Question

phosphoric acid is a tribasic form , with pka value 2.14 , 4.86, 12.4. the ionic form that predominates at pH 3.2 pH ?

Answer 1

the pKa values give a rough approximation of the pH at which ionization occurs. At pH 2.14, the first ionization is happening:\[H_3PO_4 \rightleftharpoons H_2PO_4 + H^+\] At the second pKa value, 4.86, the second ionization is happening, and so on.

When you have a pH in between these transition intervals, which species is most common? You've passed 2.14, so all the H3PO4 has been turned to H2PO4-, but haven't yet reached the second pKa, which would turn the H2PO4- to HPO4-2. So which ionic form is most common?