Potassium alum, dehydration

Potassium Aluminum Sulfate. Potassium aluminum sulfate [7784-24-9]. KAl(SO 12H20, is a white, astringent crystal known as potassium alum, ordinary alum, or potash alum. Its formula weight is 474.39 mp 92.5 °C sp gr 1.75 and solubiUty 11.4 g per 100 mL H2O at 20°C (8). It is soluble in dilute acid and insoluble in alcohol. It dehydrates at about 200 °C to porous desiccated potassium alum [10043-67-1], KAl(SO dried or burnt alum, which has a formula weight of 258.20. 

KA13(S04)2(0H)6. Based on an invention made by G.S. Tilley in 1924. The ore is first dehydrated at up to 600°C. It is then leached with a solution of sulfuric acid and potassium sulfate. After clarification of the leachate, potassium alum is crystallized out. Hydrothermal treatment of potassium alum precipitates a basic potassium alum, K2S04-3A1203 4S03-9H20, simultaneously regenerating potassium sulfate and sulfuric acid. Calcination of this potassium alum yields a mixture of alumina and potassium sulfate, which is leached out. Piloted by Kalunite in Salt Lake City, UT, in 1943 but later abandoned. See also Alumet. 

Acock et al. found that the loss of water from chrome alum when exposed to hard vacuum apparently occurs in two stages the first with a loss of 12 water molecules and the second with a loss of 4. Whether a phase change occurs in each stage is not known it is possible that the first stage is an equilibration of vacancies. The dehydration of ammonium and potassium alums may be a little more straightforward the loss of water from these salts increased from 19.8 to 20.6 of their original 24 water molecules as the temperatures was increased from 20 to 40°C. 

Gamer and Jennings [431] studied nucleation during the dehydration of potassium and ammonium chromium alums. Detailed kinetic measurements were made for the relatively enhanced rate of nucleation which followed admission of water vapour to the solid after a period of vacuum nucleation. This catalytic effect of water vapour is ascribed to its participation in the reorganization of the lattice which had collapsed during previous treatment in vacuum. 

Reference has already been made to the dehydration of alums (Sect. 1.2 and Table 10), decomposition of ammonium metal phosphates (Sect. 4.1.5) and the use of KMn04—KCIO4 solid solutions in mechanistic studies of the decomposition of potassium permanganate (Sect. 3.6). 

Hexaethylenediamino-hexol-tetrachromic Sulphate, [Cr4(OH)6en6](SO4)3.10H2O, is prepared from partially dehydrated chromic alum. The bluish-grey powder obtained is heated with ethylene-diamine monohydrate on a water-bath till a red-coloured mass is produced, which consists of a mixture of potassium sulphate, luteo-chromic sulphate, and the sulphate of the hexol compound, and from the mixture the latter compound is obtained by dissolving out the more soluble salts with water. The crude, difficultly soluble sulphate is purified by dissolving in dilute sulphuric acid and reprecipitating with excess of ammonia. It crystallises in small needles which are almost insoluble in water and soluble in dilute acids. 

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. 

A. Recoura obtained potassium chromitetrasulphate, K2[Cr2(S04)4], by evaporating on the water-bath a mol of green chromic sulphate with a mol of potassium sulphate and the tetrahydrate, K2[Cr2(S04)4].4H20, by dehydrating chrome-alum slowly at 110°. The former compound is dark green, and it is soluble in water. 

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

Fig. 12. Rate of growth of dehydration patches on copper sulphate, ammonium, potassium, and chrome alums as a func< tion of water vapor pressure.

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