3.2 pH Measurement
pH can be measured by using either pH papers/indicators or a pH meter, dependent on the level of
accuracy required. pH papers or indicators change colour as the pH level varies. These can be
used as a guide to the pH level, but can be limited in accuracy and difficult to interpret correctly in
murky or coloured samples.
For greater accuracy the use of a high impedance pH meter is recommended, together with a pH
measuring electrode and reference electrode.
Each component part of the measurement system can be described as follows:
a) the pH meter – is a high impedance amplifier used to accurately measure the minute electrode
voltages produced. The pH meter will display the results directly in pH units on either an analogue
or digital display. Voltages can also be read for special applications, ORP (Oxidation-Reduction
Potential) measurements or with Ion Selective Electrodes.
b) the pH electrode – is a hydrogen ion sensitive glass bulb, with a millivolt output that varies with
the changes in the relative hydrogen ion concentration inside and outside of the bulb. The pH
electrode has very high internal resistance, making the voltage change with pH difficult to measure.
The input impedance of the pH meter and leakage resistances are therefore important factors.
c) the reference electrode – these cells consist of an internal element, usually a silver/silver
chloride wire, electrolyte (KCl) and a liquid junction. The liquid junction provides a leak path for the
internal electrolyte to “weep” into the sample chamber and provide an electrical contact with the
liquid to be measured. If the liquid junction is inefficient then measurement will be inaccurate. It is
common for the reference electrode to be incorporated into the pH electrode. It is then called a
combination electrode. The Model 3510 meter is supplied with a combination electrode.