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Potassium Chromium Sulfate

Potassium chromium sulfate, KCr(SO4)212H2O, or chromium alum(1), forms violett octaeders and is highly soluble in water.

Potassium chromium sulfate has been used in the leather industry for tanning the hides (nowadays it is replaced by basic chromium sulfate, Cr(OH)SO4). The tanning process stabilizes the leather by cross linking the collagen fibers of the hide. The -COOH groups of the collagen are linked by chromium groups to form -COO-Cr(H2O)4-OOC- bridges enhancing temperature stability (1 m2 of cow leather contains typically about 16 g chromium).

References

Alums are sulfates belonging to the class of double salts. In general, alums consist of a monovalent and a trivalent cation, having the formula MIMIII(SO4)212H2O.
MI could be Na, K, Rb, Cs, NH4, or Tl; MIII could be Al, Ga, In, Sc, Ti, V, Cr, Mn, Fe, Co, Rh, or Ir. Please note that in contrast to the other alkali elements the lithium ion is too small to be part of an alum.

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