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