Cuprous chloride, CuCl

The pure Cuprous chloride, CuCl is more readily prepared than any other cuprous compound. A summary of the more important methods is appended.

1. The chloride is precipitated by addition of excess of water to the solution obtained by the interaction of cuprous oxide and hydrochloric acid in absence of air.
2. In a neutral atmosphere at red heat, cupric chloride decomposes with formation of cuprous chloride and evolution of chlorine.
3. In presence of a small proportion of potassium chlorate to facilitate solution, copper dissolves in hydrochloric acid, with production of cuprous chloride.
4. Sulphur dioxide precipitates the chloride from an aqueous solution of cupric sulphate and sodium chloride. After being washed with dilute sulphurous acid, and then glacial acetic acid, the salt can be dried by the aid of heat. The reduction of a solution of cupric chloride, with formation of cuprous chloride, can also be effected by means of phosphorous acid.
5. Excess of water precipitates cuprous chloride from the colourless solution produced by heating cupric sulphate with an equal weight of copper-turnings, twice the weight of sodium chloride, and ten times the weight of water.
6. Erdmann recommends boiling until colourless the mixture prepared by adding concentrated hydrochloric acid (200 c.c.) and metallic copper (32 grams) to a solution of cupric chloride (42 grams) in hot water (100 c.c.), fuming hydrochloric acid being added towards the close of the operation. The chloride is precipitated by diluting the solution with excess of water.
7. A very convenient method involves boiling copper-turnings with concentrated hydrochloric acid in a flask with a vertical air-condenser until the solution becomes clear, a few drops of concentrated nitric acid being added periodically. The solution is filtered through asbestos into a large excess of distilled water covered with a layer of ether to prevent the absorption of air. After the precipitate has settled, the supernatant liquid is syphoned off, the cuprous chloride washed quickly on a filter with water, alcohol, and ether, and dried in vacuum over sulphuric acid.

The pure salt prepared by Manchot and Friend's method is a perfectly white, crystalline powder, and in absence of moisture is unaffected by light or air. It crystallizes from hot hydrochloric acid in white tetrahedra of density 3.53. For its melting-point in absence of air Monkemeyer gives 419° C., Korreng and also Hachmeister give 425° C., and Carnelley gives 434° C. Carnelley and Williams give the boiling point as 954° to 1052° C. For the specific heat Regnault found the value 0.1383.

On exposure to light and moisture, cuprous chloride develops a violet or dark-blue tint. It also exhibits phototropy when immersed in water slightly acidified with sulphurous acid and subjected to the action of direct sunlight, the colour changing through greyish blue and dark blue to a dark-copper colour, with development of a metallic lustre after a few minutes. In the dark the original white colour is restored in about 48 hours. In absence of moisture the chloride is not sensitive to light, the phenomenon being possibly due to the light inducing the formation of a hydrate unstable in the dark. In contact with damp air cuprous chloride is converted into a dark-green mixture of cupric chloride and basic cupric chloride. Water transforms it into a mixture of copper, cuprous oxide, and cupric chloride.

Assuming the valency of copper to be unity, the formula for cuprous chloride becomes

Cu-Cl.

Without postulating the univalency of copper, the constitution of the salt can be represented by a double formula



Determinations by Victor Meyer and his collaborators of the density of gaseous cuprous chloride at 1600° to 1700° C. gave values approximately 6.5 times that of the atmosphere. Taking air as unity, the vapour-density calculated from the formula Cu₂Cl₂ is 6.83. The close agreement between the two values supports the adoption of the double formula to represent the molecular constitution of gaseous cuprous chloride.

Cryoscopic determinations in dilute solution with pyridine, quinoline, and fused bismuth chloride 9 as solvents have proved the constitution of the salt under these conditions to correspond with the simpler formula Cu-CI. Solutions in mercuric chloride consist of a mixture of single and double molecules.

The conflict of evidence as to the molecular formula of cuprous chloride precludes dogmatic generalization regarding the valency of copper in the cuprous compounds. As a matter of expediency, it seems desirable to assume the univalency of the metal in these derivatives. To explain the formation of double molecules, an interesting assumption has been made by Friend as to the tervalency of the chlorine atom in cuprous chloride. This view finds expression in the molecular formula Cu-CI = Cl-Cu.

The heat of formation of the simple molecule CuCl from solid copper and gaseous chlorine is given as 32.9 Cal. and 35.4 Cal.

Cuprous chloride is characterized by its power of absorbing carbon monoxide, its solution in either hydrochloric acid or ammonia being extensively employed for this purpose in gas-analysis. Contact of a concentrated solution of cuprous chloride in hydrochloric acid with carbon monoxide causes precipitation of the carbonyl-compound in white, crystalline flakes. The ready oxidation of the crystals by atmospheric oxygen renders direct analysis difficult, the first correct results being obtained by Jones in the year 1899. He proved the composition to correspond with the formula CuCl,CO,2H₂O, and his view has been confirmed by the work of Manchot and Friend.

Carbon monoxide is also absorbed by solutions of cuprous chloride in ammonium hydroxide, aniline, o-toluidine, pyridine, and piperidine, the maximum absorption corresponding with one molecule of the monoxide to each atom of copper. Carbon monoxide is not absorbed by mixtures of either alcohol and cuprous chloride, or alcoholic hydrogen chloride and cuprous chloride.

In contact with dilute hydrochloric acid, sulphur reacts with cuprous chloride to form cuprous sulphide, possibly in accordance with the equation

2Cu₂Cl₂ + S = Cu₂S + 2CuCl₂.

The coating of cuprous sulphide formed soon inhibits further action.

In presence of concentrated hydrochloric acid sulphur dioxide oxidizes cuprous chloride to cupric chloride, with deposition of sulphur:

2Cu₂Cl₂ + SO₂ + 4HCl = 4CuCl₂ + 2H₂O + S.

The formation of sulphuric acid in this reaction has not been detected, although in presence of concentrated, but not of dilute, hydrochloric acid sulphur can reduce cupric chloride in accordance with the equation

6CuCl₂ + S + 4H₂O = 3Cu₂Cl₂ + 6HCl + H₂SO₄.

At the boiling-point an aqueous solution of ammonium chloride reacts with finely divided copper, ammonia being evolved. On cooling, crystals of the formula CuCl,NH₃ are deposited from the solution. According to Deherain, an amorphous, black substance of similar composition is produced by the interaction of gaseous ammonia and heated cuprous chloride. The phosphorus analogue of Ritthausen's ammonia compound is formed by the absorption of phosphine by a hydrochloric acid solution of cuprous chloride, its formula being CuCl,PH₃.

Other compounds of cuprous chloride include 2CuCl,LiCl, m.p. 415° C.; CuCl,2KCl₂; 2CuCl,Ag₂S; CuCl,H₂S; CuCl,C₆H₅-NH₂, analogous to the ammonia compound cited in the preceding paragraph, and its hydrochloride, CuCl,C₆H₅NH₂,HCl₅; CuCl,2C₅H₅N and CuCl,3C₅H₅N; and CuCl,2HCl.