USING FLOW CALORIMETRY TO DETERMINE THE MOLAR HEATS OF CATION AND ANION EXCHANGE AND THE POINT OF ZERO NET CHARGE ON AMORPHOUS ALUMINUM HYDROXIDES

Abstract

A technique for determining the point of zero net charge (PZNC) using flow adsorption calorimetry was investigated. The rationale behind this method is that the heat of exchange, determined calorimetrically, is directly proportional to surface charge and that the PZNC can be equated to the pH at which the heats of cation exchange (CE) and anion exchange (AE) are equal. The technique was tested using an amorphous aluminum hydroxide (AHO) synthesized in our lab. The flow calorimetry was conducted by measuring the heat associated with CE and AE at pH 5.8, 8.0, and 10.5. Heats of AE were measured using 50 mM NaCl and NaNO3; heats of CE were measured using 50 mM NaNO3 and KNO3. CE and AE capacities were measured on the AHO samples after removal from the calorimeter. The heat associated with NO3− replacing Cl was −4.5 kJ/mol; that for K replacing Na exchange was −1.2 kJ/mol. To determine the PZNC, the calorimetric heat (mJ/mg of sample) was plotted against pH. The heat for CE was multiplied by the factor, 4.5/1.2, to account for the difference in the two molar heats of exchange. The PZNC of the AHO determined calorimetrically was about 9.5, consistent with PZNC values reported in the literature for aluminum hydroxide. In other experiments, it was shown that surface charging of the AHO was reversible with pH and that exposing the AHO to arsenate while in the flow calorimeter lowered the PZNC by 1 pH unit. The use of flow calorimetry to measure surface charge properties of soil colloids has several advantages over existing methods: (i) it requires very small sample sizes; (ii) it is quicker and easier to carry out; and (iii) it gives better control of pH and ionic strength. With flow calorimetry, it is also easy to measure the effect of specifically adsorbed species on surface charge, as was demonstrated in the case of arsenate.