Upon heating NaH2PO4 or NaNH4HPO4 to 250° and then slowly to 350°, first Na2H2P2O7 and then the insol Maddrell's salt is obtained, which is stable to about 500°; at 505° it changes to sodium trimetaphosphate, and above 607° to sodium hexametaphosphate. According to Maddrell, Ann. 61, 63 (1847) the salt may be prepd by heating two parts NaNO3 with one part H3PO4. An improved procedure is described by v. Knorre, Z. Anorg. Chem. 24, 397 (1900). Structure: Jost, Acta Crystallogr. 16, 428 (1963). Reviews: see Sodium Metaphosphate.