The Measurement of Potential Difference

The pH measurement electrode and the pH reference electrode form a so-called pH measuring chain within the liquid medium. This chain can be compared to a battery of which the voltage produced depends on the liquid medium.

The difference in voltage potential between the measurement electrode and the reference electrode is a function of the pH value of the measured medium. In theory the voltage changes by 59.16 mV per pH unit at 25°C according to the Nernst equation. The voltage produced by the pH measurement chain is large enough not to present any problem for a measurement. The measurement chain is a voltage source from which no current can be drawn, not even the low current which a DC voltmeter draws. The potential difference of the measurement chain has to be measured without drawing any current from the voltage source, otherwise the voltage would be reduced and the pH measurement would be drastically falsified. The reason is the high electrical resistance of the glass electrode which is mainly determined by the resistance of the pH sensitive glass membrane.

The resistance values of a glass membrane vary between 10 MΩ and 1000 MΩ at 25°C and increase 10 times at a temperature decrease of 25°C. The lowest possible operating temperature of a pH electrode is often determined by the resistance increase of the glass membrane at low temperatures, the internal resistance of the measuring instrument, the required accuracy of the pH measurement and the freezing point of the electrolyte. For more information on membrane glass refer to our article on pH glass membrane types.

Special electrodes are manufactured for pH measurements at very low temperatures, having especially low resistance due to a particular glass composition and reduced membrane thickness. In order to still achieve an accuracy of +/- 0.1 pH the resistance of the measurement electrode should not be higher than a hundredth of the internal resistance of the measuring instrument. The upper limit of the membrane resistance lies at 5000 MΩ (5 x 10⁹). Membrane resistance that is too high causes faults and disturbances in the electronic measuring instrument. This is often referred to as pH measurement noise.

The pH measurement system commonly deployed in laboratory and industrial applications often appear different than the illustrations in our articles. The glass membrane measurement electrode and reference electrode are built together into a single pH sensor. This is commonly referred to as a combination pH electrode.