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The anionic dye Congo red (CR) was protonated by dissolving it in an aqueous solution of HCl at pH 1.2. Potentiometric titration of CR with HCl, supplemented by previous spectrophotometric studies of CR aqueous solutions at different pH values, revealed that at this pH CR is protonated to a stage that it becomes positively charged (cationic, HCR). The visible spectrum of HCR solution was recorded immediately after its preparation and after different aging periods. Two tautomeric varieties of HCR were identified, ammonium-rich variety, where most amine groups and only part of the azonium groups were protonated and azonium-rich variety, where most azo groups and only part of the amine groups were protonated. The former absorbed at 520 nm and the latter between 600 and 800 nm. The fresh HCR solution showed the predomination of the ammonium-rich variety whereas after one hour aging the azonium-rich variety predominated. The protonated dye (between 7.5 and 75.0 mmol HCR per 100 g clay) was adsorbed by Na-montmorillonite by cation exchange. The visible absorption spectra of aqueous suspensions of HCR–montmorillonite complexes were recorded within two hours after adding the dye to the clay suspensions and after eight weeks of aging. Here also the two tautomeric varieties of HCR were identified. In the fresh suspensions the ammonium-rich variety predominated whereas in the aged suspensions the azonium-rich variety predominated. The transformation of the ammonium-rich variety to the azonium-rich variety is fast in aqueous solutions but is slow in aqueous clay suspensions. After air-drying samples of the HCR–montmorillonite complexes, their visible absorption spectra were recorded. The spectra showed that as a result of drying the azonium-rich variety predominated. The thermal behavior of the solid HCR–montmorillonite complexes was investigated by thermo-visible spectroscopy. By the thermal treatment the dye was deprotonated and converted in part to CR. The deprotonation of the adsorbed dye seems to be a zero order reaction, which does not depend on concentration but on the reaction time. The ratio between protonated azonium and non-protonated CR depended on the loading of the clay. With increasing loadings the azonium variety showed the most intense absorption bands. |
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