This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol2", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
These are compounds of sulphurous acid and salifiable bases, and are emplo37ed, almost to the exclusion of sulphur itself and sulphurous acid, as internal remedies or prophylactics, in diseases supposed to have a zymotic origin. The special salts employed are the sulphites of soda, potassa, ammonia, magnesia, and lime. These salts are prepared by passing sulphurous acid through a solution of the alkalies or their carbonates. Such at least is the case with the sulphites of soda, potassa, and ammonia; but the less soluble of these salts, as the sulphites of magnesia and lime, are more conveniently prepared in the way of double decomposition. if one of the soluble sulphites, but especially sulphite of soda or ammonia, be added to a soluble salt of the base, the sulphite of which it is designed to prepare, as the chloride of magnesium or calcium, in solution, the sulphite of the base employed, as of magnesia, or of lime, will be precipitated. The sulphite of lime, however, may be procured, like the alkaline sulphites, by passing the acid gas through lime suspended in water. (Thenard.) in all instances, the evaporation should be carefully conducted so as not to drive off the acid, and with as little exposure to the air as possible; as the sulphites in solution are strongly disposed to absorb oxygen from the air, and thus pass into the state of sulphates. Even in the solid condition, they slowly undergo the same change; and they should, therefore, when prepared, be kept, so far as can be conveniently done, from contact with the atmosphere.
These salts have in general a sulphurous taste, and, on exposure to heat, or by the addition of an acid, emit the characteristic odour of sulphurous acid, familiar to every one as that arising from a burning sulphur match. As before stated, they are disposed to absorb oxygen from the air, and are finally converted into sulphates. They are distinguishable from the hyposulphites, which, though occasionally used, are thought to be less efficient, by depositing no sulphur when their solution is treated with diluted sulphuric acid. In both kinds, sulphurous acid escapes, known by its peculiar odour; but with the hyposulphites a deposit of sulphur takes place, which the hyposulphurous acid gives up, when converted into the sulphurous. As the sulphites all operate through the qualities of their acid, they may be used indiscriminately in relation to their efficiency, though some of them are preferred for peculiar qualities, regarding convenience of administration.
The bisulphites are sometimes used indiscriminately with the sulphites, and it may not always be very easy to distinguish them. The bisulphites, however, are neutral to test-paper, while the proper sulphites have a slight alkaline reaction. They are prepared by passing sulphurous acid gas in excess through the alkaline solution. (Thenard.) They are probably quite as efficient as the sulphites, and perhaps more so, in consequence of their larger proportion of acid.
The hyposulphites consist of salifiable bases, combined in equivalent proportions with hyposulphurous acid, which, as now generally viewed, is composed of two eqs. of sulphur and two of oxygen (S2O2). This acid, however, exists only in composition; being decomposed into sulphurous acid (So2) and sulphur when separated from its salts. The hyposulphites may be prepared by boiling a sulphite or bisulphite for some time with sulphur; in the former case, the sulphurous acid taking up an additional equivalent of sulphur; in the latter, one eq. of the acid escaping, while its place is supplied with an eq. of sulphur. The hyposulphites are more stable than the sulphites, passing with greater difficulty into the state of sulphates by contact with the air. The alkaline hyposulphites, as well as those of lime and magnesia, are very soluble in water. They are known by the precipitation of sulphur when decomposed by an acid. (Thenard, 6e Úd., iii. 280.)
Sulphite of Soda. According to Berzelius (iii. 460), the salt formed by passing sulphurous acid through a solution of carbonate of soda, until the liquid shall sensibly redden litmus-paper, is the bisulphite of soda. If to this be added a quantity of soda equal to that already contained vol. ii.-60 in it, the neutral sulphite results. This salt crystallizes in prisms, which have a feeble alkaline reaction, and a taste like that of sulphurous acid, are soluble in four times their weight of cold, and in somewhat less than their weight of boiling water, and on exposure to the air are converted into sulphate of soda; while the former salt, or bisulphite, is neuter to test-paper, and by exposure becomes the bisulphate. Sulphite of soda is among the sulphites most used, and is especially preferred for external application.
Sulphite of Potassa. This is obtained in the same manner as the preceding salt. it crystallizes in plates or needles, decrepitates when heated, and effloresces in the air, at the same time absorbing oxygen. Berzelius states that there is also a bisulphite, which crystallizes more readily than the sulphite, but is usually confounded with it by authors (III. 389).
Sulphite of Ammonia. This salt is formed when the two gases are brought into contact; but may, no doubt, be prepared like the others by passing sulphurous acid gas through a solution of carbonate of ammonia. it has an acrid, sulphurous taste, becomes moist in the air, and afterward dries, having been converted into the sulphate. it is dissolved in its weight of cold, and in less than its weight of boiling water. When heated, it decrepitates, loses a part of its ammonia and water, and then sublimes as supersulphite of ammonia. (ibid., iv. 12.)
Sulphite of Magnesia. This may be prepared by double decomposition between any two soluble salts, the one of sulphurous acid, the other of magnesia; sulphite of magnesia being thrown down, when the two salts are mixed in solution. it is, as thus prepared, a white powder, but slightly soluble, of a stale, earthy taste, with a sulphurous aftertaste, but less disagreeable than that of the more soluble salts. it is soluble in a solution of sulphurous acid, which, on evaporation, yields transparent crystals. These dissolve in 20 parts of cold water, and effloresce upon the surface on exposure to the air. if the salt is exposed to a great heat in close vessels, the acid is driven off, and pure magnesia remains. (ibid., iv. 95.) it is preferred by some for internal use to all the other salts.
Sulphite of Lime. This also is prepared most conveniently by double decomposition. it is a white powder, requiring 800 parts of water for complete solution. By an excess of acid it is rendered more soluble, and from a hot saturated solution is deposited, on the cooling of the liquid, in long, six-sided needles.