We carried out potentiometric titrations of a synthetic goethite in solutions of NaCl, NaNO3, NaC1O4, and Na2SO4 to determine the intrinsic acidity constant Kintaj and intrinsic complexation constants of Cl−, NO3−, ClO4−, and S04− surface complexes formed at the goethite/aqueous electrolyte interfaces *Kintaj. The determined Kintaj values for the four electrolytes were in a narrow range of 5.9-6.2. The values of Kintaj were found to be 6.7, 6.5, and 6.2 for Cl−, NO3−, ClO4−, and S04−, respectively, and Kintaj took the value of 16.0 under the assumption that one SO42− occupies two charged sites. The values of point of zero salt effect (PZSE), which is defined as the pH value at which titration-curves in an electrolyte intersect, were 8.5, 8.6, 8.2, and 8.9 for NaCl, NaNO3, NaC1O4, and Na2SO4, respectively. There was a highly positive correlation between the PZSE and Kintaj. Selectivity coefficients for stoichiometric Cl-NO3 and NO3-SO4 exchange reactions on goethite were calculated from the Kintaj values and exchange isotherms were drawn.
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