Use buffer at pka 6.86

The glass electrode used in commercial pH meters gives an electrical response proportional to the concentration of hydrogen on. Converting these responses to a pH reading requires calibration of the electrode against standard solutions of known I+concentration. Preparation of the pH7.00 standard buffer uses sodium dihydrogen phosphate (NaH2PO4H2O,FW138) and disodium ydrogen phosphate (Na2HPO4,FW142). Phosphoric acid (H3PO4), a triprotic acid, has three pKa values: 2.14,6.86, and 2.4. Calculate the weight in grams of sodium dihydrogen phosphate and disodium hydrogen phosphate needed to prepare 0.75L tandard buffer with a total phosphate concentration of 0.13M