There can be no doubt that imperfections exist in many of the methods at present in use for the preservation of various articles of the materia medica. Wherever the fault may be in these cases, the evil is generally shared between the physicians and the patients, much the larger share of course, falling to the latter. The iodide of iron is one of these articles, and it will appear probable from the sequel that, in a multitude of cases, this remedy is administered to the patient in quantities which are inconstant and much too small to produce the effect contemplated by the physician in his prescription.
One method, extensively employed, of preserving iodide of iron, for use in medicine, is in the form of an aqueous solution in which a coil of iron wire is kept immersed. This method is given by Pereira,[27] as proposed by Hemingway. Pereira also remarks in another place that “it is important to know, that by keeping a coil of iron wire in a solution of the protiodide, as suggested by Mr. Squire, no free iodine or sesquiodide of iron is formed although the liquid may be fully exposed to air and light; sesquioxide of iron is formed, but if the solution be filtered it is found to contain protiodide only.”
[27] Materia Medica, 3rd Am. Ed. 1, 745.
In a paper previously published in this journal, I have remarked with reference to this matter, that I should strongly {354} suspect in this case a formation of a subiodide of iron and consequent abstraction of iodine from the solution.[28] Since that time I have been enabled to confirm this supposition by experiment. Pieces of iron wire placed in contact with a colorless solution of iodide of iron caused, in the course of a few hours, the deposition of a precipitate, which had a dark orange color quite distinct from the dark brown color of hydrated sesquioxide of iron precipitated from a solution of the protochloride of iron by metallic iron. This precipitate, being washed with distilled water until the washings gave no indication of the presence of iron, was still found to contain much iodine. No quantitative analysis of the precipitate, however, was attempted, for it was found that the washings which no longer contained a trace of iron still gave with nitric acid and starch, a strong iodine reaction, thus indicating that the subiodide of iron upon the filter, whatever its composition, was decomposed by the action of water and oxygen as soon as the neutral iodide of iron was washed out. This is probably the reason why previous observers have mistaken this precipitate for pure sesquioxide of iron, having continued washing the precipitate until the washing no longer gave an iodine reaction, instead of an iron reaction as in the plan adopted by me, and consequently until all the subiodide of iron was decomposed and nothing but sesquioxide of iron was actually left upon the filter.
[28] New-York Journal of Pharmacy, August, 1852.
The washings, however, after the removal of the iodide of iron, gave no iodine reaction with starch until after the addition of nitric acid; iodine, therefore, could only have been present in the form of hydriodic acid and the reaction by which the unknown subiodide of iron was decomposed may be represented as follows:—2 Fe I1x + 1xH O + (3−1x)O = Fe 2O3 + 1xHI.
Since the above experiments were made, I have found that I have, after all, merely been in a measure confirming an observation of the illustrious Berzelius. Gmelin’s Handbuch under the head of Einfachiodeisen, has the following, “Nach Berzelius ist das braune Pulver welches sich beim Aussetzen des {355} wässrigen Einfachiodeisens an die Luft absetzt, nicht reines Eisenoxyd, sondern ein basisches salz.”[29]
It appears, therefore, that the method of preserving iodide of iron in solution, in contact with metallic iron is perfectly fallacious. This remedy, if preserved in solution at all, should be kept in bottles hermetically closed.
[29] According to Berzelius, the brown powder, which is deposited upon exposure of aqueous protiodide of iron to the air, is not pure sesquioxide of iron, but a basic salt.