Isomorphous alum

Attempts at identifying the influence of structme on stability have generally been inconclusive. For example, some alkali metal permanganates with comparable stmctures show similarities of decomposition behaviour [29], while, in contrast, the decompositions of several cobalt(lll) ammine azides show little evidence of structural influences [76], Significant differences in behaviour were found for the various crystal forms of the LiK tartrate hydrates [87] and, also, for the dehydrations of the isomorphic alums [20,43], However, some reactants, for example those prepared by the dehydration of hydrated metal carboxylates [5], may be amorphous to X-rays, thus preventing recognition of any control of stability by crystal structure. 

Source of Nuclei in Contact Nucleation as Revealed by Crystallization of Isomorphous Alums... 

Potash alum, KA1(S04)2.12H20, and chrome alum KCr(S04)2.12H20 crystallize from their respective aqueous solutions as regular octahedra in almost identical forms and are known to be isomorphous. Alum crystals are also easily formed by... 

Iron(III) sulfate readily forms alums with many isomorphous compounds by mixing equimolar amounts of both the salts in aqueous solutions followed by crystaUization ... 

The normal sulphates show a marked tendency to the formation of double salts,8 the best known case being that of the alums, which are isomorphous compounds of the general formula M2(S04)3.X2S04. 24H20, where M and X represent a tervalent and univalent metal, respectively in aqueous solution these double salts are almost entirely resolved into the ions of their constituent salts,9 recombination taking place as the solution crystallises. Double salts are also formed by the crystallisation of fused mixtures of anhydrous sulphates, the freezing-... 

Certain halogen salts appear to be isomorphous with potassium sulphate and able to form alums with aluminium sulphate thus,... 

It is evident that telluric acid is notably different from what might be expected by analogy with sulphuric and selenic acids. One is hardly surprised, therefore, that isomorphism is exceedingly rare between the tellurates and the sulphates or selenates.3 The tellurates do not form alums, they do not as a rule form mixed crystals with the sulphates or selenates, the only fairly satisfactory case of mixed-crystal formation being with rubidium hydrogen sulphate and rubidium hydrogen tellurate. 

KA](S04)2- 12H20, are isomorphous with the chrome alums, KCrCSO j F O, and mixed crystals of any composition between the two extremes may be prepared by... 

The oxyacid salts of iron(III) are more numerous than those of iron(II). Among the former, the sulfates arc of interest because of the readiness with which iron(III) sulfate replaces aluminum sulfate in the alums, which are hydrated double sulfates formed by certain trivalent and alkali metal (and other monovalent) sulfates. Iron(lII) sulfate, Fe2(SOa)s, is isomorphous with aluminum sulfate, AbfSOa).), because the radius of the Fe3+ ion is so dose to that of the Al3+ ion (0.57 A). For that reason, the isomorphous relationship extends to other salts, i.e the fluorides and some of the nitrates. 

The cobalt alums are isomorphous with those of iron and aluminium, and therefore, by the application of Mitseherlich s Law, they must be assumed to contain two atoms of cobalt, their generic formula being ... 

Cobaltic sulphate, like the sulphates of rhodium and iridium, unites with the sulphates of the alkali metals to yield a series of well-defined, crystalline salts known as alums. These are isomorphous with those of iron, manganese, chromium, and aluminium, and form an interesting link between these metals and the central vertical column in Group VIII of the Periodic Table, of which column cobalt is the first member. 

Rhodium, like cobalt, iron, and aluminium, yields a well-defined series of alums. Application of Mitscherlich s Law of Isomorphism indicates that these have the general formula ... 

In crystalline salts, the Tl+ ion is usually 6- or 8-coordinate. The yellow hydroxide is thermally unstable, giving the black oxide TI2O at about 100°C. The latter and the hydroxide are readily soluble in water to give strongly basic solutions that absorb carbon dioxide from the air TlOH is a weaker base than KOH, however. Many Tl+ salts have solubilities somewhat lower than those of the corresponding alkali salts, but otherwise are similar to and quite often isomorphous with them. Examples of such salts are the cyanide, nitrate, carbonate, sulfate, phosphate, perchlorate, and alum. 

Ferric sulphate yields, with the sulphates of the alkali metals, a series of well-defined crystalline salts which are isomorphous with similar series yielded by aluminium sulphate. By the application of Mitscherlich s Law, therefore, analogous formulae are to be anticipated, so that the general formula for these iron alums is... 

The iron alums are isomorphous, not merely with one another, but with the wide series of salts known as alums of generic formula —... 

When different compounds yield almost identical crystals, the forms are referred to as isomorphs. The word almost is indicated here, as isomorphs are not exactly the same although the arrangement of atoms/ions in the lattices are identical, one or more of the atoms in the lattice have been replaced with another component. For example, alums of the general formula (M)2(S04) (M)2 (804)3 24H2O may crystallize as isomorphs where one of the monovalent or trivalent metals is substituted with another. 

Isomorphous substances often crystallize together from a mixed solution to form solid solutions, single crystals containing both substances. In a solid solution the different ions (such as Mn++ and Ca+ +) are arranged at random in the positions occupied by one kind of ion alone in a pure substance. For example, solid solu tions of chrome alum, KCr(S04)2 12HoO, and ordinary alum, KA1(S04) 12H20,... 

The chromous salts, derived from the oxide CrO, arc analogous to the salts of divalent vanadium, manganese, and iron. This is seen in the isomorphism of the sulphates of the type R" SOj-THgO. The stability of such salts increases in the order of the atomic number of the metal. The chief basic oxide of chromium is the sesquioxidc CraO, which is closely allied to ferric oxide, and, like the latter, resembles aluminium oxide. The hydroxide, Cr(OH)3, with bases yields chromites analogous to, but less stable than, the aluminates. Chromic sulphate enters into the formation of alums. The chromic salts are very stable, but in the trivaJent condition the metal shows a marked tendency to form complex ions, both anions and cations thus it resembles iron in producing complex cyanides, whilst it also yields compounds similar to the cobaltamines. 

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