Selenic Acid, H₂SeO₄

Selenic Acid, H₂SeO₄, was first prepared by Mitscherlich in 1827, who suspended lead selenate in water and precipitated the lead by means of hydrogen sulphide. By the oxidation of aqueous solutions of selenious acid with suitable reagents, e.g. chlorine or bromine, selenic acid may also be obtained. When chlorine is used, any hydrogen chloride must be removed as soon as it is formed, for selenic acid is reduced by hot hydrogen chloride to selenious acid, with liberation of chlorine; the process is best carried out by passing a current of chlorine through a solution of selenious acid in nitric acid containing lead nitrate; by this means the hydrogen chloride produced is re-oxidised to chlorine, while the selenic acid separates as lead selenate.

By passing chlorine through a suspension of basic copper carbonate in aqueous selenious acid, copper selenate and copper chloride pass into solution:

H₂SeO₃ + Cl₂ + H₂O = 2HCl + H₂SeO₄,
2CuCO₃ + 2HCl + H₂SeO₄ = CuCl₂ + CuSeO₄ + 2CO₂ + 2H₂O.

On crystallisation, copper selenate separates, contaminated with about 1 per cent, of cupric chloride. The latter may be removed by extraction with acetone, in which it is readily soluble, whereas the selenate is only very slightly soluble; after this operation the copper selenate is finally purified by recrystallisation from water. The copper may then be removed by electrolysis, using low current density, when selenic acid free from selenious acid and chlorine remains in the electrolyte. The solution may be concentrated until it contains about 82 per cent, of the acid by evaporating at 95° C. under reduced pressure. Bromine oxidises silver selenite according to the equation

Ag₂SeO₃ + Br₂ + H₂O = H₂SeO₄ + 2AgBr,

but the solution always contains a little selenious acid.

A solution of selenious acid in nitric acid can be oxidised by electrolysis, when selenic acid is produced at the anode. This is one of the best methods for the preparation of selenic acid on the larger scale, for the product is generally free from all but a trace of selenious acid. The nitric acid solution can also be oxidised by heating with potassium bromate or permanganate.

When selenious acid or a selenite is refluxed for about three hours with 30 per cent, hydrogen peroxide, oxidation proceeds to the extent of about 90 per cent.

A convenient and efficient method for preparing selenic acid is based on the fact that under suitable conditions chloric acid completely oxidises selenium or selenious acid to the required acid. If selenium is used as the starting-point, it is oxidised to selenious acid using nitric acid as oxidising agent. The selenious acid is then oxidised by heating with barium chlorate and sulphuric acid; chlorine and oxides of chlorine are expelled in the process. By concentrating the liquid in a vacuum, any perchloric acid formed is removed, and the resulting solution may contain 85 to 90 per cent, of selenic acid.

Pure selenic acid may be prepared from the solutions obtained in the foregoing processes by first neutralising with ammonia, precipitating barium selenate by the addition of barium chloride, and heating the separated precipitate with a solution containing the correct quantity of sulphuric acid. After removing the barium sulphate, a solution of pure selenic acid remains, and if this is concentrated as far as possible and then cooled to -50° C., crystallisation sets in on stirring. The same result may be obtained by heating the concentrated solution in a vacuum at 180° C. and then cooling, a crystalline mass being formed.

Selenic acid crystallises in long hexagonal prisms, apparently isomorphous with crystalline sulphuric acid, and of melting-point 57° to 58° C. The density at 15° C. in the solid state is 2.951 and in the undercooled liquid condition 2.608. The molten substance is easily cooled below its melting-point without solidification, and the presence of small quantities of water lowers the melting-point to considerably under -50° C. On heating above 160° C. the acid is slowly decomposed with the formation of selenious acid and oxygen. It can be distilled at 172° C. under a pressure of 85 mm. The following thermochemical data have been obtained:

SeO₂ (solid) + ½(O₂) + H₂O → H₂SeO₄ (liquid) + 3060 cals.
Se + 3/2 (O₂) + H₂O → H₂SeO₄ (liquid) + 59,860 cals.
H₂SeO₄ (liquid) → H₂SeO₄ (solid) + 3450 cals.

The vapour pressure of the liquid at different temperatures is as follows:

t, ° C	100	105	140	190	210
p, mm. Hg.	15.8	21.0	28.3	32.0	37.0

From the fact that in dilute solution the heats of neutralisation of selenic acid and sulphuric acid when neutralised by alkalis are approximately equal, it follows that these two acids are of comparable strength. Concentrated selenic acid resembles sulphuric acid in many of its properties, although its characteristics are rather less strongly marked. It absorbs moisture from the air, but on dilution with water it does not produce so much heat as sulphuric acid. As with sulphuric acid, determination of the freezing-point curve of mixtures with water indicates the existence of definite compounds or hydrates, viz., H₂SeO₄. H₂O and H₂SeO₄.4H₂O, the melting-points of which, +26° and -51.7° C. respectively, are given by the maxima on the freezing-point curve. The tetrahydrate appears to be isomorphous with the tetrahydrate of sulphuric acid, a crystal of the latter inducing crystallisation of the former.

Like sulphuric acid, selenic acid attacks many forms of organic matter, frequently with charring. Alcohol yields ethylene on heating, and cellulose (filter paper) is converted into a vegetable parchment. Toluene in the cold gradually undergoes concurrent oxidation and substitution with formation of tolueneseleninic acid, C₆H₄CH₃.SeO₂H, and diphenylselenone dicarboxylic acid.

Selenic acid oxidises aniline with explosive violence.

Sulphur is soluble in liquid selenic acid, forming a deep blue solution; on warming this solution to about 65° C. the selenic acid undergoes marked reduction, with formation of selenious acid and selenium. This result serves to emphasise the already observed lower stability of the oxygen compounds of selenium, and therefore their higher oxidising power, compared with the corresponding sulphur compounds. It has been suggested that the blue colour of a sulphur solution in selenic acid and the green colour of a selenium solution in sulphuric acid are due to the production of isomeric substances SSeO₃ and SeSO₃, sulphur selenoxide and selenium sulphoxide, respectively; the assumption of such isomerism is made on very slight evidence.

Selenium likewise dissolves in selenic acid, giving a deep green solution, the colour of which has been attributed to the formation of an unstable oxide, selenium sesquioxide, Se₂O₃ (analogous to S₂O₃), which, however, has not yet been isolated. The colour disappears on warming, with formation of selenious acid, but if the cold coloured solution is diluted, the selenium separates out. Tellurium gives a purple-red solution; if this colour is due to a compound TeSeO₃, analogous to the foregoing sesquioxide, the compound must be exceedingly unstable, as slight warming destroys the colour.

It might be expected by analogy with sulphuric acid that selenic acid would absorb nitrogen dioxide with the formation of a nitrosylselenic acid, NO₂.SeO₃H, but this is not the case. Nitrogen dioxide is absorbed, however, at low temperatures, the product being an unstable blue solid, probably SeO₂(NO₂)₂, which melts with decomposition at -13° C. Nitrosylselenic acid may be obtained by the action of excess of liquid nitrogen trioxide on anhydrous selenic acid which is kept ice- cold. The product is a white crystalline "snow-like" mass, having a melting-point of 80° C., with decomposition. It is unstable even at ordinary atmospheric temperatures and is immediately decomposed by water.

Aqueous solutions of selenic acid cannot, by distillation under ordinary pressure, be entirely deprived of their water without decomposition. Until a temperature of 205° C. is reached the distillate consists almost entirely of water, the residual liquid being about 88 per cent, selenic acid. Above this temperature the selenic acid is slightly volatile and a distillate of very dilute selenic acid is obtained, the residual liquid finally attaining a composition of approximately 96 per cent, and a temperature of nearly 290° C. Then decomposition begins to occur with formation of selenium dioxide, oxygen and water. By distilling under reduced pressure, the high temperature is avoided, and a residue of selenic acid of almost 100 per cent, concentration is obtained at a temperature near 180° C.

Aqueous solutions of selenie acid will, on heating, oxidise hydrochloric, hydrobromic and hydriodic acids. Hydrogen sulphide gas at any temperature above -10° C. causes decomposition of selenic acid, the rate of reaction increasing both with the temperature and the concentration of the acid. The complete reaction may be represented by the equation

H₂SeO₄ + 8H₂S = Se + 3S + 4H₂O,

but the mechanism of the reaction is more complicated. Solutions of selenic acid of all concentrations are reduced by selenium, especially on warming; with sulphur the reaction takes place more slowly and higher temperatures are necessary. The reaction with sulphur dioxide takes place in two stages and selenium dioxide may be separated as an intermediate product:

(1) H₂SeO₄ + SO₂ = SeO₂ + H₂SO₄,

(2) SeO₂ + 2H₂O + 2SO₂ = Se + 2H₂SO₄.

When the sulphuric acid reaches a certain concentration, the reducing action stops, but when the solution is diluted the reaction can proceed to completion.

By the action of hydrazine hydrate on a dilute solution of selenic acid, hydrazine hydrogen selenate may be obtained as a colourless compound which is not decomposed by boiling water, but which, when dry, explodes with great readiness when subjected to heat, to shock, or to fumes of hydrogen chloride. For this reason, before hydrazine hydrate is used in the analysis of selenium compounds, it is essential that selenic acid and selenates should be reduced to selenites by means of hydrochloric acid.

When selenic acid and molybdic acid are heated together on a water-bath for several days a compound of composition MoO₃.SeO₃ is formed, which is obtainable as a crystalline mass, and which with a little water yields a hydrate, having the formula MoO₃.SeO₃.2H₂O when dried at 110° C. This compound has the properties of a tetrabasic acid.

Aqueous selenic acid is a solvent for metals, such as magnesium and zinc, hydrogen being evolved. With metallic iron there is only slight action (contrast sulphuric acid); after some time a thin deposit of red selenium forms on the iron, due to reduction of the acid by the nascent hydrogen produced. Copper and gold are dissolved by the warm acid, selenious acid being formed simultaneously; the power of dissolving gold must be ascribed as much to the oxidising power of the acid as to its acidity. There is no appreciable action with osmium in the cold, but at about 120° C. a colourless solution is obtained containing osmium tetroxide; no selenate is formed, the other product of the reaction being selenious acid.