[K+]+[H+]=3[PO43-]+2[HPO42-]+[H2PO4-]+[OH-]
[K+]/3=[H3PO4]+[H2PO4-]+[HPO42-]+[PO43-]
[H+]=3[H3PO4]+2[H2PO4-]+[HPO42-]+[OH-]