Stannic Oxide, SnO₂

Stannic Oxide, SnO₂, occurs in crystalline and amorphous forms. As regards its crystalline forms, it is trimorphous, existing in tetragonal, hexagonal, and rhombic crystals. Native stannic oxide, cassiterite or tinstone, occurs in tetragonal crystals, which when quite pure are colourless and transparent, but are generally yellow, green, brown, or black, from the presence of oxide of iron or other impurity.

Tinstone may owe its origin to the hydrolysis of stannic fluoride or chloride by water-vapour, i.e. to so-called pneumatolytic action. Stannic oxide is infusible before the blowpipe, but it can be melted, and even vaporised in the electric furnace. Its melting-point5 is estimated to be 1127° C. Various values have been obtained for the density of crystallised stannic oxide, owing to varying degrees of purity of the substance; the mean value is 6.9 at 15° C.

Amorphous stannic oxide may be obtained in various ways. If tin is heated in the air it takes fire when near its boiling-point and burns with a bright flame, forming the dioxide. Obtained in this way stannic oxide was originally named flores Jovis; whilst the oxide formed by the superficial oxidation of molten tin, which is grey because it contains particles of finely divided metal, has been called flores stanni. Stannic oxide is also formed when stannous oxide or oxalate, or either of the sulphides is roasted in the air, and by the ignition of the hydrated oxide formed by the action of nitric acid on the metal, as well as in other ways to be noticed later. It is likewise obtained during the electrolysis of a solution of sodium or potassium chloride with a tin plate as anode, and a platinum plate as cathode. Amorphous stannic oxide is a white or cream-coloured powder, which turns yellow and brown when heated, but fades to its original colour on cooling.

The amorphous oxide is converted into the crystalline form of cassiterite when it is heated in a current of hydrogen chloride gas. The thermal relationship between crystalline and amorphous stannic oxide is represented by the equation:

SnO₂ (amorphous) = SnO₂ (cryst) + 1700 calories.

The anhydrous oxide is quite insoluble in water, and is not attacked by concentrated acids, except sulphuric acid, with which it forms an unstable sulphate. Fusion with potassium hydrogen sulphate also dissolves the oxide. A method of treatment, however, which is convenient in qualitative analysis, is to fuse the oxide with a mixture of sodium carbonate and flowers of sulphur. The fused mass dissolves in water, yielding a solution of sodium thiostannate, from which dilute acid precipitates stannic sulphide; this can then be dissolved in concentrated hydrochloric acid. Stannic oxide is employed, under the name of putty powder, for polishing glass and metal.

Berzelius, in 1817, recorded the existence of two forms of hydrated stannic oxide, or stannic acid, of the composition represented by the empirical formula H₂SnO₃; and this was the first known example of isomerism. The one form is obtained by precipitation from stannic salts or stannate solutions, the other by heating tin with moderately concentrated nitric acid. The two forms differ in solubility, and especially in the composition of the salts they yield with alkalis; salts of the former acid contain one equivalent of stannic oxide, and are of the type M'₂SnO₃; salts of the latter have five equivalents of the oxide and are, therefore, of the type M'₂Sn₅O₁₁.

The two acids have been named stannic and metastannic acid respectively; but this nomenclature is misleading, since H₂SnO₃ is properly metastannic acid. They are better known as α- and β-stannic acids; the term meta may then be employed in addition where it is appropriate. According to Mecklenburgh, the α- and β-stannic acids are to be regarded merely as colloidal substances, differing in the size of their particles.

α-Stannic Acid and its Salts. - α-Stannic acid is obtained as a white powder by precipitating stannic chloride with ammonia, or by means of calcium carbonate; by adding dilute acid to the solution of a stannate, or ammonium chloride to the same solution, since ammonium stannate, like the corresponding silicate, is immediately hydrolysed by water. The addition of sodium sulphate to stannic chloride solution also produces a precipitate of stannic hydroxide, since stannic sulphate is unstable, and is hydrolysed. These two latter reactions furnish an interesting example of the behaviour of an amphoteric hydroxide, since in the former case the hydroxide is precipitated as a weak acid, in the latter case as a weak base. When the precipitate is air-dried it contains 22.5 per cent, of water, which is father more than corresponds to the formula of the ortho-hydroxide, Sn(OH)₄. When dried in vacuo, or at 100° C., the hydroxide has approximately the composition of the meta-acid H₂SnO₃.

The precipitate before drying is slightly soluble in water, to which it imparts an acid reaction; it also dissolves in nitric, sulphuric, and hydrochloric acids, as well as in caustic soda. When the acid is dried it undergoes partial transformation into the β-form; this also occurs under water, especially on heating.

The alkali α-stannates can be obtained crystalline, and are soluble in water; other stannates are obtained from them by precipitation.

Sodium Stannate, Na₂SnO₃.3H₂O, is obtained by fusing finely divided tinstone with caustic soda, or by heating tin with caustic soda and Chili saltpetre:

Na₂O + SnO₂ (amorphous) = Na₂SnO₃ + 37,100 calories.

The trihydrated salt crystallises from aqueous solution, and is more soluble in cold than in hot water; from such solution deka-hydrated crystals, Na₂SnO₃.10H₂O, can also be obtained. When the trihydrate is heated it loses all its water, and the anhydrous stannate thus produced does not dissolve unchanged in water and cannot be rehydrated. This salt is used in calico printing, and is called preparing salts.