Prepn of dihydrate: F. Mylius, R. Funk, Ber. 30, 1716 (1897). Soly studies: J. E. Ricci, G. J. Nesse, J. Am. Chem. Soc. 64, 2305 (1942); J. N. Spencer et al., J. Chem. Eng. Data 19, 140 (1974). X-ray, IR and Raman spectroscopic study: S. Peter et al., Z. Anorg. Allg. Chem. 626, 208 (2000).