The accurate determination of the concentration of electrons incorporated into cages in 12CaO·7Al2O3 (C12A7) has been examined using iodometry. Preliminary experiments confirmed that iodine is reduced in preference to protons by electron-enriched C12A7, even in aqueous solution. The electron concentrations in several C12A7 samples are measured using a procedure where the iodine molecules in an I2-HCl solution are partially reduced by electrons released from the samples, followed by titration of the residual iodine molecules. The accuracy of this procedure is confirmed by the excellent agreement of the results with those from thermogravimetric analysis. The reevaluated maximum electron mobility is nearly twice as high as that reported previously, and the critical electron concentration for the metal-insulator transition is revised slightly because of the accuracy of the iodometry results.
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