Polyprotic acids have
one pKa for
each acidic hydrogen H.
pK1(H3PO4) = pKa(H3PO4)
pK2(H3PO4) = pKa(H2PO4–)
pK3(H3PO4) = pKa(HPO42–)
p
Ka is a measure of the acidity of the acid and the basicity of the base of a conjugate pair. This can be somewhat confusing for polyprotic acids as their
conjugates may be capable of acting as
both an acid and
a base.
Species that can act as either an acid or a base are referred to as
amphiprotic. Amphiprotic substances are in two conjugate pairs:
The acid strength is measured by pKa of the pair in which it is the acid.
The base strength is measured by pKa of the pair in which it is the base
| pK1 | pK2 | pK3 |
| acid H3PO4 | acid H2PO4– | acid HPO42– |
| base H2PO4– | base HPO42– | base PO43– |
The acid strength of HPO42– is indicated by pK3(H3PO4). which is pKa(HPO42–).
The base strength of HPO42– is indicated by pK2(H3PO4) which is pKa(H2PO4–).