Chromium alums double salts

The Cr1 ion readily forms complexes it exists in aqueous solution as CrfH 0)j. and forms other complexes wilh anions, such asCr(H 0).CI , Crother properties due to variation in the bonding. Compounds or the trichloride have been reported with the following arrangements CrtHjOlsClj. [CrfHiOlsCljCIi- HiO. and [Cr(HiO)4Cf CU2 H 0 Trivalenl chromium also forms double salts, notably the chromium alums, hydrated double salts of Cr(III) sulfate and the alkali metal (or thallium or ammonium) sulfates. 

NH4)2S04.Al2(S04)3.24H20 The name alum originally came from the presence of Al + as the trivalent ion, but is now also applied to other double salts containing trivalent ions, thus, chromium(III) potassium sulfate (chrome alum) is... 

Aluminum, chromium and iron(III) sulfates form double salts, called alums, with the alkali sulfates. Perhaps the best known of this group is the so-called common alum, KA1(S04)2-12H20 (not to be confused with commercial alum, i.e., cmde aluminum sulfate). The alums are usually less soluble than their constituent sulfates and may be crystallized on adding a saturated solution of alkali sulfate, especially of NH4 or larger IVF, to a concentrated solution of Al , Cr or Fe sulfate. 

The Preparation of Alums.—Since aluminum sulfate is a commercial chemical, the aluminum alums can be prepared by crystallizing the salt along with the double molecular weight of an alkali sulfate. Chromium and ferric sulfates are not well crystallized and have limited uses, so that they are not readily obtained. The preparation of their alums involves the preliminary preparation of the sulfates themselves. 

Chromium Chlorate, Cr(C10g)3, which is obtained by the double decomposition of barium chlorate and chrome alum, forms a violet solution which becomes green at 65° C. and decomposes on boiling, yielding chromium hydroxide and oxides of chlorine. Tire dcconr-position is facilitated by potassium ferricyanide. The salt is used iir calico printing and for increasing the permanence of aniline black. 

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