Analysis of polybasic pH titration curves

The analysis of polybasic pH titration curves is fraught with difficulties. There is no completely general analytical solution, and in these cases it is even more important to be aware of what approximations, if any, can be made, whether they are fully justified and what they correspond 

2 A dibasic acid with two apparently separated pK values, e.g. malonic acid 

In a solution of a dibasic acid there are several species which can be present H2A(aq), HA (aq), A (aq), H30 (aq) and OH (aq). Which are present in significant amounts will depend crucially on the pH. 

All these equilibria will contribute to the pH of the solution, and the likely contribution of each can be assessed. 

Depending on the situation, it may be possible to make sensible approximations to these equations. 

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