Porous Frit

The diffusion experiments were performed on a commercial porous frit composed of a sintered alumina ceramic (99.8 % alumina) measuring (50.76±0.04) mm in diameter and (6.48±0.03) mm thick; the uncertainty is an estimated standard deviation calculated from 7 frits. The porosity, estimated from mercury intrusion porosimetry analysis, was approximately 0.26. The pore size distribution was concentrated between 350 nm and 450 nm, consistent with the manufacturer's intended use and published specification of a pore size less than 500 nm.

The alumina frit was regarded as a nearly ideal porous medium for studying diffusive transport. The alumina is chemically stable when in contact with solutions having a pH near 7. In addition, conductivity measurements suggest that the frit has a relatively low surface charge. As defined in Eqn. 8, the formation factor F is calculated from the ratio of the pore fluid electrical conductivity $\sigma _p$_p to the the bulk electrical conductivity $\sigma _b$_b with that pore fluid. The calculation can be repeated for different pore solutions with varying values of $\sigma _p$_p.

Such an experiment was performed in 1979 by McDuff and Ellis [38] on a model porous material composed of a packed column of cation exchange resin. Their raw data for large pore Linear ordinary least squares regression applied to the data reveal a positive intercept of (0.68±0.17) S/m; the uncertainty is an estimated standard deviation. Similarly, one of the frits was saturated with different standard potassium chloride conductivity solutions [39], and the data are shown in Fig. 1 as filled triangles. For the frit, the intercept was only (0.00258±0.00025) S/m. Since the intercept is proportional to the surface charge of the material [40], one expects that the surface charge of the frit is a factor of 263 times smaller than that of the resin. Although this does not imply that the surface charge on the frits is negligible, the data presented herein will show that an analysis that neglects binding still captures all the salient features of the effect speciation has on the diffusive transport of ions through these ceramic frits.

Figure 1:The pore solution conductivity $\sigma _p$_p and the corresponding bulk conductivity $\sigma _b$_b for the resin system of McDuff and Ellis [38] and the frit systems used here.