Potassium chrome alum, preparation

The dichromate ion oxidises iron(II) to iron(III), sulphite to sulphate ion, iodide ion to iodine and arsenic(III) to arsenic(V) (arsenate). Reduction of dichromate by sulphite can be used to prepare chrome alum, since, if sulphur dioxide is passed into potassium dichromate acidified with sulphuric acid, potassium and chromium(III) ions formed are in the correct ratio to form the alum, which appears on crystallisation ... 

Synonyms Chrome alum, Chromium and potassium sulfate, Potassium chromium sulfate. Appearance Dark violet-red crystals (ruby red by transmitted light) or light violet powder. Uses Preparation of chrome alum fixing bath as hardening agent for gelatin. 

Chromic oxide jellies may be formed by adding sodium or potassium hydroxide or ammonia to a solution of chromic sulphate or chloride containing sodium acetate or by adding sodium or potassium hydroxide, hut not ammonia, to a solution of chrome alum. The jelly is violet if prepared by the addition of ammonia or of a slight excess of the alkali metal hydroxide if the latter is added in larger quantity the jelly is green. The jellies dissolve in hydrochloric acid, but re-form on neutralising the solution if sufficient sodium acetate is present. 

T. J. Pelouze prepared calcium oxychromite, Ca0.Ca(Cr02)2, by adding potassium hydroxide or ammonia to a mixture of a mol of chrome-alum and 2 mols of calcium chloride H. Moissan obtained calcium trioxychromite, 3Ca0.Ca(Cr02)2, in yellow... 

Two preparative methods are known for potassium hexathiocyantochromate (III). Both involve the exchange of the ligand water with thiocyanate. In both cases chrome alum, KCr(S04)2.12H20 reacts with either a solution of KSCN or it is added as a solid to molten KSCN. Since the potassium salts of the complex anions are very hygroscopic, it is advantageous analytically to transform them to the pyridinium salts. For a long time, the complexes were known to be readily partially aquated. However, interest in the spectra of their solutions in various solvents dates from the mid-fifties. 

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