Antimony Tetroxide, Sb₂O₄, or Antimony Dioxide, SbO₂

Antimony Tetroxide, Sb₂O₄, or Antimony Dioxide, SbO₂, is found naturally as the mineral cervantite. It may be obtained by a variety of methods, such as by the prolonged heating of antimony trioxide in air, or by the calcination of antimony pentoxide, or by heating the trioxide with excess of mercuric oxide. It is usually prepared by heating antimony or antimony trisulphide with nitric acid and igniting the residue at a dull red heat until the weight is constant. An impure form may be obtained by the careful roasting of antimony trisulphide, at a temperature below the melting point, until no more sulphur dioxide is evolved.

Antimony tetroxide is a refractory, white powder, massive as a mineral; it becomes yellowish on heating, reverting to white when cold. The crystal lattice is cubic, with a = 10.22 A. The density of the synthetic substance at 23.8° C. is given as 6.47. This value is considerably higher than that given for the mineral. The specific heat is 0.0951; the molar heat capacity at low temperatures is given in the following table (in gram-calories per mole):

Temp., ° C.	-200.2	-182.4	-150.4	-129.2	-79.3	-72.1	-16.6	-1.1	+11.9
Heat capacity.	8.317	10.02	14.72	16.57	21.37	21.74	25.23	26.10	27.17

The tetroxide is stable at a red heat, but loses oxygen when heated more strongly.

The crystal structure of antimony tetroxide suggests that the antimony atoms are not all equivalent, and from a comparison with the structure of the similarly constituted antimonates of lead and calcium it is suggested that the oxide may be antimony antimonate, Sb^IIISb^VO₄.

Antimony tetroxide is soluble with great difficulty in water and in acids. It is slightly acidic, imparting a faint reddish colour to moistened litmus.7 It is only slightly attacked when heated with hydrochloric acid, but dissolves in hydrochloric acid containing hydriodic acid, with liberation of iodine, according to the equation

Sb₂O₄ + 6HCl + 2HI = 2SbCl₃ + 4H₂O + I₂

This reaction may be employed for the estimation of antimony tetroxide.

When heated with a little sulphur an oxysulphide or "antimony glass" is formed; with more sulphur the trisulphide is obtained. Alkali hydrosulphides have no action in the cold, but when warmed they act as solvents, hydrogen sulphide being evolved.

The tetroxide can be reduced to the metal by heating with carbon, potassium cyanide or the alkali metals; the trioxide is obtained on heating with antimony. The tetroxide will also react with antimony trisulphide: when excess of the tetroxide is employed the trioxide is formed with liberation of sulphur dioxide:

9Sb₂O₄ + Sb₂S₃ = 10Sb₂O₃ + 3SO₂

With excess of the sulphide, "antimony glass" is formed.

The heat of formation from the elements is 209,800 gram-calories.

A hydrated form, Sb₂O₄.H₂O or H₂Sb₂O₅, is found in the mineral stibiconite. It is acidic, and the free acid, meta-hypoantimonic acid, may be prepared as a white, flocculent powder by decomposing solutions of its salts by acids. The potassium salt may be obtained by heating antimony or antimony trisulphide with potassium sulphate or bisulphate; other salts may be prepared by double decomposition. It has been suggested, however, that these salts are mixtures of antimonites and antimonates.