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a Reproduced from W. C. Bowman, M. J.

Rand and G. B. West,

Textbook of Pharmacology, Blackwell, London, 1967.

the blood and of different regions of the alimentary tract.

pH at membrane surfaces

Data on the relationship between absorption and pH of the intestinal contents on the one hand, and the percentage of drug in the unionised state on the other, can generally be rationalised if the apparent pH is reduced below the pH of the intestine. As described above, this pH 'shift' is thought to be due to the existence of a pH at the membrane surface that is lower than that of the bulk pH. One might expect that the hydrogen ion concentration at the surface would be greater (and hence pH lower) at the membrane surface as hydrogen ions would accumulate near anionic groups, leading to an effect which can be quantified by the equation

surface [H+]bulk exP

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