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Anonymous asked in Science & MathematicsChemistry · 3 months ago

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

1 Answer

  • 3 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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