The relationship among intrinsic surface reaction constant ( K ) in 1-pK model, point of zero net charge (PZNC) and structural charge density (ad for amphoteric solid with structural charges was established in order to investigate the effect of on 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 pHmc. The pK can be expressed as pK=-pHmflog[(lf2appy~)/(l-2a~~)], where %cru&NANs, in which e is the elementary charge, NA the Avogadro's constant and N, the total density of surface sites. For solids without structural charges, pK=-pH-.
The pK values of hydrotalcite-like compounds (HTlc) with general formula of [Mgl-,Fe,(OH)2](CI, OH), were evaluated. With increasing x. the pK increases, which can be explained based on the affhity of metal cations for H' or OH-and the electrostatic interaction between charging surface and H' or OH-.
Keywords
charged material surface complexation, electric double layer, points of zero charge, acid-base chemistry, structurally R&vd k h 17.2003; r e v i d August 7.2003; accepcd November 1.2003.