Reverse Current Pulse Method To Restore Uniform Concentration Profiles in Ion-Selective Membranes. 2. Comparison of the Efficiency of the Different Protocols
journal contributionposted on 01.07.2009, 00:00 by Justin M. Zook, Ernő Lindner
The membrane potential of ion-selective electrodes is measured at zero current in traditional potentiometric analysis. Recently, pulsed potentiometric methods have gained importance. In pulsed potentiometric methods, the voltage measured at the end of a current pulse is usually the analytical signal. The applied current alters the concentration profiles inside the sensing membrane. For reproducible voltage measurements the original concentration profiles must be restored in the membrane between current pulses. The simplest restoration method is the zero-current relaxation. Unfortunately, the zero-current method is very slow, which limits the frequency of measurements. In analytical practice the controlled voltage restoration method is most commonly used, but the controlled voltage method has no adequate theoretical description. This paper presents a finite element model of the controlled voltage method to predict its efficiency. The model demonstrates for the first time that increasing membrane resistance decreases the efficiency of this restoration method. The model allows estimating the necessary restoration time for voltage errors below an acceptable threshold value and provides guidance for minimizing the voltage error. The efficiency of the controlled voltage method is compared to the reverse current pulse restoration method discussed in part 1 of this set of papers. It is found that the reverse current restoration method is simpler, requires shorter restoration times (i.e., it allows higher measurement frequency), and it has 4 and 10 times smaller voltage errors compared to the controlled voltage method. These theoretical results are confirmed experimentally. The only limitation of the reverse current pulse restoration method is that it cannot be used with membranes containing a background electrolyte (R+R−) but no excess lipophilic cation exchanger (R−). However, lipophilic cation exchanger can often be added to the membrane to reduce restoration times by allowing the reverse current pulse method to be used.