Prepn of pentahydrate: Lux, Illman, Ber. 91, 2143 (1958); Holah, Fackler, Inorg. Synth. 10, 26 (1967). The common hydrated form, long considered to be the heptahydrate, has been reported to be the pentahydrate: Lux, Illman, loc. cit.; Lux et al., Ber. 97, 503 (1964). Prepn of standard soln for use as an analytical reagent: Lingane, Pecsok, Anal. Chem. 20, 425 (1948).