Abstract: Hydrogen hectorite-water sol was prepared using mixed-bed ion-exchange resins. At 25°C, the rate of change of concentration with respect to time was determined for (a) strong- and weak-acid hydrogen ions; (b) low molecular weight, unassociated silicates; and (c) soluble, unassociated magnesium ion.
The experimental data indicate that two consecutive first-order reactions are occurring: (1) strong acid is undergoing a rapid, spontaneous reaction to yield weak acid, and (2) the resulting weak acid is undergoing a slow, spontaneous reaction to yield a neutral clay. The equivalent weight of the strong acid is apparently one-half that of the weak acid. For each milliequivalent of strong-acid hydrogen ion undergoing reaction, one milliequivalent of magnesium ion is released from the crystal lattice. Thus, the acid clay is eventually converted to magnesium clay. In addition, for each milliequivalent of weak-acid hydrogen ion undergoing reaction, one millimole of silica is released from the lattice. Since the release of magnesium ion precedes the release of silica, proton attack is probably occurring at the edges of the crystals. The exact nature of the weak acid is unknown and detailed study would probably be difficult in view of its transitory existence.
Freshly prepared acid hectorite contains approximately 40 meq of hydrogen ion per 100 g clay. Therefore, only 3 percent of the total magnesium is released from the lattice. The lattice structure of the resulting magnesium clay is identical with that of natural hectorite as shown by x-ray diffraction.