case with transport phenomena and equilibrium between phases, where it results in simple quantitative expressions. The phenomena of interest when dealing with the exchange of odorous compounds and oxygen between wastewater and a sewer atmosphere are, in this respect, relevant examples. 

The fundamentals of mole fraction can be illustrated for a binary system consisting of two components, A and B. The mole fraction of A (considering a gas phase) is defined as follows  

The mass balance (in the actual case for the gas phase) is, therefore, as follows  

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