Intrinsic Surface Reaction Constant in 1‐pK Model of Mg‐Fe Hydrotalcite‐like Compounds

Abstract

AbstractThe relationship among intrinsic surface reaction constant (K) in 1‐pK model, point of zero net charge (PZNC) and structural charge density (σst) for amphoteric solid with structural charges was established in order to investigate the effect of σst on pK. The theoretical analysis based on 1‐pK model indicates that the independent PZNC of electrolyte concenmtion (c) exists for amphoteric solid with structural charges. A common intersection point (CIP) should appear on the acid‐base titration curves at different c. and the pH at the CIP is pHPZNC. The pK can be expressed as pK=‐pHPZNC+log[(l+2αPZNC)/(1–2αPZNC)], where α‐PZNC σ/eNANs, in which e is the elementary charge, Na the Avogadro's constant and NS the total density of surface sites. For solids without structural charges, pK=‐ pHPZNC. The pK values of hydrotalcite‐like compounds (HTlc) with general formula of [Mg1‐x Fex(OH)2](Cl, OH)x were evaluated. With increasing x. the pK increases, which can be explained based on the affinity of metal cations for H− or OH− and the electrostatic interaction between charging surface and H− or OH−.