Abstract: Fe(ClO4)3 solutions of different concentration and acidity were studied to search for the factors governing their appearance and stability during aging and to elucidate the mechanisms for the formation of amorphous and crystalline iron(III) hydroxides from slow hydrolysis. It was found that dilute solutions (0·001M or lower) rapidly hydrolyzed to clear sols after a brief induction period. With an increase in iron(III) concentration, one could notice gradual increases in the induction period, turbidity and particle size. Thus, solutions 0·006M to 0·01M in iron(III) developed into dense, cloudy, yellow suspensions impervious to light. The polymeric iron(III) species in the aged 0·02M solution were primarily large particles that settled under gravity, resulting in a suspension of low turbidity. It was also observed that in dilute solutions, the initial polymerization product was amorphous iron(III) hydroxide which yielded X-ray diffraction peaks for FeOOH only after prolonged aging or not at all. In 0·01 and 0·02M solutions, however, crystalline FeOOH was the major product from the very beginning of polymerization. The results suggest that the initial degree of supersaturation with respect to amorphous iron(III) hydroxide is the key factor governing particle size distribution, which in turn governs the appearance, stability and crystallinity of the hydrolyzed product during aging.
The addition of HClO4 to dilute Fe(ClO4)3 solutions decreased, whereas the addition of NaOH to concentrated solutions increased, the degree of supersaturation and, thus, induced changes in induction period, appearance, stability and crystallinity of the hydrolyzed product accordingly.