Precipitation in hydrolysed iron(III) solutions

Abstract

The time for a visible precipitate to form in iron(III) nitrate, perchlorate, and chloride solutions was measured at 25°C as a function of the base (NaHCO3) to iron(III) concentration ratio (OH/Fe) for solutions containing insufficient base (OH/Fe < 2·8) to cause immediate precipitation. The effects of iron(III) concentration (0·01–0·3 M) and solvent (H2O and D2O) on precipitation times for the iron(III) nitrate system were investigated. The precipitation time decreases initially with increasing base addition until a minimum is reached. The precipitation time for nitrate and perchlorate solutions becomes nearly invariant with OH/Fe at high base additions while chloride solutions are very stable with respect to precipitation at base additions slightly greater than that required for the minimum precipitation time. For 0·125 M iron(III) nitrate solutions, the pH passes through a maximum with increasing base addition and the solution colour changes from the yellow characteristic of the low molecular weight species FeOH2+ and Fe2(OH)24+ to the deep red-brown of high molecular weight polynuclear species. The precipitates produced were examined by X-ray diffraction and electron microscopy. In the nitrate system the precipitate is α-FeOOH (goethite) and the particle size is smaller the larger the base addition. In perchlorate solutions containing low base additions and in chloride solutions, the precipitates are λ-FeOOH (lepidocrocite) and β-FeOOH, respectively. At high base additions perchlorate solutions produce α-FeOOH. An overall mechanism involving competition between the precursors of precipitate formation by crystal growth, and polycation formation, is consistent with the observations made at low base additions (OH/Fe < 1). A proposal of Fe(OH)2+ as the precursor of crystalline material and Fe2(OH)24+ as the precursor of polycations is examined. The near invariance of precipitation time with base at high base addition is attributed to coagulation of polymeric units as the dominant process of precipitate formation.