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Selenious Acid, H2SeO3

If a concentrated solution of selenium dioxide in water is evaporated in a vacuum over sulphuric acid, crystals can be obtained having the composition Selenious Acid, H2SeO3. Similar crystals can be obtained by recrystallising selenium dioxide from aqueous acetic acid, when the dioxide separates in the hydrated condition. Moist benzene may also be used as solvent.

Selenious acid may be prepared directly from "anode slimes" by oxidising with nitric acid; on evaporating down to a syrupy consistency and allowing to cool, crystals of the crude acid may be obtained, which can be purified by repeated recrystallisation from water.

Selenious acid is a crystalline solid having a density of 3.00 at 15° C. The crystals are deliquescent in moist air, whilst in dry air they gradually lose their combined water, with the formation of the dioxide; this loss of water is accelerated by heat. The acid is very soluble in water, solutions of it being obtained if selenium is oxidised in the presence of water.

Towards oxidising agents selenious acid is much more stable than sulphurous acid. This is in accordance with the observation that in a series of related elements, with increasing atomic weight there is generally a decrease in the relative stability of the higher oxidation products, as is illustrated by the groups N2O3, N2O5; As2O3, As2O5; Bi2O3, Bi2O5; and CO, CO2; SnO, SnO2; PbO, PbO2. The acid can be oxidised to selenic acid by strong oxidising agents such as chlorine, hypochlorites, potassium dichromate or permanganate. Nitric acid oxidises selenium only as far as selenious acid. There is no formation of intermediate compounds analogous to the dithionates in the oxidation of selenious acid.

On reduction, selenious acid readily gives selenium; electrolysis of the aqueous solution therefore yields selenium at the cathode and selenic acid at the anode. With nascent hydrogen an aqueous solution of selenious acid will yield hydrogen selenide. Sulphurous acid, preferably in the presence of hydrochloric acid, also reduces selenious acid to selenium, which separates as a red precipitate:

SeO2 + 2H2SO3 = 2H2SO4 + Se.

When this reduction is carried out in the presence of copper sulphate, as well as hydrochloric acid, the composition of the precipitate obtained varies from that of cuprous selenide in slightly acid solution, to pure selenium when the concentration of hydrochloric acid is high. It is probable that reduction to the element first occurs under all conditions, the subsequent formation of selenide depending on the hydrogen-ion concentration of the solution. Other reducing agents cause a similar separation of selenium, for example, hydriodic acid, phosphorous acid, hypophosphorous acid, hydroxylamine and hydrazine, and such organic substances as glucose, formic acid and formaldehyde. Sodium hydrosulphite also reduces a selenious acid solution, and if used in the presence of a little sodium carbonate to remove any free acid, the formation of a red colloidal solution of selenium is so easily observed that a test will detect one part of selenious acid in 20,000 of water.

Hydrogen sulphide reduces selenious acid solution with formation of a mixture of selenium and sulphur. The proportion of the two elements in the precipitate varies with the conditions. At one time this precipitate was regarded as a definite sulphide of selenium, but the sulphur can be entirely extracted by a mixture of benzene and carbon disulphide. The so-called "sulphur selenide" obtained by fusing together sulphur and selenium probably is also not a true compound, but only "mixed crystals" of the elements.

The behaviour of manganese dioxide towards selenious acid solution at 140° C. in a sealed tube is interesting. A yellow crystalline compound, MnSe2O6, is obtained, not a selenium analogue of manganese dithionate, but a selenite of quadrivalent manganese. The substance is an oxidising agent, converting hydrochloric and arsenious acids into chlorine and arsenic acid, respectively.

Selenious acid forms a sparingly soluble crystalline additive compound with thiocyanic acid, known as "thiocyanoselenious acid", H2SeO3.2HCNS. This is formed when a mixture of an alkali selenite and ammonium thiocyanate in aqueous solution is treated with fairly concentrated hydrochloric acid. When warmed with water it decomposes, liberating selenium.

The Selenites

Selenious acid is a weak acid but neutralises hydroxides and carbonates with the production of selenites. Ammonium selenite is formed by dissolving selenious acid in aqueous ammonia. The selenites can be obtained in an anhydrous condition by heating the metal oxides in a current of selenium dioxide vapour or with selenium dioxide in a sealed tube; in the latter case the products can be obtained crystalline if the selenium dioxide is used in excess.

By treating a base with more than the equivalent amount of selenious acid, acid selenites may be obtained.

The normal salts, on account of the feeble nature of selenious acid, are alkaline in solution. They exhibit a marked tendency to form complex salts; for example a series of selenito salts having bivalent central atoms with four co-ordination positions has been prepared by dissolving the normal selenites of cobalt, nickel, zinc, cadmium, manganese and copper in warm concentrated solutions of ammonium selenite and allowing to crystallise. Molybdoselenites and vanadoselenites have also been prepared, and a series of crystalline uranyl salts has been described. Alkali salts of the type MX.2SeO2.2H2O (where M = K or NH4 and X = Cl or Br) are known, and the potassium- chlorine salt when treated in solution with silver oxide yields silver chloride and a solution from which the tetraselenite, KHSeO3.H2SeO3 or K2O.4SeO2.3H2O, may be crystallised. This substance at 100° C. loses only two-thirds of its water and may be considered as the hydrogen pyroselenite, KO.SeO.O.SeO.OH + H2O, the chlorine compound from which it was derived then being the chloropyroselenite, KO.SeO.O.SeO.Cl.

Mercuric selenite, HgSeO3, prepared by the action of sodium hydrogen selenite on mercuric oxide, forms double salts of the types Na2Hg(SeO3)2 and HgCl.SeO3Na.2H2O.

The selenites are readily attacked by micro-organisms with the formation of a red substance, probably "metallic" selenium. The alkali selenites act on the animal organism as irritant poisons. Chabrie has observed that the presence of 0.2 per cent, of selenious acid prevents the putrefaction of beef.

Constitution of Selenious Acid

As with sulphurous acid and the sulphites, a decision is necessary between alternative formulae for selenious acid and the selenites, the possible structures being the symmetrical and the unsymmetrical . The non-formation of a selenium analogue of thiosulphuric acid on heating selenium with a selenite solution may be regarded as slight evidence in favour of the symmetrical formula, which also receives a little support by the production of a symmetrical ethyl selenite, SeO(OC2H5)2, by the action of ethyl iodide on silver selenite.

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