Polyprotic buffers deal mainly with triprotic acids and their different forms. 

Polyprotic Buffers

Concept #1: A triprotic buffer can be approached in a way similar to diprotic buffers. The key difference is that a triprotic acid has 3 pKa values. 

Polyprotic buffers usually deal with triprotic buffers. It is imperative we remember the relationships between Ka and the different forms of a triprotic acid. 

Depending on the forms used the format of the Henderson Hasselbalch equation can change.