Studies on Zero Point of Charge and Permanent Charge Density of Mg−Fe Hydrotalcite-like Compounds

The theoretical analysis on the zero point of charge (ZPC) and charge density of colloidal particles possessing permanent charges indicates that ZPC determined directly by means of potentiometric titration (PT) should be zero point of net charge (ZPNC) and the permanent charge density (σ P ) can be obtained from the adsorption amounts of H + and OH - (Γ OH − − Γ H + ) at ZPNC. ZPNC does not change with the electrolyte concentration, whereas the zero point of variable charge (ZPVC) changes with the electrolyte concentration. When σ P is zero, ZPNC equals ZPVC, and only under this condition is ZPC measured directly by PT equal to ZPVC. The relationship among ZPNC, ZNVC, σ P, the variable charge density (σ V ), the total net surface charge density (σ T ), pH, and the electrolyte concentration is derived. ZPNC and σ P of Mg−Fe hydrotalcite-like compounds (HTlc) were determined by the PT method. It was found that the ZPNC of Mg−Fe THlc increases with the decrease of the content of Fe 3+ in the HTlc samples. The σ P value measured by PT is lower than the theoretical permanent charge density (σ P,T ) calculated from the crystal structure of HTlc because of the fact that the gallery anions OH - adsorbed before titration neutralize some of the permanent charges. The permanent charge density obtained by PT is an apparent value. The σ P value obtained by PT increases with the decrease of the Fe 3+ content of HTlc because the gallery anion Cl - content increases with the decrease of the Fe 3+ content of the HTlc samples.