For a tribasic acid like H3PO4 has three ionisation constant. The overall ionisation constants (K) will be product of ionisation constants of three steps, i.e. K=K1×K2×K3 H3PO4⇌H++H2PO4− K1=‌
[H+][H2PO4−]
[H3PO4]
. . . (i) H2PO4−⇌H++HPO42− K2=‌
[H+][HPO42−]
[H2PO4−]
. . . (ii) HPO42−⇌H++PO43− K3=‌
[H+][PO43−]
[HPO42−]
. . . (iii) Adding Eqs. (i), (ii) and (iii), we get H3PO4‌⇌3H++PO43− K‌‌=‌
[PO43−][H+]3
[H3PO4]
Multiply mathematical Eqs. (i), (ii) and (iii), we get K1×K2×K3=‌