This section is from the book "Materia Medica And Therapeutics Inorganic Substances", by Charles D. F. Phillips. Also available from Amazon: Materia medica and therapeutics.
Iron, the most abundant and the most useful of metals, occurs extensively in the mineral kingdom, its principal ores being either oxides, as the magnetic iron ore, or carbonates, as clay iron-stone. It occurs also in many mineral, so-called chalybeate waters, generally as carbonate with excess of carbonic acid, sometimes as ferrous chloride or sulphate. In the animal kingdom it is an essential constituent of blood, being contained, though only in minute quantity, in the haemoglobin of the red corpuscles. It occurs largely also in the vegetable kingdom, and may be traced in the ashes of almost all plants. Sometimes the pure metal is found native, and is then commonly supposed to be of meteoric origin.
Iron is hard, malleable, ductile, and of great tenacity; sp. gr. 7.7. Exposed to moist air, it becomes covered with a reddish layer - rust - which is mainly hydrated sesquioxide. It forms two distinct classes of compounds known as proto- or ferrous salts, and per- or ferric salts; in the former, it combines with not more than two atoms of a monad, as Cl or I; in the latter, it requires three, or, as most consider, six atoms of a monad for saturation ("Smith's Commentary"). The ferrous or proto-salts are commonly lighter in color, less astringent, and less soluble in alcohol; they have a marked tendency to absorb oxygen, and to become ferric compounds, hence most of the officinal ferrous salts are in a partially oxidized state, but to some, sugar is added to prevent such change as in syrupus ferri iodidi, and ferri carbonas saccharata. Ferric or per-salts are generally brownish-yellow, astringent, and soluble in alcohol, and are not prone to change: within the body, however, they are probably reduced to proto-salts.
The general tests for iron are - (1) the color test, with tannic or gallic acid; (2) the precipitate and blue color produced by ferro-cyanide; and (3) by ferrid-cyanide of potash. (1) Tannins change the per-salts of iron bluish-black, and act similarly, though more slowly, with proto-salts. (2) The yellow prussiate of potash (ferro-cyanide) gives a deep blue precipitate with per-salts of iron, and a whitish or light blue one with proto-salts. (3) The red prussiate (ferrid-cyanide) gives no precipitate with the per-salts, but the liquid becomes of a dark color: a deep blue precipitate with proto-salts (Turnbull's blue).
Sulphuretted hydrogen and ammonium sulphide are also used as tests for iron salts; thus, in acid solutions of pure ferrous salts, the former gives no precipitate, while with ferric salts it throws down a nearly white precipitate of sulphur, with reduction to the ferrous state: Fe2Cl6 + H1S = 2FeCl2 + 2HC1 + S. The same tests will also precipitate any copper contained in acid solutions of iron salts.
By acids iron is readily dissolved, with formation of metallic salts and evolution of hydrogen.