Ammonium chromium alum

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. 

Ammonium Chromium Alum, (NH4)2S04.Cr2(S04)3.24H20, crystallises from a suitable mixture of solutions of its component salts in efflorescent octahedra of density 1-73, melting at 100° C. with loss of ISHjO. 

Ammonium chromium alum is soluble in water to the extent, at 25° C., of 21-21 grams of the hydrated salt in 100 c.c. of water, yielding a bluish- iolet solution which becomes green at 70° to 80° C. A study of the equilibria existing in such a solution makes it appear that the change from the violet to the green modification d.oes not take place at all at 0° C., but that at 40° C. the solution contains about 40 per cent, of the green alum. ... 

Finally, chromium metal can be obtained by electrowinning from chromium-rich liquors by two electrochemical processes chromium-alum electrolysis and chromic acid electrolysis. In chromium-alum electrolysis, the chromium-rich liquor is obtained by leaching high-carbon ferrochrome with recycled spent catholyte containing chromium alum [(NHJCr(S0J.12H30] and makeup sulfuric acid. It contains ammonium chromium alum. In chromic acid electrolysis, chromium trioxide is dissolved in deionized water acidified with sulfuric acid. The performance of each electrowinning process is presented in Table 4.85. 

All compounds of chromium are colored the most important are the chromates of sodium and potassium and the dichromates and the potassium and ammonium chrome alums. The dichromates are used as oxidizing agents in quantitative analysis, also in tanning leather. 

The mother Hquor from the cmde ferrous sulfate crystallisation contains neady all the chromium. It is clarified and aged with agitation at 30°C for a considerable period to reverse the reactions of the conditioning step. Hydrolysis reactions are being reversed therefore, the pH increases. Also, sulfate ions are released from complexes and the chromium is converted largely to the hexaaquo ion. Ammonium chrome alum then precipitates as a fine crystal slurry. It is filtered and washed and the filtrate sent to the leach circuit the chrome alum is dissolved in hot water, and the solution is used as cell feed. 

Biiltemann 2 observed that vanadium ammonium alum separates out in blue crystals from a solution containing sulphuric acid, but from solutions containing a weak acid, or from neutral solutions, red crystals are obtained. (The chromium alums can also be prepared in differently coloured modifications.) The analytical data, melting-point, electrical conductivity, rate of efflorescence, and general behaviour of both kinds of crystals are identical, so that it is difficult to ascribe different constitutions to them. Meyer and Markowitz3 have shown that both forms separate out when the molecular proportion of sulphuric acid in the solution is less than that theoretically required, and attribute the red colour to the presence of traces of vanadous oxide, V203, or its hydroxide, V(OH)3. Vanadium rubidium and vanadium ciesium alums behave in the same way. A vanadium guanidine alum has also been prepared.4... 

The Cr1 ion readily forms complexes it exists in aqueous solution as CrfH 0)j. and forms other complexes wilh anions, such asCr(H 0).CI , Crother properties due to variation in the bonding. Compounds or the trichloride have been reported with the following arrangements CrtHjOlsClj. [CrfHiOlsCljCIi- HiO. and [Cr(HiO)4Cf CU2 H 0 Trivalenl chromium also forms double salts, notably the chromium alums, hydrated double salts of Cr(III) sulfate and the alkali metal (or thallium or ammonium) sulfates. 

Chromium Ammonium Alum of Chromium Ammonium Sulfate. See Ammonium-Chrome Alum in Vol 1, pA156-L... 

T. Klobb obtained mixed crystals of the anhydrous sodium and ammonium chromium sulphates, (NH4,Na)S04.Cr2(S04)3, by melting together chromic sulphate or chrome-alum with ammonium and sodium sulphates. The hexagonal crystals are isomorphous with the anhydrous alums. A. N. Bach found the transition temp, to be between 60° and 70°. 

Ohio, high carbon ferrochromium is leached with a hot solution of reduced anolyte plus chrome alum mother Hquor and makeup sulfuric acid. The slurry is then cooled to 80°C by the addition of cold mother Hquor from the ferrous ammonium sulfate circuit, and the undissolved soHds, mostiy siHca, are separated by filtration. The chromium in the filtrate is then converted to the nonalum form by conditioning treatment for several hours at elevated temperature. 

Although the latter had previously overlooked the new earth because of its similarity to alumina, he found in 1798 that the hydroxide that precipitates when caustic potash is added to an acid solution of the beryl does not dissolve in an excess of the alkali. It also differs from alumina in other respects, for it forms no alum, it dissolves in ammonium carbonate, and its salts have a sweet taste. Vauquelin s paper read before the French Academy on le 26 pluviose an VI of the Revolutionary Calendar, or the fifteenth of February, 1798 (6, 23), proved that, except for a little chromium in the emerald, the two gems have the same composition and that they contain a new earth, a sample of which he presented to the Academy. At the suggestion of the editors of the Annales de Chimie et de Physique, he called the new earth glucina, meaning sweet. The specimen of beryl that Vauquelin analyzed was presented to him by... 

Alum, KAl(S0i)i-l2H20.—Ammonium, rubidium, cesium, univalent thallium, and in some cases sodium may replace potassium, while the aluminum may be replaced by trivalent iron, chromium, indium, gallium, titanium, vanadium but not by the rare-earth metals. 

Alum Ammonium alums are those alums, such as aluminum ammonium sulfate A12(NH4)2 (804)4 24H20, ferric ammonium sulfate Fe2(NH4)2< S04)4 - 24H2O, chromium ammonium sulfate Cr2(NH4)2 (S04)4 24H2O where ammonium sulfate is crystallized with the heavier metal sulfate. 

Add a little, and then an excess, of ammonium sulphide to a solution of chrome alum. Compare the result with that in ( ). Docs chromium form a sulphide Do aluminium and zinc ... 

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