COMPOUNDS OF ALUMINIUM Aluminium and Oxygen.—Aluminium (which is trivalent) forms one oxide—alumina
. A film of this oxide normally covers the metal and upon this its character, its behaviour under chemical attack, indeed its whole utility may be said to depend. Generally invisible, the film may be seen if a thin foil of metal be heated for a prolonged period and then placed in hot nitric acid. The metal dissolves leaving the skin as glistening scales. Normally the thickness of the film is o.000oImm., increasing twentyfold to 0.000 2mm. after prolonged heating of the metal. It is impervious to many reagents and it is due to this fact that so highly reactive a material as aluminium resists attack by many agencies to which, if this film be perforated, it immediately suc cumbs. Most important is the protection given against further attack by oxygen itself, but for which aluminium would oxidize in air as readily as sodium or potassium. This may be appreciated if a strip of aluminium be amalgamated with mercury. Some aluminium dissolves in the latter and, being no longer protected, immediately combines with oxygen and water vapour to form hydrated oxide, long fibres of which grow from the amalgamated surface. At high temperatures aluminium burns in air with great evolution of heat and the finely divided metal ignites readily, burning with a brilliant flame or even explosively, whilst, mixed with oxides of other metals and ignited, violent interaction takes place, the aluminium depriving the other metal of its oxygen and itself burning to alumina. (See THERMIT.) In pure oxygen aluminium ignites at about 58o° C. As we have seen, alumina occurs i .i many forms in nature. It is manufactured on a very large scale from the hydrate as a white crystalline powder for use in the production of aluminium. Fused in electric furnaces it forms alundum or artificial corundum, a valuable abrasive ; whilst mixed with colour-giving oxides it yields artificial rubies, sap phires, etc.
Distilled water and the soft waters of peaty districts dissolve aluminium extremely slowly, but hard fresh waters and sea water cause rapid local corrosion of com mercial metal unless air be entirely excluded. Recent work on the corrosion of aluminium seems to show it clearly due to local breakdown of the protective film and accumulation in the minute recesses formed in the surface of the metal, of hydrated oxide. This prevents access of the oxygen to the points it covers. Such differences are known to promote solution of metals. Corrosion by water may be slight or very serious, according to the state of the metal, its purity, temper and the perfection of its surface; the composition of the water; and the ease with which the prod ucts of corrosion can accumulate. Damp air merely causes a roughening of the surface. Stagnant or flowing water may cause superficial blisters, beneath which in time deep-seated cavities may form. These may, in turn, perforate the metal or disinte grate its surface.
Many methods have been suggested for obviating what is one of the chief disadvantages from which aluminium and its alloys suffer. Against atmospheric corrosion painting or varnishing is usually effective. Pure aluminium immersed in water may be effectively protected if connected to one of its more easily cor rodible alloys, as iron boilers are protected by zinc plates. Air craft parts are largely protected by strengthening the natural film upon them, to which end they are made the anodes in an electro lytic bath containing chromic acid.
All three possible hydrated oxides occur in nature, the monohydrate
as diaspore, the dehydrate
as bauxite and the trihydrate
as gibb site or hydrargillite. These hydrates behave both as bases, dissolv ing in acids to form salts, and as weak acids, dissolving in strong bases to form the aluminates. On this property the process most frequently used for the preparation of pure alumina depends. Bauxites, which contain iron and other impurities, are heated with caustic soda solution, whereby the aluminium hydrate is alone dis solved to form sodium aluminate. From the filtered solution the purified trihydrate is precipitated by various methods. On being heated aluminium hydrates give up water and are converted into alumina. When crystallized the trihydrate is insoluble in water but readily assumes the state of extremely fine division known as collodial solution. In this state it absorbs many other bodies, notably dyestuffs, and its use as a mordant in dyeing, as well as for other purposes, is due to this property. Produced within the fibres of textiles the hydrate serves to fix the colouring matters which the fibres themselves are not capable of retaining.
In the absence of water aluminium is not attacked by the fixed alkalis, but it dissolves in their caustic solutions to form aluminates, hydrogen being evolved. Solutions of sodium and potassium carbonate ' rapidly attack the metal but silicate of soda solutions do not and, moreover, prevent attack by the carbonates. For this reason commercial mixtures of carbonate and silicate of soda form excellent detergents for aluminium do mestic utensils and are widely used for this purpose. Ammonium hydrate attacks aluminium much less actively than do the fixed alkalis and the only product is aluminium hydrate. To ammonium carbonate, aluminium is entirely resistant. At high temperatures gaseous ammonia is decomposed by aluminium, yielding aluminium nitride (AIN) which is also formed when aluminium burns in air.
As might be assumed of so active a substance, aluminium may be caused to unite with almost all acids. Hydrochloric and the other halogen acids readily dissolve the metal under all conditions, although even to hydrochloric acid the very purest metal shows considerable resistance. Aluminium is but very slowly dissolved by cold, fully concentrated nitric and sulphuric acids ; rise of temperature and increasing dilution how ever, increase the rate of attack, especially by sulphuric acid. Dilute organic acids dissolve aluminium very slowly, those occur ring naturally in foodstuffs so slowly that attack is generally negligible. It is particularly noteworthy that even where the pure acids dissolve the metal comparatively readily, the presence of other bodies in the foodstuff frequently inhibits their action alto gether. Thus, a hot o.2% solution of lactic acid attacks aluminium, but to milk of similar acidity aluminium is completely resistant. On the other hand, the presence of salts of those acids, which themselves readily attack aluminium, naturally enhances attack by organic acids. Vinegar mixed with common salt is much more active than vinegar alone. To concentrated organic acids alumin ium is generally indifferent, but in the complete absence of water, boiling fatty acids as well as many alcohols and phenols rapidly dissolve aluminium, regardless of its purity. Minute traces of water suffice to prevent this form of attack, the cause of which has not yet been ascertained.
Aluminium is not attacked by sulphuretted hydrogen, ammo nium sulphide or sulphur dioxide, but combines directly with fluorine, chlorine, bromine or iodine, to form the respective halides. At ordinary temperatures it is not attacked by oxides of carbon, but at high temperatures forms with them or with carbon the carbide
with nitrogen the nitride (A1N) and with sulphur the sulphide
: these are decomposed by water yielding methane
ammonia
and sulphuretted- hydrogen
respectively. Similar combinations are formed with phos phorus, arsenic, etc.
As its position in the electrochemical series indicates, aluminium decomposes many metallic salts, replacing the metal in them. Thus copper, silver, gold, etc., may be precipitated from solutions of their salts by aluminium. Finally, reactions at high tempera tures similar to those described as occurring with metallic oxides take place with sulphides, sulphates and other metallic salts.
Saltsof a great variety of acids are known, some only in aqueous solution. All are, when pure, colour less and have an astringent taste, and readily form mixed salts with the corresponding salts of other metals. In aqueous solution aluminium salts are hydrolysed (or partially resolved into hydrate and acid) to a very marked extent, the hydrate remaining colloid ally dissolved. To this property the industrial importance of many aluminium salts is due. As the hydrate is a weak base these solu tions show an acid reaction. The more important aluminium salts and their uses are : aluminium chloride
crystals, readily decomposed by water vapour, of great importance in syn thetic organic chemistry; aluminium sodium fluoride or cryolite
used as a solvent for alumina in the production of aluminium as well as in the manufacture of opaque glass and enamels; aluminium sulphate (Al2[SO4]3)—widely used in the dyeing industry for mordanting, for coagulating impurities in water and in paper making, also as an astringent in medicine; the alums or mixed sulphates of aluminium and alkali metals— used for similar purposes; aluminium silicates and mixed silicates (see CLAY and FELSPAR) ; aluminium acetate (A1[C2H302]3) existing only in solutions more strongly hydrolysed than those of any other aluminium salts used technically and for this reason of particular value to the dyer, to whom they are known as red liquor, also used as an antiseptic ; aluminium oleate, palmitate, etc., used as thickeners for lubricating oils ; ultramarine—a sul phide containing sodium and aluminium with silica occurring in nature as the much prized lapis lazuli, also manufactured on a large scale as a colouring matter, the laundry blue.
Aluminiummay be taken into the stomach without ill effect. Long series of experiments made with aluminium salts on animals and men indicate that, when ingested, aluminium does not pass through the wall of the stom ach, is never found in the urine, but is totally rejected with the faeces. Under correct experimental conditions the organs of ani mals treated are found completely free from aluminium com pounds.
Injected subcutaneously, however, aluminium salts, like those of other metals, are toxic, although poisoning develops slowly and discontinuation of the injections is followed by recovery and clearance of the system. From the earliest times aluminium salts have been used in therapy, generally as astringents, but\ also as antiseptics and as mild caustics.
