The determination of redox potentials of soils
1939
Volk, N.J.
The purpose of this investigation was to find a suitable method of preserving samples of soil for redox measurements and to develop an accurate method for determining the oxidation-reduction potentials of soils. The results are summarized as follows: 1. All chemical preservatives tested which effectively inhibited bacterial action in soils also altered the Eh of the soil to such an extent that the results were no longer comparable. 2. Cooling the soils to just above the freezing point was found to be a very effective preservative method. 3. Oxidation of reduced compounds in the soil was prevented by the use of water saturated with nitrogen and by performing analytical operations in an atmosphere of nitrogen; 4. The Eh/pH relationship, even within close range of the initial pH, was found to vary from 58 to 101 millivolts for different soils, thus the Eh/pH relationship was determined for each soil studied and correction in Eh made accordingly for differences in pH. 5. Potential drift was found to be due to previous treatment of the blank electrodes and not to the agar bridge. 6. Smooth straight wire platinum electrodes 2 cm in length were found superior to foil since roots clinging to the foil electrode caused misleading results. 7. Details are given for the analytical procedure finally adopted. Results agreeing within 5 millivolts by this method are considered satisfactory. 8. A total of 15 to 20 soils can be analyzed per hour for Eh values by using a battery of 30 aspirators involving 30 glass electrodes and 120 blank electrodes.
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