Oxidation-reduction potential may be expressed either in pe values (-log electron activity) or in terms of mV. The pe values are usually completed with a pH value. The first description of the redox potential determination appeared in Bekmann's Instruments Bull (1951) and then completed in pH values by Langmuir (1971). The physicochemical measurements, described in this report, are based on the apparatus used in the Laboratory for Wetland Soils and Sediments (Louisiana State University, Baton Rouge, USA). During my stage there I had a possibility to work with this control system to measure Eh and pH values. Oxidation-reduction potential (Eh) is expressed in terms of Eh. This is a potential of an inert metal electrode (platinum) assumes in soil or solution relative to a reference electrode. In practice, the preference electrode is a saturated calomel electrode connected with the soil system by a salt bridge. Platinum electrode (used in the laboratory experiment) is prepared by sealing 2 cm piece of 18 gauge platinum wire in the end of a section of the glass tubing. Sealing may be done either by fusing the glass around the base of the tube or by sealing with epoxy glue. Mercury is placed in the tubing to connect the platinum with a 20 gauge copper wire which is extended down in the inside of the tubing. The top of the tube is sealed with epoxy glue (Fig. 1). Calomel electrodes are checked by substituting a standard saturated calomel electrodes for Pt-electrode and checking for a zero potential difference between the two half cells. Platinum electrode (used for field measurements) is prepared by fusing a 2.5 cm piece of 18 gauge platinum wire to one end of a 12 gauge section of copper wire of the pre-determined length (Fig. 1a). Copper is not exposed and platinum only is in the contact with the soil. A plastic insulation covering the wire is removed and approximately 1 cm of the exposed copper is removed prior to fusing copper and platinum in a small flame. After fusion, the plastic insulation is slipped back onto the copper wire. The insulation extends over the platinum-copper junction approximately 1 cm. Epoxy adhesive is applied to the edge of insulation and tapered to the point halfway down the platinum wire. Adhesive is applied also to the area where the section of insulation joins the insulation remaining on the copper wire. The epoxy coating on the electrode is allowed to harden for 24 hours before use. Performance of platinum electrode may be checked by immersing an electrode along with the salt bridge which is connected to the calomel electrode in pH 4.0 and pH 7.0 buffer to which a small amount of Quinhydrone has been added. Reading the Eh meter in millivolts should agree with that shown in Tab. 1. The meter reading is corrected relative to hydrogen by adding the calomel potential. The potential is measured by inserting the electrode in substance to be measured and read on the pH meter the mV scale. Since the calomel electrode is used as a reference (+ 244.3 mV) were added to the recorded potential for reference to the hydrogen electrode. The electrods are placed at various depths in the field. These electrodes may be constructed several meters long to facilitate Eh measuring in soils. The electrodes are usually left in the field for at least 24 hours before measuring alone. The laboratory performance for Eh measurements is given in Fig. 2. The measuring system described was used to control the redox potential (-200, -100, 0, +100, +200, +300 and + 400 mV) in the soil suspension of clay loam Crowley soil (U.S.A.) and to measure N2/N2O production under different redox. Nitrous oxide production was reduced to zero at Eh of -200 mV. The N2/N2O ratio was approximately 1:1 at Eh equal to 0 mV. The Eh increase to +400 mV caused any other nitrate reduction, nitrification started. Publishing the redox potential article is done with the agreement of prof. W. H. P A T R I C K Jr.
