Modern inorganic chemistry

THE TRANSITION ELEMENTS 41 5

white copper(I) chloride separates, but if air is present in the water,

it rapidly turns blue owing to the formation of the eopperfll) ion.

2. By the reduction of copper(II) chloride or a mixed solution of
   copper(II) sulphate and common salt by sulphur dioxide :

2Cu2+ + 2CT + 2H 2 O + SO 2 -> SO|~ -f 4H+ 4- 2CuCl|

In both cases, the precipitate must be filtered and dried quickly,

by washing first with alcohol and then with ether (to prevent

formation of the eopper(II) compound).

Measurements on copper(I) chloride show the vapour to be the

dimer of formula Cu 2 Cl 2 , but molecular weight determinations in

certain solvents such as pyridine show it to be present in solution

as single molecules, probably because coordination compounds

such as py -> CuCl (py = pyridine) are formed.

The solid readily dissolves chemically in concentrated hydro-

chloric acid, forming a complex, and in ammonia as the colourless,

linear, complex cation [H 3 N -* Cu <- NH 3 ]+ (cf. AgCl) if air is

absent (in the presence of air, this is oxidised to a blue ammino-

copper(II) complex). This solution of ammoniacal copper(I) chloride

is a good solvent for carbon monoxide, forming an addition com-

pound CuCl. CO. H 2 O, and as such is used in gas analysis. On

passing ethyne through the ammoniacal solution, a red-brown

precipitate of hydrated copper(I) dicarbide (explosive when dry) is

obtained :

2[Cu(NH 3 ) 2 ]+ + HC=CH -* Cu 2 [C^C]j + 2NH + + 2NH 3

Coppeiil) iodide, Cul, is obtained as a white precipitate on addition
of potassium iodide to a solution containing copper(II) :

2Cu2+ 4-41" ->2CuIi + I 2 |
white
The reaction provides a method of estimating copper(II) since the
liberated iodine may be titrated with sodium thiosulphate :

Copper(I) iodide is used in the extraction of iodine (p. 320).

Copperil] cyanide, CuCN (and copper(I) thiocyanate], are similarly
obtained as white precipitates on adding cyanide and thiocyanate
ions (not in excess) respectively to copper(II) salts :

2Cu2+ -f 4CN" -> 2CuCN -f C 2 N 2 T

+ 4SCN~ -» 2CuSCN -f (SCN 2 )T