Aluminium Alums

An intimate mixture ot 274 grms. of rubidium iron alum, or 260 grms. of rubidium aluminium alum with 100 grms. of calcium carbonate, and 27 grms. of ammonium chloride, is heated in a nickel crucible to a dull red heat until ammonia vapours are no longer evolved, and then the temp, is raised to redness. The product is ground with a litre of cold water for 15 minutes filtered by suction and washed with 400 c.c. of water, added in small portions at a time. The combined sulphuric acid is precipitated by the addition of barium hydroxide, and the filtered liquid boiled while a stream of carbon dioxide is passed through the soln. If the soln. loses its alkaline reaction, and yet retains some calcium, a little rubidium carbonate must be added to precipitate calcium carbonate. The soln. is then treated with hydrochloric acid and evaporated. 

Filter off the formed crystals and dry them on filter paper. Choose a well formed crystal of the alum, fasten it with the aid of a thin thread or hair to a glass rod and lower it into the mother liquor. Watch how the crystal grows several days. Examine the shape of the chromium potassium alum crystals under a microscope and compare it with that of potassium aluminium alum crystals. What will happen if crystals of potassium aluminium alum are lowered into a saturated solution of chromium potassium alum. 

Rhodium sulphate, like its analogues the sulphates of cobalt and iridium, yields stable salts with sulphates of the alkali metals known as alums. These are well-defined crystalline salts, isomorphous with the better known iron and aluminium alums. They thus form an interesting link between these metals and the central vertical column in Group VIII, of which rhodium is the middle member. These alums... 

Chromium in its Compounds is sometimes metallic and sometimes non-metallic toward the other components. Thus, in chromium trioxide, CrOg, the anhydride of the hypothetical chromic acid, H2Cr04, chromium acts as a non-metal, just like sulphur in sulphuric acid hence in chromates the chromium is acidic, or non-metallic, in its chemical relations with the other elements. In chromic compounds, however, chromium acts as a metal. Thus chromium hydroxide, Cr(OH)3, is analogous to aluminium hydroxide chrome alum is potassium chromium sulphate, and is analogous to aluminium alum. The two classes pass into each other by appropriate operations,... 

Arrhenius parameters for the dehydrations of ammonium and potassium aliuninium alums [38] were in agreement with the Polanyi-Wigner equation. Comparisons of the shapes of nuclei on different crystal surfaces indicated that reaction proceeds along (100) planes. The observed decrease of the rate in water vapour is attributed to the blocking of pores by adsorbed molecules. No intranuclear cracking was apparent and the product-reactant boimdaries became irregular because of the influence of water on reorganization of the product phase. The appearance of these nuclei contrasted markedly with those in mixed potassium chromium/aluminium alums, where there is an approximately concentric structure. 

Thermogravimetric and X-ray powder diffraction methods have revealed"" the dehydration of sodium aluminium alum to proceed by reactions (43)—(45). 

Materials Potassium aluminium alum, potassium chromium alum. 

Directions (a) Dissolve 16 grams of potassium aluminium alum in 100 c.c. of hot water, filter if necessary, and cool the solution to room temperature in running water. Shake occasionally for five minutes 8 grams of finely powdered potassium chromium alum with 100 c.c. of water at about 30°. The temperature should not be allowed to go higher than 35° in order to prevent the change of the chromium salt to the green modification. Filter the solution. In four beakers place the following (1) 40 c.c. of the solution of aluminium alum and 10 c.c. of the chromium alum, (2) 20 c.c. of the former and 10 c.c. of the latter, (3) the rest of the solution of the former and (4) the rest of the solution of the latter. 

Materials Sodium bicarbonate, potassium acid tartrate, sodium aluminium alum, primary calcium phosphate, starch, commercial baking powder. 

Fig. 81.—Composition of Solution and Solid Mixtures of Potassium Chromium and Potassium Aluminium Alums.

Fig. 82.—Vapour Pressures of Mixtures of Potassium Chromium in Potassium Aluminium Alums.

The octahedral, cubic crystals are violet to black in colour, and appear ruby-red in thin layers and J. H. Kastle found that the intensity of the colour is very much reduced at liquid air temp. F. Klocke, C. F. Rammelsberg, C. von Hauer, and J. W. Retgers showed that the crystals are isomorphous with other alums, for they show similar corrosion figures. E. Dittler obtained overgrowth with potassium aluminium sulphate. T. V. Barker found a close connection between parallel overgrowths in chrome-alum, potash-alum, and ammonia-alum and the mol. vols which are respectively 542-2, 541-6, and 552-2. C. von Hauer found that with the introduction of a crystal of iron-alum in a sat. soln. of chrome-alum nearly all the latter separates out while L. de Boisbaudran found that a sat. soln. of basic ammonium aluminium alum does not affect the octahedral faces of the... 

E. Hollmann gave y = 13-2 mm. at 17° to 19° and he also obtained values for solid soln. of the chromium and aluminium alums. H. Lescoeur and D. Mathurin found that over cone, sulphuric acid the alum loses half its water of crystallization and passes into a lilac-coloured powder which does not lose its colour at 75°. At 20°, the dissociation press., f mm. of the moist alum, KCr(S04)2 WH2O, is 15-25 mm. for m=13-05, and 15-0 for n=12-3 for w=12 l, p=9-8, and the violet powder produced when w=ll-8, has p=9-l. This value... 

The complex dehydration processes of potassium aluminium alum have been followed by t.g.a., A -ray diffraction, and i.r. methods. ... 

Nishide, T. and Tsuchiya, R. (1965) Formation of Al " -SO ion-pair in an aqueous solution of potassium aluminium alum. Bull. Chem. Soc. Jpn., 38, 1398-1400. 

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