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Aluminium sulphate forms double salts with the sulphates of sodium, potassium, rubidium, caesium, ammonium, and thallium (thallous sulphate), of the type R2ISO4.Al2(SO4)3.24H2O. These double salts are called alums, the salt Cs2SO4.Al2(SO4)3.24H2O, for example, being called caesium aluminium alum, or simply caesium alum. All these salts are isomorphous with one another, usually crystallising in regular octahedra or combinations of the octahedron and cube. Further, isomorphous series of double sulphates of the same type, R2ISO4.R2III(SO4)3.24H2O, are known, in which R may, as before, be Na, K, Rb, Cs, NH4, or Tl, but in which RIII is not Al, but Ga, In, Cr, Fe, V, Co, Ti, Mn, or Rh. All these salts crystallise in the regular system in crystals of octahedral habit, and, like the double aluminium sulphates, are also called alums. In naming one of these alums, both metals must be mentioned, the salt Rb2SO4.Co2(SO4)3.24H2O, for example, being called rubidium cobalt alum, while with an aluminium alum it is usual to mention only the univalent metal present. Finally, analogous and isomorphous double selenates are known, and called selenium alums; e.g., the salt Tl2SeO4.Cr2(SeO4)3.24H2O is called thallium chromium selenium alum; and mixed alums, in which one component salt is a sulphate and the other a selenate, have also been prepared.

The alums are described in this series of text-books under the heading of the tervalent metals present. Hence only the aluminium alums are described in this chapter.

The alums are readily prepared in the crystalline form by allowing aqueous solutions of the requisite sulphates, mixed in the ratio of their molecular weights, to evaporate slowly at the ordinary temperature. The crystals which then separate almost invariably possess an octahedral habit, but in the presence of small quantities of foreign substances, the habit may undergo considerable modification. Thus, it has long been known that below 45°, and in the presence of a trace of basic alum, potassium alum crystallises in cubes and not in octahedra. Moreover, it is an old observation that sodium alum may separate out in monoclinic crystals; but according to Surgunoff the monoclinic form (a:b:c = 2.510:1:0.908, β = 109°1') has the formula Na2SO4.Al2(SO4)3.22H2O and separates from supersaturated solutions at temperatures above 20°.

The densities of the aluminium alums at 18° to 20°, according to the careful determinations of Pettersson, are as follows: -


while the refractive indices for the D-line are given by Soret as follows: -


The solubilities of the alums are given in the accompanying table:

Temp. °CSodiumPotassiumRubidiumCaesiumAmmoniumThallium

It will be noticed that rubidium and caesium alums are only sparingly soluble in water. The alums are insoluble in alcohol. They melt in their water of crystallisation at the following temperatures (Locke): -

Temp. °C63°91°109°122°95°91°

The alums are completely dehydrated at 200°; information concerning lower hydrates is not very definite. The porous mass obtained by dehydrating potassium alum is called burnt alum, and dissolves slowly but completely in water. The ignition of pure ammonium alum leaves a residue of pure alumina, which is conveniently prepared in this manner.

Aqueous solutions of the alums have an acid reaction and in dilute solution behave as would be expected of solutions of mixed sulphates which did not interact to any appreciable extent to produce complexes.

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