Potential measurement alternating currents, influence

Potential Measurement under the Influence of Alternating Currents... 

A method of avoiding the effect of potential differences arising at the electrodesolution interface is to take advantage of the capacitive behavior of the double layer at the electrode surface to make ac (alternating current) contact with the solution. To understand how this may be accomplished, it is necessary to consider a basic model of a conductance cell and examine its behavior under the influence of ac excitation. A review of ac circuit principles at a level sufficient for understanding the behavior of conductance cells and the instrumentation for conductance measurement is presented. The reader who desires a more thorough study of this topic is directed to material contained in the references [4-7]. 

Conductometry is an electrochemical technique used to determine the quantity of an analyte present in a mixture by measurement of its effect on the electrical conductivity of the mixture. It is the measure of the ability of ions in solution to carry current under the influence of a potential difference. In a conductometric cell, potential is applied between two inert metal electrodes. An alternating potential with a frequency between 100 and 3000 Hz is used to prevent polarization of the electrodes. A decrease in solution resistance results in an increase in conductance and more current is passed between the electrodes. The resulting current flow is also alternating. The current is directly proportional to solution conductance. Conductance is considered the inverse of resistance and may be expressed in units of ohm (siemens). In clinical analysis, conductometry is frequently used for the measurement of the volume fraction of erythrocytes in whole blood (hematocrit) and as the transduction mechanism for some biosensors. 

Earlier sections of this chapter have illustrated potentiometric methods for the selective determination of ions and molecules. An alternative, elegant, more recent adaptation in which the currents flowing in tiny field effect transistors (FETs) can be used to probe ion concentrations is shown schematically in Fig. 5.8. The FET is made of n-p-n semiconductors which form respectively the source, channel and the drain. In a conventional FET the current flowing between the source and the drain is influenced by a voltage applied to the (metallic) gate (Fig. 5.8). In an ISFET an ion-selective membrane acts as the gate instead of a metal and the electrical potential it exerts on the source-to-drain current depends on the extent to which substrate ions have entered the membrane. The current therefore measures the ionic concentrations. Typical ion-membrane combinations for use in ISFETs are... 

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