Telluric Acids

Tellurium trioxide gives rise to several hydration products which can all be regarded as telluric acids, but which are more conveniently considered as products of the further hydration of telluric acid, H₂TeO₄, the compound H₂TeO₄.2H₂O (possibly an "ortho-" telluric acid, H₆TeO₆) being the most stable.

1. Powerful oxidising agents are able to convert tellurium into the sexavalent condition, the product obtained in the presence of water being telluric acid, whereas in the presence of an alkali a tellurate is obtained.
   1. Aqua regia oxidises tellurium, incompletely, as far as telluric acid. Nitric acid gives a solution of tellurous acid, which by the action of lead dioxide or chromium trioxide produces telluric acid.
   2. A still better process is the passage of chlorine into an aqueous suspension of tellurium until the clear solution no longer gives a precipitate of tellurous acid when made alkaline and subsequently acidified with acetic acid. The solution obtained, when evaporated to small bulk and treated with alcohol, gives a crystalline precipitate of the compound H₂TeO₄.2H₂O.
   3. Sodium peroxide oxidises an aqueous suspension of tellurium to sodium tellurate.
2. Instead of using tellurium as the starting-point, the dioxide can be employed, either as such or in the form of tellurite. Method (1) (a) involves such oxidation of the dioxide, which is formed as an intermediate product. The dioxide can also be oxidised by fusing with potassium nitrate or potassium chlorate; it can also be replaced for this purpose by a tellurite; the product in each case is a tellurate, from which the free acid is obtainable by precipitation as barium tellurate and treatment of this with the calculated quantity of sulphuric acid. The oxidation of the alkali tellurite can be effected still more satisfactorily in solution by hydrogen peroxide; in this case the relative amount of alkali metal present is so small that the concentrated solution on acidification with nitric acid deposits crystals of the acid, H₂TeO₄.2H₂O, which can be recrystallised from a little water, separation being aided by the addition of nitric acid.
3. A very pure product and almost theoretical yield may be obtained by the oxidation of tellurium tetrachloride with chloric acid. A slight excess of chloric acid is used to avoid the formation of any explosive oxides of chlorine. The addition of concentrated nitric acid causes precipitation of the telluric acid, which may be freed from chlorine and nitrogen oxides by drying in a vacuum. Prepared by this method the acid has the composition H₂TeO₄.2H₂O.

Two different crystalline forms of the dihydrate, H₂TeO₄.2H₂O, are known. The acid separating from hot concentrated solution in nitric acid crystallises in octahedra of the regular system, having a density of 3.035. The more usual and more stable form consists of prismatic crystals of the monoclinic system, having a density of 3.071, and obtained on gradual evaporation of an aqueous solution or on cooling a dilute solution in warm dilute nitric acid.

By very cautious heating at 140° C. the dihydrate very gradually loses a bimolecular proportion of water, forming allotelluric acid, H₂TeO₄, which is a loose powder of density 3.43 at 19° C.; on further heating it passes successively into tellurium trioxide and tellurium dioxide.

The dihydrated acid is a snow-white crystalline powder, easily soluble in water but not deliquescent. The addition of nitric acid or of alcohol diminishes its solubility in water. When its aqueous solution is evaporated at 0° C., tetragonal crystals of a hexahydrate, H₂TeO₄.6H₂O, are obtained.

In aqueous solution the acid has a metallic taste and only feebly affects litmus; the latter characteristic is in accord with the evidence of its electrical conductivity, which is small and indicates relatively slight dissociation, comparable with that of hydrogen sulphide rather than with that of sulphuric acid. Measurements of the hydrogen-ion concentration of normal telluric acid solution gave [H^•] = 4×10^-5 gram-ion per litre. On the assumption that the acid is ionised in accordance with the equation

H₆TeO₆ = H^• + H₅TeO₆',

this gives the ionisation constant K = l.6×10^-4. According to this result telluric acid is a very weak acid.

Examination of the solubility curves of the dihydrate and the hexahydrate shows that these intersect at 10° C.; this temperature therefore is the highest at which the hexahydrate is stable with reference to the dihydrate. The heat of solution of the dihydrate is negative, the value per gram-molecule being -3.35 Calories.

Allotelluric acid, H₂TeO₄, judged by its electrical conductivity in aqueous solution, is a decidedly stronger acid than the dihydrate, H₂TeO₄.2H₂O, but in the course of three days conversion into the latter acid is complete. Cryoscopic examination of the aqueous solution shows that the molecule of allotelluric acid is more complex than that of the dihydrate, the change (H₂TeO₄)_x → H₂TeO₄.2H₂O being one of concurrent depolymerisation and hydration. It has been suggested that allotelluric acid is not a homogeneous substance, but a mixture of polymerised forms of the crystalline acid.

Telluric acid (dihydrate), when heated, loses water and becomes orange in colour, then gradually changing to white, owing to the successive formation of trioxide and dioxide. The residue is soluble in hydrochloric acid, forming an amber-coloured solution.

Telluric acid may readily be reduced, although not so easily as selenic acid. Hydrochloric acid on warming effects a partial reduction to the dioxide, and on boiling, chlorine is evolved; the latter reaction is characteristic of tellurium in the sexavalent form. Hydrobromic and hydriodic acids can carry the reduction to tellurium. Anhydrous hydrogen chloride converts heated telluric acid into tellurium tetrachloride, which sublimes away from the heated mass. Sulphurous acid causes a partial reduction to tellurium. Hypophosphorous acid and hydrazine can effect quantitative reduction to tellurium. The reducing action of hydrogen sulphide on telluric acid may be due to the formation of unstable sulphoxytelluric acid, which breaks down with the liberation of tellurium and sulphur. Telluric acid in 30 per cent, solution is reduced to tellurous acid by sulphur, selenium or tellurium. A concentrated solution of telluric acid boiled with a solution of potassium thiocyanate yields a yellow, heterogeneous, amorphous precipitate, containing tellurium, carbon and nitrogen.

Possibly owing to its oxidising power, telluric acid in warm aqueous solution is more active towards metals than might be expected from its feeble acidity, even lead, tin, silver and mercury being attacked.

Telluric acid is able to form well-defined crystalline additive compounds with the salts of such acids as iodic, arsenic, phosphoric, molybdic and tungstic acids. It also forms large, well-defined crystals with potassium nitrate of the composition 2KNO₃.H₂TeO₄.4H₂O. With silver nitrate telluric acid forms the compound AgNO₃.H₂TeO₄.2H₂O.

When telluric acid or an alkali tellurate, or even tellurium dioxide, is heated with ammonium chloride, the mixture changes in colour from yellow to orange with the formation of a white sublimate. On further heating the mixture darkens in colour, while a yellow sublimate is formed which in turn blackens on careful heating. If the chloride is replaced by other common salts of ammonium, such as the nitrate, carbonate, sulphate, phosphate, acetate or molybdate, these changes do not occur. It has been suggested that the white sublimate consists partly of ammonium chloride and partly of the additive product TeO₂.2HCl, and that the black sublimate is probably an ammoniated tellurous chloride, possibly TeCl₂.2NH₃.

Solutions of telluric acid give a quantitative precipitation of barium tellurate, BaTeO₄.3H₂O, on the addition of barium hydroxide solution, and the use of a standard barium hydroxide solution, followed by titration of the excess of alkali with a standard solution of oxalic acid, using phenolphthalein as indicator, forms a convenient process for the determination of the acid. With a solution of mercurous nitrate telluric acid and the tellurates yield a yellow crystalline precipitate of mercurous tellurate; the crystals may either take the form of triclinic plates or spheroidal masses.

The oxyacids of tellurium, like those of selenium, have an inhibitory action on the growth of many bacilli in cultures. The ions of selenious and tellurous acids seem to be much more inhibitory than those of selenic and telluric acids. In the case of the growth of diphtheria bacilli it has been shown that the active concentrations of selenium and tellurium are, for selenites, 1:1160, for selenates, 1:666, for tellurites, 1:420, and for tellurates, 1:125.