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→Physiological Variables
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.
The ionisation of a weak acid is
The ionisation reaction of a weak acid is
AH<sup>+</sup> =<sup>'''Ka'''</sup> A<sup>-</sup> + H<sup>-</sup>
AH =<sup>'''Ka'''</sup> A<sup>-</sup> + H<sup>+</sup>
and its dissociation constant '''pK<sub>a</sub> is given by the Henderson-Hasselbach equation:
and its dissociation constant '''pK<sub>a</sub> is given by the Henderson-Hasselbach equation:
pK<sub>a</sub> = pH + log<sub>10</sub> ([AH]/[A<sup>-</sup>)
pK<sub>a</sub> = pH + log<sub>10</sub> ([AH]/[A<sup>-</sup>])
The ionisation reaction of a weak base is
BH<sup>+</sup> =<sup>'''Ka'''</sup> B + H<sup>+</sup>
and its dissociation constant '''pK<sub>a</sub> is given by the Henderson-Hasselbach equation:
pK<sub>a</sub> = pH + log<sub>10</sub> ([BH<sup>+</sup>]/[B])