Commercial product is 98-99% pure. Prepn of monohydrate from aq oxalic acid and NH3 or ammonium carbonate: Bérard, Ann. Chim. 73, 277 (1810). Review: Gmelins, Ammonium (8th ed.) 23, pp 400-407 (1936). Crystal structure of monohydrate: Robertson, Acta Crystallogr. 18, 410, 417 (1965). Photodecomposition study: M. N. R. Nair, V. R. P. Verneker, J. Phys. Chem. 80, 2552 (1976). Heat capacity determn: S. V. Dalidovich et al., Thermochim. Acta 89, 387 (1985). Dehydration of monohydrate: J. E. House, Jr., R. P. Ralston, ibid. 214, 255 (1993). Physical properties of aqueous solns: H. Frej et al., J. Chem. Eng. Data 45, 415 (2000).