Chromium

Chromium is a grayish crystalline metal, which when produced in a suitable way, can have a very high polish. For this reason, it is used for decorative purposes on vehicles, in houses and certain ornaments.

Chromium is fairly corrosion resistant, but concentrated non-oxidizing acids are capable of dissolving it at a moderate rate.

Metallic chromium can be interesting for the home lab, but for experiments with chromium in aqueous environments, it is easier to use soluble salts of chromium as a starting point. Elementary chromium of high purity frequently is available on eBay.

Chromium exists in several ionic compounds in oxidation states +2, +3, +4, +5 and +6. Most stable are the +3 and +6 oxidation states.

The name of the element is derived from the latin word 'chromos', meaning 'color'. Indeed, chromium forms a remarkable number of colorful compounds in its several oxidation states:

	+2 : blue
	+3 : almost every color in the range from purple to green
	+5 : red tones and brown tones
	+6 : red, orange and yellow

Chromium in its +3 oxidation state by far has the richest aqueous chemistry. It forms complexes with virtually everything. These complexes also are remarkably inert. Transformation from one coordination complex to another coordination complex can be very slow. Frequently, preparation of complexes of chromium in its +3 oxidation state is done through the element in another oxidation state. The complex then is formed in a combined redox/coordination reaction, where the newly formed chromium (III) is coordinated immediately on formation.

The following compounds of chromium are available for the general public:

• chrome alum, KCr(SO₄)₂·12H₂O
• chromium (III) oxide, also called chromium sequioxide, Cr₂O₃
• chromium (VI) oxide, also called chromium trioxide, CrO₃
• potassium dichromate, K₂Cr₂O₇
• sodium dichromate, Na₂Cr₂O₇·2H₂O
• ammonium dichromate, (NH₄)₂Cr₂O₇

Chrome alum is a crystalline dark purple compound, available at many photography raw chemical suppliers. It is used as part of certain hardening fixers. This compound dissolves in water fairly well and is isomorphic with plain alum. A mixture of a small amount of chrome alum and ordinary alum can form beautiful reddish/purple crystals.

Chrome alum can serve as the basis for several interesting experiments, related to coordination chemistry. It is remarkably difficult to reduce this compound to chromium in its +2 oxidation state. Oxidation to the +6 oxidation state is accomplished fairly easily with bleach or with hydrogen peroxide in an alkaline environment.

Chromium (III) oxide is available as an inert green powder, used as a pigment in ceramics. It is available from ceramics and pottery stores. This powder is so inert, that it cannot be dissolved in either concentrated acids, nor in concentrated basic solutions. The only way of dissolving this is adding it to molten alkalies. For the home chemist, this compound is not interesting.

Chromium (VI) oxide is available as dark red/brown lumps, which dissolve in water very easily to a reddish or orange liquid, depending on concentration. This compound is very hygroscopic. It is available from a limited number of photography raw chemical suppliers.

Chromium (VI) oxide is a really dangerous compound, which is very reactive, very corrosive and an absolute pain to the skin. When it is brought in contact with organic solvents (alcohol, acetone), then it might cause spontaneous ignition of the liquid. It certainly is not a compound to start with in a home lab! As all hexavalent chromium compounds, it probably is a carcinogen.

Potassium dichromate, sodium dichromate and ammonium dichromate all are similar looking bright orange crystalline solids. For aqueous chemistry experiments it is best to have either potassium dichromate or sodium dichromate, because the aqueous properties of these compounds are only determined by the anion. Sodium dichromate is hygroscopic, which is a disadvantage, but it allows solutions to be prepared at much higher concentration than potassium dichromate.

Dichromates are fairly strong oxidizers, but they are much safer on storage and handling than chromium (VI) oxide. An acidified solution of one of the dichromates for most experiments works equally well as a solution of chromium (VI) oxide, so working with dichromates is strongly preferred over working with chromium (VI) oxide. The only situation where the oxide may have an advantage over the use of dichromates is where the reaction is carried out in a non-aqueous solvent, in which the dichromates do not dissolve.

Ammonium dichromate can be used for carrying out the well-known 'volcano' experiment. This is a nice visual effect, google the following set of keywords for tons of info about this: volcano ammonium dichromate. In this reaction, the ammonium dichromate is decomposed to water, nitrogen gas and chromium (III) oxide. However, be very careful not to breathe any of the small green/grey ashes, which are blown into the air! Frequently the green/grey powder still contains left-over ammonium dichromate!

Dichromates are skin sensitizers. When the skin comes in contact with them, then the skin should be rinsed well immediately.

Dichromates probably also are carcinogens. Incidental exposure to some dichromate will not lead to cancer immediately, but when working with them more frequently, it is really important to pay extra attention to not getting exposed to them and to keep the working place clean.

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