Cell constant studies of bipolar and tetrapolar electrode systems for impedance measurement

In comparison to the bipolar or two-electrode system, the tetrapolar or four-electrode arrangement is a well-known technique to reduce electrode polarisation just as effectively. However, recent studies have showed that phenomena such as negative sensitivity and multiple current paths can compromise the advantages of the tetrapolar electrode arrangement, thereby potentially limiting its applications. This paper reveals a novel method to evaluate the performance of the different electrode systems in which the concept of cell constant is extended to an impedance measurement system. We employ it as a standardised parameter to quantitatively analyse planar electrode systems in bipolar and tetrapolar measurement modes. Indeed, the cell constant is a key parameter in conductivity sensors to evaluate electrodes designs since it is independent of any readout electronics. A comparison of measurement modes using finite element methods (FEM) simulations and measurements for sodium chloride solutions is presented. While the cell constant of the bipolar electrode system is one order of magnitude greater than that of the tetrapolar arrangement, it shows large discrepancy over the measured frequency range. In contrast, despite the existence of measurement errors, the tetrapolar arrangement yields a uniform cell constant and good agreement with the simulations