Cox–Yates equation

A modification of the Bunnett–Olsen equation of the form:

$log 10 ( [ SH + ] [ S ] ) − log 10 [ H + ] = m * X + pK SH +$

is the activity function

$log 10 ( γ S γ H + γ SH + )$

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

$X = − ( H 0 + log 10 [ H + ] )$

and

$m * = 1 − Φ$ .

Source:

PAC, 1994, 66, 1077 (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1101