Buffer made of salts of polyprotic acid: reactions
For use in a bacterial culture, you no doubt want a buffer solution with a $\mathrm{pH = 7.20}$, which corresponds to the $pK_{a2}$ for phosphoric acid. At that pH $\ce{[H2PO4-] = [HPO4^2-]}$. The $\ce{K+}$ ion is basically a spectator ion and doesn't influence the pH directly. So typically you'd start with a solution which has equal molarities of $\ce{KH2PO4}$ and $\ce{K2HPO4}$ and then adjust the pH with $\ce{H3PO4}$ and $\ce{KOH}$ using a pH meter to get $7.20$ "exactly."