Hydrolysis of ferric salts

Waters of pH less than 6 may be expected to be corrosive, but, because any weak acids present in the solution may not be fully ionised, it does not follow that water of pH greater than 7 will not be corrosive. Mine waters are particularly corrosive to cast iron, often to such an extent as to preclude its use with them, because of their relatively high acid content, derived from the hydrolysis of ferric salts of the strong acids, mainly sulphate, and because the ferric ion can act as a powerful cathodic depolariser. 

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. 

Daichuan, D., Pinjie, H., and Shushan, D. Preparation of uniform a-FeO(OH) colloidal particles by hydrolysis of ferric salts under microwave irradiation. Materials Research Bulletin, 30(5), 537-541 (1995). 

The need for anoxic conditions is to prevent hydrolysis of ferric salts which would mantle the limestone chips reducing the ALD efficiency. Elevated dissolved oxygen levels (>2mgl ) or presence of Fe+ will cause precipitation of ferric salts, mantling limestone and reducing ALD efficiency. Removal of other metals will require an anaerobic wetland to encourage precipitation of insoluble sulphides. 

Experiment.—Dissolve 1 c.c. of nitromethane in water and test the solution with litmus paper. Then add some phenolphthalein and, drop by drop from a burette, OliV-sodium hydroxide solution. Before a permanent pink colour develops about 2 c.c. of the alkali will be added—a sign that an acid, aci-nitromethane, H2C NOOH, has been formed from the neutral nitromethane. A small sample of this solution gives with ferric chloride a blood-red colour, characteristic of aci-nitro-compounds. The salts of the oci-compound undergo extensive hydrolysis. This is shown by further addition of 0-1 N-alkali which produces a deep red colour. If 10 c.c. of alkali were added and 5 c.c. of 0-1 JV-hydrochloric acid are now run in the solution is decolorised because the liberated oci-compound restricts the hydrolysis of its salt. But the conversion of H2C N02H into H3C.N02 proceeds so rapidly that the red colour reappears in a few moments. 

An interesting lecture experiment to illustrate suppression of hydrolysis of ferric chloride under certain conditions consists in diluting a solution of the salt until it is practically colourless. Concentrated hydrochloric acid is now added, and the solution assumes a yellow colour, characteristic of the un-ionised FeCl3-molecule.2 The addition of glycerol to a solution likewise intensifies the colour, and this is attributed to diminished dissociation consequent upon the introduction of a substance possessing a lower dielectric constant. 

If, on the other hand, the free acid or base is removed in some manner, the extent of hydrolysis of the salt must increase in order to maintain the hydrolytic equilibrium. For example, if a solution of potassium cyanide is heated or if a current of air is passed through it, the hydrogen cyanide formed by hydrolysis can be volatilized as it is removed, however, more is regenerated by the continued hydrolysis of the potassium cyanide. When a solution of ferric chloride is heated, the hydrogen chloride is removed and hence the hydrolytic process continues the hydrated ferric oxide which is formed remains in colloidal solution and imparts a dark brown color to the system. 

Nanometer-sized quasicubic and spindle a-Fe203 particles were prepared by microwave heating from Fe+ salt solutions [184]. The obtained a-Fe203 particles formed by MWH have smaller size and more uniform distribution than with conventional heating. Inorganic ions such as H+, OH and NaF, were found to affect the precipitating rate of spindle a-Fe203 and accelerate the hydrolysis of ferric ions. 

We now consider Fe hydrolysis. The hexaaquaflFerric cation[Fe(H20)e] is more acid than hexaaquaferrous cation [Fe(H20)g]. The equilibrium constant of hydrolysis is approximately one order lower than that in phosphoric acid, whereas the equilibrium constant of the hydrolysis of Fe " is approximately one order higher than that in boric add. During the hydrolysis the following essentially mononuclear complexes are produced [FeOH] ", [Fe(OH)2]" , [Fe(OH)3(aq)]° and [Fe(OH)4]. By other reactions a series of polynuclear complexes is formed, for example, [Fe2(OH)2], [Fe3(OH)4] , [Fe4(OH)g] , etc. (for simplicity, the coordinated water molecules are omitted). First, colloid hydroxo complexes are formed and finally there is a precipitate of hydrated ferric oxide which is in fact a mixture of different polynuclear complexes. The distribution of polynudear complexes depends not only on pH, but also on the initial concentration of iron. In diluted solutions of ferric salts a precipitate of hydrated Fe203 is separated only at a higher pH. The equilibrium between particular polynuclear complexes is established only very slowly. 

Ferric Oxide. - Hydrolysis of Fe " salts may lead either to a-FeaOa (haematite) or FeOOH (both in the )3- and a-form), depending on the nature of precipitating agent, the pH and the type of salt. a-FeOOH (goethite) was studied in the mid-seventies to some extent but its interest as a catalyst is very limited, as it transforms into haematite at 180°C. Crystallites of goethite terminate by the (100) face, as confirmed by the adsorption of phosphate and sulphate groups. The adsorption of some test molecules has been studied. 

F. W. Grover obtained a little by the dialysis of the commercial ferric chlorides. W. Biltz obtained no coDoid by the hydrolysis of soln. of chromic nitrate, owing to the small hydrolysis of the salt as observed by H. W. Woudstra. S. Takegami obtained the colloid at the cathode during the electrolytic reduction of chromic acid and B. Kandelaky, by the hydrolysis of chromic ethylate. 

Bottero, J.Y, Tchoubar, D., Amaud, M. and Quienne, P. (1991). Partial hydrolysis of ferric nitrate salt. Structural investigation by dynamic light scattering and small angle X-ray scattering. Langmuir, 7,1365-1369. 

Iron oxide yellows can also be produced by the direct hydrolysis of various ferric solutions with alkahes such as NaOH, Ca(OH)2, and NH. To make this process economical, ferric solutions are prepared by the oxidation of ferrous salts, eg, ferrous chloride and sulfate, that are available as waste from metallurgical operations. The produced precipitate is washed, separated by sedimentation, and dried at about 120°C. Pigments prepared by this method have lower coverage, and because of their high surface area have a high oil absorption. 

The nature of the products is strongly dependent on the anion of the ferric salt used in the forced hydrolysis processes. For this reason, this review first considers results obtained with ferric chloride, followed by the studies involving ferric sulfate solutions. 

Commercial zinc sulphate invariably contains a small amount of iron as an impurity. Since FeS04-7H20 crystallizes isomor-phously with ZnS04-7H20 a preparation of the latter cannot be freed of the former by recrystallization. By addition of chlorine, or its equivalent, to the solution of zinc sulphate, the iron is oxidized to ferric salt the ferric salt hydrolyzes somewhat, and, if the acid produced by the hydrolysis is neutralized as fast as formed, the hydrolysis proceeds to completion and all the iron is precipitated as Fe(OH)3. In this case, the reagent used to bring about the exact neutrality of the solution is a suspension of basic zinc carbonate. (Compare the similar procedure for removing traces of iron in the preparation of strontium chloride, Preparation 21.)... 

The ions of ferric carbonate are brought together in the solution, but this salt evidently cannot exist. Its hydrolysis products, ferric hydroxide and carbonic acid (C02), are obtained. The behavior of the red precipitate when treated with HC1 shows that it is ferric hydroxide and not ferric carbonate. 

---