Chemical elements
  Chromium
    Isotopes
    Energy
    Occurrence
    Preparation
    Physical Properties
    Chemical Properties
    Alloys
    Amalgams
    Compounds
      Chromous Fluoride
      Chromic Fluoride
      Chromyl Fluoride
      Chromous Chloride
      Chromic Chloride
      Oxychlorides
      Chromyl Chloride
      Trichromyl Chloride
      Chromium Chlorate
      Chromium Perchlorate
      Chromous Bromide
      Chromic Bromide
      Complex Halogen-halides
      Chromous Iodide
      Chromic Iodide
      Chromium Iodate
      Chromous Oxide
      Chromo-chromic Oxides
      Chromic Oxide
      Chromic Hydroxide
      Barium Chromite
      Cadmium Chromite
      Calcium Chromite
      Cobalt Chromite
      Cupric Chromite
      Cuprous Chromite
      Ferrous Chromite
      Lithium Chromite
      Magnesium Chromite
      Manganese Chromite
      Zinc Chromite
      Chromium Dioxide
      Chromium Trioxide
      Aluminium Chromate
      Ammonium Chromate
      Ammonium Lithium Chromate
      Ammonium Potassium Chromate
      Ammonium Sodium Chromate
      Ammonium Dichromate
      Ammonium Fluochromate
      Barium Chromate
      Barium Dichromate
      Barium Potassium Trichromate
      Beryllium Chromate
      Bismuth Chromate
      Bismuth Potassium Chromates
      Cadmium Chromate
      Cadmium Dichromate
      Cadmium Trichromate
      Caesium Chromate
      Caesium Dichromate
      Calcium Chromate
      Calcium Dichromate
      Cobalt Chromate
      Cobalt Dichromate
      Copper Chromates
      Cupric Dichromate
      Gold Chromates
      Iron Chromates
      Ferric Chromate
      Ferric Ammonium Chromate
      Lead Chromate
      Basic Lead Chromates
      Lead Dichromate
      Lithium Chromate
      Lithium Chlorochromate
      Magnesium Chromates
      Manganese Chromates
      Mercuric Chromate
      Mercuric Dichromate
      Nickel Chromate
      Nickel Dichromate
      Potassium Chromate
      Potassium Dichromate
      Potassium Trichromate
      Potassium Tetrachromate
      Potassium Fluochromate
      Potassium Chlorochromate
      Rubidium Dichromate
      Silver Chromate
      Silver Dichromate
      Sodium Chromate
      Sodium Dichromate
      Sodium Trichromate
      Sodium Chlorochromate
      Stannic Chromate
      Strontium Chromate
      Strontium Dichromate
      Strontium Trichromate
      Thallous Chromate
      Thallic Chromate
      Thallous Dichromate
      Thallous Trichromate
      Thallous Chlorochromate
      Uranyl Chromate
      Zinc Chromate
      Zinc Dichromate
      Zinc Trichromate
      Perchromic Acid
      Chromium Tetroxide Triammine
      Chromous Sulphide
      Chromium Tetrasulphide
      Chromic Sulphide
      Sodium Thiochromite
      Potassium Thiodichromite
      Chromic Sulphite
      Chromous Sulphate
      Chromic Sulphate
      Lithium Chromic Sulphate
      Sodium Chromic Sulphates
      Potassium Chromic Sulphates
      Potassium Chromium Alum
      Ammonium Chromium Alum
      Hydrazine Chromium Alum
      Sulphochromic Acid
      Chromous Selenide
      Chromic Selenide
      Chromic Selenite
      Chromium Nitrides
      Chromium Azide
      Chromic Nitrate
      Chromium Monophosphide
      Chromium Sesquiphosphide
      Chromic Hypophosphite
      Chromous Orthophosphate
      Chromic Orthophosphates
      Chromous Metaphosphate
      Chromic Metaphosphate
      Chromic Pyrophosphate
      Ammonium Chromi-pyrophosphate
      Potassium Chromi-pyrophosphate
      Sodium Chromi-pyrophosphate
      Chromous Thiophosphite
      Chromous Thiopyrophosphite
      Chromous Thiopyrophosphate
      Chromous Arsenide
      Chromium Sesqui-arsenide
      Chromic Arsenite
      Chromic Arsenate
      Chromium Pyroarsenate
      Chromic Thioarsenite
      Chromium Chlorantimonate
      Chromium Orthoantimonichloride
      Tetrachromium Carbide
      Pentachromium Dicarbide
      Tetrachromium Dicarbide
      Chromium Tungsten Carbide
      Chromous Carbonate
      Ammonium Chromous Carbonate
      Potassium Chromous Carbonate
      Sodium Chromous Carbonate
      Chromic Carbonates
      Chromium Thiocarbonate
      Chromous Cyanide
      Chromic Cyanide
      Potassium Chromocyanide
      Hydrogen Chromicyanide
      Ammonium Chromicyanide
      Lithium Chromicyanide
      Sodium Chromicyanide
      Potassium Chromicyanide
      Cobaltous Chromicyanide
      Cupric Chromicyanide
      Lead Chromicyanide
      Manganous Chromicyanide
      Mercury Chromicyanide
      Nickel Chromicyanide
      Silver Chromicyanide
      Zinc Chromicyanide
      Chromous Thiocyanate
      Chromic Thiocyanate
      Chromithiocyanic Acid
      Ammonium Chromithiocyanate
      Sodium Chromithiocyanate
      Potassium Chromithiocyanate
      Barium Chromithiocyanate
      Silver Chromithiocyanate
      Lead Chromithiocyanate
      Chromium Ferrocyanide
      Trichromium Silicide
      Dichromium Silicide
      Trichromium Disilicide
      Chromium Disilicide
      Chromium Aluminium Silicide
      Sodium Chromisilicates
      Chromium Silicofluoride
      Chromium Boride
      Trichromium Diboride
      Chromous Borate
      Chromic Borate
    Detection
    Estimation
    PDB 1huo-9icc

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:

2CrO3Cr2O3 + 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.


Chromium and Oxygen

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.

Chromites

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, Dichromates, and Polychromates

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' + H2OHCrO4' + OH',

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

2HCrO4H2O + 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.

Perchromic Acid and Perchromates

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 R3CrO8, reddish brown in colour;
  2. Alkali salts of the type RH2CrO7 or RH2CrO5.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.

Double Compounds with the Halides of Phosphorus

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.

Chromium and Silicon

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.

Chromium and Boron

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.
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