Crystal formation growth medium

Precipitation of ferric hydroxide gel was also observed in the preparation of spindlelike hematite (a-Fe203) particles in a dilute ferric chloride solution in the presence of phosphate (9). In this case, however, the positive role of the gel was not definite since similar uniform hematite paricles were obtained as well in homogeneous systems in the presence of the same anions (9). Also, Hamada and Matijevic (10) prepared uniform particles of pseudocubic hematite by hydrolysis of ferric chloride in aqueous solutions of alcohol (10-50%) at I00°C for several days. In this reaction, it was observed that acicular crystals of (3-FeOOH precipitated first, and then they dissolved with formation of the pseudocubic particles of hematite. The intermediate P-FeOOH appears to work as a reservoir of the solute to maintain an ideal supersaturation for the nucleation and growth of the hematite. Since the (3-FeOOH as an intermediate and the pseudocubic shape tire not peculiar to the alcohol/water medium... 

At very low concentrations, c. 10 4 to 10 3 mol dm-3, the supersaturation is sufficient for extensive nucleation to occur, but crystal growth is limited by the availability of material, with the result that a sol is formed. At moderate concentrations, c. 10 2 to 10 1 mol dm 3, the extent of nucleation is not much greater, so that more material is available for crystal growth and a coarse filterable precipitate is formed. At very high concentrations, c. 2 to 3 mol dm-3, the high viscosity of the medium slows down the rate of crystal growth sufficiently to allow time for much more extensive nucleation and the formation of very many small particles. Owing to their... 

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