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Zinc pigments react in aqueous alkaline media (e.g. water-borne paints) by the evolution of hydrogen; at pH 10 more hydrogen is evolved than at pH 8. With the addition of phenol derivatives the opposite effect is observed: hydrogen volumes evolved are higher at pH 8 than at pH 10. These results can be explained with regard to the isoelectric point (IEP) of zinc oxide (pH 9). It is supposed that at pH 8 (below the IEP) more negatively charged phenolates are adsorbed than at pH 10 (above the IEP); the adsorption could be followed by a chemical reaction of the adsorbed phenolate with zinc which causes the formation of hydrogen.