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# Which is the best estimate for the proportion of a basic drug (pKa = 8.2) that will be charged at pH 7.4?

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- hcbiochemLv 73 months ago
In general, at 1 pH unit below the pKa, 90% of the molecules are in the protonated (charged) form. Since this pH is not quite 1 pH unit below the pKa, a little bit less than 90% of the molecules will be charged.

If you want a more accurate number, you can calculate this using the Henderson-Hasselbalch equation:

pH = pKa + log [uncharged]/[charged]

7.4 = 8.2 + log [uncharged]/[charged]

[uncharged]/[charged] = 0.156

Inverting this gives [charged]/[uncharged] = 6.31/1

So, out of every 7.31 molecules, 6.31 are charged, or 6.3/7.3 = 0.86.

86% of the molecules are charged.

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