Anionic components of wood preservatives and fire retardants such, as arsenates and phosphates, equilibrate at much lower concentrations in the cell wall bound water than in the free solution in the cell lumens. A degree of anion exclusion is expected in wood, due to the Donnan membrane effect. Fixed anions in the wood cell wall, produced by pH-dependent dissociation of the weak acid groups in wood, are not free to diffuse into the lumens, resulting in a chemical potential that limits migration of the mobile anions into the cell wall. The extent of exclusion, measured as "solute free water" (d) or by a molal partitioning coefficient (Km), decreases with increasing solute concentration, as expected from Donnan exclusion effects. However, the expected pH dependence (lincreased exclusion with increased pH) is not observed. In fact, there is an apparent anomalous effect of high cell wall penetration at pH's > 9.

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