PBS - pKa of PBS (Aug/03/2010 )

Does anyone know the pKa of PBS?

It'll have more than one, as it is composed of H2PO4 and HPO4, but I don't know the pKa of either of them off the top of my head.

phosphoric acid has 3 pKs: 1.0 (may be wrong or in wrong place in my table), 6.8 and 12.3.