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PK and PH

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					           Relationship between pKa and pH
The correct operational relationship between pKa and
pH is that:
A. both are log functions.
B. both are always < 7 for acids, and >7 for bases.
C. These two concepts are not operationally related in any way since biological fluids
   contains mixtures of too many acids and bases.
D. When pH = pKa , the compound in question will have a charge of +0.5.
E. When pH = pKa , the ionizable compound in question (whether acid or base) will be
   half protonated and half deprotonated.




pKa and pH
The "operational" relationship between pKa and pH is mathematically represented by
Henderson-Hasselbach equation:

                              pH = pKa + log [A-] / [HA]

where [A-] represents the deprotonated form and [HA] represents the protonated form.
One oft-cited solution to this equation is obtained by arbitrarily setting pH = pKa.

                 In this case, log([A-] / [HA]) = 0, and [A-] / [HA] = 1.

In words, this means that when the pH is equal to the pKa of the acid, there are equal
amounts of protonated and deprotonated acid molecules. This same relationship holds for
bases as well, with [B] substituting for [A-] as the deprotonated form, and [HB+]
substituting for [HA] as the protonated form. It should be emphasized that the
Henderson-Hasselbach relationship holds for a specified acid or base even if multiple
acids or bases are present.

Other solutions to the Henderson-Hasselbach equation over a range of pH values are
displayed in the following figure (for acetic acid).
                                                      "Net charge on acid" refers to the
                                                      average of all acid molecules in
                                                      the solution.




Several points about this graph deserve mention:

      At the pKa , the acid is 50% deprotonated.
      At the 1 pH unit above (below) the pKa , the acid is 90% deprotonated
       (protonated).
      At 2 pH unit above (below) the pKa , the acid is 99% deprotonated (protonated).
      At 3 pH unit above (below) the pKa , the acid is 99.9% deprotonated (protonated).
      When fully protonated, charge on acetic acid is 0.
      When fully deprotonated, charge on acetate is -1.