Trivalent metals, hydrous oxides

Hydrolysis and adsorption. Some years ago, a theory was advanced that hydrolyzed metal species, rather than free metal ions, are adsorbed to hydrous oxides. Spectroscopic information is in accord with the reaction of (free) metal ions with the surface. There is, however, the possibility, especially with trivalent ions and within given pH ranges, that surface hydroxo species can be formed. 

Regardless of the method of treatment, a coagulated colloid is always contaminated to some degree, even after extensive washing. The error introduced into the analysis from this source can be as low as 1 to 2 ppt, as in the coprecipitation of silver nitrate on silver chloride. In contrast, coprecipitation of heavy-metal hydroxides on the hydrous oxides of trivalent iron or aluminum can result in errors as large as several percent, which is generally intolerable. 

Aluminum is a hard trivalent ion, and is usually found as an aquo- or hydroxy-complex or hydrous oxide solid (Martell and Motekaitis 1989). Aluminum can act as a complexing agent of other metals, or as a metal center to other inorganic and organic ligands (Buffle 1990). As the hexa-coordinated hydrolyzed cation, aluminum is octahedrally coordinated by six waters (Fig. 6), and as such acts as a multiprotic acid (Stumm and Morgan 1981). 

Metal hydroxides in general are anion-selective in acid solution and turn to be cation-selective beyond a certain pH, called the point of the iso-selectivity, pHpjS it is pHpjS = 10.3 for ferric oxide and pHpis = 5.8 for ferric-ferrous oxide [72]. Adsorption of multivalent ions may also control the ion selectivity of hydrous metal oxides because of its effect on the fixed charge in the oxides. For instance, hydrous ferric oxide, which is anion-selective in neutral sodium chloride solution, turns to be cation-selective by the adsorption of such ions as divalent sulfate ions, divalent molybdate ions, and trivalent phosphate ions [70,73]. It is worth emphasizing that such an ion-selectivity change due to the adsorption of multivalent ions frequently plays a decisive role in the corrosion of metals. 

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