Stannous Chloride, SnCl₂

Stannous Chloride, SnCl₂, is formed by heating tin in hydrogen chloride gas, or by distilling the metal or its amalgam with the requisite amount of calomel or corrosive sublimate. It also resujts from the dehydration of the hydrate SnCl₂.2H₂O, by leaving it over sulphuric acid in a vacuum, or by heating it in a stream of hydrogen chloride gas. Anhydrous stannous chloride forms a transparent mass with a fatty lustre and a conchoidal fracture. It melts at 250° C., and boils at 606° C. or 603° C., and is useful for obtaining a vapour bath of constant high temperature. Fused stannous chloride has remarkable reducing properties which have been studied by Sandonnini and Aureggi. Biltz and Meyer found the vapour densities at various temperatures to be as follow:

Temperature ° C.	639	678	699	759.6	790.2	1113
Vapour Density (Air = 1)	8.55	8.57	8.49	8.26	7.7	7.08
Vapour Density (H = 1)	123	123	122	119	111	102

The theoretical value for stannous chloride, SnCl₂, is 6.53 (Air = 1) or 94.0 (H = 1); hence it is inferred that some associated molecules, Sn₂Cl₄, exist in the state of vapour, but that they are dissociated as the temperature rises. When dissolved in urethane, however, stannous chloride depresses the freezing-point of this substance by an amount which corresponds with the formula SnCl₂. The heat of formation of stannous chloride from its elements is:

[Sn] + 2(Cl) = [SnCl₂] + 80,790 calories;

its specific heat between 20° C. and 99° C. is 0.1016. Stannous chloride forms several compounds with ammonia. Dry ammonia reacts with dry stannous chloride in a freezing mixture forming a yellow powder having the composition SnCl₂.2NH₃, which blackens on exposure to light, and is decomposed by moisture into stannous oxide and ammonium chloride. At ordinary temperatures a mixture of the foregoing compound with SnCl₂.NH₃ is produced, whilst at 100° C. only the latter compound is formed. At 120°-300° C. a brownish red crystalline substance having the composition 3SnCl₂.2NH₃ results; this is the most stable of the three compounds, and is only slowly decomposed by water.

Stannous chloride is said to form hydrates with one and four molecules of water, but the dihydrate SnCl₂.2H₂O is the compound best known. This compound, known commercially as Tin Salt, is prepared by dissolving tin in hydrochloric acid, and may also be obtained by dissolving the anhydrous salt in water, or stannous hydroxide in hydrochloric acid. It separates from its concentrated solution in transparent monoclinic prisms, or in octahedra, which have a density of 2.71, melt at 40.5° C., and when further heated lose water and hydrogen chloride, leaving a basic chloride. This salt is very soluble in water; 100 grams of water dissolve 83.9 grams SnCl₂ at 0° C., and 269.8 grams at 15° C. When stannous chloride solution is diluted, hydrolysis takes place with separation of the insoluble basic chloride, 2Sn(OH)Cl.H₂O, which redissolves in hydrochloric acid; the same precipitate is produced on account of atmospheric oxidation, with the simultaneous formation of stannic chloride, thus:

6SnCl₂ + 2H₂O + O₂ = 2SnCl₄ + 4Sn(OH)Cl.

This precipitation may be prevented by keeping the acidified solution over granulated tin, and also by adding tartaric acid, which forms a complex ion.

The action of excess of concentrated sulphuric acid on hydrated stannous chloride takes place in three successive stages:

(i) Dehydration; (ii) liberation of hydrogen chloride between 20° C. and 90° C.; (iii) the reaction

SnSO₄ + 2H₂SO₄ = Sn(SO₄)₂ + 2H₂O + SO₂

between 130° C. and 200° C. The following reactions take place between stannous chloride and sulphurous acid at atmospheric temperatures:

1. Primary 3SnCl₂ + 6HCl + H₂SO₃ = 3SnCl₄ + 3H₂O + H₂S.
2. Consequent 2H₂S + H₂SO₃ = 3H₂O + 3S.
3. Consequent 2H₂S + SnCl₄ = 4HCl + SnS₂.
4. Consequent H₂S + SnCl₂ = 2HCl + SnS.
5. Retarding SnCl₄ + H₂SO₃ + H₂O = SnCl₂ + 2HCl + H₂SO₄.

Stannous chloride combines with hydrochloric acid and with potassium chloride and iodide in aqueous solution to form complex ions. This is proved by the reduction of electric conductivity. The following complex salts, chlorostannites, have been isolated:

KSNCl₃.H₂O, K₂SnCl₄.H₂O, K₂SnCl₄.2H₂O, (NH₄)₂SnCl₄.2H₂O, BaSnCl₄.4H₂O,

as well as the free acid HSnCl₃.3H₂O, which melts at -27° C. Stannous chloride is readily soluble in alcohol; it also dissolves in acetone, ether, and ethyl acetate. An aqueous solution of stannous chloride absorbs free oxygen; the rate of absorption is roughly proportioned to the concentration of hydrochloric acid present, and is much influenced by certain catalysers. Connected with this ability to absorb oxygen is the reducing action of stannous chloride, whereby it is converted into stannic salt. Stannous solutions are powerful reducing agents, and find varied application. Thus an acid solution of stannous chloride reduces ferric to ferrous chloride, cupric to cuprous chloride, and mercuric chloride to mercurous chloride and metallic mercury; it also reduces dilute nitric acid solution to hydroxyl- amine, bleaches indigo, and converts benzene diazonium chloride into phenyl hydrazine hydrochloride. Alkaline stannous solution reduces bismuthous hydroxide to the black suboxide, BiO, and metallic bismuth. Tin-salt is used as a wool mordant for cochineal scarlet, for dyeing silk black; and for weighting silk. It is also employed for reducing indigo, in preparing "purple of Cassius," and in tinplating.