Molar parts, to calculate portions of His and HisHCl
from total His equivalents.

            (pH-pKa)
           10                            1
P_His = --------------;  P_HisHCl = ---------------
              (pH-pKa)                    (pH-pKa)
         1 + 10                      1 + 10

General formula
               / [A-] \
pH = pKa + log | ---- |
               \ [HA] /