Polyprotic acids

A polyprotic acid is a substance which can donate more than one proton. So they have successive acidity constants. One example is hydrogen sulphide:

SH₂ is a flammable poisonous gas with the odor of rotten eggs - produced by the anaerobic breakdown of proteins by bacteria.

Using the acidity constants and initial concentrations of the acid, many questions will ask for the concentration of a particular ion. These usually require clever approximations. Example:

It is interesting that this approximation gives an answer independent of the initial acid concentration.

Obviously we can change the concentration of a specific species by changing pH, which changes [H₃O⁺] in the acidity constant equations. In distribution diagrams, the fraction of a solute present as a specific species, is shown as a function of pH.

For example, consider the triprotic acid H₃PO₄. The fraction of molecules present as intact H₃PO₄ is:

And here is the distribution diagram: