Chrome alum, crystallization

It is possible to model such crystals by using spheres to represent the particles (Figure 1.3a). If the spheres are built up in a regular way then the shape compares very closely with that of a part of a chrome alum crystal (Figure 1.3b). 

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. 

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... 

Synonyms Chrome alum, Chromium and potassium sulfate, Potassium chromium sulfate. Appearance Dark violet-red crystals (ruby red by transmitted light) or light violet powder. Uses Preparation of chrome alum fixing bath as hardening agent for gelatin. 

SYNS CHROME ALUM CHROME POTASH ALUM CHROMIC POTASSIUM SULFATE CHROMIC POTASSIUM SULPHATE CHROMIUM POTASSIUM SULFATE (1 1 2) CHROMIUM POTASSIUM SULPHATE CRYSTAL CHROME ALUM POTASSIUM CHROMIC SULPHATE POTASSIUM CHROMIUM ALUM POTASSIUM DISULPHATOCHROMATE(III) SULFURIC ACID, CHROMIUM (3+) POTASSIUM SALT (2 1 1)... 

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 aqueous solution (blue, with a reddish tinge, but becoming green at 50° to 70° C.) deposits violet octahedra of the several forms in which chrome alum crystallises, including the rhombic dodecahedron, cube, and trapezohedron, the octahedron is the most stable. The crystals have a density of about 1-84, a specific heat of 0-324, and a coefficient of expansion of 0-0000246. The transformation of the violet to the green variety takes place at 78° C., - the substance melting at 89° C.ii to a green mass. Other physical constants have been determined, and the absorption spectra of its aqueous solution, studied. ... 

Acid Clirmnic Orthophosphate, CrH8(P04)2.8H20, has been described. It forms asymmetric crystals of the colour of the chrome alums stable in air. Yauquelin obtained an emerald green uncrystallisable solution by acting upon the hydrated sesquioxide of chromium with aqueous phosphoric acid. 

FIGURE 8.24. A view of (a) potash alum (transparent) growing on a single crystal of isomorphous chrome alum (purple interior). Diffraction photographs of (b) potash alum and (c) chrome alum. The radial streaks are from white radiation and can be ignored. Note some differences in intensity. The arrows indicate two differences that are visible to the naked eye. (Photographs courtesy Maria Flocco and Henry Katz.)... 

A curious analogy to the boiling to string proof of third molasses is found in the manufacture of chrome alum. It is well known that if molasses be boiled to a high temperature and allowed to cool taffy results, a vitreous noncrystalline mass. In like manner if in concentrating a solution of chrome alum, the evaporation passes a certain point, the mass on cooling assumes a glue-like consistency and refuses to crystallize. In such a case it is necessary to redissolve and reconcentrate carefully. 

Ill) The formation of alum crystals such as chrome alum is carried out as described in (1) above. 40 g of chrome alum or 30 g of KA1(S04)2 12 H2O suffice when 100 mL of water are used. The octahedral crystals (see figure) are formed within several hours. If the isomorphous iron (III) alum is required, 20 g of FeS04 7 H2O are stirred with 30 mL of half-concentrated nitric acid the solution is kept at boiling point for 15 minutes and a hot solution of 30 g (NH4)2S04 in 30 mL of water is added. The resulting solution is stirred for a... 

Dodecahydrate, chrome alum ammonium. Small dark-violet Or violet-blue, octahedral, cubic crystals ruby-red by transmitted light, mp 94 loses 9H20 on melting and the remaining H.O by 300". tl25 1.72. Readily sol in water slightly sol in alcohol. Aq soln is violet when cold, green when hot. 

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