Xerogel of SiO2/ZrO2 with Zr in amounts of 17.4, 25.4, and 32.8 wt% were prepared by the sol-gel processing method. The oxide particles were homogeneously distributed in the xerogel and the Cr(VI) adsorption capacities were 5.2, 7.0, and 7.8 wt%, following the increasing order of the ZrO2 loading. The potentiometric response was better for samples with higher ZrO2 loading, and the plot of E vs -log aCr(VI), where a is the activity, showed a linear correlation with an angular coefficent of 57.2 mV, very close to the 59 mV required for nernstian behavior. For the other two samples with lower metal oxide loadings, the angular coefficients were lower, showing a larger deviation from ideal behavior. The potentiometric selectivity constants, Kpotx,y, where x is the primary ion (Cr(VI) in the present case) and y is the interferent ion, were calculated. The following values of Kpotx,y were found for the different interferent ions: Cl-, 0.24; NO-3, 2 x 10(-3); HPO2-4, 2 x 10(-4); SO2-4, 3 x 10(-5); Cr3+, 6 x 10(-9). Copyright 1999 Academic Press.209428-434