Ferric alums hydrolysis

The hydrolysis of ferric salts is so common that the color of ferric ion, Fe(H20)g+ + , is usually masked by that of the hydroxide complexes. Ferric ion is nearly colorless it eems to have a very pale violet color, seen in crystals of ferric alum, KFe(SO )o I2H.2O, and ferric nitrate, Fe(N03)3 9H20, and in ferric solutions strongly acidified with nitric or perchloric acid. Solutions of ferric salts ordinarily have the characteristic yellow to brown color of the hydroxide coniplexe Fe(H20)50H + + and Fe(H20)4(OH)o+, or even the red-brown color of colloidal particles of hydrated ferric hydroxide. 

Water treatment by either direct or contact filtration has become common practice for raw water with low turbidity [<3NTU] and low colour. Simple metal salts such as alum or ferric chloride are added to plant inlet water. Hydrolysis takes place with the formation of hydroxylated species, which adsorb, reducing or neutralizing the charge on the colloidal particles in the raw water, promoting their collision and the formation of floes that settle or can be filtered out. 

The sesquioxide, Cr Oa, containing trivalent chromium, is an amphoteric oxide. It yields chromic salts, such as chromic chloride, CrCla, and sulphate, Cr2(S04)a, which are very stable and show great similarity to the ferric salts and to salts of aluminium as, for example, in the formation of alums. Since, however, chromic oxide functions as a weaker base than chromous oxide, the latter having a lower oxygen content, the chromic salts are more liable to hydrolysis than the chromous salts. This is well marked in the case of the chlorides. Again, in spite of the stability of chromic salts, only a slight tendency to form simple Cr " ions is exhibited, whilst complex ions are formed much more readily, not only complex anions, as in the case of iron and aluminium, but also complex cations, as in the extensive chromammine series. In this respect chromium resembles cobalt and platinum. 

Destabilisation may be achieved by the enmeshment of the colloid in a precipitate. In this process a metal salt such as aluminium sulphate (alum) or ferric chloride, is added to the water forming positively charged species in the typical pH range of 6 - 7 for clarification. The hydrolysis reaction produces an insoluble gelatinous hydroxide according to the following equations ... 

The removal of negatively charged particles is commonly accomplished via coagulation with ferric or aluminum salts (Stumm and O Melia, 1968). Common ferric salts include ferric sulfate and ferric chloride, both of which hydrolyze in water to form the aqueous species Fe, Fe(OH) ", Fe(OH)2, Fe(OH)3, and Fe(OH)4, as well as the solid precipitant Fe(OH)3(s). Common aluminum salts include alum, PACl, and AICI3, which hydrolyze to Al +, Al(OH) +, Al(OH)2+, Al(OH)3°, and Al(OH)4, as well as the solid precipitant Al(OH)3(s). In addition to the common hydrolysis species, aluminum is prone to the formation of polymeric aluminum species. Alum is the aluminum sulfate (Al2(S04)3 18H20) and contains 0.081 (g Al/g alum). 

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