β-Stannic Acid and its Derivatives

Boyle, in 1670, stated that aqua fortis destroys more tin than it dissolves, and also observed that a solution of tin in the same liquid easily becomes gelatinous. Kunkel likewise recorded the fact that to dissolve tin the nitric acid employed must be cold, or calx of tin would be precipitated. The explanation of these observations is that whilst tin dissolves slowly, in very dilute nitric acid to produce stannous nitrate, the stannous nitrate first formed, when hot and more concentrated acid is employed, is very unstable and quickly decomposes, yielding the form of hydrated stannic oxide known as β-stannic acid. Probably α-stannic acid is first produced from stannic nitrate, which then passes into the β-form. The product is, therefore, liable to contain both α- and β-forms, and the pure β-acid is obtained by dissolving this product in sodium hydroxide solution, and then adding excess of concentrated soda, which precipitates sodium p-stannate whilst the α-salt remains in solution. Pure β-stannic acid, or β-metastannic acid, is then obtained by decomposing the sodium salt with acid. It is also formed by the hydrolysis of its sodium salt at 60° C., and by boiling a dilute solution of stannic chloride, with or without the addition of nitric acid. Moreover, when a solution of a-stannic acid in hydrochloric or hydrobromic acid is allowed to stand, α-stannic acid gradually separates as an opalescent precipitate, and the process of transformation can be followed quantitatively.

A hydrosol of stannic acid was obtained by Graham by the dialysis of a mixture of stannic chloride and alkali, or of sodium stannate and hydrochloric acid. The liquid at first contained a gelatinous precipitate which gradually dissolved. The hydrosol was gelatinised by a trace of hydrochloric acid or of a salt, and on heating yielded colloidal β-stannic acid. Gelatinous, precipitated β-stannic acid has the empirical composition SnO₂.4H₂O, when air-dried SnO₂.2H₂O, and when dried in a vacuum, SnO₂.H₂O; These formulae do not, however, convey a just idea of the nature of the β-acid, which is gained from a study of its salts and other derivatives.

Sodium β-stannate, prepared by the action of cold sodium hydroxide solution on β-stannic acid, is a sparingly soluble crystalline powder, having the composition Na₂Sn₅O₁₁.4H₂O. Similarly the potassium salt is K₂Sn₅O₁₁.4H₂O. Thus the molecule of β-stannic acid appears to contain five tin atoms; and the air-dried acid becomes H₂Sn₅O₁₁.9H₂O instead of SnO₂.2H₂O, whilst the acid dried in a vacuum is H₂Sn₅O₁₁.4H₂O instead of simply SnO₂.H₂O. Alternative formulae are Sn₅O₅(OH)₁₀.5H₂O and Sn₅O₅(OH)₁₀ respectively, which suggest that β-stannic acid may possibly contain a ten-membered ring of alternated tin and oxygen atoms. At least an analogy is suggested between β-stannic acid and the polymerised silicic acids.

β-stannic acid is distinguished from the a-acid by its insolubility in dilute nitric and sulphuric acids. It does not dissolve in cold concentrated sulphuric acid, but with the hot acid forms stannic sulphate, which by hydrolysis yields a-stannic acid. Hydrochloric acid dissolves the p-acid, but excess of it precipitates β-stannyl chloride (q.v.), which was formed in solution. The β-acid may be converted into a salt of the a-acid by fusion with potash or by long-continued boiling with concentrated potash solution. Evaporation to dryness with hydrochloric acid also yields the α-acid.