Chromium Compounds

The chemistry of chromium is somewhat complicated owing to the varying degrees of valency exhibited by the element, wich results in meny different Chromium Compounds. In its three most important oxides, all of which give rise to corresponding series of salts, it functions respectively as a di-, tri-, and hexa-valent element, while in some of its compounds it behaves as a hepta-, penta-, or even tetra- valent element.

The oxide CrO, chromous oxide, containing divalent chromium, is strongly basic and gives rise to the chromous salts, for example, chromous chloride, CrCl₂, which are similar in character to the ferrous and manganous salts except that they show a greater tendency to become oxidised by the air, or other oxidising agents, to the chromic condition. They therefore act as powerful reducers. The analogy to iron and manganese, and also to vanadium, its congener in Group V, is further shown in the sulphate, CrSO₄.7H₂O, and its double salts. The stability of these isomorphous sulphates increases in the order V-Cr-Mn.

The sesquioxide, Cr₂O₃, containing trivalent chromium, is an amphoteric oxide. It yields chromic salts, such as chromic chloride, CrCl₃, and sulphate, Cr₂(SO₄)₃, which are very stable and show great similarity to the ferric salts and to salts of aluminium as, for example, in the formation of alums. Since, however, chromic oxide functions as a weaker base than chromous oxide, the latter having a lower oxygen content, the chromic salts are more liable to hydrolysis than the chromous salts. This is well marked in the case of the chlorides. Again, in spite of the stability of chromic salts, only a slight tendency to form simple Cr^••• ions is exhibited, whilst complex ions are formed much more readily, not only complex anions, as in the case of iron and aluminium, but also complex cations, as in the extensive chromammine series.1 In this respect chromium resembles cobalt and platinum.

An interesting form of isomerism, dependent on the formation of such cations, is exhibited by chromic salts, which usually exist in at least two modifications, the one green and the other violet or dark blue. In both varieties the chromium is in the same state of oxidation, but the non-metallic radicle, while apparently freely ionised in a violet solution, is only partly active in the green. Thus the chlorine in violet chromic chloride, CrCl₃.6H₂O, is completely precipitated by the addition of a soluble silver salt, but in the ordinary green variety only one-third of the chlorine can be so precipitated; a third isomeride, green in colour, in which two-thirds of the chlorine can be precipitated, is also known. The probable constitution of these isomers is discussed later under the respective Chromium Compounds. In a solution of a chromic salt equilibrium is gradually set up between the violet and green varieties, the proportion of each present depending on the temperature and the total concentration. The violet solutions on heating usually turn green, the violet form of the salt being less stable at higher temperatures. The formation of the green variety is also favoured by concentration. Nevertheless, evaporation of a green solution generally leads to the formation of a basic salt due to hydrolysis. The less soluble violet salts are therefore more readily obtained in the crystalline form than are the green. The rate at which equilibrium sets up varies with different salts. Solutions of the chloride, nitrate, and acetate readily become green when heated to 95° C., and return to violet on cooling; the sulphate, however, changes much more slowly. Owing to the difference in the constitution of the two types of Chromium Compounds, and its effect on the nature of their ionisation, solutions of the violet salts have different electrical conductivity from that of solutions of the corresponding green salts of the same concentration, and use has been made of this fact in determining the rate of change. The two modifications also act as hydrolytic catalysts showing distinct differences in their degree of activity.

Chromic oxide also exhibits acidic properties, combining with strong bases to form chromites.

Chromium trioxide, CrO₃, or chromic anhydride, is a strong acid- forming oxide, producing chromic acid and the chromates analogous to sulphuric acid and the sulphates. The position of chromium as the first member of the A subgroup of Group VI. of the Periodic Table explains the extreme stability of this oxide and its derivatives, in which the metal figures as a hexavalent element. There is a far-reaching analogy between these Chromium Compounds and the corresponding compounds of the other members of the group, as may be seen, for example, in the isomorphism of the sulphates, chromates, selenates, molybdates, and tungstates. The closer relation between chromium, molybdenum, and tungsten shows itself in the formation of condensed poly-acids, whereas similar compounds of sulphuric acid are not known.

Chromium trioxide and the salts of chromic acid are powerful oxidising agents. The action depends upon the reduction of chromium to the trivalent condition thus:

2CrO₃ → Cr₂O₃ + 3O,

so that chromic compounds result, and the reaction is accompanied by a colour change from yellow to green.

It is thought by some that the metal acts as a tetravalent element in chromium dioxide, CrO₂, but this compound may also be considered as a basic chromic chromate, Cr₂O₃.CrO₃. Chromium appears to function as a pentavalent element in the oxychloride, CrOCl₃, and its derivatives. The perchromic acids and perchromates have long been thought to contain heptavalent chromium, but it would appear that some of these compounds at least are derived from a hypothetical chromium tetroxide, CrO₄, in which the element is hexavalentt, thus:



These compounds are extremely unstable.

Four well-defined oxides of chromium are known: - chromous oxide, CrO; chromic oxide, Cr₂O₃; chromium dioxide, CrO₂; and chromium trioxide, CrO₃. Chromous oxide, in which the metal is divalent, is basic in character and gives rise to the chromous series of salts; chromic oxide, containing trivalent chromium, has both basic and acidic properties, since, on the one hand, it gives rise to chromic salts, and on the other, it is soluble in alkalies with the formation of chromites; the dioxide, CrO₂, may be regarded as chromic chromate, Cr₂O₃.CrO₃; chromium trioxide, which contains hexavalent chromium, possesses only acidic characters, combining with alkalies to form chromates. A number of other oxides, which may be regarded as compounds of the above, have also been described. The heptoxide, Cr₂O₇, regarded as the basis of some of the perchromates, has not been isolated.

Chromic hydroxide is an amphoteric compound and exhibits acidic properties in combining with basic oxides to form chromites, to which the general formula M₂O.Cr₂O₃ is given, and which are isomorphous with the corresponding aluminium compounds known as "spinels." They may be considered as derived from an acid, HCrO₂; the monohydrate, Cr₂O₃.H₂O, has this empirical formula. From a study of the action of sodium hydroxide on chromium hydroxide for prolonged periods and the rate of the formation of chromate by the oxidation of dissolved chromite, it would appear that the chromic hydroxide acts as a polybasic acid.

Chromates are usually yellow or red in colour, and, except those of ammonium, the alkali metals, calcium, strontium, and magnesium, are practically insoluble in water. They are obtained by oxidation of chromites, by fusion of chromium sesquioxide with the appropriate base in presence of air or of an oxidising agent; by oxidation of chromium salts in solution; or by double decomposition. Normal, di-, and tri-chromates, etc., are derived from one and the same acid oxide; K₂CrO₄ behaves like an alkali towards CrO₃, since it is quantitatively converted into dichromate. A large number of complex double chromates are known.

Chromates, dichromates, etc., are readily reduced, e.g. by hydrochloric acid (with evolution of chlorine), by sulphurous acid (with formation of sulphate and dithionate), by hydrogen sulphide (with separation of sulphur), by ferrous salts, by alcohol, etc., the solution simultaneously becoming green owing to the formation of a chromic salt. The chromates of the more feebly electro-positive elements decompose when strongly heated, with formation of chromium sesquioxide; dichromates of other metals yield normal chromates, chromium sesquioxide, and oxygen.

In aqueous solution normal chromates are yellow in colour; on treatment with acid they are converted into the orange-red dichromates; the yellow chromate is regenerated on treatment of an aqueous solution of a dichromate with an alkali. Alkali chromates and dichromates may be supposed to dissociate in solution primarily in accordance with the equations:

M₂CrO₄ ⇔ 2M^•+ CrO₄''
M₂Cr₂O₇ ⇔ 2M^•+ Cr₂O₇''.

Chromate solutions, however, undergo hydrolysis, which may be represented thus:

CrO₄'' + H₂O ⇔ HCrO₄' + OH',

the hydrolysis proceeding further than would ordinarily be the case, owing to the dehydration of part of the hydrochromate ion:

2HCrO₄ ⇔ H₂O + Cr₂O₇''.

Accordingly, normal chromate solutions are alkaline to the usual indicators. On the other hand, the dichromates react acid, since the dichromate ion, Cr₂O₇'', is partly hydrated, with the formation of 2HCrO₄', which in turn is slightly dissociated into 2H^• and 2CrO₄''. Thus, in the equilibria prevailing in chromate and dichromate solutions, the intermediate hydrochromate ion, HCrO₄', plays an important part. Chromates, if soluble in the gastric juices, exert a poisonous action on the human system; they also possess antiseptic and preservative properties.

When hydrogen peroxide is added to an acidified aqueous solution of a chromate, oxidation occurs and a deep indigo-blue colour results. The reaction is extremely delicate, and may be used as a test for either reactant. The blue product is very unstable, rapidly losing oxygen and yielding a chromium salt, but it remains undecomposed for a longer time if dissolved in ether, amyl alcohol, or ethyl acetate. The isolation of definite "perchromic" compounds from such solutions has been attended with great difficulty. By evaporation of the ethereal solution at -20° C., Moissan obtained a blue oily substance, which he formulated as CrO₃.H₂O₂, and salts of composition BaCrO₅ and Na₆Cr₂O₅.28H₂O, prepared by suitable neutralisation of the blue solution, have been described. Definite perchromates, however, were not isolated until 1897. A free perchromic acid, of composition H₃CrO₈.2H₂O, has now been prepared, and four types of derivatives have been shown, with reasonable certainty, to exist:

1. Alkali perchromates of the type R^•₃CrO₈, reddish brown in colour;
2. Alkali salts of the type R^•H₂CrO₇ or R^•H₂CrO₅.H₂O₂, blue in colour;
3. Derivatives of chromium tetroxide; for example, chromium tetroxide triammine, CrO₄.3NH₃;
4. Perchromates of organic bases of the type HCrO₅.X.

When chromic chloride or chromyl chloride is heated with excess of phosphorus pentachloride in a sealed tube, a violet crystalline compound, having the composition PCl₅.CrCl₃, results.

Michaelis noticed that when small quantities of chromyl chloride and phosphorus trichloride were brought together, a vigorous reaction occurred accompanied by a hissing noise and evolution of light. He represented the change by the following equation:

4CrO₂Cl₂ + 6PCl₃ = 4CrCl₃ + PCl₅ + 3POCl₃ + P₂O₅.

On heating potassium dichromate with phosphorus trichloride in a sealed tube at 166° C. the following reaction occurred:

30K₂Cr₂O₇ + 42PCl₃ = 18CrO₃.KCl.15PO₃K + 42CrO₂ + 27KCl + 27POCl₃'

The action of the phosphorus halides on chromyl chloride has been studied more recently by Fry and Donnelly, who worked with nonaqueous solvents. The explosive nature of the reactions was moderated by bringing the substances together in solutions of 0.2 molecular concentration in anhydrous carbon tetrachloride. With phosphorus trichloride and tribromide, solid double compounds were produced according to the equations:

2CrO₂Cl₂ + 3PCl₃ = 2(CrOCl.POCl₈) + PCl₅;
2CrO₂Cl₂ + 3PBr₃ = 2 (CrOCl.POBr₃) + PBr₃Cl₂.

The double compounds are extremely deliquescent, and react with water, with development of heat, according to the equation:

CrOCl.POCl₃ + 2H₂O = CrCl₃ + HCl + H₃PO₄.

On ignition, the compounds CrOCl (or Cr₂O₃.CrCl₃) and CrOBr (or Cr₂O₃.CrBr₃) are produced.

Chromyl chloride and phosphorus pentachloride, under the same conditions, yield an additive compound, CrO₂Cl₂.P₂O₅, as a yellowish-red powder, which is easily decomposed by water.

With phosphorus pentabromide a substance is obtained which appears to be a mixture of the compounds CrOCl.POBr₃ and CrO₂Cl₂.PBr₅. This is probably due to the fact that phosphorus pentabromide is partly dissociated to tribromide in carbon tetrachloride solution.

With phosphorus di-iodide a brown additive compound, CrO₂Cl₂.PI₂, is obtained. It is readily decomposed by water, giving free iodine and a solution containing chromic, phosphate, chloride, and iodide ions.

Phosphorus tri-iodide under similar conditions gives the additive compound CrO₂Cl₂.PI₃, which is a purplish-red powder when dry. It is decomposed by water, thus:

2CrO₂Cl₂.PI₃ + 4H₂O = 4HCl + 4HI + 2CrPO₄ + I₂.

Alloys of chromium and silicon are readily obtained by heating chromium sesquioxide with excess of silicon at full white heat, or with silicon carbide, or silicon carbide and carbon, in the electric furnace; or by strongly heating chromium sesquioxide, silica, and aluminium. From these alloys several definite silicides have been isolated, which are usually grey in colour, hard and brittle, and very resistant to acids, except hydrofluoric acid, which readily decomposes them. The silicides, Cr₃Si, Cr₂Si, Cr₃Si₂, and CrSi₂, have been obtained in a state of comparative purity by special methods of preparation.

When chromium, or chromium sesquioxide, is heated with boron in an electric furnace, borides are formed. If the operation is carried out in a carbon crucible, the product always contains free carbon which cannot be completely separated. By reducing the sesquioxide by heating with boron in magnesia crucibles in an electric furnace, du Jassonneix obtained a series of alloys containing 5 to 17 per cent, of combined boron, and succeeded in isolating two definite borides of composition CrB and Cr₃B₂.