Semiquinone Cation Adsorption on Montmorillonite as a Function of Surface Acidity
Semiquinone cations derived from aromatic diamines by one-electron oxidation in aqueous solution possess characteristic colors that depend on hydrogen ion concentration. The colored free radicals are formed by oxidation on the surface of certain mont-morillonite clay minerals, where they are stabilized by adsorption, apparently in cation exchange positions, but many clays react anomalously when the procedure is employed as a diagnostic test for montmorillonite. One of the problems involved is the chemical nature of the various semiquinones. Another is the effect of clay mineral crystal structure on the color that is produced.
The dihydrochlorides of benzidine, p-phenylenediamine, and N,N-dimethyl-p-phenylenediamine in aqueous solution have been used with a centrifuged Wyoming bentonite (montmorillonite) and a centrifuged hectorite clay, each suspended in various buffered solutions, to study the pH effect and the quantity and nature of the semiquinones formed. Hypothetical structures differing by one hydrogen (or hydronium) ion have been assumed for two semiquinone forms of each diamine, each formula representing a distinct hue, with the divalent cation formed at low pH and the monovalent cation at a higher pH. Mixtures of the two cations were assumed to exist at intermediate acidities corresponding to mixtures of the two hues. A simple set of equations was postulated to describe these reactions, and the theory was tested experimentally by determining the amount of nitrogen adsorbed on each clay sample at a series of fixed pH values ranging from 0.5 to 6.5.
The milligram atoms (mga) of nitrogen (or 2 X millimoles of diamine) adsorbed were found to increase with increasing pH in approximately the amount predicted in the range 1.0-4.2. In this pH range, however, the calculated base exchange capacity (BEC) for each clay was lower than the accepted value, presumably because an approximately constant fraction of the base exchange sites was occupied by nonexchangeable cations, probably aluminum and hydrogen ions. Amounts of base adsorbed were the same whether sodium or calcium ions were initially in exchange positions. At pH values of 5.5 and 6.5 the mga of nitrogen adsorbed showed a sharp increase, apparently because colloidally dispersed reagent diamine was formed as the free base and no aluminum or hydronium cations were adsorbed. When corrections were made for nonadsorbed free base (not adsorbed on exchange positions but physically adsorbed to the floccules) and, in the case of benzidine, for base lost in filtration, the calculated BEC’s for each clay were in satisfactory agreement with accepted values.
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