Chromium(III) potassium sulfate dodecahydrate belongs to a group of chemical compounds called alums with an analogical composition of MIMIII(SO4)2·12H2O (where MI is usually monovalent cation such as potassium K+ or ammonium NH4+ cation and MIII is represented by trivalent cation – Al3+, Cr3+, Fe3+. All alums are isostructural and crystallize in cubic system. The crystals are octahedrally shaped. The loss of water of crystallization is followed by frothing and swelling leaves amorphous powder. The alums can be easily prepared mixing the solutions of sulphates containing desired metal cations in ratio of metal atoms 1:1. The alums are highly water soluble and crystallize well also from the solutions with a little deviation from ratio 1:1. For chrom alum there is a special way it can be prepared. Reducing potassium dichromate by sulphur dioxide in the presence of sulphuric acid leaves in the solution its basic structure units K+, Cr3+ and SO42-.
K2Cr2O7 + 3 SO2 + H2SO4 -> K2SO4 + Cr2(SO4)3
K2SO4 + Cr2(SO4)3 + 12 H2O -> 2 CrK(SO4)2·12H2O
Potassium dichromate (1.767 g, 6 mmol) was dissolved in 20 mL of distilled water and placed into the gas washing bottle. The apparatus shown on the picture below was set up. The content of gas washing bottles: 1. empty, 2. concentrated sulfuric acid (drying agent), 3. empty, 4. potassium dichromate water solution, 5. empty, 6. NaOH 20% water solution (neutralizing agent), 6. empty (adapted to the vacuum pump).
To the three-neck round bottom flask copper (ca. 5 g) was placed and the funnel containing concentrated sulfuric acid was mounted to the flask. Sulfuric acid was slowly added to the flask and the mixture was heated over the flame. Sulfur dioxide that started to evolve was directed to the potassium dichromate solution. After a change in color from orange to dark green, the reaction was over. The mixture that resulted was let freely to crystallize. Dark violet octahedral crystals were collected after a week.