Prepd by treating lead selenate with hydrogen sulfide and concentrating the filtered soln by evaporation: Mitscherlich, Pogg. Ann. 9, 623 (1827); 11, 327 (1827); Ann. Chim. Phys. [2] 36, 100 (1827); by treating calcium selenate with cadmium oxalate and hydrogen sulfide: von Hauer, J. Prakt. Chem. [1] 80, 214, 317 (1860); by treating a soln of silver selenite with bromine and evaporating the filtered soln: Thomsen, Ber. 2, 598 (1869); by oxidizing selenium oxide with 30% hydrogen peroxide and removing water: Gilbertson, King, J. Am. Chem. Soc. 58, 180 (1936); Inorg. Synth. 3, 137 (1950).