A modification of the Bunnett–Olsen equation of the form: \[\log _{10}(\frac{[\text{SH}^{+}]}{[\text{S}]})- \log _{10}[\text{H}^{+}] = m^{*}\ X + \text{pK}_{\text{SH}^{+}}\] is the activity function \[\log _{10}(\frac{\gamma _{\text{S}}\ \gamma _{\text{H}^{+}}}{\gamma _{\text{SH}^{+}}})\] for an arbitrary reference base. The function X is called the excess acidity because it gives a measure of the difference between the acidity of a solution and that of an ideal solution of the same concentration. In practice \[X=- (\mathrm{H}_{0}+\log _{10}[\mathrm{H}^{+}])\] and \[m^{*}=1- \mathit{\Phi }\]