A modification of the Bunnett–Olsen equation of the form: $${\log }_{10}(\frac{[{\text{SH}}^{+}]}{[\text{S}]})-{\log }_{10}[{\text{H}}^{+}]=m^{\ast }X+{\text{pK}}_{{\text{SH}}^{+}}$$ 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 $$m^{\ast }=1-\mathit{\Phi }$$