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Why do we only consider the $\mathrm{p}K_\text{a}$ value of a drug regardless of whether it is an acid or base? Why do we not use the $\mathrm{p}K_\text{b}$ value?

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  • $\begingroup$ Can you provide an example? pkb can be obtained readily from pka and pkw. $\endgroup$ Dec 27, 2013 at 19:19

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For easier comparability:

Comparing two pKa values is much easier than comparing a pKa with a pKb value.

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The reason we only usually consider the $\mathrm p K_\mathrm a$ is that in a constant medium (in the case of drugs, water) there is a simple relationship between $\mathrm p K_\mathrm b$ and $\mathrm p K_\mathrm a$ so we don't need two scales for acidity and basicity and can define a continuous scale from strong acid to strong base using just one number.

$$\mathrm p K_\mathrm b = \mathrm p K_\mathrm w - \mathrm p K_\mathrm a$$ where $\mathrm p K_\mathrm w = -\log_{10}([\ce{H+}][\ce{OH-}])$, roughly equal to 14 under normal conditions.

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