Rotating Disk Potentiometry for Inner Solution Optimization of Low-Detection-Limit Ion-Selective Electrodes

Abstract

The extent of optimization of the lower detection limit of ion-selective electrodes (ISEs) can be assessed with an elegant new method. At the detection limit (i.e., in the absence of primary ions in the sample), one can observe a reproducible change in the membrane potential upon alteration of the aqueous diffusion layer thickness. This stir effect is predicted to depend on the composition of the inner solution, which is known to influence the lower detection limit of the potentiometric sensor dramatically. For an optimized electrode, the stir effect is calculated to be exactly one-half the value of the case when substantial coextraction occurs at the inner membrane side. In contrast, there is no stir effect when substantial ion exchange occurs at the inner membrane side. Consequently, this experimental method can be used to determine how well the inner filling solution has been optimized. A rotating disk electrode was used in this study because it provides adequate control of the aqueous diffusion layer thickness. Various ion-selective membranes with a variety of inner solutions that gave different calculated concentrations of the complex at the inner membrane side were studied to evaluate this principle. They contained the well-examined silver ionophore O,O' '-bis[2-(methylthio)ethyl]-tert-butylcalix[4]arene, the potassium ionophore valinomycin, or the iodide carrier [9]mercuracarborand-3. Stir effects were determined in different background solutions and compared to theoretical expectations. Correlations were good, and the results encourage the use of such stir-effect measurements to optimize ISE compositions for real-world applications. The technique was also found to be useful in estimating the level of primary ion impurities in the sample. For an iodide-selective electrode measured in phosphoric acid, for example, apparent iodide impurity levels were calculated as 5 x 10(-10) M.

Sensitivity and Working Range of Backside Calibration Potentiometry

“…Changes in the diffusion layer thickness have no effect on the phase boundary concentrations because they are now fully dictated by the level of ion-exchange at the interface, see also the reference. 21 At the maximum stir effect shown in Figure 1B, the concentration gradients are found to proportionally change with the change in the Nernst diffusion layer thickness, yielding the largest phase boundary concentration changes.…”

Computer simulation of ion-selective membrane electrodes and related systems by finite-difference procedures

“…A series of numerical simulations have been reported in the past for membranes of ion-selective electrodes [18][19][20][21][22][23][24][25], for liquid junctions of reference electrodes [22,24] and for diffusion layers of other electrode systems [26][27][28][29]. In an early contribution, Buck's group introduced digital simulations for investigating the electrical behavior of membranes based on liquid ionexchangers [18,19].…”

Potentiometric sensors for trace-level analysis

“…Later, ion-exchange resins were used for the same purpose [20]. Such resins are not extracted into the membrane phase and bind to a wide variety of ions for which no chelators are available, and the resulting LODs have been found to correlate quantitatively with theoretical predictions [13,41]. More recently, a lipophilic interfering ion in the inner solution was used together with a salt of the analyte ion [31,33].…”

“…Note that the achievable LODs of sensors with an aqueous inner contact may be conveniently predicted if one assumes a steady-state concentration gradient across the membrane and if the selectivity and the composition of the membrane are known [13]. Moreover, the optimal inner solution composition may be predicted for well-behaved cases [13,41] and the level of optimization of the electrode may be assessed by performing stir experiments [41]. These advances are likely soon to lead to a unified approach to the development of such sensors with low LODs, and that may be easily adopted by researchers less familiar with the technology.…”