Chrome alum or Chromium(III) potassium sulfate is the
potassium
Potassium is a chemical element; it has Symbol (chemistry), symbol K (from Neo-Latin ) and atomic number19. It is a silvery white metal that is soft enough to easily cut with a knife. Potassium metal reacts rapidly with atmospheric oxygen to ...
double
sulfate of
chromium. Its chemical formula is KCr(SO
4)
2 and it is commonly found in its
dodecahydrate form as KCr(SO
4)
2·12(H
2O). It is used in
leather
Leather is a strong, flexible and durable material obtained from the tanning (leather), tanning, or chemical treatment, of animal skins and hides to prevent decay. The most common leathers come from cattle, sheep, goats, equine animals, buffal ...
tanning.
Production and properties
Chromium alum is produced from
chromate salts or from ferrochromium alloys. Concentrated aqueous solutions of potassium dichromate can be reduced, usually with
sulfur dioxide
Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a colorless gas with a pungent smell that is responsible for the odor of burnt matches. It is r ...
but also with
alcohol
Alcohol may refer to:
Common uses
* Alcohol (chemistry), a class of compounds
* Ethanol, one of several alcohols, commonly known as alcohol in everyday life
** Alcohol (drug), intoxicant found in alcoholic beverages
** Alcoholic beverage, an alco ...
s or
formaldehyde, in the presence of sulfuric acid at temperatures <40 °C. Alternatively and less commonly, ferrochromium alloys can be dissolved in sulfuric acid and, after precipitation of the ferrous sulfate, the chrome alum crystallizes upon addition of
potassium sulfate. Chromium alum crystallizes in regular
octahedra with flattened corners and is very soluble in water. The solution reddens
litmus and is an
astringent. Aqueous solutions are dark violet and turns green when it is heated above 50 °C.
In addition to the
dodecahydrate, the hexahydrate KCr(SO
4)
2·6H
2O, dihydrate KCr(SO
4)
2·2H
2O, and the monohydrate KCr(SO
4)
2·H
2O are known.
[Gerd Anger, Jost Halstenberg, Klaus Hochgeschwender, Christoph Scherhag, Ulrich Korallus, Herbert Knopf, Peter Schmidt, Manfred Ohlinger, "Chromium Compounds" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2005. ]
Uses
Chromium alum is used in the
tanning of
leather
Leather is a strong, flexible and durable material obtained from the tanning (leather), tanning, or chemical treatment, of animal skins and hides to prevent decay. The most common leathers come from cattle, sheep, goats, equine animals, buffal ...
as chromium(III) stabilizes the leather by cross linking the
collagen
Collagen () is the main structural protein in the extracellular matrix of the connective tissues of many animals. It is the most abundant protein in mammals, making up 25% to 35% of protein content. Amino acids are bound together to form a trip ...
fibers within the leather. However, this application is obsolete because the simpler
chromium(III) sulfate is preferred.
[
It was also used in gelatine emulsions in photographic film as hardener.][British Journal of Photography, vol 23 https://archive.org/details/britishjournalof35unse/page/478/mode/2up ]
References
{{Reflist, 2
Sulfates
Potassium compounds
Chromium(III) compounds
Coordination complexes
Double salts