Redox electrode

One important characteristic of redox electrodes is, the material used to measure the E_H 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 E_H-pH conditions is limited by the stability of water. High E_H values would oxidize water to O₂ and low E_H 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 E_H 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⁺ + 2^e− ⇌ Fe) is way below H₂ (0 V; 2H⁺ + 2^e− ⇌ 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⁺ + 2^e− ⇌ Pt), since under E_H 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 E_H 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.

Figure 1: Scheme of the platinum surface in contact with the soil matrix and one continous pore filled with air. Presumably, the tourtous path enable O₂ to diffuse towards the Pt surface. The electrodes are scaled for comparison with a sand grain with the reactive surface area from three studies. Source: The scheme on the left is adapted from Reddy and DeLaune (2008) and on the right my own illustration.

The E_H determined from the reactive surface area of the redox electrode is characteristic for the average E_H and the participating redox couples of the soil volume in contact with Pt surface. E_H 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.