Cox–Yates equation
A modification of the Bunnett–Olsen equation of the form:
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is the activity function
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for an arbitrary reference base. The function Math - text 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
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PAC, 1994, 66, 1101 (Glossary of terms used in physical organic chemistry.)