A solution of a neutral seleniate, or of one with excess of base, is soon rendered turbid by having sulphuretted hydrogen passed through it. At first pure selenium separates; afterwards sulphuret of selenium; and, lastly, mere sulphur. The solution should be considerably diluted; when concentrated, the precipitate formed is of a flame yellow colour, but soon becomes brownish-black, and sulphur is deposited, sometimes crystallizing at the surface of the deposite.—Phil. Mag., N. S., ii. 390.
13. On a new Compound of Selenium and Oxygen—Selenic Acid, by MM. Mitscherlich and Nitzsch.—This acid contains half as much more oxygen as that discovered by M. Berzelius, and with potash forms a neutral salt, having the same form and optical properties as sulphate of potash, containing no water when crystallized, and producing insoluble precipitates with barytic salts. The acid is isomorphous with the sulphuric, and may with propriety be called selenic acid, that described by M. Berzelius being considered as the selenious acid.
The new acid is easily prepared: for this purpose selenium, selenious acid, a selenite or a metallic selenuret is to be fused with nitre. Selenuret of lead, being the most abundant source, has been used for this purpose, but being accompanied by sulphuret, the selenic acid is usually contaminated by sulphuric acid. The selenuret of lead is to be freed from carbonates by muriatic acid, and the residue mixed with its weight of nitrate of soda, and thrown gradually into a red-hot crucible. Water then dissolves out seleniate nitrate and nitrite of soda, no selenium remaining in the residue. The solution quickly boiled, deposits anhydrous seleniate of soda, and this being separated, by cooling crystals of nitrate of soda are formed; these being removed, ebullition again causes more seleniate to fall down, and proceeding in this way an imperfect separation is effected. The seleniate, like the sulphate of soda, is most soluble in water at 181°. To purify the salt completely, the nitrite should be changed into nitrate by nitric acid; but then sulphate of soda would remain as an impurity formed from sulphuret in the ore, and no attempt to separate this has as yet succeeded.
But if the seleniate of soda be mixed with muriate of ammonia and heated, selenium, nitrogen and water come over, no trace of sulphur appearing. The selenium may, however, be dissolved in excess of nitric acid, and the selenious acid produced tested by [p472] muriate of baryta, which would then separate sulphuric acid if present; the clear solution is to be saturated with carbonate of soda, evaporated to dryness, and the mixture of selenite and nitrate of soda obtained, fuzed in a porcelain crucible over a spirit-lamp. Then proceed by crystallization as before, and a pure seleniate of soda will be produced.
To separate the selenic acid, the solution is to be decomposed by nitrate of lead; the seleniate of lead is as insoluble as the sulphate, and being well washed, is to be decomposed by a current of sulphuretted hydrogen, which has no action on the selenic acid; the solution being filtered, is to be boiled, and is then diluted selenic acid. Its purity, as respects fixed bodies, is ascertained by its entire volatility; if sulphuric acid be present, it may be ascertained by boiling a portion with muriatic acid, which produces selenious acid, and then testing by muriate of baryta, a precipitate indicates sulphuric acid.
From the isomorphism of selenic acid and its salts with sulphuric acid and its salts, M. Mitscherlich concluded, that the oxygen in the acid should be to that in selenious acid as 3 to 2; and to that in bases when it forms salt, as 3 to 1. These views were confirmed by experiments. From the decomposition of seleniate of potash by muriate of baryta, it appeared that the seleniate was composed of
| Potash | 42.16 | oxygen | 7.15 | |
| Selenic acid | 57.84 | —— | 21.79 | |
| 100.00 |
The composition of the acid was determined by boiling a certain weight of the seleniate of soda with muriatic acid in excess, and decomposing the selenious acid formed by sulphite of soda; 4.88 of the salt gave 2.02 of selenium, from which, and the above result, it would appear that the acid is formed of
| Selenium | 61.4 |
| Oxygen | 38.6 |
| 100.0 |
According to Berzelius, selenious acid consists of 100 selenium, and 40.33 oxygen; and supposing this contains two-thirds the oxygen in selenic acid, the latter should consist of 62.32 and 37.68. From the analysis above given of the seleniate of potash, it is evident that 100 of selenic acid saturates a quantity of base captaining 12.56 of oxygen, which would agree with the latter estimate of selenic acid.