solves it freely and forms a compound soluble both in water and in alcohol; by boiling the solution it is decomposed, and the two acids are separated. Hydrochloric acid combines with it, but does not dissolve it; the compound is soluble in pure water, but is reprecipitated on the addition of acid in excess, or on boiling the solution. Metastannic acid is freely soluble in solutions of potash and of soda, as well as in solutions of their carbonates, but it is not dissolved by ammonia, unless recently precipitated from a cold solution of its salts by the addition of an acid; the precipitate is not soluble in ammonia after it has been boiled. The metastannates are not crystallizable, and are precipitated by adding caustic potash to their aqueous solution; the granular precipitate may be drained upon a tile, and dried at 260°; their normal formula is M',Sn.11 with usually 4 H,→ ; or (M'O,Sn¡O10, 4 HO). The potassium salt has a strongly alkaline reaction; it consists of (K2Sn¡Ð11, 4 H‚Ð). The metastannates of the alkalies can only exist in the hydrated condition; if strongly heated they are decomposed and become insoluble; when the residue, after ignition, is treated with water, metastannic acid is left, whilst the alkali is dissolved. Metastannic acid may be recognized by the beautiful golden-yellow colour which it yields when its hydrate is moistened with protochloride of tin, owing to the formation of metastannate of tin (Sn,Sn.11, 4 H2O). The only metastannates which are soluble are those of potassium and sodium; they are precipitated in the gelatinous state from their solutions by the addition of almost any of the neutral salts of sodium, potassium, or ammonium.

5

11

(815) Stannie Acid (H,Sne, or HO,SnO).—This variety of the hydrated oxide of tin may be procured by precipitating a solution of tetrachloride of tin by ammonia, or still better by adding to the solution of the tetrachloride a quantity of an insoluble carbonate, such as chalk or carbonate of barium, insufficient for its entire decomposition; it is thus separated as a gelatinous precipitate, which may be readily washed clean: when dried in vacuo, the composition of the hydrate is (H,SnO). In this state it is freely soluble in hydrochloric acid, with which it reproduces tetrachloride of tin; it is also soluble even in diluted sulphuric acid, but the stannic acid is separated on boiling. Nitric acid dissolves it freely. Stannic acid is soluble in the cold in solutions of potash and of soda, but not in ammonia; by a heat of 284° it is converted into metastannic acid. In combination with the alkalies it forms compounds which crystallize readily, especially from solu

II.

U U

658

STANNIC ACID-STANNATES.

tions which contain an excess of alkali.

M',Sne, or MO,SnO,.

The general formula is

The soluble stannates have a powerfully alkaline reaction; they absorb carbonic acid from the air when in solution, and are precipitated by solutions of most of the salts of potassium, sodium, and ammonium.

Stannate of potassium (K2SnÐ ̧,4 H2O) is easily prepared by heating any form of peroxide of tin with excess of caustic potash; on dissolving and evaporating the product, transparent oblique rhombic prisms are formed. When heated to redness, the stannate of potassium may be rendered anhydrous. Stannate of sodium (Na,Sn0,4 H2O) may be prepared in the same way as the stannate of potassium. It crystallizes with facility in sixsided tables, when a solution saturated at about 100° F. is heated to the boiling-point, as it is more soluble in cold than in hot water. This stannate is now largely prepared as a mordant for the use of the dyer and calico-printer. It forms the basis of what is technically known as tin-prepare liquor. Copper is quickly tinned by a solution of this salt.

Sesquioxide of tin, or stannate of tin as it is often called (Sn,,, or SnO,SnO2), may be prepared as a slimy grey hydrate, soluble in ammonia, by boiling pure hydrated sesquioxide of iron with a solution of stannous chloride; ferrous chloride remains in solution, 2 SnCl,+Fe,O=2 FeCl2+SnO,Sne,. It is soluble in hydrochloric acid and also in ammonia, which latter reaction seems to indicate that it is really a distinct oxide; the hydrochloric solution gives a purple precipitate with salts of gold.

3

(816) The SULPHIDES OF TIN are three in number,—the protosulphide, the bisulphide, and the sesquisulphide: the latter is unimportant.

The Protosulphide (SnS=150, or SnS=75) may be procured by fusing the metal with sulphur, when it forms a bluish-grey crystalline mass, easily dissolved by melted tin; it may also be obtained by passing sulphuretted hydrogen through a stannous salt in solution, when it falls as a chocolate-brown hydrate. It is soluble in solution of bisulphide of ammonium, and in the sulphides of the alkaline metals, if they contain an excess of sulphur. Protosulphide of tin combines with the sulphides of the electronegative metals, such as arsenic and antimony. Hydrochloric acid dissolves it with extrication of sulphuretted hydrogen.

The Sesquisulphide (Sn,S,) may be prepared by mixing the protosulphide with one-third of its weight of sulphur, and heating to dull redness; it is only partially soluble in hydrochloric acid.

SULPHIDES AND CHLORIDES OF TIN.

659

The Bisulphide of tin (SnS, 182, or SnS,=91) is known as mosaic gold; it forms a beautiful yellow flaky compound, which is obtained by preparing an amalgam of 12 parts of tin and 6 of mercury this is reduced to powder and mixed with 7 parts of sublimed sulphur and 6 of sal ammoniac. This mixture is introduced into a flask with a long neck, and is heated gently so long as any smell of sulphuretted hydrogen is perceptible; the temperature is then raised to low redness; calomel and cinnabar are sublimed, and a scaly mass of bisulphide of tin remains. If the heat be pushed too far, part of the sulphur is expelled, and the operation fails: the sal ammoniac appears by its volatilization to moderate the heat produced during the sulphuration of the tin, which would otherwise rise so high as to decompose the bisulphide, and mechanically preserves the requisite flaky structure of the compound. Bisulphide of tin is used in the arts to imitate bronze. Aqua regia is the only acid that decomposes it, but it is readily soluble in the alkalies. A hydrated bisulphide of tin, of a dingy yellow, is produced by passing sulphuretted hydrogen through a solution of one of the stannic salts. This hydrate is readily dissolved by hydrosulphate of ammonium, evolving sulphuretted hydrogen: it is also soluble in the alkalies, and in hot hydrochloric acid. With sulphide of sodium it forms a salt which may be obtained in yellow crystals, consisting of 2 Na,S,SnS, .12 H2O.

The bisulphide fuses when chlorine is passed over it; 6 atoms of the gas are absorbed, without the aid of heat, by each atom of bisulphide, and a yellow crystalline compound is obtained which may be considered as a combination of 1 atom of tetrachloride of tin with 2 atoms of tetrachloride of sulphur, SnCl4,2 SCI.

(817) CHLORIDES OF TIN.-Tin forms with chlorine two compounds, SnCl2, and SnCl,, formerly termed the chloride and bichloride of the metal, but they are now better distinguished as stannous, and stannic chloride.

=

Stannous chloride (SnCl, 189), or protochloride of tin (SnCl =945). The hydrate of this salt may be obtained by dissolving tin in hydrochloric acid. This solution is usually effected on the large scale in copper vessels, since the voltaic opposition of the two metals favours the solution of the tin: on evaporating the liquid till it crystallizes, prismatic needles are formed (SnCl2, 2 H2O; sp. gr. 2759); by a heat of 212° it may be rendered anhydrous, but it generally loses a portion of hydrochloric acid at the same time. Stannous chloride is decomposed if mixed with a large quantity of water, hydrochloric acid remains in solution, and a white hydrated oxychloride (SnCl,,SnO,2 H2O)

660

STANNOUS CHLORIDE, OR PROTOCHLORIDE OF TIN.

subsides. When exposed to the air, in crystals or in solution, stannous chloride absorbs oxygen and forms a mixture of perchloride and oxychloride of tin. Stannous chloride has a strong attraction both for chlorine and for oxygen; it therefore acts as a powerful reducing agent. Thus it deoxidizes completely the salts of mercury, of silver, and of gold. Advantage is sometimes taken of this circumstance in the analytical determination of the quantity of mercury, since all the salts of mercury, when boiled with the stannous chloride, are decomposed, and yield their mercury in a metallic form. Sulphurous acid is likewise deprived by it of its oxygen, producing a yellow precipitate of bisulphide of tin when mixed with a solution of the salt. Stannous chloride

reduces the metallic acids in the salts of chromic, tungstic, molybdic, arsenic, antimonic, and manganic acids to a lower state of oxidation; it also converts the ferric into ferrous salts, and the cupric into cupreous salts. Stannous chloride is extensively employed as a mordant by the dyer and calico-printer, under the name of salts of tin, and they also use it for deoxidizing indigo and the peroxides of iron and manganese.* It forms double chlorides with many of the chlorides of the metals of the alkalies and alkaline earths; these double salts are capable of crystallization.

The anhydrous stannous chloride, or butter of tin, may be procured by distilling a mixture of equal weights of tin filings and corrosive sublimate; HgCl2+Sn=SnCl2+Hg: it remains behind as a grey brilliant mass with a vitreous fracture; at a full red heat it may be distilled. On passing a current of chlorine over it, heat and light are evolved, and the tetrachloride of tin is formed.

(818) Stannic Chloride, or Tetrachloride of Tin (SnCl1 = 260); Sp. Gr. of vapour, 9'2; of liquid, 2.367 at 32°; Mol. Vol. Boiling-pt. 239°5; or Bichloride of Tin (SnCl,=130). This compound may be prepared either by passing dry chlorine over melted tin, or by mixing 4 parts of corrosive sublimate with I part of tin filings on the application of heat a colourless liquid distils 2 HgCl2+Sn yielding SnCl4 +2 Hg. It emits dense white fumes when exposed to the air: when mixed with water, intense heat is

:

;

The proportion of stannous chloride available for this purpose in any commercial sample may be determined by Penny's method:-A solution of a weighed quantity of stannous chloride in hydrochloric acid is taken, and a standard solution of anhydro-chromate of potassium is added until a drop of the liquid when mixed with acetate of lead gives a yellow precipitate, showing that the chromic acid is no longer reduced: 3 SnCl2+K2¤r2+7+14 HCl= 3 SaCl,+2 KCl+er2Cl2+7 H20.

6

STANNIC CHLORIDE OR PERCHLORIDE OF TIN-TESTS FOR TIN. 661

evolved, and a hydrate is formed; this compound crystallizes in rhombohedra (SnCl4, 5 H2O; Lewy) when it is allowed to form spontaneously, by attracting moisture from the air; in vacuo it loses 3 H: but though freely soluble in a small quantity of water, copious dilution causes the precipitation of hydrated stannic acid, and hydrochloric acid is set free. Tetrachloride of tin is readily soluble in water acidulated with hydrochloric acid. When its aqueous solution is mixed with a solution of the sulphate of one of the alkali-metals, hydrated binoxide of tin is precipitated; SnCl + 4 H2O + 4 Na2SO becoming Sne,, 2 H ̧Ð +4 NaCl + 4 NaHSO4, acid sulphate of the alkaline metal remaining in solution.

Tetrachloride of tin forms numerous double salts with the soluble chlorides; the compound with chloride of potassium crystallizes in anhydrous octohedra, 2 KCl,SnCl; a similar constitution holds in the corresponding ammoniacal salt (2 H,NCI,SnCl), which is the pink salt of the dyer. An impure tetrachloride of tin is largely used by the dyers under the name of nitromuriate of tin, or composition; it is generally prepared by dissolving tin at a gentle heat in a mixture of nitric acid and sal ammoniac.

The other salts of tin are unimportant. Stannic sulphate is soluble in water strongly acidulated by sulphuric acid, but is precipitated to a large extent on copious dilution with water.

(819) CHARACTERS OF THE SALTS OF TIN.-Tin forms two series of salts, the salts formed from the protoxide and the salts formed from the binoxide: the tetrachloride of tin is the only salt of the latter class that has been minutely examined.

1.—The stannous salts, or protosalts of tin, are nearly colourless; with the exception of the chloride, they are not often prepared they have a powerfully astringent taste; when in solution they absorb oxygen rapidly from the air; when largely diluted with water the solution becomes milky, but it is rendered clear by a small excess of hydrochloric acid. The hydrates of the fixed alkalies produce a white precipitate of hydrated protoxide of tin, which is soluble in excess of the alkali, but on boiling, part of the oxide is deposited as a black crystalline powder. Ammonia gives a white hydrated oxide of tin, but the precipitate is not redissolved by an excess of ammonia. The carbonates of the alkali-metals give a similar precipitate, whilst carbonic anhydride escapes with effervescence. A very characteristic reaction is the production, with sulphuretted hydrogen, of a chocolate-brown precipitate of hydrated protosulphide of tin. With sulphide of ammonium, a similar precipitate is formed, which is soluble in excess of the pre