This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol1", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
This is prepared by dissolving sulphate of iron in water, adding sulphuric acid, and boiling; then adding nitric acid in small portions successively, boiling after each addition, until a dark colour is no longer produced; and, finally, precipitating with ammonia in excess, and washing the precipitate with water. The object of the first part of the pro-cess is to convert the protoxide of the sulphate completely into sesquioxide, which is done at the expense of the oxygen of the nitric acid. The addition of sulphuric acid is necessary to saturate the sesquioxide formed, which requires more acid than the protoxide in the proportion of its excess of oxygen. The precipitated sesquioxide, after having been washed, is introduced into a bottle, and kept in a moist state under water.
This preparation consists of one eq. of the sesquioxide, and two of water of combination, which it retains when carefully dried.
Hydrated sesquioxide of iron, as officinallv prepared, is a moist, reddish-brown mass or pulp, inodorous, of a slightly styptic taste, nearly insoluble in water, but readily dissolved by most acids. Dried carefully, it is still dissolved by acids, though less rapidly. By standing long, even under water, it acquires a new molecular condition, which, though it does not render it absolutely insoluble, very much impairs its solubility. Heated to redness, so as to be deprived of all its water, it is dissolved very slowly by the dilute acids.
As this preparation, in the moist state, or when carefully dried, is dissolved with considerable facility by acids, it would no doubt act efficiently as a chalybeate; but it is not used for this purpose. It was introduced into the Pharmacopoeias as an antidote for arsenious acid; and there can be little doubt that it possesses great efficiency in this respect; often saving life, and perhaps always, if properly employed, unless the injury already done is fatal. It acts by converting the poisonous arsenious acid into an insoluble and inert sub-arseniate of the sesquioxide of iron. It is true that it will not produce this effect on the undissolved arsenious acid; but it is not in this condi-tion that the poison acts; and if, as it dissolves, there is enough of the antidote present to neutralize the dissolved portion, it prevents evil effects, until the whole of the arsenic can be evacuated from the stomach, and subsequently from the bowels. To prove successful, however, it is neces-sary that the antidote should be employed in great excess; not less than twelve times as much as the arsenious acid taken being required; and some advise thirty times as much, or even more. As the chalybeate produces no injury, except perhaps a slight irritation, infinitely less deleterious than the effects of the poison, it should be given very freely; and attention to precise quantity is unnecessary. It becomes much less efficacious when dried, or long kept even in the moist state; and should, therefore, if possible, be obtained freshly precipitated when wanted for use. Hence the propriety of keeping a portion of the solution of the sesquisulphate (tersulphate of the sesquioxide, Fe2O3,3So3), as prepared in the process, and precipitating the oxide by ammonia at the time it may be wanted.