Chromium III

24 CHROMIUM, Cr (At 51-996) - CHROMIUM(III) Chromium is a white, crystalline metal and is not appreciably ductile or malleable. It melts at 1765°C. The metal is soluble in dilute or concentrated hydrochloric acid. If air is excluded, chromium(II) ions are formed  

In the presence of atmospheric oxygen chromium gets partly or wholly oxidized to the tervalent state  

Dilute sulphuric acid attacks chromium slowly, with the formation of hydrogen. In hot, concentrated sulphuric acid chromium dissolves readily, when chromium(III) ions and sulphur dioxide are formed  

Both dilute and concentrated nitric acid render chromium passive, so does cold, concentrated sulphuric acid and aqua regia. 

In aqueous solutions chromium forms three types of ions the chromium(II) and chromium(III) cations and the chromate (and dichromate) anion, in which chromium has an oxidation state of + 6. 

---

In acid solution, dichromates(VI) (and also chromates(VI) which are converted to dichromates) are reduced to chromium(III) salts ... 

Hence the orange colour of a dichromate is converted to the green colour of the hydrated chromium(III) ion, Cr ", and sulphur is precipitated when hydrogen sulphide is passed through an acid solution.)... 

The reaction provides a valuable method of preparing anhydrous chlorides of metals. It has been used to prepare the anhydrous chlorides of copper(II), zinc, cadmium, chromium(III), iron(III). cobalt(Il) and nickel. 

Sulphur dichloride oxide (thionyl chloride) on the hydrated chloride can also be used to produce the anhydrous chloride in certain cases, for example copper(II) chloride and chromium(III) chloride ... 

The chromates of the alkali metals and of magnesium and calcium are soluble in water the other chromates are insoluble. The chromate ion is yellow, but some insoluble chromates are red (for example silver chromate, Ag2Cr04). Chromates are often isomorph-ous with sulphates, which suggests that the chromate ion, CrO has a tetrahedral structure similar to that of the sulphate ion, SO4 Chromates may be prepared by oxidising chromium(III) salts the oxidation can be carried out by fusion with sodium peroxide, or by adding sodium peroxide to a solution of the chromium(IIl) salt. The use of sodium peroxide ensures an alkaline solution otherwise, under acid conditions, the chromate ion is converted into the orange-coloured dichromate ion ... 

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 ... 

This is the most common and stable state of chromium in aqueous solution. The Cr ion, with 2d electrons, forms mainly octahedral complexes [CrX ], which are usually coloured, and are kweticallv inert, i.e. the rate of substitution of X by another hgand is very slow consequently a large number of such complexes have been isolated (see below, under chromium(III) chloride). 

Chromium(III) hydroxide is obtained as a light green gelatinous precipitate when an alkali or ammonia is added to a chromium(III) salt ... 

Chromium(III) hydroxide, like aluminium hydroxide, possesses adsorptive power, and the use of ehromium compounds as mordants is due to this property. 

Chromium(III) hydroxide dissolves in acids to form hydrated chromium(lll) salts in concentrated alkali, hydroxo-complexes [Cr(OH)g] are formed. 

Hydrated chromium(III) sulphate exhibits different colours and different forms from which varying amounts of sulphate ion can be precipitated by barium chloride, due to the formation of sulphato-complexes. Chromium(III) sulphate can form alums. 

A reducing agent (for exampie sulphur dioxide) reduces the yellow chromate or orange dichromate to the green chromium(III) state. 

Addition of alkali gives a green gelatinous precipitate of chromium(III) hydroxide, soluble in a large excess of strong alkali. 

However, compounds known to be double oxides in the solid state are named as such for example, Cr2Cu04 (actually Cr203 CuO) is chromium(III) copper(II) oxide (and not copper chromite). 

BackTitrations. In the performance of aback titration, a known, but excess quantity of EDTA or other chelon is added, the pH is now properly adjusted, and the excess of the chelon is titrated with a suitable standard metal salt solution. Back titration procedures are especially useful when the metal ion to be determined cannot be kept in solution under the titration conditions or where the reaction of the metal ion with the chelon occurs too slowly to permit a direct titration, as in the titration of chromium(III) with EDTA. Back titration procedures sometimes permit a metal ion to be determined by the use of a metal indicator that is blocked by that ion in a direct titration. Eor example, nickel, cobalt, or aluminum form such stable complexes with Eriochrome Black T that the direct titration would fail. However, if an excess of EDTA is added before the indicator, no blocking occurs in the back titration with a magnesium or zinc salt solution. These metal ion titrants are chosen because they form EDTA complexes of relatively low stability, thereby avoiding the possible titration of EDTA bound by the sample metal ion. 

Chromium(III) Chemistry. The most characteristic reactions of Cr(III) in aqueous solution at >4 pH, eg, in the intestine and blood, and hydrolysis and olation (147). As a consequence, inorganic polymeric molecules form that probably are not able to diffuse through membranes. This may be prevented by ligands capable of competing for coordination sites on Cr(III) (see Coordination compounds) (147). Thus any large fraction of ingested Cr(III) should be absorbed. Chromium (ITT) in the form of GTF may be more efficiendy absorbed. 

---