Stannic Chloride, SnCl₄

Stannic Chloride, SnCl₄, was first obtained by Libavius in 1597, by distilling tin or its amalgam with corrosive sublimate. The liquid thus obtained was termed by its discoverer Spiritus argenti vivi sublimati, but was known later as Spiritus fumans Libavii. It was known in 1630 that a solution of this substance is obtained by dissolving tin in aqua regia; and a Dutchman named Drebbel discovered its value as a mordant by which the red colour of cochineal could be fixed on cloth.

Stannic chloride is prepared by passing chlorine over molten tin, tin-foil, or tin-plate. This latter method is employed in the recovery of tin from tin-plate waste. The reaction is accompanied by the evolution of heat and sometimes light. Stannic chloride may also be obtained by the action of chlorine on stannous chloride, and of chloroform vapour on the dioxide. It is reduced to stannous chloride by hydrogen at 1000° C. Stannic chloride is a thin, colourless, fuming liquid, which has a density of 2.2788 at 0° C.; it forms a solid at low temperature, which melts at -33° C., and boils at 113.89° C. under 750 mm., 114° C. under 757 mm., and 114.1° C. under 760 mm. pressure; its critical temperature is 318.7° C., and critical pressure 36.95 atmospheres. The heat of formation of this compound from its elements is 127,250 calories, and its heat of solution 29,920 calories. Stannic chloride dissolves very readily in alcohol, and also in ether, benzene, nitrobenzene, carbon disulphide, and other non-hydroxylic solvents. By the extent to which the freezing-points of such solvents are depressed when stannic chloride is dissolved in them, it is concluded that this compound undergoes molecular association in such solutions. Although a solution of stannic chloride in benzene is non-conducting, and so contains no ions, it precipitates cupric chloride from a benzene solution of cupric oleate.

Stannic chloride dissolves rhombic sulphur, yellow phosphorus, iodine, and bromine; with turpentine it reacts so vigorously as to set fire to the hydrocarbon. It dissolves in excess of water, forming a solution from which hydrated stannic oxide separates on boiling or standing; nevertheless it unites with smaller quantities of water to form various solid hydrates. Demarchy first observed, in 1770, that with one-third of its weight of water stannic chloride forms a crystalline mass, which was termed butter of tin. This hydrate, which is SnCl₄.5H₂O, is also called oxymuriate of tin. Hydrates containing 3, 4, and 8 molecules of water are also known, and separate from the saturated solution with which they are in equilibrium at different temperatures. Thus SnCl₄.8H₂O is stable up to 19° C., SnCl₄.5H₂O is stable between 19° C. and 56° C., SnCl₄.4H₂O between 56° C. and 63° C., SnCl₄.3H₂O between 63° C. and 83° C.

The hydrolysis of stannic chloride proceeds according to the scheme:

SnCl₄ + 4H₂O ⇔ Sn(OH)₄ + 4HCl;

the extent of the hydrolysis as measured by conductivity when equilibrium is attained depends, not only on the amount of water present, but also on the temperature, the raising of the temperature tending to retard the hydrolysis which is an exothermic reaction. The combination of stannic chloride with hydrochloric acid to form H₂SnCl₆ also influences the equilibrium, the conductivity being less than corresponds to the hydrochloric acid formed by hydrolysis. It has been shown by Pfeiffer that the first product of the hydrolysis of a 50 per cent, solution of stannic chloride, externally cooled by ice, is the hydroxy trichloride SnCl₃OH, since the compound SnCl₃OH.H₂O.(C₂H₅)₂O can be isolated after shaking out with ether. Pfeiffer suggests that hydrolysis proceeds according to the scheme:

→ + HCl

Alcoholysis yields the compound SnCl₃OC₂H₅.

The hydroxychloride SnCl₃OH.2H₂O may be written , according to Werner's system, so as to correspond with the acid [SnCl₆]H₂, to which also the hydrated potassium stannate SnO₃K₂.3H₂O or [Sn(OH)₆]K₂ appears to be related.

Stannic chloride forms various additive compounds. Thus with ammonia it yields a soluble, white powder, which, according to Rose, is SnCl₄.2NH₃. Persoz, however, obtained a solid of the composition SnCl₄.4NH₃, which could be sublimed unchanged in an atmosphere of hydrogen. The compound with phosphine, 3SnCl₄.2PH₃, is a yellow, fuming solid. With sulphur tetrachloride, stannic chloride forms the compound SnCl₄.2SCl₄, which is also obtained in yellow crystals by the action of chlorine on stannic sulphide; these melt below 30° C. and decompose above 40° C. With phosphorus pentachloride the compound SnCl₄.PCl₅ results; it forms colourless, glistening crystals, which fume strongly in the air and have a pungent odour; whilst with phosphoryl chloride the crystalline compound SnCl₄.POCl₃, melting at 58° C. and distilling at 180° C., is formed.

Stannic chloride combines with dry nitrous fumes to form SnCl₄.N₂O₃, a yellow amorphous mass; and when this substance is sublimed, or the dry vapours from aqua regia are passed into the chloride, SnCl₄.2NOCl is formed, and crystallises in shining octahedra; if, however, nitrogen peroxide is passed into a chloroform solution of stannic chloride the compound 3SnCl₄.4NOCl is produced; and when a chloroform solution of nitrogen sulphide is added to a similar solution of stannic chloride, red crystals of SnCl₄.2N₄S₄ separate. With sulphur trioxide the white solid SnCl₄.SO₃ is formed.

Stannic chloride is employed in dyeing, as a mordant, and for weighting silk. Formerly a preparation known as tin-composition, physic, or dyers' spirit was employed; this was made by dissolving tin with common salt or sal ammoniac in nitric acid, or the metal alone in aqua regia; but now the pentahydrate, SnCl₄.5H₂O, oxy-muriate of tin, is generally used.