Hydrogen ions montmorillonite

When studying the surface acid-base properties of montmorillonite, it is essential to understand that hydrogen ions and cations of the support electrolyte can also participate in cation-exchange processes. The processes on the internal and external surfaces have to be taken into consideration simultaneously, and they both have to be included into the equilibrium thermodynamical models. 

The Quantity of Zinc and Calcium Ions in the Process Zn2+/ Ca-Montmorillonite and the Equilibrium pH of the Solution, the Difference Between the Quantities of Zinc Entering and Calcium Ions Leaving the Montmorillonite Phase, and the Quantity of Dissolved Hydrogen Ions... 

ROLE OF HYDROGEN IONS IN THE INTERFACIAL AND DISSOLUTION PROCESSES OF MONTMORILLONITE... 

The other interfacial process involving hydrogen ion is the cation-exchange process in the interlayer space. When montmorillonite is suspended in water or in an electrolyte solution, a part of exchangeable cations can be dissolved. In Table 2.7, the relative quantity of calcium ions dissolved in water or in acidic solutions is shown. 

As seen in Table 2.7, the lower the pH, the greater quantity of calcium ions dissolved. The dissolved calcium ions are substituted by hydrogen ions in the interlayer space. In other words, the cations in the interlayer space of montmorillonite react with water itself, and a hydrogen-calcium cation-exchange reaction takes place ... 

FIGURE 2.6 Hydrogen ion-exchange isotherm in Ca-montmorillonite/hydrogen-ion exchange. T = 20°C. 

FIGURE 2.7 The selectivity coefficients of hydrogen ion/calcium-montmorillonite cation exchange calculated from the experimental data and on the basis of Equation 2.24. 

Equivalent Fractions of Cobalt, Calcium, and Hydrogen Ions on Montmorillonite at Different Initial Cobalt Concentrations... 

When the pH of the solution is in the strongly acidic range, that is, the hydrogen ion concentration of the solution increases, the crystal lattice of montmorillonite starts to disintegrate, and the cations dissolved from the crystal lattice... 

The acidic destruction of montmorillonite results in the release of silicon and aluminum. The initial fast exchange of surface cations by hydrogen ions is followed by the release of aluminum and silicon. The dissolution rate of Si is higher than that of A1 and is influenced by the relative ratios of basal siloxane and edge surfaces. The shift of pH to more basic values by the ion-exchange processes and the hydrolysis of dissolved species induce the formation of secondary amorphous solids, initiating the formation of amorphous aluminosilicates (Sondi et al. 2008). 

The exchange of hydrogen ions between the solution and montmorillonite is also possible (Section 2.7.1) ... 

To summarize, calcium ions in the solution can be present as hydrated Ca2+ions, and CaHEDTA and CaEDTA2 complexes, of which only the positive calcium aqua complexes (Ca2+) participate in the ion-exchange reaction. In the process, calcium ions dissolve from montmorillonite to the solution, and mostly hydrogen ions get into the interlayer space of montmorillonite. In addition, sodium ions are also present in the system (EDTA is added as disodium salt H4EDTA is hardly soluble in water), which also affects the ion-exchange process. The ratio of cCa aCa can be plotted as a function of the concentration of Ca2+ (Figure 2.11). 

FIGURE 2.13 The equivalent fractions of manganese(II) (XMn), calcium (XCa), hydrogen (XH), sodium ions (XNa), and their sum (X total) in the interlayer space of montmorillonite as a function of pH of the solution. The ratio of the total concentrations of EDTA, calcium, and manganese ions is 1 1 1. The equivalent fractions of calcium and hydrogen ions as a function of pH without the complex-forming agent is also shown. 

H30 per liter, but for a solid acid such as acid-activated clay a sharp distinction must be made between soluble acidity and local acid strength . The soluble acidity can be readily measured by convential techniques such as titration or gas volumeter analysis. As to titration, the clay can be dispersed in water, and any acidity thus liberated can be neutralized. On this basis, Thomas, Hickey, and Stecker [89] found that raw montmorillonite yielded 0.41 milliequivalents of acid per gram of dry clay, while after acid treatment (removal of half of the aluminum) this value rose to only 1 milliequivalent per gram. If the clay were a liquid with the density of water, these results would mean hydrogen ion concentrations of 0.41 x 10 and 1 X 10 mole per liter, which corresponds to pH values of 6.39 and 6.00, respectively. Thus, even for the acid-activated clay the soluble acidity is extremely small, and cannot possibly explain the proven catalytic effect of this material. It does, however, explain the fact that TONSIL can be swallowed without harm. 

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