Commercial prepn from waste gases of phosphate fertilizer production: Molstad, US 2833628 (1958 to W. R. Grace). Lab prepns: Booth, Swinehart, J. Am. Chem. Soc. 57, 1337 (1935); Hoffman, Gutowski, Inorg. Synth. 4, 145 (1953); Belf, Chem. Ind. (London) 1955, 1296.