Note 16 — PART I. THE CHANGES UNDERGONE BY ANTIQUITIES IN EARTH AND IN AIR.
George A. (George Augustus) Auden translation
[It may not be out of place here to give the main conclusions, drawn from a long series of experiments by Prof. W. R. Dunstan (“Proc. Chem. Soc.,” XIX. 150, 1903).
(a) Pure iron is not oxidised in the presence of gases and water-vapour only, but for the appearance of rust the presence of water in the liquid state is necessary.
(b) The reagents which prevent the rusting of iron are those in the presence of which hydrogen peroxide is decomposed, and which are consequently inimical to its formation: among such reagents the following are given—sodium chloride, sodium sulphate, ferrous sulphate and potassium nitrate.
(c) The action of H2O2 on metallic iron leads to the production of red basic ferric hydroxide, which is identical with ordinary rust. The composition of rust may therefore be represented by the formula Fe2O2(OH)2, the reaction being represented by the equations:
Fe + O2 + H2O = FeO + H2O2. 2FeO + H2O2 = Fe2O2(OH)2.
These views are however combated by Moody (“Proc. Chem. Soc.,” XIX. 157 and 239) who concludes that aerial rusting must be regarded as a change involving the interaction of iron and carbonic acid and the subsequent formation of rust by oxidation of the ferrous salt.
He also states that those salts which do not combine with and which are not decomposed by CO2 have no retarding influence on the formation of rust, e.g. sodium chloride, sodium sulphate, etc.
On the other hand substances which absorb and combine with carbonic oxide (e.g. sodium carbonate or hydroxide, ammonium carbonate, calcium hydroxide), or which are decomposed by carbonic acid (potassium and sodium nitrites), inhibit rusting, which may therefore be regarded as a change involving the interaction of iron and acid and the subsequent formation of rust by the oxidation of the ferrous salt.
O. Kröhnke (“Wochensch. Brauerei,” XVII. 233) gives the following equations:
Fe + 2CO2 + H2O = Fe(HCO3)2 + H2. 2Fe(HCO3)2 + O + H2O = 2Fe(OH)2 + 4CO2.
Comp. also Dammer, “Handbuch der anorg. Chem.,” Vols. III. and IV. (supplement).
Considerable attention has also been directed to the influence of bacteria upon iron. Thus the growth of Crenothrix may cause much trouble in waterworks, vide “Centralblatt für Bakterien und Parasitenkunde,” II. 12, 681. A variety, Chlamydothrix (Gallionella) ferruginea (Mig.) appears to play an important part in the formation of rust (comp. Zopf, “Crenothrix polyspora die Ursache der Berliner Wasser-Calamität,” Berlin, 1879. De Vries, “Unter. der Crenothrix Commission,” Rotterdam, 1887 and 1890).
Neufeld (“Chem. Centralblatt,” 1904, I. 1621—abstracted from “Zeitschrift für Untersuchung der Nährungs- und Genussmittel,” VII. 478) gives particulars of three varieties: Crenothrix polyspora, which separates iron; Cr. ochracea, which separates aluminium and some iron; and Cr. manganifera, which separates manganese.
Jackson (“Journal of Society of Chemical Industry,” 1902, p. 681) gives micro-photographs of these varieties. Microscopically the masses of Crenothrix are seen enclosed in a gelatinous sheath, in which is imbedded the precipitated metallic hydrate. It is anaërobic and its action is favoured by absence of light. In the absence of dissolved oxygen, the bacillus appears to take its iron from the pipes. Cr. polyspora is found however (“Zeitschrift für analytische Chemie,” XLII. 590) to separate the iron not from the ferrous carbonate (FeCO3) but from iron organically combined. See also Winogradsky, Ueber Eisenbakterien, “Bot. Zeit.,” 1888, and “Chem. Centralbl.” 1904, II. 1332. Transl.]