Antimony Pentoxide, Sb₂O₅

Antimony pentoxide and its derivatives were employed in the sixteenth and seventeenth centuries as diaphoretics. A potassium salt was described by Basil Valentine by the name of "antimonium diaphoreticum ablutum"; it was prepared by deflagrating a mixture of antimony and saltpetre and washing the residue with water and alcohol. A similar substance was "antimonium diaphoreticum non ablutum". The acid, or oxide, was probably obtained by the action of acids upon the above substances, and was used medicinally under the name of "materia perlata Kerkringii." Glauber obtained a similar product, which he called "benzoardicum miner ale" by the action of nitric acid upon antimony trichloride.

Antimony pentoxide may be prepared by heating antimony, or a lower oxide of antimony, with nitric acid or aqua regia. It may also be obtained by ignition of the hydrated pentoxide (antimonic acid), which may, in its turn, be prepared by the hydrolysis of antimony pentachloride. It is formed in a variety of reactions, many of which, however, are not suitable for its preparation. Among these reactions may be mentioned the action of alkaline hydrogen peroxide upon antimony or antimony trioxide; and the deflagration of a mixture of antimony, antimony trioxide, antimony trisulphide or potassium antimonyl tartrate with potassium nitrate.

Antimony pentoxide is a pale, lemon-yellow powder, without taste. Its density is 3.78. It crystallises in the cubic system, with a lattice constant similar to that of the tetroxide. The arrangement of the oxygen atoms in the lattice, and the effect of adding oxygen atoms within the range Sb₂O₄ to Sb₂O₅, affords a possible explanation for the development of colour in the neighbourhood of Sb₆O₁₃. The pentoxide is insoluble in alcohol, but soluble in tartaric acid.

The heat of formation, calculated from thermal data obtained in the oxidation of the element or lower oxides by means of sodium peroxide, is 229,600 gram-calories. The molar heat capacity at low temperatures (in gram-calories per mole) calculated from determinations of specific heat on two hydrated forms is given in the following table:

Temperature, °C.	-213	-203	-173	-123	-103	-73	-3	+ 17	+ 27
Heat capacity	5.75	7.83	12.66	18.46	20.32	22.67	26.54	27.62	28.23

The pentoxide is decomposed on heating, decomposition beginning at about 300° C. with the formation of antimony tetroxide, and also by hydrogen under the influence of the silent electric discharge. With chlorine it forms antimony trichloride; hydrochloric acid has a slight solvent action, but no chlorine is evolved. It is reduced by phosphorus trichloride. Hydriodic acid reduces it to antimony trioxide with liberation of iodine. This reaction affords a delicate qualitative test for antimony. The reaction

Sb₂O₅ + 4HI ⇔ Sb₂O₃ + 2H₂O + 2I₂

is reversible, and the equilibrium conditions have been studied with reference to the influence of varying concentrations of the reacting substances and of the presence of certain other substances. Reduction of the antimony pentoxide is practically complete in the presence of excess of potassium iodide or hydrochloric acid, the latter being the more effective. The presence of tartaric acid reduces the amount of iodine set free; cadmium iodide acts similarly; but the presence of neutral salts, or a rise in temperature, increases the amount.

Antimony pentoxide reacts with sulphur to form antimony trioxide, or antimony trisulphide, according to the proportion of sulphur, the reactions being represented by the following equations:

Sb₂O₅ + S = Sb₂O₃ + SO₂
2Sb₂O₅ + 11S = 2Sb₂S₃ + 5SO₂

When heated in a current of hydrogen sulphide a black oxysulphide, Sb₄OS₅, is formed; an orange-red precipitate of antimony pentasulphide is obtained with a solution of hydrogen sulphide. This precipitate is soluble in warm alkali sulphides, and very slowly soluble in ammonium sulphide. Sulphuric acid, both dilute and concentrated, dissolves antimony pentoxide only slowly and after prolonged action. The pentoxide reacts with sulphur monochloride with the formation of antimony trichloride:

6S₂Cl + 2Sb₂O₅ = 4SbCl₃ + 5SO₂ + 7S

Antimony pentoxide is partially reduced when heated with carbon in the blowpipe flame, but for complete reduction admixture with sodium carbonate is necessary. Carbon disulphide and silicon tetrachloride also act as reducing agents, chlorine being evolved in the latter case.

Aqueous alkali solutions have only a slight solvent action, but antimonates are formed on fusion with alkalis. Reduction occurs on fusion with potassium cyanide, potassium formate, the sulphides of lead, copper and silver, antimony and antimony trisulphide.

Antimony pentoxide liberates chlorine from potassium chloride and iodine from potassium iodide, in both cases on heating in the presence of oxygen.

Stannous chloride produces a darkening of the colour of antimony pentoxide, the resulting product containing stannous oxide. The pentoxide is reduced to antimony by the action of tin and hydrochloric acid.

The hydration of antimony pentoxide has been studied by many investigators, and, while earlier workers reported a number of definite hydrates, more recent work suggests that the hydrates of antimony pentoxide resemble those of stannic oxide in being colloidal. Three of the so-called hydrates have been studied, being prepared respectively by (1) the hydrolysis of antimony pentachloride at 0° to 1° C., (2) the hydrolysis of antimony pentachloride at 100° C., and (3) the oxidation of antimony trichloride by nitric acid and hydrolysis of the product at 60° C. The results suggested that gels were formed in each case, the behaviour of these depending upon grain size, this in turn varying with the method of preparation. The three products contained the following molecules of water per molecule of antimony pentoxide after treatment as described:

	(1)	(2)	(3)
Dried on porous plate	30.57	9.97	7.91
Dried over sulphuric acid	3.68	2.17	0.60
Dried at 105° C.	2.43	1.02	0.45

Alcogels of antimony pentoxide have also been prepared, and their de-alcoholation curves were found to be similar.

Sols have been obtained by the hydrolysis of concentrated aqueous solutions of antimony pentachloride at 0° C. Freezing point determinations of these solutions suggest analogies with soap solutions, while pH values indicate that, on subsequent dilution, the micelles decompose further and ionise. Certain of the more stable solutions suggest a molecular weight of a very high order. The soluble products are acidic, and are probably hydrosols of low stability. It is probable that the ortho-, pyro- and meta-antimonic acids have no free existence. Towards alkalis, these hydrates show marked selective adsorption, forming amorphous substances, probably antimonates, of indefinite composition.

Certain crystalline antimonates may also be obtained by dissolution in concentrated alkali solutions, followed by careful evaporation at low temperature.

Evidence has been obtained for the existence of the definite hydrate 3Sb₂O₅.5H₂O from a study of the behaviour of gels obtained by the hydrolysis of antimony pentachloride, and also for the existence of a dihydrate and a hemihydrate. The formula HSb(OH)₆ has also been suggested for antimonic acid. Earlier investigators have described several hydrates obtained by the following methods: by the decomposition of a solution of potassium antimonate by nitric acid; by the hydrolysis of antimony pentachloride; by the repeated action of aqua regia on antimony; by the action of nitric acid upon antimony trichloride.

Much confusion exists as to the nature of the many antimonates that have been obtained and described. It seems fairly certain that only in a few instances - notably the salts of iron and aluminium - are normal ortho-antimonates obtained, the majority of the salts being either acid ortho-antimonates of the type KH₂SbO₄, or meta-antimonates of the type KSbO₃. The elucidation of the constitution of these salts is handicapped by the difficulty experienced in determining the true water content of the solid substances. From conductivity determinations in solution it appears probable that both the potassium and the sodium salts are acid ortho-antimonates of the above type. The following compounds have been obtained by the interaction of a concentrated solution of a salt of the metal with a concentrated solution of sodium antimonate:

Ortho-antimonates - Fe₂O₃.Sb₂O₅.7H₂O; Fe₂O₃.Sb₂O₅.9H₂O.

Meta-antimonates - CuSb₂O₆.5H₂O; Ag₂Sb₂O₆.3H₂O; BeSb₂O₆.6H₂O; BaSb₂O₆.5H₂O; ZnSb₂O₆.5H₂O; CdSb₂O₆.6H₂O; PbSb₉O₆.5H₂O; MnSb₂O₆.5H₂O; NiSb₂O₆.6H₂O; CoSb₂O₆.6H₂O. The last two have also been obtained with 12 molecules of water of hydration.

In many cases the proportion of water retained in the molecule depends upon the conditions of preparation. In general, this water is removed completely by heating to 100° C. or by drying over concentrated sulphuric acid. In the case of potassium antimonate it is much more difficult to remove the water, and the suggestion has been put forward that in this case a pyro-antimonate may be formed.

Alkali antimonates are to some extent soluble in water, but antimonates of the heavy metals are, in general, soluble only with difficulty. The solubility of sodium meta-antimonate (expressed in milligrams Na₂O.Sb₂O₅.6H₂O in 100 c.c. of solution) is 56.4 in water at 18° C., 0.1 in a mixture of equal volumes of water and ethyl alcohol, and 3.1 in a 2.5 per cent, solution of sodium acetate.

Most antimonates are decomposed by concentrated acids with the formation of hydrated antimony pentoxide. Solutions of alkali antimonates react slowly with sulphuretted hydrogen in the absence of other alkali salts, an orange-red precipitate of antimony pentasulphide being formed; they react with carbon dioxide to form a white precipitate of an acid alkali antimonate.

A number of naturally occurring antimonates has been examined and the constitutions have been discussed.