Element Chromium, Cr, Transition Metal
|In many of its chemical peculiarities chromium is closely allied to the metals of the iron group, especially to iron and manganese. On the other hand, it is related to the elements molybdenum, tungsten, and uranium, which must be placed in the last group of metals, so that chromium could he grouped just as well with them. It is somewhat arbitrary, therefore, in which group it is placed; that it is here classed in the iron group is done for didactic reasons.|
Chromium is closely related to manganese in the number of the series of compounds which it forms. Whereas, however, in the case of manganese, the acids corresponding to the higher stages of oxidation were somewhat unstable, these belong in the case of chromium to the most important and best-known compounds.
Chromium forms the following series of compounds:
Salts of the divalent dichromion, Cr••.
Salts of the trivalent trichromion, Cr•••. and complex compounds derived from it.
Chromium trioxide, CrO3, and acids derived from it.
Chromium peroxide, whose composition is not yet known with certainty.
The combining weight of chromium is Cr = 52.1.
Early History of Chromium
|Lehmann, writing in 1762 to de Buffon, described a new mineral from Siberia, now known as crocoite. On examination of this mineral by Vauquelin and Macquart in 1789, lead, iron, and alumina were reported; chromium was not detected till 1797 when, on reinvestigation, Vauquelin1 observed that the lead was combined with a new acid derived from the oxide of a hitherto unknown metallic element. On account of the fact that its compounds are all coloured, the name chromium (Greek chroma, colour) was given to the element in question. In 1798, Vauquelin detected the new element in spinel and in smaragdite, and the following year Tassaert found it in chrome iron ore. The discovery of chromium in crocoite was made simultaneously and independently by Klaproth. More recent work has dealt largely with the isomerism exhibited by chromium compounds, and the relation between their colour and constitution.|
|Metallic Chromium was for long known only in the form of an impure product containing carbon, since the fusion of pure chromium could not be effected on account of its high melting-point. By the reduction of chromium oxide with aluminium, in accordance with a general method given by H. Goldschmidt, very pure metallic chromium is now manufactured in large quantities. It is used in the iron industry to add to steel (chromium steel).|
This preparation is carried out by mixing chromium oxide with aluminium powder, both carefully dried, and initiating the reaction with a small quantity of the mixture. For this a very high temperature is necessary; this is produced by mixing aluminium powder with barium peroxide, forming a pill of this, and sticking a piece of magnesium ribbon into it. If the magnesium ribbon is ignited, which can be done with a match, the combustion of the aluminium with the oxygen of the barium peroxide begins; the mass thereby becomes white-hot, and initiates the reaction in the neighbouring portions of the chromium mixture. In proportion as this undergoes transformation, more of the mixture is added; the temperature thereby soon rises so high that the chromium fuses.
The method has the advantage that it does not require a specially built furnace, but can be carried out in an ordinary crucible, preferably one of magnesia; the outside of the crucible becomes only slowly warm. On account of the high temperature of fusion of chromium, the preparation of the fused metals is successful only when fairly large quantities are employed, but then with ease.
A similar method is used for preparing other metals, and also for the production of very high temperatures. In the latter case cheap oxides, generally iron oxide, are used. With such mixtures fusions, welding, etc., can be carried out on the spot with great ease and certainty, so that the method is of great technical importance.
Chromium is a lustrous white, very hard metal, the melting-point of which is about 2000°. Its density is 6.8. It remains unchanged in the air; even at a red heat it becomes only slowly coated with a thin layer of oxide, which exhibits the colours of thin plates. It is dissolved by dilute hydrochloric and sulphuric acids, with evolution of hydrogen. Nitric acid does not attack it, since it becomes " passive " in that acid.
Chromium passes into this passive state, i.e. ceases to be attacked by acids, even by lying in the air. The metal when in this state is not attacked at the ordinary temperature by dilute acids. Treatment of the metal with strong oxidising agents has the same effect. If the passive metal is allowed to lie for a fairly long time under acid, or if this is warmed, solution with evolution of hydrogen suddenly commences. If the metal is used as an anode in dilute acid, it is converted by weak currents into its lowest compound, a chromous salt. If, however, the strength of the current is increased, the metal suddenly begins to dissolve in the form of its highest stage of combination as chromic acid.. The passive metal likewise becomes active, i.e. becomes soluble in acids when it is touched with a piece of zinc or similar metal under acid.