CHROMIC ACID, OR CHROMIC ANHYDRIDE.

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of the iron is, however, generally displaced by the isomorphous metal magnesium, and part of the chromium by aluminum. Like magnetic iron ore, chrome iron-stone often crystallizes in octohedra, which have about the same hardness as felspar. Chrome ironstone is scarcely attacked by any of the acids. It is infusible in the furnace, and when heated absorbs oxygen from the air; this oxidation takes place rapidly when it is powdered and mixed with a carbonate of one of the alkalies or alkaline earths, chromic anhydride being formed. 100 parts of this ore, if pure, contain 48 27 of chromium, and yield 89.6 of chromic acid.

(784) Fremy believes in the existence of a series of ammoniated compounds of chromium presenting some analogy with those of cobalt, but other chemists have not confirmed his results. If a solution of sal ammoniac containing free ammonia be digested on the hydrated oxide precipitated by ammonia from one of the violet salts of chromium, the oxide is dissolved, and a fine violet-coloured solution is formed, owing to the formation of a double salt of ammonium and chromium. If the solution be evaporated to dryness, it furnishes a fine violet compound, which is very soluble in water, has scarcely any alkaline reaction, and gives no precipitate with nitrate of silver; the ordinary tests of chromium do not show the presence of the metal. When the solution is boiled, ammonia is expelled and hydrated oxide of chromium is precipitated.

(785) CHROMIC ANHYDRIDE, or Chromic Acid (Ereg=100'5, or Cro1=503); Sp. Gr. 2·676.-There are several modes of obtaining this compound. 1. The simplest consists in mixing 4 measures of a cold saturated solution of anhydro-chromate of potassium with 5 of oil of vitriol: as the liquid cools, the chromic anhydride separates in beautiful crimson needles; for though very soluble in water, this compound has the peculiarity of being nearly insoluble in sulphuric acid of sp. gr. 155, but is freely dissolved by it either in a more concentrated or in a more dilute condition. The crystals are allowed to dry upon a porous tile, under a bellglass. A good deal of sulphuric acid, however, still adheres to them; in order to remove it, the crystals should be dissolved in water, and a solution of acid-chromate of barium should be added in quantity just sufficient to throw down the whole of the sulphuric acid as sulphate of barium: the solution may be recrystallized by evaporation in vacuo. 2.-Chromic anhydride may also be prepared from the fluoride of chromium, by decomposing this compound with water (790).

Chromic anhydride is easily freed from water by drying it at

a gentle heat. While hot it is black, but it becomes dark red on cooling at about 400° it fuses, and if heated more strongly, becomes vividly incandescent, and is converted into the sesquioxide with disengagement of oxygen. The anhydride deliquesces when exposed to the air. Its solution has a sour, metallic taste, and possesses considerable oxidizing power, from the facility with which it is reduced to sesquioxide of chromium. When heated with hydrochloric acid, chlorine is evolved and chloride of chromium is formed; 2 ¤rÐ ̧+ 12 HCl=Єr2Cl ̧+ 6 H2→+ 3 Cl2. The anhydride forms more than one crystalline compound with sulphuric acid; these compounds are decomposed by water.

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(786) Chromates.-Chromic acid forms 3 classes of saltsbasic, normal, and acid.* The chromates of the alkali-metals are soluble in water; the normal salts have the general formula M'¤re; they have a yellow colour; the acid salts are of a bright orange: the most important of these salts are the chromate and anhydro-chromate of potassium, from which the other chromates are generally obtained.

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Anhydro-chromate of potassium (K‚Ð 2 ¤r✪ ̧, or K‚¤Ã‚Ð, =295); or Bichromate of potash (KO, 2 CrO,=147'5); Sp. Gr. 2624: Comp. in 100 parts, K., 31.86; Ere, 68·14.—This salt crystallizes in large red, transparent, anhydrous four-sided tables. It fuses below redness, and as it cools splits to pieces from the inequality of its contraction. By heating the anhydro-chromate to bright redness, normal chromate and green oxide of chromium are formed, whilst oxygen escapes. It requires about 10 times its weight of water at 60° for its solution.

In order to procure the anhydro-chromate of potassium, chrome iron-stone is heated to redness and quenched in cold water, by which means it is rendered friable, and is then reduced to an extremely fine powder, and heated to bright redness in a current of air in a reverberatory furnace with a mixture of chalk and carbonate of potassium, the mixture being constantly stirred to hasten the oxidation. When this is complete, the product is digested in water, carbonate of potassium being added, if necessary, to decompose any chromate of calcium which may have been formed, and the yellow solution is drawn off from the insoluble matter; it is then supersaturated with nitric acid; a portion of silica is thus precipitated, and after this has been sepa

*Chromic acid is remarkable for the anhydro-salts which it forms with potash ; like iodic acid, it yields three salts with this base: 1, that known as the chromate (K,0,¤re,); 2, the bichromate (K,→, 2 ¤r→ ̧); and 3, the terchromate of potash (K,, 3 ere,).

rated, the liquid, on evaporation, yields crystals of anhydrochromate of potassium, which are purified by recrystallization. The addition of chalk in the furnace favours the oxidation by preserving the mass in a porous condition: if potash alone were used, it would fuse, and the chrome ore would fall to the bottom.

According to Schweitzer several double salts may be formed by digesting anhydro-chromate of potassium with an equivalent of some base, such as lime or magnesia. The chromate of magnesium and potassium crystallizes in oblique rhombic prisms (K2Mg 2 ¤r✪ ̧ . 2 H,→), and a salt of similar composition may be obtained with calcium; but there is no analogy between these double chromates and the magnesian double sulphates.

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If a solution of carbonate of potassium be added to the anhydro-chromate, until it becomes of a pale yellow colour, carbonic anhydride is expelled, and the normal or neutral chromate (K,Єre=1945, or KO,CrO,=97'3; sp. gr. 2.682) is formed. This salt is soluble in about twice its weight of cold water, and still more freely so in boiling water; it has a pure and intense yellow colour. A very small quantity of the salt suffices to impart a yellow tinge to a considerable volume of water. evaporation of its solution chromate of potassium may be obtained with some difficulty, in transparent, yellow, anhydrous prisms, which are isomorphous with those of sulphate of potassium: at a red heat it fuses without undergoing decomposition. A terchromate of potassium (K,→, 3 ¤r→ ̧, or KO, 3 CrO3), also anhydrous, was obtained by Mitscherlich in deep red crystals, by adding an excess of nitric acid to a solution of the anhydro-chromate, and allowing it to evaporate.

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Chromate of sodium (Na¤r✪4.10 H2=162°5+180, or NaO,CrO ̧ . 10 Aq=81·3+90) may be obtained by a process similar to that employed in preparing chromate of potassium: it forms efflorescent crystals: an acid-chromate of sodium may likewise be formed.

Chromate of calcium is soluble, as is also the acid-chromate, which is formed in many chrome works as a preliminary stage in the manufacture of the chromates. Jacquelain decomposes chrome ore by roasting it in fine powder intimately mixed with chalk, grinds the roasted mass with water, and adds sulphuric acid till the liquid has an acid reaction, in which case acid-chromate of calcium is formed, and remains in solution. Chromate of barium (BaЄre; sp. gr. 3'90) is a canary-yellow insoluble powder. Chromate of strontium is yellow and but slightly soluble.

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Chromate of lead (Pb¤r✪=324°5, or PbO,CrO。=162'3 ;

sp. gr. 5653) forms the pigment called 'chrome yellow.'

It

is obtained by precipitating a somewhat dilute solution of acetate of lead by one of chromate or of anhydro-chromate of potassium. Chromate of lead is insoluble in water and in acids, but, like all the insoluble salts of lead, it is dissolved by a large excess of hydrate of potash or of soda. When heated to 400° or 50c°, its colour becomes reddish brown; at a higher temperature it fuses, and when heated still more strongly it gives off about 4 per cent. of oxygen, sesquioxide of chromium and basic chromate of lead being formed; 8 PbЄr0=4(PbЄre,,PbO) +2Єr,03 +30. Fused chromate of lead when reduced to powder is sometimes advantageously substituted for oxide of copper in the combustion and analysis of organic substances very rich in carbon. A dibasic chromate of lead (2 Pb,ЄrÐ ̧; sp. gr. 6·266), of a splendid scarlet colour, may be obtained by boiling a solution of the yellow chromate of lead with half an equivalent of lime, or by adding to a solution of nitrate of lead a solution of chromate of potassium, with which an additional equivalent of hydrate of potash has been previously mixed. It may be obtained of a still more brilliant colour by fusing one part of the normal chromate of lead with 5 parts of nitre; chromate of potassium and dibasic chromate of lead are formed; the salt of potassium may be removed by washing. This salt is used to impart a permanent orange to calico: it is easily fixed upon the fabric by dyeing it yellow with chromate of lead and then boiling it with limewater, by which half the chromic acid is abstracted, and the dibasic chromate is left attached to the fibre.

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Basic mercurous chromate (3 Hg,Єre, Hg,O) falls as an orange-coloured insoluble precipitate on adding basic mercurous nitrate to a soluble chromate. Chromate of silver (Ag2¤rÐ ; sp. gr. 5'770) is of a dark-red colour, the tint of which is deeper if the solutions be mixed whilst hot: it is crystalline, and sparingly soluble.

An anhydro-chromate of silver is obtained in beautiful crimson tables by heating metallic silver with anhydro-chromate of potassium and sulphuric acid, chrome-alum being formed during the process :

3 Ag2+ 4 K2¤r2+7+ 7 H2SO1= 3·Ag2¤r2Ð,+2 (K¤r 2 SO1) + 3 K2SO4+7 H2O.

Chromic acid is the colouring matter of the ruby. Bichromate of Chloride of Potassium (KCl,Єre,? or K,Єre Ere Cl; Sp. Gr. 2'466).—This remarkable compound may be

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SULPHIDE OF CHROMIUM- CHLORIDES OF CHROMIUM.

631

obtained crystallized in orange-coloured needles, by dissolving 3 parts of anhydro-chromate of potassium and 4 of hydrochloric acid in a little water at a gentle heat, and allowing it to cool: a large quantity of water decomposes the salt.

(787) Sesquisulphide of Chromium (Er,S,=201). This compound may be obtained in black shining scales, resembling plumbago in appearance, when the vapour of bisulphide of carbon is transmitted over sesquioxide of chromium strongly heated in a porcelain tube. The attraction of chromium for sulphur is but slight. If the sulphide of ammonium be mixed with a chromic salt, the hydrated sesquioxide of the metal is precipitated, whilst sulphuretted hydrogen is evolved.

(788) COMPOUNDS OF CHROMIUM WITH CHLORINE.-Chromium forms two chlorides, chromous chloride, ErCl,, and chromic chloride, Er,Cl: the latter is the more important. It also forms an oxychloride (ErCl,,), frequently termed chlorochromic acid.

Chromous chloride (Péligot's protochloride, ErCl2 = 123'5, or CrCl=61°7) is obtained by heating the chromic chloride to redness in a current of dry hydrogen (carefully freed from every trace of oxygen); it is a white substance which is readily dissolved by water, forming a blue solution which rapidly absorbs oxygen, and becomes green: like ferrous chloride, it absorbs nitric oxide quickly and becomes brown.

Chromic Chloride, or Sesquichloride of Chromium (Er,Cl=318, or Cr2Cl=159).—When a current of dry chlorine is transmitted over an intimate mixture of finely divided sesquioxide of chromium and charcoal, heated to redness in a glass tube, beautiful pale violet-coloured scales of anhydrous chromic chloride sublime. When rubbed upon the skin they have a soapy feel; they are quite insoluble in cold water, but by boiling them with water for some time a green solution is gradually formed. Sulphuric and hydrochloric acids, and even aqua regia, do not dissolve them. It is, however, very remarkable that the change from this insoluble to the soluble green variety is effected in a few moments with extrication of heat, by the addition of a minute quantity of the chromous chloride to the insoluble chloride when it is suspended in water. When the green hydrated sesquioxide of chromium is dissolved in hydrochloric acid a similar green solution is formed: the liquid furnishes, by spontaneous evaporation, green crystals, which may be represented, according to Péligot, as consisting of (Er,,Cl, 4 HCl, 10 H,O); for it is singular that only two-thirds of the chlorine which this solution contains is precipitated when it is mixed with nitrate of silver (Ann. de Chimie, III. xii. 536,

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