Soils redox probe

Oxidation-reduction potential Because of the interest in bacterial corrosion under anaerobic conditions, the oxidation-reduction situation in the soil was suggested as an indication of expected corrosion rates. The work of Starkey and Wight , McVey , and others led to the development and testing of the so-called redox probe. The probe with platinum electrodes and copper sulphate reference cells has been described as difficult to clean. Hence, results are difficult to reproduce. At the present time this procedure does not seem adapted to use in field tests. Of more importance is the fact that the data obtained by the redox method simply indicate anaerobic situations in the soil. Such data would be effective in predicting anaerobic corrosion by sulphate-reducing bacteria, but would fail to give any information regarding other types of corrosion. 

The presence of bacterial activity can be qualitatively estabfished by placing a few drops of hydrochloric acid on the corrosion products. The evolution of H2S indicates the existence of anaerobic bacteriological activity. The bacteriological existence is also established by measuring the redox potential. This can be done by placing a redox probe into freshly dug soil at the pipeline depth. The potential is measured between a clean platinum surface and a saturated calomel reference electrode [49—53]. The redox potential measurement data and the tentative anticipated bacteriological activity are shown in Table 15.4. 

An indicator of soil corrosivity is the value of soil oxidation-reduction (ORP) or redox potential. It is calculated from the potential difference measured with a probe that contains an inert platinirm (Pt) electrode and a saturated calomel electrode (Hg/HgjClj/KCl, +0.241 V versus SHE) as a reference electrode. The value of this soil redox piotential depends on the dissolved oxygen content in the piore water and provides some information on the conditions under which sulfate-reducing bacteria could grow. The use of redox potentials to predict soil corrosivity is presented in Table 4. 

The redox potential is determined with a probe consisting of a platinum electrode and a Hg/Hg2Cl2 Cl or Ag/Ag Cl—Cl reference electrode. If Ep is the potential of the platinum probe, Ej the potential of the reference electrode and tne redox potential of the soil (in mV on the hydrogen scale) then... 

Obtaining a reliable measure of the number of unpaired electrons ( spin concentration ) in a sample is often extremely useful. Even reliable relative values measured across a series of samples can often provide useful information. There are various important applications, as may be illustrated by the following incomplete list ESR dating, the determination of oxidized polycyclic aromatic hydrocarbons and of environmental carbon in samples of ambient air, the influence of air pollution (e.g., SO2 and NO2) on plants and soils, quantification of NO2, RO2, and HOI radicals in air samples, radiation dosimetry, redox activities of zeolite catalysts, and the metabolism of spin probes in cells and tissues. 

The oxidation-reduction behavior of a soil is not frequently measured, because it is difficult to obtain reliable data in the field. Extraction of soil samples for laboratory analysis is likely to introduce oxygen into the sample that will adversely affect the accuracy of the data. Accurate data can be obtained with the soil in situ, but special probes are required. The use of redox potentials in evaluating the corrosivity of soils is discussed in Refs 20 and 21. 

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