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Anal. Chem. 1982, 5 4 , 1022-1023
Two- to Four-Electrode Conversion Module for Conductometric Measurements Alicla Brestovisky Department of Chemistry, Tel-Aviv University, Ramat A viv, Israel
Chalm Yarnltzky" Department of Chemistry, Technlon-Israel Institute of Technology, Haifa, Israel
It is well-known that accurate measurements of the specific conductance of a solution can be carried out with a conductometric bridge, in which high precision capacitors and conductors are used for the complete compensation of the electrical double layer and cell conductance, respectively. With the introduction of direct reading conductometersthe error caused by the double layer of the electrodes became apparent, especially in the case of solutions with high conductivities. To overcome this problem some instruments utilize frequencies as high as 50 kHz (Model CDM-3 made by Radiometer), at which the double layer behaves like a dead short. Others use a bipolar pulse (1) or sophisticated bridge measurement (2). The oscillometric probe used in a previous work (3)is suitable for solutions with very high conductivities. The four electrode cell offers an alternative approach: one electrode pair supplies the current required for the measurement while another is used to sense the potential difference between two points in the cell ( 4 ) . For a well-defined difference of potential between the two sense points, a controlled current is imposed on the cell. This current is proportional to solution conductance. Several commercial companies manufacture four-electrode conductometers (e.g., Polymetron conductivity instrument, type 8263);however, due to their high cost, these are mainly employed for industrial purposes. The converter described below is connected to a four-electrode system and can be used in conjunction with a simple two-electrode conductometer. Is supplies the current flowing through the cell by means of two counterelectrodes and senses the difference of potential of the other two, unaffected by the periodic potential applied by the conductometer (sine or square wave). The inexpensive converter (
