Dissociation constants of phosphoric acid at 25°C and the ion pairing of sodium with orthophosphate ligands at 25°C

The first and second acidity constants of phosphoric acid were determined in a 0.70 ionic strength solution of both a sodium and a potassium ion background salt at 25°C. These experimentally determined constants were compared to literature values as well as values calculated from a theoretical model. Assuming negligible ion pairing of the phosphate ligand with the potassium ions, the stability constants of the NaHPO ₄ ^- and NaH₂PO ₄ ⁰ complexes were determined to be 1.3±0.3 and 0.49±0.07 (mol-L^–1), respectively. These constants were used to model the speciation of orthophosphate in a background of a sodium salt from pH 3 to 10.