Interpretation of potentiometric measurements in suspensions of solid particles

Abstract

In potentiometric measurements, suspensions of solid charged particles influence the potential of the indicator electrode ( PWSE), and in combination with the commercially available reference electrodes, they cause the occurrence of a new potential difference ( OSE) and in this way influence the response of the measuring galvanic cell ( PWSE + OSE = ODSE). A valid interpretation, as with direct potentiometric measurements is not possible. Only when OSE is negligible or avoided, eg with the use of a modified reference electrode, can the directly measured voltages be interpreted in terms of the activity of measured ions in the equilibrium solution, or in the particle double layers, or both. This depends on the size and concentration of particles and on the concentration of electrolytes in the suspension. The contribution of the particle double layers to the measured ion activity depends on the particles; it is greater the smaller the particles are and—to a certain extent—the higher their concentration. In the limit that is a sediment of very small particles, the measurements could be considered as dependent on the average activity of measured ions in the nearest vicinity of particles. If the suspension can be separated into sediment and equilibrium solution, measurements interpreted as above can be obtained with the standard, correspondingly placed electrodes and values of OSE, PWSE and ODSE can be determined.