Capacity factor Poise

Redox reactions describe the coupling of reduction reactions to oxidation reactions in free soil. Electrons are transferred from reductors (e donors or reducing agents) to oxidants (e acceptors or oxidising agents) (James Bartlett 2000). 

Electron activity is a function of the ratio of reduced-to-oxidised species activity and of the equilibrium constant (or Gibbs free energy) for the reaction, since AG°=-RT InK. 

Two concepts having similarity of response from soils are those of proton activity and electron activity as the thermodynamic activity is defined as the ratio of H+ and e activity under standard state conditions (James Bartlett 2000). 

The capacity factor in redox is referred to as poise and is defined as the change in added equivalents of reductant or oxidant to bring about a one unit change in pe (or Eh change of 59 mv). The concept is similar to that of buffer capacity for pH (Stumm Morgan 1996). However, poise in soils has been less studied than pH buffering. 

The measurement of pe together with pH can describe the equilibrium condition of a soil through the master variable, pH and pe and their sum (Lindsay 

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