Ionisation of amphoteric drugs

Ampholytes (amphoteric electrolytes) can function as either weak acids or weak bases in aqueous solution and have plC values corresponding to the ionisation of each group. They may be conveniently divided into two categories - ordinary ampholytes and zwitterionic ampholytes - depending on the relative acidity of the two ionisable groups. 

In this category of ampholytes, the pK of the acidic group, plCf is higher than that of the basic group, and consequently the first 

The steps of the ionisation on increasing pH are shown in the following equilibria  

This compound can exist as a cation, as an unionised form, or as an anion depending on the pH of the solution, but because the difference between pfCfand is 2, there 

Redrawn from A. Pagliara, P.-A. Corrupt, G. Caron, P. Gaillard and B. Testa, 

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