How to make 20mM disodium phophate buffer (pH=7), excluding monosodium phosphate.?
The buffer can not be make by disodium phosphate alone ,we have to add some other phosphate compound. For buffer of pH=7 and 20mM concentration the ration of two phosphates will be : Monosodium phosphate, monohydrate = 0.1167% Disodium phosphate, heptahydrate = 0.3093% Calcultion method : Using the three pK’s of phosphoric acid and the pH, the ratio’s of each of the phosphoric pairs is calculated. For example, the first ionization is given by: H3PO4 –> H+ + H2PO4- K1 = [H+][H2PO4-] / [H3PO4], or [H2PO4-] / [H3PO4] = K1 / [H+] Normalizing the amount of each of the four moieties, their relative amounts are calculated. The molar amounts of monosodium phosphate and disodium phosphate are derived from the following two equations. Monosodium Phosphate = [H3PO4] + [H2PO4-] Disodium Phosphate = [PO43-] + [HPO42-] Using the molecular weights for monosodium phosphate, monohydrate and disodium phosphate, heptahydrate (the typical lab forms) together with the buffer strength, the actual percentag
