1 + 3.162 4.162 In the same way, we find 3.07% unionised at pH 5; 0.032% at pH 7; and less than 0.009% unionised at pH 9. Absorption is much greater than one would expect, being 41% at pH 4 and 27% at pH 5, although the trend is as predicted.

pH is around 7 the surface pH is nearer 6. One can calculate a 'virtual' surface pH which will allow results such as those in Table 9.2 to be explained. If we take as another example salicylic acid, again calculating the percentage of unionised species by equation (9.1), we obtain the following figures (taken from Table 9.2):


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