HETEROGENEITY IN SILICON-IRON MIXED HYDROXIDES

Several mixed hydroxides obtained by co-precipitation of FeCl₂ or FeCl₂ with Na₂SiO₂ followed by ageing and purification were studied by X-ray diffraction, infra-red spectroscopy, and surface-area measurements. The compounds with molecular ratios SiO₂/Fe₂O₃ ranging from 0·1 to 1·17 are essentially amorphous. Silica, which is not to be considered as an integral part of the gel, is chemi-sorbed on the ferric or‘ferrosic’ phase. The good agreement between IR, DTA, and surface-area measurements with increasing molecular ratios permits the distinction between two ranges of silica contents. In the first range, MR <·1, silica is directly chemisorbed on the adsorbant. In the MR >0·1 range, silica polymerizes as a separate phase growing upon the‘ferrosic’ or‘ferric’ phase.