The following is adapted from an Environmental Engineering P.E. Exam. The total phosphate concentration (TOTPO4) in a river is 5.8 mg/L as P. The pH of the river is 7.6. What is the concentration of H2PO4- and HPO42-, expressed as mg P/L? The equilibrium constants needed to work this problem are given in Table 2-3 (under Phosphoric Acid). Hint: use the mass balance equation for phosphate species and equilibria expressions to solve this problem: TOTPO4 = [H3PO4] + [H2PO4-] + [HPO42-] + [PO43-]