... but why stop at dirprotic acids?

With the phosphate system we have three pKs, and thus two ampholytes H2PO4– and HPO42–.

A glance at the graph allows one to estimate the relative amounts of the two major species at any pH.

The pH of a solution of the acid H3PO4 or of the base PO43– is found from the system points 1 and 4 which are constsructed in the same way as those for a monoprotic acid.

System points 2 and 3 for solutions of the two ampholytes provide only approximate values pH values because of the effects of competing equilibria, but the accuracy is good enough for most applications.

>>>