COMPOUNDS OF TIN Tin forms two well-marked series of salts, in one of which it is bivalent, these salts being derived from stannous oxide, SnO, in the other it is quadrivalent, this series being derived from stannic oxide, Sn02.
Stannous Oxide, SnO, is obtained in the hydrated form from a solution of stannous chloride by addition of sodium carbonate ; it forms a white precipitate, which can be washed with air-free water and dried at 8o° C without much change by oxidation; if it be heated in carbon dioxide the black SnO remains. Precipitated stannous hydrate dissolves readily in caustic potash; if the solution is evaporated quickly it suffers decomposition, with formation of metal and stannate, 2SnO 2KOH=K2Sn03 + Sn + H20.
If it is evaporated slowly, anhydrous stannous oxide crystallizes out in forms which are combinations of the cube and dodecahe dron. Dry stannous oxide, if touched with a glowing body, catches fire and burns to stannic oxide, Sn02. Stannous oxalate when heated by itself in a tube leaves stannous oxide.
Stannic Oxide, if the term is taken to include the hydrates, exists in a variety of forms. (I) Tinstone (see above and also CASSITERITE) is proof against all acids. Its disintegra tion for analytical purposes can be effected by fusion with caustic alkali in silver basins, with the formation of soluble stannate, or by fusion with sulphur and sodium carbonate, with the formation of a soluble thiostannate. (2) A similar oxide (flares Jovis) is produced by burning tin in air at high tempera tures or exposing any of the hydrates to a strong red heat. Such tin-ash, as it is called, is used for the polishing of optical glasses; "putty powder" is another name for a similar preparation used for polishing. Flores stanni is a finely divided mixture of the metal and oxide obtained by fusing the metal in the presence of air for some time. (3) Metastannic acid (generally written to account for the complicated composition of meta stannates, e.g., the sodium salt is the white com pound produced from the metal by means of nitric acid. It is not the true meta-acid, however, and is therefore better called j3-stannic acid (see below). It is insoluble in water and in nitric acid and apparently so in hydrochloric acid ; but if heated with this latter for some time it passes into a more soluble compound, which is a basic chloride, j3-stannyl chloride. 0-stannic acid is distinguished from ortho- or a-stannic acid by its insolubility in nitric and sulphuric acids. The salts are obtained by the action of alkalis on the acid. (4) Orthostannic acid, so-called, is really the meta-acid and is better called a-stannic acid; it is obtained as a white precipitate on the addition of sodium carbonate or the exact quantity of precipitated calcium carbonate to a solution of the chloride. This acid, is readily soluble in acids forming stannic salts, and in caustic potash and soda, with the formation of metastannates. Of these sodium stannate, is produced industrially by heating tin with Chile saltpetre and caustic soda, or by fusing very finely powdered tinstone with caustic soda in iron vessels. A solution of the pure salt yields
fine prisms of the composition Na2Sn03-1- ioH20, which effloresce in the air. The salt is used as a mordant in dyeing and calico printing, being called "preparing salts" in the latter industry. Alkaline and other stannates when treated with aqueous hydro fluoric acid are converted into fluostannates or stannifluorides (e.g., into K2SnF6), which are closely analogous to, and isomorphous with, fluosilicates or silicifluorides.
A colloidal or soluble stannic acid is obtained by dialysing a mixture of tin tetrachloride and alkali, or of sodium stannate and hydrochloric acid. On heating it is converted into colloidal 0-stannic acid. A hydrated tin trioxide, has been described by Spring.
Stannous chloride, SnC12, can only be obtained pure by heat ing pure tin in a current of pure dry hydrogen chloride. It is a white solid, fusing at 250° C to an oily liquid which boils at 6o6°, and volatilizing at a red heat in nitrogen, a vacuum or hydrogen chloride, without decomposition. The vapour density below 700° C is somewhat high, but above 800° C corresponds to nearly The chloride readily combines with water to form a crystallizable hydrate known as "tin salt" or "tin crystals." This salt is also formed by dissolving tin in strong hydrochloric acid and allowing it to crystallize, and is industrially prepared by passing sufficiently hydrated hydrogen chloride over granulated tin contained in stoneware bottles and evaporat ing the concentrated solution produced in tin basins over granu lated tin. The basin itself is not attacked. The crystals are very soluble in cold water, and if the salt is really pure a small pro portion of water forms a clear solution ; but on adding much water most of the salt is decomposed, with the formation of a precipitate of oxychloride, 2Sn(OH)C1.H20. The same oxychloride is pro duced when the moist crystals are exposed to the air. Hence all tin crystals as kept in the laboratory give with water a turbid solution, which contains stannic in addition to stannous chloride. The complete conversion of stannous into stannic chloride may be effected by a great many reagents—for instance, by chlorine (bromine, iodine) readily ; by mercuric chloride, with precipita tion of calomel or metallic mercury; by ferric chloride on warm ing, with formation of ferrous chloride ; by arsenious chloride in strong hydrochloric acid solutions, with precipitation of brown metallic arsenic. All these reactions are available as tests for "stannosum" or, alternatively for the respective agents. A strip of metallic zinc when placed in a solution of stannous chloride pre cipitates the tin in crystals and takes its place in the solution. Stannous chloride is largely used in the laboratory as a reducing agent, in dyeing as a mordant.