Chemical elements
  Tin
    Isotopes
    Energy
    Production
    Application
    Physical Properties
    Chemical Properties
      Tetramethyl Stannane
      Methyl stannic chloride
      Tin Tetra-ethyl
      Tin Tri-ethyl
      Stannous Fluoride
      Stannic Fluoride
      Sodium Stannifluoride
      Potassium Stannifluoride
      Ammonium Stannifluoride
      Stannous Chloride
      Stannic Chloride
      Chlorostannates
      Stannous Bromide
      Stannic Bromide
      Stannous Iodide
      Stannic Iodide
      Mixed Stannic Halides
      Stannous Oxide
      Stannous Hydroxide
      Stannic Oxide
      Potassium Stannate
      Stannic Acid and its Derivatives
      Parastannic Acid
      Stannyl Chloride
      Parastannyl Chloride
      Stannous Sulphide
      Stannic Sulphide
      Stannic Oxysulphide
      Stannic Iodosulphide
      Stannous Sulphate
      Stannic Sulphate
      Stannic Nitrate
      Stannous Nitrate
      Phosphor-tin
      Stannioxalic Acid
      Stannous Tartrate
      Tin and Silicon
      Stannous Tungstate
    PDB 3e94-3kwy

\xCE\xB2-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.


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