Linear sweep voltammetry—compensation of cell resistance and stability Determination of the residual uncompensated resistance

Abstract

The problem of cell resitance compensation with a system comprising a potentiostat, a current measurer and a positive feedback device is discussed in relation to stability in the case of linear sweep voltammetry. The part of the cell resistance that remains uncompensated by the action of the potentiostat may be quite significant, particularly in non-aqueous solvents. Reduction of this resitance by means of a positive feedback compensation allows first a better separation of the double layer charging current from the faradaic current. Also the effect of ohmic drop on faradaic polarization curves is minimized. “Unconditional” stability by means of phase shifting devices has not been attempted. On the contrary, the approach was to perform a purely ohmic compensation involving a real positive feedback rate so that a mathematical correction can be effected after measuring of the residual uncompensated resistance. It is shown that these conditions can be met practically for systems characterized by a second order transfer function. A controlling and measuring instrument composed of operational amplifiers is described which fulfils the preceding requirements. General procedures are proposed for the verification of the second order character of a given controlling and measuring system and for the measurement of the residual uncompensated resistance. They are applied experimentally to the particular instrument considered here, with a dummy cell in which no faradaic reaction occurs at the working electrode, and with another one involving a simulation of the faradaic current. Lastly, the question of complex positive feedback is briefly discussed.