Determining the redox potential requires some tools, and some care. Care is needed to not influence the redox potential itself. In water, this is rather easy. Water will flow freely around the electrode, and the supply of fresh medium is always possible. There are many good probes available for ORP measurements (Oxidation Reduction Potential measurements in water). Measurements in solid media as soil, sediments or others requires more specialistic tools.
Measuring the redox potential requires three items: (1) a non-reacting contact with the medium, (2) a stable potential source, the socalled reference, and (3) a stable measurement device that will not influence the measurement. For calculations of a more standardized redox potential, it is also advised to measure temperature and the pH of the medium.
- A non-reacting contact to the medium
The electrons in the medium need to be able to contact the measurement device via a good and solid contact. Normally, the very stable metal platinum (Pt) is used for this. Pt is solid and unlikely to react with the surroundings. If you, for instance, would use INOX or another iron based material, this material would in itself be an oxidator or reducer of the suroundings. Those reactions will influence the redox potential of the medium, and that is what you do not want.
Most available probes have either a larger diameter Pt wire extruding from the tip, or Pt wires embedded on the side. The latter makes it possible to measure at multiple depths simultaneously.
Any nobel metal will do the same in a redox probe. Most are however harder to handle than Pt. Gold (Au) is used as well, as it is easier to deposit on probes. But Au will dissolve and scratch much easier than Pt.
A typical thing of solid media is that there is usually a strong layering. The top is closer to air, and the bottom is closer to water. Grain size can influence water movement, and create niches for microbes to grow. It is important to take this layering into account, and use probes that can measure at multiple depths.
- A stable potential, the reference probe
Most reference probes nowadays have a Ag/AgCl combination liquid. The toxic calomel type should not be used anymore. This stable standard is needed as a fixed value, to compare the measurement potential against. Without it, you would compare two unknown values, rendering only the difference between two points in space. With one fixed value, you can calculate the redox potential as a known potential. Most standards dictate a convertion to a value compared to the H2 reference electrode (hence the Eh).
When a standard reference electrode is used, you should always look up its values for specific temperatures. Temperature measurements are adviced.
- A measurement device
A very important part of the measurement is the measurement device. It is very common to use a pH meter for Eh measurements, as the two determinations are greatly alike. But, there is a problem when measuring in solid media for long times: the electrons cannot always move freely around, and the reactants are always on the same spot near the probe. A typical pH meter allows a relatively large current to have the pH probe respond fast. As said, in water this shouldn't be a problem. But in soils, many researchers have found that if they connect a Pt probe to the pH meter, the dial changes over time, and never really becomes stable.
For solid media, the measurement device should have the highest possible resistance, thus limiting the flowing current as much as possible. In general: the higher the better, but also the slower the measurement device.
The option to measure at multiple depths requires multiple channels. Vorenhout et al., 2004 published a first device that had multiple channels. MVH Consult has improved that device, and now sells the Hypnos IV with 48 channels for simultaneous and continuous measurements. That version was first mentioned in Vorenhout et al., 2011