ABSTRACT
Determination of redox potential ƒ The redox potential generates a current if we use an apparatus such as the one represented in Fig. 12.2, the oxidation of iron being taken as example.
The redox potential of oxygen being the highest, electrons will flow in the direction indicated. Iron is spontaneously oxidized on contact with water and oxygen. It has been accepted that the normal hydrogen electrode, H2/H
+, will serve as standard (zero potential). But in the field, the manipulation of this hydrogen electrode is practically impossible. A calomel Hg/Hg2Cl2 or Ag/AgCl electrode is used instead. This leads to much confusion (Boivin 2000) and the following equation has to be applied:
Eh = E1 + E2 where, Eh is the redox potential in volts or millivolts, E1 is the potential difference measured in the field between a platinum electrode and a calomel electrode or even an electrode of the Ag/AgCl type, E2 is a correction factor corresponding to the potential difference between the electrode used for E1 and the standard hydrogen electrode. E2 = 1000A – B(t – 25°)
where A = 0.2444 (Hg2Cl2) or 0.2047 (Ag/AgCl) and B = 0.0007588. The apparatus is standardized using a 0.1 M KCl solution containing an equimolar (3 mM) mixture of potassium ferricyanide and potassium ferrocyanide. The potential is then 0.428 V at 25° C. The redox potential in soil solutions is far from the expected values. For example, in aerated conditions, it would theoretically reach or surpass 1000 mV. But only 500 or 600 mV is measured. The difference is explained by defects in principle or practical difficulties:
(1) too low a quantity of reactants to effectively generate a current, (2) slowness of the reactions whereby the system is prevented from attaining equilibrium, (3) asymmetry of the processes in the direction of oxidation or reduction, (4) presence of inorganic ions and living species so that we are not dealing with a pure O2/H2O system, and (5) difficult determinations because oxygen does not diffuse well and the medium is thereby heterogeneous. For example, wetted soil clods exposed to air may have an anoxic centre if their diameter exceeds 3 to 7 mm (Renault et al. 1997). Consequently, some authors go as far as to state that rule of thumb is as good as all the methods for determination of redox potential! This justifies the creation of classes of degree of water saturation based on study of plants and morphological features of soils.
