Activity of ionised drugs

The activity of each ion is the product of its activity coefficient and its concentration, that is 

The anion and cation may each have a different ionic activity in solution and it is not possible to determine individual ionic activities experimentally. It is therefore necessary to use combined terms, for example the combined activity term is the mean ionic activity, a . Similarly, we have the mean ion activity coefficient, and the mean ionic molality, m+. The relationship between the mean ionic parameters is then 

The Debye-Hiickel expression as given by equation (3.36) is valid only in dilute solution (/ 0.02 mol kg ). At higher concentrations a modified expression has been proposed  

More details of these combined terms are given in Box 3.4. 

Values of the mean ion activity coefficient may be determined experimentally using several methods, including electromotive force measurement, solubility determinations and colligative properties. It is possible, however, to calculate y+ in very dilute solution 

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