Effect of Stirring on PH Measurements

Abstract

Potential variations in pH measurements using a pH glass electrode and SCE during agitation by a magnetic bar, compressed gas, a glass rod, and ultrasound in the presence and absence of colloidal particles have been studied. The so-called streaming potential is partly related to the disturbance of the charge density of glass electrode double layer as a result of stirring. The observed potential under stirring (observed disturbed potential) is the potential difference between the undisturbed potential and the stirring potential which may be more positive or negative than the undisturbed potential depending on the testing conditions. The stirring effect derived from both glass electrode and SCE have been studied individually. The junction potential of SCE is affected by stirring or ultrasound for different reasons, chiefly for changes in charge density of the electrode interface. Electrolyte solution flowing in a vortex mode at the interface carries part of double layer charges and produces an electric field. The stirring of an electrolyte solution may also be viewed as a reversed electro-osmosis. That the stirring may result in the change of the double layer capacitance is suggested. An accurate pH measurement using a glass electrode should be done in a quiet solution containing no colloidal particles. For the potentiometric titration using a pH glass electrode as an indicator electrode should have no adverse effect, because only the relative potential jump signifies the end point.