Soil Science Society of America Journal 66:1813-1820 (2002)
© 2002 Soil Science Society of America
DIVISION S-2—SOIL CHEMISTRY
Continuous Multiple Measurement of Soil Redox Potential Using Platinum Microelectrodes
Eric van Bochove*,
Suzanne Beauchemin and
Georges Thériault
Soils and Crops Research and Development Centre, Agriculture and Agri-Food Canada, 2560 Hochelaga Blvd., Sainte-Foy, QC, Canada G1V 2J3
* Corresponding author (vanbochovee{at}agr.gc.ca)
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ABSTRACT
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Redox potential (EH) measurement is a reading of voltage difference between a working electrode such as a Pt electrode and a reference electrode inserted into the soil or various substrates. This study was conducted to develop a method for continuous, autonomous, and multiple EH measurements using Pt microelectrodes connected to a data logger. A preliminary field experiment was carried out to assess the long-term viability of Pt microelectrodes installed in situ. A second experiment was conducted in the laboratory to test an interface designed to allow the stabilization of EH measurements. The Pt microelectrodes and reference electrode showed reliable readings during the field trial and generally tested viable at the end of the four month experiment. However, discrepancies between logged EH measurements and manually stabilized readings, particularly under moderate reductive conditions, emphasized the necessity to adapt the principle of manually stabilized readings to the use of a data logger. Laboratory data obtained from pairs of Pt microelectrodes and reference electrodes connected to a new homemade interface confirmed that instantaneous logged measurements led to the underestimation of EH values by 140 mV in the critical and unstable range of 0 to 200 mV. Continuous, multiple measurements of stabilized EH using Pt microelectrodes is now feasible using a stabilization interface placed between the microelectrode and the data logger.
Abbreviations: EH, redox potential • NHE, normal H electrode • PVC, polyvinyl chloride
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INTRODUCTION
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PLATINUM ELECTRODES and Pt microelectrodes have been extensively used for in situ (McKenzie and Erickson, 1954; McKeague, 1965a,b; Flühler et al., 1976; Faulkner et al., 1989) and laboratory measurements (Flessa and Beese, 1995) of EH of soils. Redox potential measurements offer a semiquantitative assessment of the intensity of oxidation or reduction of a soil, which reflects many redox couples operating simultaneously in a dynamic system (Patrick et al., 1996). The assessment of soil redox potential is particularly useful for characterizing the onset of reducing conditions in a soil caused by a lack of O2 and for partly interpreting their associated biogeochemical processes such as denitrification (Flühler et al., 1976; Flessa and Beese, 1995) or bacterial degradation processes (Crawford et al., 2000).
In natural systems, the EH readings initially drift for awhile (several minutes) before reaching a relative equilibrium or until the rate of drifting slows down considerably. The phenomenon of drift of measured potentials is proportional to the redox stability of the systems and may be attributed to oxygen desorption, platinum reduction, instability of low concentration of redox couples (Bohn, 1971) and electrode depolarization (Böttcher and Strebel, 1988). In highly reduced systems such as flooded soils or sediments, the reading usually stabilizes quickly (several seconds). However, in transitional system turning from oxidized to more reduced conditions (350 to -100 mV) and vice versa, the drift is significant and may persist over a longer period. In such systems, Patrick et al. (1996) suggest an overnight stabilization period prior to recording the measurement.
Long-term, continuous measurements of redox potential would provide insight of the dynamics of changes in a soil profile when environmental conditions are drastically modified (e.g., after addition of organic matter, flooding, and fertilization). To achieve this goal, the long-term viability of electrodes installed in situ needs to be assessed and an automated, continuous recording of stabilized EH values must be accessible. Although Pt electrodes installed in situ over a long time may become contaminated by surface reactions and lose their efficiency and accuracy (Bohn, 1971; Devitt et al., 1989), some studies demonstrated that electrode performance was not significantly impaired by poisoning even after many months of use (Smith et al., 1978; Austin and Huddleston, 1999). The automated recording of continuous, multiple measurements of stabilized EH values still needs to be addressed.
Soil EH measurements in the field have usually been made at weekly intervals using a portable meter (Olness et al., 1989). Automated measurements for hourly monitored EH readings have also been carried out in long-term studies (Smith et al., 1978; Fiedler and Fischer, 1994). Fiedler and Fischer (1994) collected continuous, automated multiple EH readings using a data logger and multiplexer. Data logging of EH is particularly convenient for remote study sites and simultaneous measurements of other critical environmental parameters such as pH, soil water content, O2, and temperature. However, in the case of simultaneous and continuous multiple EH logging, the actual electronic configurations of the multiplexer and the data logger allow to keep an electrode circuit closed no longer than 5.9 x 10-3 s during the channel switching (CR-10 Meassurement and control system operator's manual, Campbell Scientific Inc., Logan, UT). Consequently, a stabilized EH value may not be reached before recording the measurement for a given electrode circuit. In the set-up of Flessa and Fischer (1992), it remains unclear whether stabilization of readings was achieved prior to recording the measurements but initial drift would probably be negligible in their well reduced systems. This aspect needs to be considered in less well-poised systems.
Degueldre et al. (1999) adapted an in situ polishing treatment of a flat Pt-disc electrode to achieve a quick (instant) measurement of continuously monitored redox potential of groundwaters. Their design, however, is not applicable to soil EH measurement. For soils, Böttcher and Strebel (1988) showed that the initial drift or depolarization kinetics corresponded to a first-order process and could be described by an exponential function. Based on this function, stabilized EH values could be calculated from the short time depolarization curves. Other works proposed to apply a polarizing voltage to accelerate the setting of a constant equilibrium potential (Liu, 1981; Liu and Yu, 1984). It was later shown that electrode polarization may affect the measured EH values (Böttcher and Strebel, 1988).
In this study, our objective is to develop an automated method for continuous, multiple, in situ measurements of soil EH using Pt microelectrodes connected to a multiplexer and a data logger. We propose to adapt an interface between each pair of Pt microelectrode and reference electrode that will overcome the actual configuration of the multiplexer and the data logger and allow the stabilization of EH readings. The interface does not impose a polarizing voltage and simply allow the soil current to keep circulating in the electrode circuit even when the data logger channel is turned off and switched to another one. In the first part of the study, we first tested the long-term viability of the Pt microelectrodes used. The specific objectives were thus to (i) test Pt microelectrodes long-term viability during a 4-mo field experiment, and (ii) develop and test in laboratory an interface that permits the continuous monitoring of multiple, stabilized EH readings.
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MATERIALS AND METHODS
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Field Evaluation of Platinum Microelectrodes Viability
Experimental Setup
The surface horizon of an orthic humic gleysol (pH 5.8) was excavated in June to a depth of 0.4 m by layers of 0.2 m and was then replaced in the same order into a white polyvinyl chloride (PVC) box (length, 0.91 m; width, 0.53 m; height, 0.4 m, thickness, 6.3 mm) closed with a gastight PVC cover. The box was placed horizontally in the trench flush with the ground for the experiment duration to circumvent the effects of daily temperature variations. Three sets of triplicate Pt microelectrodes (Jensen Instruments, Tacoma, WA) of different lengths (0.15, 0.30, and 0.60 m) were installed through the PVC cover at three depths (0.05, 0.20, and 0.30 m) in the soil profile and sealed into the cover with silicone. The Pt microelectrodes consist of a 4.0-mm length of Pt wire mounted in ceramic with flexible epoxy on the end of a stainless steel tube of 3.05-mm in diameter. The electrode wire is a spring loaded contact pin connected to an insulated Cu wire that extends to an insulated terminal at the other end of the tube. The microelectrodes were connected to the data logger by single shielded cables (20 ga; length, 12 m) using pin sockets (AMP part No. 205090-1, Digi-Key Corp., Thief River Falls, MN). One Ag/AgCl reference electrode (RC5, Jensen Instruments, Tacoma, WA) was installed through the cover in the middle of the box at a 0.10-m depth. The reference electrode was not connected to the soil using a salt bridge as described by Veneman and Pickering (1983). The reference electrode was inserted directly into the soil as currently done with pinpoint field measurements (Faulkner et al., 1989).
The soil EH was successively reduced during the experiment by adding deionized water while restricting the O2 entrance from outside the box during periods between water additions. The soil water content at the beginning of the experiment (8 June) was 0.199 m3 m-3 from 0- to 0.2-m depth and 0.292 m3 m-3 from 0.2- to 0.4-m depth. Continuous redox readings were started on 12 June at noon. The soil was then brought to field capacity (0.470 m3 m-3) on 16 June (Hour 96 of the experiment) by adding 14 L of distilled water through a removable window (width, 0.04 m) at one side of the cover. On 19 June at Hour 167, an additional 7 L of water was added to bring the soil to saturation (0.565 m3 m-3) in the box. Soil volumetric water content was measured using water content reflectometers (CS615, Campbell Scientific, Logan, UT). On 27 June at Hour 355, an additionnal 7 L of water containing 500 g of glucose was added through the removable window to further reduce redox potentials. The glucose addition aimed at increasing soil microbial respiration to simultaneously favor O2 depletion in the system (0.2% fresh soil weight, Mahapatra and Patrick, 1969). The removable window was then sealed with silicone to prevent any O2 from entering.
Platinum Microelectrodes Viability
Before installation and in situ experimentation, the microelectrodes were tested in quinhydrone solution at pH 4 and 7 using the procedure described by Patrick et al. (1996). After removal from the field site, the microelectrodes were first tested in a slurry using the procedure described by Austin and Huddleston (1999), then in quinhydrone solutions as previously mentioned, and again in quinhydrone solutions after brushing the electrode Pt tips with a scrubbing agent (commercial household cleanser) to remove any surface coatings and then soaking overnight the electrodes in distilled water prior to testing (Patrick et al., 1996).
Reference Electrode Viability
It was also necessary to verify the viability of the long-term installed reference electrode even though its 4 M KCl solution level remained fairly constant. To test the reference electrode (No. 1), a second similar reference electrode (No. 2) was installed through the PVC cover of the box at Hour 1913. Both electrodes were left in the soil until Hour 2057 when Reference 2 was connected to the data logger and Reference 1 was removed to be tested and cleaned in the laboratory. This time lag between insertion and connection of Reference 2 was necessary to avoid any potential impact of O2 input on the EH readings in the box during the insertion of the second reference electrode. Reference 2 had to be removed because of the risk of freezing at Hour 2521, and Reference 1 was replaced into the box at Hour 2707 until the end of experiment (Hour 3000).
Data Acquisition
The nine Pt microelectrodes were individually connected to the high terminals of differential channels of a relay multiplexer (AM416, Campbell Scientific, Logan, UT) which was itself linked by its common high line to the high terminal of a differential channel of a CR-10 data logger. Connecting probes to a multiplexer increases the number of available channels on the data logger. To be common for all microelectrodes, the reference electrode was connected to the low terminal of the same differential channel of the data logger.
Redox potentials (mV) were logged every 2 min and hourly average values were recorded in a storage module. The raw EH values were then corrected relative to the normal H electrode (NHE) by adding 207 mV (average soil temperature in the box over the experiment duration = 16.8°C, Standard Deviation 4.6). To verify whether the multiplexer created interference (cross talk) on redox readings during the switching of its differential channels, the signal of the three microelectrodes installed at 0.2-m depth was shunted in two distinct wires. The first wire was connected to a differential channel of the multiplexer as previously described, and the second wire was directly connected to the high terminal of another differential channel of the CR-10 data logger. The signal of the reference electrode was shunted also to the low terminals of the same differential channels. Continuous EH readings from the data logger were compared with manual readings using a portable voltmeter (ORP meter P5E, Jensen Instruments, Tacoma, WA).
Continuous Redox Potential Measurements
During the first field experiment, a distinct and removable microelectrode (20D) was used in the vicinity of the three buried microelectrodes at 0.2 m (20A, 20B, and 20C) to assess independent redox measurements using the portable voltmeter and the data logger. These measurements were performed at four intervals (Hours 517, 540, 565, and 588) by carefully inserting successively the 20D microelectrode through rubber septa (Suba Seal, England) preinstalled in the box cover equidistant (0.05 m) of the buried microelectrodes. After having measured EH, septa were sealed with silicone. Manual readings of the buried microelectrodes and the logged data were compared by calculating Spearman rank order correlations (Statistica 5.1, Statsoft, Tulsa, OK).
A second experiment was performed in laboratory to test EH monitoring by using an interface (Fig. 1)
placed between the data logger and the microelectrodes. The interface was designed to stabilize EH readings through a resistor that allows the soil current to keep flowing between each pair of Pt microelectrode and reference electrode. The interface consisted of a circuitry of six 10 x 106 ohm resistors connected between six pairs of microelectrode and reference electrode under measurement (1 to 6). In contrast, six control Pt microelectrodes (7 to 12) were directly connected to the datalogger without stabilization interface. The control Pt microelectrodes shared a common reference electrode as previously described. The 12 microelectrodes and the seven reference electrodes were installed in a closed cubic acrylic box (0.008 m3) containing 6 kg of an orthic humic gleysol at field capacity (water content [WC], 0.34 m3 m-3). To enhance reducing conditions, a solution of glucose (0.5 L, 10 mM) was spread at the soil surface using a syringe needle through a rubber septum installed on the side of the box. The headspace of the sealed box was flushed with dinitrogen in an effort to induce the system under anaerobic conditions and further reduce soil EH. Redox potential readings were logged following a logarithmic sampling design as soon as the program was started, that is, when Circuits 1 to 6 were closed simultaneously. Redox potential values were logged every 2 s during 30 min and then every 30 s until the end of data recording. Redox values were corrected by adding 203 mV for NHE at 21°C.
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Fig. 1. Schema of the electronic circuitry used as interface between the pairs of Pt microelectrodes and reference electrodes and the multiplexer-data logger assembly. The set of six microelectrodes and reference electrodes connected to the interface includes resistors (R) of 10 x 106 ohms (1%). The seven reference electrodes are connected to ON/OFF switch (S). H1 means high terminal of Channel 1 and L1 means low terminal of Channel 1. H1 COM means high terminal of Common Line 1 and L1 COM means low terminal of Common Line 1 of the multiplexer. 1H1 and 1L1 are the differential channels of the multiplexer sequentially connected to the Common Line 1.
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RESULTS AND DISCUSSION
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Electrode Viability
As shown in Fig. 2
, soil EH decreased over time in the field experiment and consistent patterns were observed among replicates at a given depth. These data suggest that Pt microelectrodes were, overall, properly reading redox potentials during the long-term experiment. Postexperimental tests in laboratory confirmed that most of the buried microelectrodes gave reliable EH readings throughout the 4-mo experiment. As expected, results from electrode testing showed more variable results in a soil slurry than in quinhydrone solution (Table 1). Austin and Huddleston (1999) proposed the slurry test as a system closer to real soil conditions than the quinhydrone test, which is a highly poised system. By constant stirring of the slurry solution, the slurry test removes the spatial variability affecting field replicates. Electrode readings are retained as reliable when the readings of different replicates of a same set of electrodes are within an arbitrarily close interval from each others (e.g., 20 to 40 mV; Austin and Huddleston, 1999). According to this test, electrode 30B was clearly out of range compared with the corresponding replicates. For the quinhydrone test, the reference EH values at 20°C with an Ag/AgCl reference electrode are 268 mV at pH 4 and 92 mV at pH 7. A faulty electrode is noted if its reading differs more than 5 to 10 mV from the reference value (Patrick et al., 1996; Austin and Huddleston, 1999). Except for electrode 30B, all Pt microelectrodes after field removal were within 10 of the 92 mV reference value for the quinhydrone test at pH 7. Although all readings at pH 4 failed to be within 10 mV, they were all within 20 mV of the 268 mV reference value. Also, compared with their corresponding values at pH 4 before experiment, the readings were usually within 10 mV except from electrode 30B (Table 1). Cleaning the Pt tip after field removal did not significantly improve the values (Table 1), indicating that most Pt microelectrodes were properly reading despite sligthly lower values in the post-removal test with quinhydrone at pH 4. Even though results from the slurry and quinhydrone tests did not show obvious failure of the microelectrode 5A, data from this microelectrode were discarded because EH values from Hour 750 to 1500 were abnormally high compared with other electrodes (data not shown). These outlier data may have been caused by a loose contact of the Pt tip with the soil. In contrast, data from the Pt microelectrode 30B, which notably failed the postremoval slurry and quinhydrone tests, were retained because the field values observed were consistent (Fig. 2) with those of electrodes 30A and 30C. Electrode 30B regained its accuracy after replacing the O-ring between the electrode shaft and the Pt tip. The O-ring displacement probably occured when the Pt microelectrode was removed from the box at the end of experiment.
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Table 1. Before experiment and post removal tests at 21°C of Pt microelectrodes using a 1:5 slurry of silt loam soil/water, a quinhydrone solution in buffer at pH 7 and 4, and the same quinhydrone solutions after microelectrode tip gentle brushing.
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When testing the buried reference electrode, drilling the box cover to introduce a second electrode had no impact on the EH (Fig. 2), even though some O2 entered in the anaerobic system during the operation which took <2 min. However, as soon as the second reference electrode was operated, the average EH increased by about 100 mV. After reconditioning the first reference electrode (4 M KCl) and replacing it into the soil, it appeared that there was no difference between both reference electrodes. The discrepancy in the EH values noted between the use of Reference 1 and 2 at Hour 2057 of the experiment (Fig. 2) was attributed to desaturation of the KCl solution of the Reference 1 electrode. Using a salt bridge (Veneman and Pickering, 1983) would resolved such problems in future applications of continuous redox measurements.
Continuous Redox Potential Measurement
During the field experiment, the EH values at each depth declined rapidly after glucose addition at Hour 355 from an oxidated state (150 to 500 mV) to moderate-strong reductive conditions (-300 to 150 mV) in approximately 20 h (Fig. 2). Initial soil pHs were not significantly different across the three depths (pH 5.90, SD 0.10). From Hour 1000 to the end of experiment, the oxidation-reduction state into the box remained quite constant around -200 mV, except when the Reference Electrode 1 was replaced by the Reference Electrode 2 (Hour 2057), as discussed previously.
Connecting the Pt microelectrodes to either the multiplexer or directly to the CR-10 data logger did not alter the EH readings (data not shown). A series of selective measurements of the EH performed around the three buried microelectrodes (20A, 20B, and 20C) with a readily removable Pt microelectrode (20D), showed substantial spatio-temporal variation of EH readings (Table 2). The average coefficient of variation (CV) on six measurements was 376% for instantaneous readings and 46% for 1-min readings.
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Table 2. Comparison of redox potentials (EH, mV) between readily removable Pt microelectrode (20D) and long-term installed Pt microelectrodes (20A, 20B, 20C) at different intervals. Readings were recorded manually (instantaneous and 1 min) using a voltmeter for the microelectrode 20D and automatically logged for the long-term installed ones. 20A, 20B, and 20C were triplicate microelectrodes buried at 0.20-m depth. Microelectrode 20D was punctually used to make two measurements of EH at 0.20-m depth in the vicinity of microelectrodes 20A, 20B, and 20C. Coefficients of variation were calculated for instant and 1-min readings of microelectrode 20D.
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In contrast to the generally good agreement between 1-min and instant EH values for the well oxidized soil at the beginning of the experiment (<Hour 355; Table 3), results from Table 2 indicate that instantaneous readings differed much from 1-min readings following glucose addition (>Hour 355). Differences between instant and 1-min readings ranged from few millivolts to as high as 400 mV in some cases. These differences are related to the drift of the readings that became significant as the soil slowly shifted towards lower EH. Although 1-min readings had a much lower coefficient of variation than instant readings, the variation among replicates was still important. A 1-min stabilization period was arbitrarily chosen in the present study but the stabilization might have required a longer period when the soil started to get more reduced after glucose input. Therefore, part of the variation in 1-min EH readings may be because of a time effect but spatial variation is also known to be important with EH measurements. Under field conditions, Flühler et al. (1976) concluded that a fairly large number of replicates (six to nine replicates) was required to get a representative mean value of the soil EH.
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Table 3. Comparison of redox potentials (EH, mV) at different intervals between manual (instantaneous and 1-min) readings using a voltmeter and automatically recorded hourly value (data logger). A, B, and C were triplicate microelectrodes long-term installed at 0.05-, 0.20-, 0.30-m depth.
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Spearman rank order correlations calculated from EH data (all data set) were higher between manual instant readings and logged data (0.84, p < 0.001) than between manual 1-min readings and logged data (0.76, p < 0.01), reflecting that logged values were closer to instant than to 1-min reading values.
Stabilization of the Redox Potential Readings using a Data Logger
Stabilization of EH measurement is achieved when the manual voltmeter is switched ON for a given time period, which means that the circuit between the microelectrode and its reference electrode is closed. To resolve the particular data logger configuration, we designed an interface (patent pending) to be added between the microelectrodes and the data logger (Fig. 1). The added interface provides a closed loop by using 10 x 106 ohm resistors that allow the soil current (
20 mA for a potential of 200 mV) to keep flowing between pairs of microelectrodes and the reference electrodes. The adapted interface simulates the stabilization period as when setting a portable voltmeter at ON. It draws a soil current comparable with a voltmeter which is considered as negligeable in the electrode circuit to avoid polarization of the electrode (Bohn, 1971). Commercial voltmeters have a standard resistor of 10 x 106 ohms. The measured value is a function of this resistor. The closest readily available commercial resistor value to that of the P5E ORP meter (11 x 106 ohms) is 10 x 106 ohms (1% precision), introducing a difference of 10%. Tests in quinhydrone solutions did not shown differences between the readings made using the interface connected to the data logger compared with the ones recorded directly from the ORP meter (data not shown).
At the start of the laboratory continuous measurements, the mean EH values of the stabilized (26.8 mV; SD 40.6) and the nonstabilized (25.7 mV; SD 80.2) sets of microelectrodes were equivalent (Fig. 3)
. After 600 s, the mean EH readings stabilized at 162.4 mV (SD 13.4) as showed by the Pt microelectrodes 1 to 5 connected to the interface, while the mean nonstabilized EH measured by the Pt microlectrodes 7 to 12 remained fairly constant at 23.0 mV (SD 78.3). The results illustrate clearly the drift phenomenon which characterized soils with medium oxidation and reduction state and the need to achieve a stabilized reading before recording the EH value (Patrick et al., 1996). Redox potentials recorded from the six pairs of microlectrodes and one reference electrode not connected to the interface (Fig. 1, Microelectrodes 7 to 12) were instantaneous (nonstabilized) and led to an underestimation of the soil redox potential by approximately 140 mV. During the experiment, the signal from the Pt microelectrode 6 became defective and the corresponding data were discarded.
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Fig. 3. Redox potentials (EH, mV) continuously logged in a laboratory experiment using a stabilization interface. Measurements of Pt microelectrodes (1 to 5) connected to the interface are indicated by cross symbol and thin black lines, noninterfaced Pt microelectrodes (7 to 12) are indicated by open triangles wide gray lines.
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CONCLUSIONS
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Data from buried Pt microelectrodes installed in the field experiment showed decreasing redox potentials following glucose addition, which indicated proper functioning under continuous use and data logging. Laboratory quinhydrone and soil slurry tests confirmed the long-term viability of the Pt microelectrodes after 4 mo of continuous use. The actual configuration of the data logger used to monitor a voltage difference between high and low inputs of numerous differential channels only permits instantaneous readings. However, EH stabilized values are expected to better reflect the real soil redox conditions, particularly in medium reductive conditions where the drift phenomenon is important. The new interface developed allowed continuous and autonomous measurements of soil EH and permitted the monitoring of multiple, stabilized EH. Future applications of continuous EH measurements should improve the knowledge on temporal variations in different environments and allow the multiplication of redox potential measurements.
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ACKNOWLEDGMENTS
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This study was supported by the Program for Energy Research and Development (PERD) and Agriculture and Agri-Food Canada (Contribution Number 730). The technical assistance of Sylvie Côté, Nadia Goussard, and Michel Lemieux is also gratefully acknowledged. We thank anonymous reviewers for constructive comments that help improving the manuscript. We also thank Dr. Clarke Topp for valuable comments on an earlier version of this manuscript.
Received for publication September 7, 2001.
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M. Vorenhout, H. G. van der Geest, D. van Marum, K. Wattel, and H. J. P. Eijsackers
Automated and Continuous Redox Potential Measurements in Soil
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ABSTRACT
INTRODUCTION
