Non-Faradaic electrochemical sensors: principles and practice

In this paper the principles of non-faradaic electrochemistry are outlined, and non-faradaic electrochemical methods are described which provide: (i) a solution to the problem of devising a real-time biomass probe; and (ii) in certain instances a means of measuring enzymic behaviour in situ. The linear, non-faradaic or 'passive' electrical properties of a biological or other system are completely characterized by its conductance (siemens) and capacitance (farads), reflecting, respectively, the inand out-of-phase portions of the alternating potential difference caused in response to the application of an alternating current. These macroscopic properties depend in part on the size and geometry of the electrodes, and reflect the intrinsic properties of the system, permittivity and conductivity (see, for example, references 1-3). For plane-parallel electrodes of area A separated by a distanced, the relationship between the conductivity a' and conductance G is a'= G(d/A), where ~d/A) is known as the cell constant and has units of length. The capacitance Cis related to the permittivity E' by E' = C(d/Ac0J, where Eo is an experimental constant equal to 8.854 x 10F m, such that a cubic electrochemical cell of unit dimensions containing water (which has a permittivity of 78.4 at 298 K) has a capacitance of some 6.94 pF. By 'linear', we mean that the measured conductance and capacitance are independent of the magnitude of the exciting field, and that excitation by a current at a frequency f Hz leads only to an alternating potential difference at the same frequency.