Properties of Iron Powder

Iron powder, as obtained by the reduction of ferrous or ferric salts, is considerably more chemically reactive than the compact metal. Thus, the powder obtained by reduction of iron oxide, carbonate, or oxalate in a current of hydrogen at 440° C. is pyrophoric, becoming incandescent upon exposure to moist air. It decomposes acetylene with incandescence, depositing free carbon and yielding small quantities of benzene. Reduced iron absorbs some 2 per cent, of nitrogen at atmospheric temperature, the solubility of the gas being proportional to the square root of the pressure. According to Sieverts, Kahlbaum's reduced iron absorbs nitrogen at 900° C., and this is quantitatively released on cooling.

When maintained at 310° to 320° C. for about forty-eight hours, pyrophoric iron is transformed into the non-pyrophoric form, the change being accompanied by an increase in volume. Probably the pyrophoric form consists of a mixture of molecules of different kinds which are not in a state of equilibrium.

Nitrogen peroxide is decomposed by reduced iron at the ordinary temperature, the metal becoming incandescent and yielding a mass of ferric oxide, Fe₂O₃. Nitrous oxide is reduced at 170° C., and nitric oxide at 200° C., ferrous oxide resulting.

Nitric oxide is almost quantitatively converted into ammonia when mixed with hydrogen and passed over the warmed metal. The reaction begins at about 300° C., and is very rapid at 350° C.

When heated in a continuous current of carbon monoxide at 650° C., a considerable quantity of carbon is deposited. If, however, the carbon monoxide is admitted to a closed vessel containing the heated metal, absorption of the gas takes place, possibly with the formation of cementite - that is, iron carbide, Fe₃C.

In contact with air and water iron powder readily rusts at ordinary temperature. When warmed with water, hydrogen gas is evolved. Dilute solutions of sodium and potassium hydroxides are decomposed at their boiling-points in a similar manner. Under the influence of gentle heat iron powder decomposes steam - a reaction that has been recommended as a most convenient one for rapidly obtaining small quantities of pure hydrogen. The gas is evolved at considerably lower temperatures than when compact iron is used, decomposition proceeding slowly, in the presence of Kahlbaum's reduced iron, at about 250° C.

Finely divided iron decomposes sulphuric acid of density 1.75 at 200° C., yielding sulphur dioxide.

It is spontaneously inflammable in sulphur vapour, the ignition temperature lying below 448° C.

The exceptionally pure reduced metal obtained by Lambert and Thomson possessed several unusual properties. Thus, it exhibited remarkable inertness or " passivity," remaining free from rust upon prolonged exposure to air and tapwater.

Cold, dilute sulphuric and nitric acids had very little action on the metal, but on warming the iron readily dissolved. Aqueous hydrogen chloride attacked the metal even in the cold. Saturated solutions of the sulphate or nitrate of copper exerted no action at ordinary temperatures; even after an exposure of several months to copper sulphate solution, no change could be detected in the iron when examined under the microscope. On raising to 100° C., however, the iron gradually dissolved, copper being simultaneously deposited.

Solutions of copper chloride, when concentrated, immediately attacked the metal, depositing copper. Even dilute solutions (less than one per cent.) attacked the iron, although slowly.

Copper was also deposited on the metal if the latter was subjected to pressure in an agate mortar prior to being placed in the copper sulphate solution. Pressure with a quartz rod whilst immersed in the solution had a like effect.

These remarkable results might be attributed to a film of hydrogen protecting the metal in the first experiments from attack in copper sulphate solution, the film being disrupted in the later experiments by the pressure; but specimens of the metal which had been heated for several hours at 1000° C. in a vacuum, until spectroscopic tests showed that all hydrogen had been removed, behaved in precisely the same manner.

When the pure metal was treated with ferroxyl, unlike ordinary iron, it remained quite bright for an indefinite time, manifesting no tendency to corrode. On applying pressure locally, however, corrosion set in, a pink colour developing round the pressed portion, and Turnbull's blue appearing round the unpressed parts, indicating solution of the metal.