DETERMINATION OF THE APPARENT DISSOCIATION CONSTANTS OF PHOSPHORIC ACID IN SEAWATER1

Abstract

A method was developed for determining the apparent dissociation constants of a tribasic acid in complex concentrated ionic solutions using potentiometric pH measurements and acid‐base titrations. The apparent dissociation constants of phosphoric acid were determined in artificial seawater over the ranges of temperature and salinity normally encountered in the ocean. These apparent dissociation constants were also measured in 0.68 m NaCl, which has the same ionic strength as seawater of 33‰ salinity. It was estimated that 99.6% of the PO 4 3− species and 44% of the HPO 4 2− species are complexed by cations in seawater other than Na + . The values of the apparent dissociation constants reveal that in seawater of 33‰ salinity at 20C at p H 8.0, 87% of the inorganic phosphate exists as HPO 4 2− , 12% as PO 4 3− , and 1% as H 2 PO 4 − .