The post was last updated on 2022-02-28.
One important characteristic of redox electrodes is, the material used to measure the EH must be non-selective. This means, it is able to accept or release electrons depending on the conditions of the soil without participating itself in electron transfer reactions. In soils, the theoretical range of EH-pH conditions is limited by the stability of water. High EH values would oxidize water to O2 and low EH would reduce H+ to H2. Thus, the upper and the lower end are always buffered since water molecules are always present in natural systems and H+ are always present in water (even at high pH). This determines the EH range found in soils (Strawn et al. 2015).
A redox electrode with the tip made of iron would be a bad choice because in the electropotential series that lists standard electrode potentials (E0) elemental Fe (-0.44 V; Fe2+ + 2e− ⇌ Fe) is way below H2 (0 V; 2H+ + 2e− ⇌ H2). Since the E0 of the redox couple Fe-Fe2+ is lower than the H2-H+ couple, elemental Fe would be oxidized to Fe2+ and further hydrolyse and oxidize to form Fe(III) oxides and H+ would be reduced to H2.
This is the main reason platinum is used with a E0 of +1.20 V (Pt2+ + 2e− ⇌ Pt), since under EH conditions found in soils it will always be on the right hand side of the equation as elemental Pt and not participate itself in electron transfer reactions (it will not oxidize). Other noble elements could also be employed but this is typically not done (e.g., gold, wax impregnated graphite, glassy carbon) (Mansfeldt 2019).
Redox electrodes with an active Pt surface area are different in construction and incorporated in electrode housings (PVC or fiberglass). However, the bottleneck which determines the durability of redox electrodes is the junction between the Pt wire solded onto a Cu wire. If water gets in contact with the Cu an electrical shortcut occurs with erratic readings being the result. The best way recommended by Pfisterer and Gribbohm (1989) is to use a ceramic jacket to make the housing water-proof. At Polder Speicherkoog we continuously measure EH in situ since April 2010 on hourly basis and since then, no electrode showed a faulty reading that could be related to a short circuit. Obviously, with ~2 million readings the way of constructing electrodes is recommended.
The EH determined from the reactive surface area of the redox electrode is characteristic for the average EH and the participating redox couples of the soil volume in contact with Pt surface. EH from individual electrodes are therefore single-point measurements of only a few mm³. The pore size distribution and connected pores filled with O2, even within a predominantly reducing soil environment, might change at small spatial and temporal scale. Thus, the likelihood of the redox electrode embedded within a micro aggregate featuring reducing conditions or a few mm apart from its present position, e.g. within a connected coarse pore filled with O2, has great impact on the reading. Very recently we highlighted this by using µ-computed-tomography imaging and 3D reconstruction of air-filled pores (see: Dorau, Uteau et al. 2022). Using electrodes with various active surface area are therefore neither worse not better, but they are characteristic for different soil domains. If the focus is on aggregation of the soil, certainly miniaturized redox electrodes as presented by Jang et al. (2005) have advantages over electrodes with larger contact area to investigate very small domains such as anoxic microsites.
Measurements should be performed with at least 2-4 replicates per depth or soil horizon to circumvent difficulties with spatial variability.