Redox probe cleaning, maintenance and calibration

Cleaning and polishing


Redox probes do not normally need maintenance, but should be cleaned before use and whenever retrieved from the soil. Use soft cloth and general cleaning agents, no more aggressive than necessary: water, water and soap, ethanol or mild acid (e.g. 0.1M HCl). Rinse with distilled water after cleaning.

Probes that have been used in an oxidising environment may respond slowly to reducing environments. Probes that have been in contact with sulfides may be 'poisoned', meaning a thin layer of PtS may have formed. In both cases, polishing the Pt surface with abrasive paper P1200 grit or higher exposes a fresh Pt surface. Always wet polish as fiberglass dust is not healthy.


Calibration of a redox probe in practice boils down to calibration of a set consisting of a mV meter, a reference electrode and a redox probe. The reference electrode and redox probe are placed in a calibration solution and the resulting voltage is measured with the mV meter. If the mV meter reads the expected value for the calibration solution, the set calibrates ok and it is reasonable to assume all parts work. However, this is not 100% sure. It is possible the mV meter has an offset opposite to a reference electrode error.

mV meter (or data logger)

It is recommended to run a few simple tests on the mV meter before starting the actual calibration. Although thorough testing of the mV meter requires specialized equipment, the following simple tests will give a quick indication whether your mV meter is working fine.

Wiring and connectors

Redox measurements can easily fail due to sloppy wiring. Make sure all wires and connectors are in good shape:

Redox probe calibration

To calibrate a redox probe, connect a clean redox probe and reference electrode to a mV meter and place both the redox probe and reference electrode in a beaker filled with calibration solution. The mV meter should display a stable result within 10 seconds. The value obtained should lie within ± 10 mV from the expected value. This value depends on the calibration solution used, the type of reference electrode used and temperature (see tables below).

If the calibration is not ok, the error usually originates from the redox probe and/or from the reference electrode. In rarer cases, the mV meter or wiring may cause an error. Assuming the mV meter has passed the simple tests above and the wiring is fine, first try cleaning and/or polishing of the redox probe. If calibration still fails, check the reference electrode (see below).

Four common calibration solutions are Light's Solution, Zobell's Solution, Quinhydrone in pH 7.0 and Quinhydrone in pH 4.0. These solutions differ in toxicity and 'redox strength' (poise). Toxicity must be considered when handling and disposing of the solutions. Poise is important because an unsatisfactory redox probe may give accurate readings in a highly poised solution, whereas the same probe may fail calibration in a lightly poised solution. Many soil systems do not have a high poise, so it is important to calibrate a redox probe in a lightly poised solution.

Calibration values for Zobell's Solution, quinhydrone in pH 7 and quinhydrone in pH 4
Reference electrode Temperature Zobell's Solution Quinhydrone in pH 7 Quinhydrone in pH 4
Ag|AgCl 3M KCl 20°C 229 mV 87 mV 265 mV
Ag|AgCl 3M KCl 25°C 221 mV 82 mV 261 mV
Ag|AgCl sat'd KCl 20°C 239 mV 92 mV 268 mV
Ag|AgCl sat'd KCl 25°C 231 mV 86 mV 263 mV

Reference electrode

The easiest way to calibrate a reference electrode is to compare it with another reference electrode that is known to be good. In practice this means it is a good idea to get an extra reference electrode that is kept for calibrating other reference electrodes only.