1. J. W. Mellor, A Comprehensive Treatise on Inorganic and Theoretical Chemistry, Vol. X. London: Longmans Green. 1930. p. 563.
2. Gmelins Handbuch der anorganischen Chemie, Schwefel System, No. 9. Teil B2. New York: Wiley. 1960. p. 1041.
3. E. M. Chamot and C. W. Mason, Handbook of Chemical Microscopy, 2nd. Ed. Vol. 2. Chemical Methods and Inorganic Qualitative Analysis. New York: Wiley. 1940. p. 349.
4. J. W. Mellor, l. c. 1 p. 166.
5. R. C. Brasted, Comprehensive Inorganic Chemistry, Vol. 8. Princeton: Van Nostrand. 1961. p. 149.