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→Physiological Variables
* '''pH at site of absorption'''
* '''pH at site of absorption'''
Many drugs are either a '''weak base''' or a '''weak acid''' and so will exist in both an unionised and ionised form in the same solution. The ratio of the two forms is dependent upon the pH of the location of the drug.
Many drugs are either a '''weak base''' or a '''weak acid''' and so will exist in both an unionised and ionised form in the same solution. The ratio of the two forms is dependent upon the pH of the location of the drug. It must be remembered that ionised molecules don't pass easily through lipid membranes. Unionised molecules will diffuse easily as long as they are lipid soluble.
The ionisation reaction of a weak acid is
The ionisation reaction of a weak acid is
pK<sub>a</sub> = pH + log<sub>10</sub> ([BH<sup>+</sup>]/[B])
pK<sub>a</sub> = pH + log<sub>10</sub> ([BH<sup>+</sup>]/[B])
The dissociaton constant allows one to measure the strength of an acid or base and to determine the charge on a molecule in any givem pH.
Thus the extent of ionisation of a drug and so it's extent of absorbtion, depneds upon its '''pK<sub>a</sub>''' and the pH within the body compartment.