Determination of the Amount of the Surface Hydroxyl Groups on Iron Oxide(III) by Various Met h ods

Abstract

The amounts of the surface OH groups on α- and γ-Fe₂O₃ were determined by exchanging the OH groups with F^- or Na⁺ in an aqueous solution and by the reactions with Al(C2H5)3and some polar organic reagents such as 1-butanol, butylamine and butyric acid.
The amounts determined by exchanging with F^- and Na⁺ remarkably decreas ed or increased with an increase in pH in both cases, respectively (Figs.2 and 3). The fact showed that the F^- and Na⁺-exchanging methods determined the amounts of the basic or acidic surface OH groups, respectively. On the other hand, the reaction with Al(C₂H₅)₃ is considered to be suitable for the estimation of the amount of the total surface OH groups (Fig.4). The reactions with polar organic reagents are not applicable to estimate the amount of the total OH groups, nevertheless they are useful to measure the amount of the active sites being available for the surface modification using these organic reagents.
In general, the determination of the exact amount of the surface OH groups on γ-Fe₂O₃ is difficult because of the high reactivity of γ-Fe₂O₃ with various reagents used for the determination (Table 2).