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Cobalt alums. A demonstration experiment

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Claus E. Schäffer and Poul Steenberg

Department of Inorganic Chemistry, University of Copenhagen, DK 2100 Copenhagen, Denmark

Wayne Wolsey and Erica Nostrom

Department of Chemistry, Macalaster College, St. Paul, MN 55105-1899

J. Chem. Educ., 2002, 79 (8), p 958

DOI: 10.1021/ed079p958

Publication Date (Web): August 1, 2002

Abstract

The demonstration experiment describes the isolation of [Co(H₂O)₆]³⁺ both as the pure blue crystalline cesium cobalt alum, CsCo(SO₄)₂·12H₂O, and as a light greenish-blue solid solution of ammonium cobalt alum in (NH₄)Al(SO₄)₂·12H₂O. The hexaaquacobalt(III) ion is prepared chemically by oxidation with hydrogen peroxide, taking advantage of the stabilization of Co^III relative to Co^II by complexation with the carbonate ligand. A brief description of alum structure and a characterization of alum subclasses are included.