pH Measurements

Abstract

To measure the pH in vitro an indicator electrode and a reference electrode are put in the vessel containing the fluid to be examined. The glass of a glass electrode is sensitive to hydrogen ions. The potential difference between the glass and the solution is determined by the concentration of hydrogen ions in the solution when the other variables are constant. By definition the pH is equal to the negative logarithm of the concentration of the hydrogen ions; therefore the electromotive force of this electrode will depend on the pH. The reference electrode (e.g. calomel electrode) builds an electromotive force which is independent of the pH and which remains constant. Thus, the potential dif ference between these two electrodes is determined by the pH of the fluid. It can be measured electronically and the results can be directly read from the scale of a pH meter; if so desired they can be registered on an appropriate recording system. In order to achieve correct measurements all constituents of the solution must have the same potential. If electromechanical reasons prohibit the direct contact of the reference electrode with the solution the circuit can be closed with e.g. a salt bridge consisting of saturated KC1.