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Aluminium hydroxides

Two hydrated oxides of aluminium are found in nature in the crystalline state, namely, diaspore, Al2O3.H2O, which occurs in orthorhombic crystals (holohedral; a:b:c = 0.9372:1:1.6038) of density 3.30-3.45, and hydrargillite (or gibbsite), Al2O3.3H2O, which occurs in fibrous, monoclinic crystals (holohedral; a: b: c = 0.7089:1:1.9184, β = 85°29') of density 2.42. The most important naturally occurring hydrated oxide of aluminium, however, is bauxite, a white, yellowish, red, or brown clay-like, amorphous material originally found at Les Beaux near Aries in the south of France. Bauxite varies widely in composition, and consists of amorphous, colloidal, hydrated alumina (with perhaps a little diaspore and hydrargillite) associated with varying amounts of ferric hydroxide, clay, quartz, sand, etc. It is therefore better regarded as a rock than as a mineral. Formerly, bauxite was regarded as a mineral of the formula Al2O3.2H2O, but most bauxites more nearly approach the ratio Al2O3: H2O than Al2O3:2H2O. Bauxite is a very valuable source of aluminium; it occurs mainly in the department Yar (France), in County Antrim (Ireland), and in the states of Alabama, Georgia, and Arkansas (America).

When excess of ammonium hydroxide is added to an aqueous solution of an aluminium salt, a precipitate is obtained, white, opaque, and amorphous at 100°, transparent and gelatinous at ordinary temperatures. The precipitate has a pronounced tendency to pass into colloidal solution when washed with water. Air-dried in hot weather, its composition corresponds to the formula Al2O3.3H2O or Al(OH)3; dried at 100°, or at the ordinary temperature over concentrated sulphuric acid, the composition is that of a dihydrate, Al2O3.2H2O. The amorphous trihydrate is also obtained by heating an alkali aluminate with ammonium chloride, or by boiling basic aluminium carbonate with water; when an alkali aluminate solution is boiled, the trihydrate slowly separates in a crystalline form. A mono-hydrate, Al2O3.H2O, is said to be obtained by heating amorphous alumina with water in a closed tube to 250°.

The amorphous mono- and di-hydrates are very hygroscopic substances, absorbing water with the formation of the trihydrate or normal aluminium hydroxide. At a red heat all the hydrates are converted into alumina.

A colloidal solution of aluminium hydroxide was obtained by Crum from aluminium acetate solution. This was heated to obtain a precipitate of basic acetate, and the precipitate dissolved in 200 times its weight of boiling water. The solution was then maintained at 100° for some days, when complete hydrolysis occurred. The liquid was diluted and heated to 100° until all the acetic acid had been volatilised, fresh water being added from time to time. A colourless, tasteless, neutral solution of aluminium hydroxide was thus obtained, readily coagulated by salts and a number of acids, and the gel thus obtained dissolving only in concentrated acids. The solution did not act as a mordant, and when evaporated at 100°, left a residue difficultly soluble in acids. Graham obtained aluminium hydroxide in colloidal solution by dialysing a solution of aluminium chloride saturated with aluminium hydroxide. The colloidal solution so obtained acted as a mordant, and was readily coagulated by acids, bases, and salts to a gel soluble in dilute acids. Colloidal aluminium hydroxide exhibits anodic cataphoresis; it is seen to be a suspension when examined in the ultra-microscope.

In the absence of salts, freshly precipitated aluminium hydroxide is perceptibly soluble in ammonia, and much more so in methylamine and other organic bases. It is also readily soluble in acids and alkali hydroxides. When kept under water for several months, it becomes difficultly soluble in acids and alkalies, concentrated sulphuric acid excepted. The naturally occurring hydroxides are not readily attacked by acids.

Precipitated aluminium hydroxide assumes a bright red colour, not destroyed by dilute acetic acid, when boiled with water containing a drop or two of 1 per cent, alizarin solution. This test readily distinguishes it from gelatinous hydrated silica. Aluminium hydroxide also forms soluble complex substances with many organic hydroxy-compounds. Further, it enters into combination with many organic colouring matters, producing coloured, insoluble lakes. Upon this property depends the use of aluminium salts as mordants in dyeing.

The solubility of aluminium hydroxide in acids is due to the fact that it acts as a weak base and reacts with acids to produce aluminium salts. In the same way, the solubility of aluminium hydroxide in alkali hydroxides is attributed to the feeble acidic character of the hydroxide. It is, in fact, an amphoteric hydroxide. The minute amount of aluminium hydroxide present in aqueous solution in equilibrium with the solid phase must be supposed to dissociate in two ways: -

Al••• + 3OH' ⇔ Al(OH)3H + H2AlO3' (or AlO2' and H2O).

The hydroxide is weaker as an acid than as a base, and the affinity constant for the acid dissociation represented above has been estimated to be approximately 1×10-10, i.e. the acid is of the same order of strength as boric acid and its alkali salts must be perceptibly hydrolysed in aqueous solution.

hydrogen ion concentration
Change of hydrogen ion concentration during precipitation of aluminium hydroxide from aluminium chloride and solution of the precipitate in sodium hydroxide.
From solutions of aluminium hydroxide in alkali hydroxides a number of solid substances, some of them crystalline, have been isolated, which must be regarded as salts of aluminium hydroxide (aluminic acid). These salts are called aluminates. The nature of the solutions obtained by dissolving aluminium hydroxide in alkali hydroxides has been the subject of much discussion. The freezing-point of a dilute solution of sodium hydroxide is not changed by dissolving aluminium hydroxide in it. This would be expected if each OH' ion used up leads to the production of one aluminate anion, i.e. the result points very clearly to the presence of a meta-aluminate, NaAlO2, in solution. Determinations of the ratio Al2O3: Na2O (or K2O) in solutions saturated with aluminium hydroxide are not conclusive, since the above ratio is much greater when aluminium is dissolved in alkali than when aluminium hydroxide is dissolved. Moreover, unless the atomic ratio Na (or K) to Al exceeds the value two, the solutions are unstable. The equivalent conductivity gradually rises, and aluminium hydroxide is slowly precipitated. The change in conductivity is in harmony with the view that the salt of a monobasic acid is undergoing hydrolysis. Moreover, no change in conductivity is observed without the simultaneous deposition of aluminium hydroxide, so that the latter, when produced by hydrolysis, does not first pass into colloidal solution. These results negative the suggestion that while some of the aluminium hydroxide dissolves in alkali hydroxides with the formation of alkali aluminates, most of the hydroxide merely passes into solution as a colloidal hydrosol, which, on standing, slowly reverts either to a crystalloidal form or to a colloidal hydrogel, and so precipitates. A study of the variation of the hydrogen ion concentration in an aluminium chloride solution, as sodium or potassium hydroxide is gradually added to the liquid, has been made by Hildebrand and by Blum. The general nature of their results may be seen by reference to fig.; the points A, B, C are points of inflexion on the curve, and correspond respectively to the commencement of precipitation of the hydroxide, the completion of the precipitation, and the completion of the solution of the precipitate in sodium hydroxide. It is found that be is one-third of ab; i.e. the results point clearly to the existence of aluminates NaAlO2 and KAlO2 in aqueous solution: -

AlCl3 + 3NaOH = Al(OH)3 + 3NaCl
Al(OH)3 + NaOH = NaAlO2 + 2H2O

and are opposed to the view that colloidal solution occurs to any appreciable extent. Moreover, observations with the ultra-microscope fail to indicate that the solutions are suspension-colloids. The microscopic evidence, however, is not very conclusive.

The following hydrated aluminates have been obtained in the solid state, several of them in the crystalline form, by the interaction of aluminium with concentrated alkali hydroxides: -

K(AlO2).1.5H2O; Ca2Al2O5.7H2O; Ca3(AlO3)2.6H2O; Na(AlO2).2H2O; Ba2Al2O5.5H2O; Sr3(AlO3)2.6H2O; LiH(AlO2)2.5H2O; Tl4Al2O5.7H2O; Ba3(AlO3)2.7H2O; Ba(AlO2)2.5H2O; Sr(AlO2)2.4H2O?
A number of anhydrous, crystalline alluminates occur as minerals; e.g: -

Magnesia spinel or spinel ruby - Mg(AlO2)2
Magnesia iron spinel or pleonaste - (Mg,Fe)(AlO2)2
Iron spinel or hercynite - Fe(AlO2)2
Zinc spinel or gahnite – Zn(AlO2)2

Equilibrium lime - alumina
Equilibrium diagram for the system lime - alumina.
These four minerals crystallise in regular octahedra, and are isomorphous with magnelite, Fe(FeO2)2, and chromite, Fe(CrO2)2. The mineral chrysoberyl, or beryllium aluminate, Be(AlO2)2, is not isomorphous with the preceding, but crystallises in the orthorhombic system (a:b:c = 0.470:1:0.580) and is isomorphous with olivine, Mg2SiO4. Magnesia spinel is often found in beautiful red crystals, which are used as gem-stones; such crystals contain a little chromic oxide. Beautiful yellowish-green crystals of chrysoberyl, found in Ceylon, are also used as gem-stones. The aluminates of barium, calcium, beryllium, magnesium, zinc, manganese, and cobalt were prepared in the crystalline form by Ebelmen by intensely heating alumina and the requisite oxide with boron sesqui-oxide until the latter substance, which initially acts as a solvent, had been largely volatilised. The blue mass obtained in qualitative analysis when testing on charcoal for aluminium contains cobalt aluminate.

A thermal study of the system lime - alumina has shown that four anhydrous calcium aluminates can be obtained, of the formulae 3CaO.Al2O3, CaO.Al2O3, 5CaO.3Al2O3, and 3CaO.5Al2O3. The second and third melt at 1587° and 1387° respectively; the others have no melting-point. The third and fourth compounds are dimorphous. The nature of the equilibrium diagram is indicated in fig. Only one magnesium aluminate has been obtained, namely, spinel, MgO.Al2O3.

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