Determination of the Ion-Pair Constant between Chloride Ion and Cationic-Coated, Silver SERS Substrates Using Competitive Complexation

The interaction of chloride ion with cationic-coated silver surface-enhanced Raman scattering (SERS) substrates was monitored using a competitive complexation technique. In this approach, the nitrate concentration is kept constant and the chloride ion concentration is varied. The change in the nitrate peak area, ΔA NO 3, is measured as a function of chloride ion concentration, C Cl. The measured response is described by the following relationship: ΔA NO 3 = VC Cl/(K + C Cl). Using the value of Κ and the value of the ion-pair constant between the cationic coatings and nitrate ion, it is possible to extract the value of the ion-pair constant for chloride ion. The selectivities of the cationic coatings for nitrate, sulfate, and chloride ions are discussed.