Instrumentation for Measuring Potential

Understanding electrochemical instrumentation requires a basic knowledge of electricity and basic electronics. Coverage of these fundamentals is impossible in a text of this size. The student is advised to review the concepts of electricity learned in general physics and to understand the definitions of current, voltage, resistance, and similar basic terms. The texts by Kissinger and Heineman, Malmstadt et al., or Diefenderfer and Holton, listed in the bibliography, are excellent sources of information on electronics used in instrumentation. The electrochemical cell is one circuit element with specific electrical properties in the complete instrumentation circuit. 

Notice that E is fed back into the noninverting input (E-). This is termed voltage feedback. 

Commercial solid-state potential measuring devices based on the type of op-amp described are often called pH or plon meters and are designed to work with glass pH electrodes, ISEs, and other indicator electrodes described earlier. Research quality plon meters have built-in temperature measurement and compensation, autocalibration routines for a three-point (or more) calibration curve, recognition of electrodes (so you do not try measuring fluoride ion with your pH electrode ), and the ability to download data to computer data collection programs. The relative accuracy of pH measurements with such a meter is about 0.005 pH units. Meters are available as handheld portable devices for field or plant use these meters have less accuracy, on the order of 0.02 pH units, but are much less expensive. The field-portable meters are battery powered and waterproof. Commercial meters will read out in mV, pH, concentration of ion for an ISE, and mS for conductivity measurements, and many will also read out the temperature. 

A second important use of op-amps and feedback circuits is to control the potential at a WE. The circuit that performs this task is called a potentiostat. Potentiostats and modern polarog-raphy equipment are called three-electrode devices because they attach to a WE, a reference electrode, and an auxiliary electrode, also called a CE (Eigure 15.14). The principle of operation is that the potential between the WE and the reference electrode is maintained by a feedback circuit. The applied voltage E ppi be a constant voltage or some signal generator voltage. 

On modern potentiostats, the applied voltage is given as the actual sign of the voltage on the WE, and there is no need to assume a potential reversal. In other words, if you set the potentiostat at -0.500 V, WE is at -0.500 V versus whatever reference electrode you have in the actual cell. If, for example, this electrode was an SCE, then on the SHE scale at 25°C, 

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