Disodium phosphate. Disodium phosphate (DSP), or sodium hydrogen phosphate, is the inorganic compound with the formula Na 2 HPO 4. It is one of several sodium phosphates. The salt is known in anhydrous form as well as forms with 2, 7, 8, and 12 hydrates.All are water-soluble white powders; the anhydrous salt being hygroscopic.isodium phosphate (DSP), or sodium hydrogen phosphate, is the inorganic compound with the formula Na 2 HPO 4.