Selectivity of Membranes - Potentiometric Methods of Analysis

Selectivity of Membranes

Membrane potentials result from a chemical interac- tion between the analyte and active sites on the membrane’s surface. Because the signal depends on a chemical process, most membranes are not selective toward a single analyte. Instead, the membrane potential is proportional to the concen- tration of all ions in the sample solution capable of interacting at the mem- brane’s active sites. Equation 11.8 can be generalized to include the contribution of an interferent, I,

where z_A and z_I are the charges of the analyte and interferent, and K_A,I is a selectivity coefficient accounting for the relative response of the interferent.* The selectivity coefficient is defined as

where [A]_E and [I]_E are the concentrations of analyte and interferent yielding identical cell potentials. When the selectivity coefficient is 1.00, the membrane responds equally to the analyte and interferent. A membrane shows good se- lectivity for the analyte when K_A,I is significantly less than 1.00.

Selectivity coefficients for most commercially available ion-selective elec- trodes are provided by the manufacturer. If the selectivity coefficient is un- known, it can be determined experimentally. The easiest method for determining K_A,I is to prepare a series of solutions, each of which contains the same concentration of interferent, [I]_add, but a different concentration of analyte. A plot of cell potential versus the log of the analyte’s concentration has two dis- tinct linear regions (Figure 11.11). When the analyte’s concentration is signif- icantly larger than K_A,I[I]_add, the potential is a linear function of log [A], as given by equation 11.8. If K_A,I[I]_add is significantly larger than the analyte’s concentration, however, the cell potential remains constant. The concentra- tion of analyte and interferent at the intersection of these two linear regions is used to calculate K_A,I.