NUCLEATION, POLYMERIZATION, AND PRECIPITATION OF FeOOH 1

Summary Fresh Fe(ClO 4 ) 3 solutions, 0.01 M or 0.001 M in iron (III), were seeded with iron (III) hydroxide brown sols and the reaction processes were investigated. In two experiments, HC1O 4 was added to fresh Fe(ClO 4 ) 3 solutions to lower their degrees of super saturation with respect to iron (III) hydroxide. Depending upon the quantity of seeding solution added, it was found that samples differed greatly in appearance and in the stability during ageing. With a small amount of seeding solution added and following a relatively long induction period, the sample rapidly developed to a dense, cloudy yellow suspension with FeOOH precipitate settled shortly afterwards. With an increased amount of seeding solution added, the sample gradually became more brownish and less turbid and eventually a clear brown sol was observed. Although α‐FeOOH was the major hydrolysed species in all samples, a yellow precipitate which settled under gravity was found only in those containing a small quantity of seeding solution. The above results provide experimental evidence supporting the hypothesis (Hsu and Ragone, 1972) that the initial number of nuclei relative to the concentration of mono‐meric species is the key factor governing the appearance and the stability of an hydrolysed iron (III) solution. In another experiment, varying amounts of a seeding solution were added to a fresh, pure 0.001 M Fe(ClO 4 ) 3 solution to which no HC1O 4 was added. All samples rapidly hydrolysed to clear brown sols in less than 6 hours and no noticeable seeding effect was observed. It is suggested that a pure 0.001 M Fe(ClO 4 ) 3 solution is highly supersaturated with respect to iron (III) hydroxide and a large number of nuclei spontaneously form in situ shortly after preparation.