Further information 4.2 The pH of an amphiprotic salt solution

The aim is to calculate the pH of a solution of a salt MHA, where HA is an amphiprotic anion (HCOj is an example). We consider the foUowing two equilibria  

Multiphcation through by [HsO JlCi turns this expression into 

The mass conservation expression can also be written in terms oflHjOl  

When this expression is rearranged for [HA ] in terms of S and substituted into eqn 4.46 we find 

Of course, H depends on [H3O ], but we take care of that feature later (it will turn out that under the conditions to be specified, H l). The solution is 

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