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Construction of Platinum-Tipped Redox Probes for Determining Soil Redox Potential

Department of Soil Science, Box 7619, North Carolina State University, Raleigh, NC 27696-7619

^* Corresponding author (deanna_osmond{at}ncsu.edu)

Received for publication February 17, 2004.

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
Redox probes are typically constructed by soldering Pt wire to a metal wire or rod, such as copper or brass. The junction between the Pt and wire or rod is often sealed with an epoxy resin and hardener or with heat-shrink tubing. Microcracks (small cracks invisible to the unaided eye) can form in the hardened resin and result in incorrect readings. The hardened resin is not easily removed, making repairs difficult. Heat-shrink tubing is thin, lacks rigidity, and can be damaged in the soil. The method described in this paper used a thick-walled, adhesive-lined terminal insulator to seal the junction. The terminal insulators were easily applied and removed, which made faulty probes easy to repair. Two-hundred forty probes were made with this method and eight were made with a marine epoxy resin. The probes were tested with a redox buffer solution (Light Solution) and were usable if they read +476 ± 10 mV. The probes were installed 0.76 and 1.5 m deep in the soil. The ability of the probes to provide reliable redox readings was examined by testing selected probes after 10 mo of use and testing all of the probes after completion of the study (19 mo). Ten of the twelve probes tested after 10 mo worked satisfactorily, while the other two clearly malfunctioned before testing. After the study was completed, 236 of the 240 of the probes worked satisfactorily. These results indicate that the construction method presented produces reliable, long-lasting probes.

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
REDOX POTENTIAL is a measure of electron availability and is a result of electron transfer between oxidized (chemical species that have lost electrons) and reduced (chemical species that have gained electrons) chemical species (Gambrell et al., 1991; Gambrell and Patrick, 1980). The measurements indicate the likelihood of electron donation to or loss from a chemical species and are used to assess characteristics of the soil environment (Gambrell et al., 1991; Pang and Zhang, 1998). Redox potential measurements provide an indication of the soil aeration status and are often used in conjunction with ground water data to determine the most probable chemical species of ground water constituents. For example, redox potential measurements below a certain value indicate that denitrification is likely to occur and decreases of ground water nitrate N (NO^–₃–N) could be attributed to denitrification. On the other hand, redox potential measurements above a certain value indicate that the soil is sufficiently aerated for ground water NO^–₃–N to not be denitrified and remain available to crops.

Platinum-tipped redox probes that can be easily made and repaired are an asset to researchers who often require numerous probes for studies of soil redox potential. Platinum is used because it is nonselective, meaning it will accept electrons from all redox reactions, and it is stable, meaning it will accept or release electrons, but will not release chemical species that will influence redox reactions (Bohn, 1971). Various redox probe construction methods have been used during the past 40 yr. The Hg junction method fused Pt to the melted end of Pyrex glass tubing that was filled with liquid Hg after the junction had cooled (Bohn, 1971; Ponnamperuma, 1972; Gambrell et al., 1975; Faulkner et al., 1989). Glass breakage and the health risks associated with liquid Hg were concerns related to the use of this redox probe construction method (Mueller et al., 1985).

Another method fused or soldered Pt to Cu wire (Letey and Stolzy, 1964; Meek et al., 1968; Bohn, 1971; Faulkner et al., 1989; Jordan et al., 1993) or to brass alloy brazing rod (Mann and Stolzy, 1972; Mueller et al., 1985; Jacobs and Gilliam, 1983). Glass tubing was melted at the Pt-Cu/brass junction to waterproof some of the earlier probes (Letey and Stolzy, 1964) but this method was also time consuming and there was a risk of glass breakage (Mueller et al., 1985).

Other methods covered the Cu/brass rod with heat-shrink tubing and waterproofed the Pt-Cu/brass junction with epoxy resin and hardener (Bohn, 1971; Mann and Stolzy, 1972; Mueller et al., 1985; Faulkner et al., 1989) or with additional heat-shrink tubing (Jacobs and Gilliam, 1983). Problems resulted from the use of epoxy resin because some of the resins deteriorated under saturated conditions (Mueller et al., 1985), although marine epoxy has been used because it is resistant to deterioration in saturated conditions. In either case, microcracks were found to develop in the epoxy and result in incorrect readings; the epoxy was also difficult to remove, thus hindering repair of the probes (J.W. Gilliam, personal communication, 2003). Care had to be taken to not over apply the epoxy on the Pt-Cu/brass junction and to prevent epoxy from getting on the Pt. The epoxy also had to be sanded when it dried, which made this a time-consuming method.

The heat-shrink tubing, on the other hand, was easy to apply. There were also fewer concerns about the heat-shrink tubing deteriorating in the soil, which provided the durability needed for long-term studies (Jacobs and Gilliam, 1983). The construction method presented in our paper is a modification of the method used by Jacobs and Gilliam (1983). We employed a thick-walled heat-shrink insulator, which was thicker, more rigid, and easier to remove to fix faulty probes than the regular heat-shrink tubing used by Jacobs and Gilliam (1983). Our objective for this study was to determine the effectiveness of this modified construction method by testing all of the probes pre-installation, testing selected probes after 10 mo of readings and testing all probes after 19 mo of data collection.

	Materials and Methods

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
Redox Probe Manufacturing
The body of the redox probes was 0.32-cm-diameter, nonflux, low-fuming bronze brazing rod cut to a length of 122 cm. A 1-cm-deep hole was made in both ends of the brazing rod using a small #1 combined drill and counter sink on a metal working lathe. One of the holes was deepened to 1.3 cm using a 0.12-cm-diameter drill bit. Air was blown into the holes to remove any metal shavings before soldering.

A freestanding, flat-bottom torch was used for soldering, which allowed the materials to be held by both hands. Rosin core solder (Dutch Boy; Taracorp, Winston-Salem, NC) was melted into the 1-cm-deep hole by heating the end of the rod with the torch and touching the solder to the hole until it filled with solder (Fig. 1) . All soldering was done in the fume hood; goggles and heat-resistant gloves were worn because the brazing rod became very hot. The soldered end was reheated to melt the solder and a 1.5-cm piece of 18-gauge 98.95% pure Pt wire from Fisher Scientific (Hampton, NH) was inserted into the molten solder. The heat was then removed and the solder was allowed to solidify.

View larger version (24K): [in this window] [in a new window]   Fig. 1. Diagram of a completed redox probe.

Shrink-wrap, adhesive-lined tubing (Catalog #74965K22) was purchased in 122-cm sections from McMaster-Carr (Atlanta, GA). The tubing was slid over the probe, starting at the end with the Pt tip, and positioned so that one end was approximately 0.5 cm from the Pt–brazing rod connection. From that point the tubing covered the brazing rod, Cu wire-brazing rod connection, and a portion of the insulated copper wire. The tubing was shrunk by slowly heating it with a Milwaukee paint stripper heat gun, set on low heat (Milwaukee Electric Tool Corporation, Brookfield, WI). It was essential to not heat an area too long, because it would cause blistering or air bubble formation.

A rigid terminal insulator (1.3-cm-long, adhesive-lined, thick-walled heat-shrink tubing; McMaster-Carr Catalog #72675K51, color code red) was slid over the Pt, leaving approximately 0.32 cm of the Pt exposed. The insulator covered a portion of the Pt and the exposed brazing rod, and slightly overlapped the cooled and hardened heat-shrink tubing. The adhesive inside the insulators increased the contact with the Pt–brazing rod connection. Insulators without adhesive were tried as well, but they melted and pulled away from the heat-shrink tubing, exposing bare brazing rod. Probes also were constructed by putting heat-shrink tubing over the terminal insulator. This proved undesirable, however, because the heat-shrink tubing did not seal tightly on the brazing rod near the insulator. There also was concern that the heat-shrink tubing covering the insulator would have been pulled off during installation.

The insulators were applied by heating them at high heat with the heat gun, as long as the redox probe was rotated to heat the insulator evenly and reduce the potential for boiling the adhesive. All air bubbles between the insulator and Pt–brazing rod junction were removed by reheating the insulator and lightly squeezing it as the insulator cooled. This prevented water from getting under the insulator and contacting the brazing rod, which could cause false readings. If an insulator had air bubbles that could not be removed, the insulator was removed by either stripping it off with a sharp knife or box cutter or heating it with the heat gun and pulling it off. A new insulator was then installed and heated on low heat to further reduce the chance of boiling. Probe construction took about 20 min per probe from the time soldering began until the terminal insulator was applied. Each probe was numbered before testing.

Testing Redox Probes
Redox measurements were made with a KCl-saturated Ag/AgCl reference probe, which was purchased from Jensen Instruments (Tacoma, WA), and an Accumet portable pH/mV meter (Fisher Scientific). The redox potential of each completed probe was tested after the probes had cooled at least 4 h. If 0.32 cm of Pt was not exposed before testing, the terminal insulator was cut with a sharp knife or box cutter to expose the Pt. The Pt on each probe was lightly sanded before testing to remove any oxidized layer that may have formed on the Pt.

Two methods were used to test the redox probes, and all test measurements for each probe were recorded. Initially, probes were tested in a redox buffer solution to assess the accuracy of measurements made with the probes in an environment that would donate or accept electrons from the Pt. The redox buffer solution was a ferrous/ferric Fe solution (Light Solution) that was made according to the procedure described by Light (1972). The expected redox potential in the redox buffer solution was +476 mV because the KCl-saturated Ag/AgCl reference probe was used (Light, 1972). To test the probes, five redox probes and the reference probe were placed in the redox buffer solution in an acid-washed Nalgene beaker (Nalge Nunc International, Rochester, NY). The redox measurements often stabilized (did not change) within 30 s of being connected to the pH/mV meter. The redox probes were left in the solution until three readings were made with each probe, which took 5 to 10 min. During that period the probes were in the solution long enough to observe any drift (gradual increase or decrease of the redox potentials equal to or greater than 5 mV) of the redox values. Probes were acceptable if they read ±10 mV from the expected redox potential. If the measured redox potentials drifted the probe was reconstructed.

One measurement was then made with the redox probes and reference electrode in tap water to assess whether or not the probes would work in a saturated, but less-buffered environment. The measured redox potentials in tap water were dependent on the source of the tap water (J.W. Gilliam, personal communication, 2003), but were generally in the range of +300 to +600 mV for the tap water used. Thus, acceptable probes were those that measured redox potentials in this range. Probes that measured redox potentials less than +300 mV, or potentials that fluctuated greatly, often failed a second test in the redox buffer solution. It is possible that the lack of chemicals in the tap water made it less viscous than the redox buffer solution, which could have allowed the water to more readily reach the brazing rod through any holes. The differences between the tap water and redox buffer solution were not examined, making any explanation of the observed testing difference speculative at best. Regardless of the process that caused the difference, the tap water method helped detect unacceptable probes that would have otherwise gone undetected.

If a probe was unacceptable, the terminal insulator around the Pt–brazing rod connection was reheated and pressure was applied as it cooled, which closed air bubbles that may have formed. The probe was tested again in tap water. If the readings remained unsuitable, the insulator was reheated with the heat gun and pulled off or it was stripped off with a box cutter or knife. A new insulator was then applied by previously described methods and the Pt was sanded more vigorously before testing in tap water. If the probe was still unacceptable, it was sanded more heavily and tested in tap water. If one of the repairs produced probes that had results in tap water similar to the previously tested and acceptable probes, they were then tested in redox buffer solution. If those probes failed in redox buffer solution, the repairs were made again. Those that passed in tap water and redox buffer solution were used in the field. If none of the repairs worked, the insulator and Pt were removed and the Pt was used to make a new probe following the previously described method.

Installation and Subsequent Testing
All acceptable probes made with the terminal insulator were inserted into PVC pipe to provide additional protection and rigidity when the probes were in the soil (Fig. 2) . The Cu wire and brazing rod were inserted through a hole in a PVC cap that was narrower than the terminal insulator, which allowed the insulator to be flush with the cap. Epoxy putty was put in the cap to hold the probe in place. The Cu wire and brazing rod were then inserted into a PVC pipe long enough to contain the brazing rod and some Cu wire. The PVC cap was permanently affixed to the PVC pipe with PVC primer and glue. A second PVC cap with a hole was slid over the remaining Cu wire and placed on the other end of the PVC pipe. The hole around the wire was sealed with silicone.

View larger version (21K): [in this window] [in a new window]   Fig. 2. Diagram of a completed redox probe in protective PVC pipe.

Select redox probes were examined in October 2002, 10 mo after installation. Probes that had suspect readings as well as a few probes with more typical readings were removed from the soil and tested in the redox buffer solution. Probes that measured unacceptable redox potentials were replaced with properly functioning probes. Probes that were tested and found to be acceptable were returned to their original location. All the probes were removed on completion of the study (19 mo after installation) and tested again in the redox buffer solution to determine how well they functioned.

Epoxy versus Terminal Insulators
A set of eight (three deep and five shallow) redox probes also was made with marine epoxy resin and hardener. The epoxy was made according to the package directions and applied so that it was not much wider than the brazing rod. After 24 h of drying time, the epoxy was sanded enough to allow the flexible heat-shrink tubing to be slid over the epoxy. The epoxy probes were tested in the same manner as the terminal insulator probes. Epoxy probes that failed testing were disposed of because the epoxy could not be easily removed to salvage the Pt.

The epoxy probes were installed in December 2002, within 1 m of one set of terminal insulator probes. The installation method used for the terminal insulator probes also was used for the epoxy probes. The average deep and shallow redox potentials measured with the epoxy and terminal insulator probes were compared to determine if the different methods produced similar results. All eight epoxy probes were removed on completion of the study (7 mo after installation) and tested again in the redox buffer solution to determine how well they functioned.

	Results and Discussion

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
Probe Construction and Testing
A total of 260 redox probes were made. Fifty-seven percent (151) of the probes were acceptable after initial construction and required no modification. The terminal insulator on each of the 109 unacceptable probes was reheated and squeezed, which made 67 more probes (218 total) acceptable. The terminal insulator was replaced on each of the 42 remaining unacceptable probes, which produced 21 additional (239 total probes) acceptable probes. Heavy sanding of the Pt made 14 of the remaining 21 unacceptable probes acceptable. This brought the total number of acceptable probes to 253, or 95% of the total 260 probes made.

The quick testing methods and the relative ease of repair allowed for testing and repairing of unacceptable probes multiple times a day. The successful repairs involving the terminal insulators were probably a result of eliminating air bubbles under the insulator so the redox buffer solution contacted the Pt and not the brazing rod. We did not examine the terminal insulators to determine if air bubbles formed channels by which the redox buffer solution could contact the brazing rod or to see if the insulators were porous and the bubbles provided a point of contact to the brazing rod. The latter possibility was unlikely, because the same terminal insulators were installed on the probes that properly functioned.

It was possible that the heavy sanding improved some of the unacceptable probes because an oxidized layer may have formed on the Pt during reheating when making repairs. The seven probes that were not successfully repaired were disassembled by reheating and removing the terminal insulator, which could not be easily done with the epoxy-covered junction. The soldered Pt–brazing rod connections were then reheated to remove the Pt and the Pt was reused to make new probes, which saved money.

Probe Installation and Subsequent Testing
The installation of 240 probes (90 at 1.5 m deep, 150 at 0.76 cm deep) was completed in December 2001. In October 2002, probes that malfunctioned, probes suspect of malfunctioning, and a select group of well-functioning probes were tested in the redox buffer solution. Two deep probes that clearly malfunctioned based on field measurements also malfunctioned in the redox buffer solution and were replaced with new probes. Two deep probes had readings that fluctuated more than the majority of the readings and two deep probes that had similar readings to the majority of the probes were tested. All four probes were acceptable in the redox buffer solution. Three shallow probes with widely fluctuating redox potentials and three shallow probes that read similar to the majority of the probes were examined. All six probes were also acceptable in the redox buffer solution. Probes that were acceptable were returned to their original location.

After the study was completed, all of the redox probes were removed and tested in the redox buffer solution. After being in the field for 19 mo, 89 of the 90 deep probes and 147 of the 150 shallow probes were acceptable. These results demonstrate the long-term usability of the probes with little equipment failure. These results also indicate that the thick-walled terminal insulators provided a resistant, water-tight seal because the deep probes were saturated for the entire study period and only three (two at 10 mo, one at 19 mo) showed signs of failure.

Epoxy versus Terminal Insulator
The epoxy probes also were tested after the study was completed and all three deep and five shallow probes were acceptable after 7 mo of use. The redox potentials measured with the epoxy probes and adjacent terminal insulator probes were compared over the time period that the epoxy probes were in use (December 2002–June 2003) (Fig. 3) . The average deep redox potentials measured with the epoxy probes were 23 mV (±85 mV) (n = 3) and –152 mV (±49 mV) (n = 3) with the terminal insulator probes. The average shallow redox potentials measured with the epoxy probes were 241 mV (±189 mV) (n = 5) and 161 mV (±65 mV) (n = 5) with the terminal insulator probes. These results show a slight difference between redox potential measurements made with the deep epoxy probes and the terminal insulator probes; however, the redox potentials were similar between the methods at the shallow depth. The small sample sizes prohibited the use of statistics to determine the significance of differences between the construction methods.

View larger version (30K): [in this window] [in a new window]   Fig. 3. Average redox potential (±1 standard deviation) of the epoxy and terminal insulator methods for the (a) deep and (b) shallow redox probes.

	Conclusions

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

	ACKNOWLEDGMENTS

 
We want to thank the Water Resources Research Institute of the University of North Carolina and the Neuse Crop Management Project (Pew Charitable Trust Foundation and the USEPA) for providing the resources for this research. Dr. Wendell Gilliam's advice was greatly appreciated.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
The use of trade names in this publication does not imply endorsement by North Carolina State University of the products named, or criticism of similar ones not mentioned.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 

• Bohn, H.L. 1971. Redox potentials. Soil Sci. 112:39–45.
• Faulkner, S.P., H.W. Patrick, Jr., and R.P. Gambrell. 1989. Field techniques for measuring wetland soil parameters. Soil Sci. Soc. Am. J. 53:883–890.[Abstract/Free Full Text]
• Gambrell, R.P., R.D. Delaune, and W.H. Patrick, Jr. 1991. Plant life under oxygen deprivation. p. 101–117. In M.B. Jackson et al. (ed.) Plant life under oxygen deprivation: Ecology, physiology and biochemistry. SPB Academic Publ., The Hague, the Netherlands.
• Gambrell, R.P., J.W. Gilliam, and S.B. Weed. 1975. Denitrification in subsoils of the North Carolina Coastal Plain as affected by soil drainage. J. Environ. Qual. 4:311–316.[Abstract/Free Full Text]
• Gambrell, R.P., and W.H. Patrick, Jr. 1980. Chemical and microbiological properties of anaerobic soils and sediments. p. 375–413. In D.D. Hook et al. (ed.) Plant life in anaerobic soils and sediments. Ann Arbor Science Publ., Ann Arbor, MI.
• Jacobs, T.C., and J.W. Gilliam. 1983. Nitrate loss from agricultural drainage waters: Implications for non-point source control. Rep. 209. Water Resour. Res. Inst. of the Univ. of North Carolina, Raleigh.
• Jordan, T.E., D.L. Corell, and D.E. Weller. 1993. Nutrient interception by a riparian forest receiving inputs from adjacent cropland. J. Environ. Qual. 22:467–473.[Abstract/Free Full Text]
• Letey, J., and L.H. Stolzy. 1964. Measurement of oxygen diffusion rates with the platinum microelectrode. I. Theory and equipment. Hilgardia 35(20):545–554.
• Light, T.S. 1972. Standard solution for redox potential measurements. Anal. Chem. 44:1038–1039.
• Mann, L.D., and L.H. Stolzy. 1972. An improved construction method for platinum microelectrodes. Soil Sci. Soc. Am. Proc. 36:853–854.
• Meek, B.D., A.J. MacKenzie, and L.B. Grass. 1968. Effects of organic matter, flooding time, and temperature on the dissolution of iron and manganese from soil in situ. Soil Sci. Soc. Am. Proc. 32:634–638.
• Mueller, S.C., L.H. Stolzy, and G.W. Fick. 1985. Constructing and screening platinum microelectrodes for measuring soil redox potential. Soil Sci. 139:558–560.
• Pang, H., and T.C. Zhang. 1998. Fabrication of redox potential microelectrodes for studies in vegetated soils or biofilm systems. Environ. Sci. Technol. 32:3646–3652.
• Ponnamperuma, F.N. 1972. The chemistry of submerged soils. Adv. Agron. 24:29–96.
• Wafer, C.C. 2004. Effectiveness of groundwater nitrate-N removal by shrub buffers. M.S. thesis. North Carolina State Univ., Raleigh.

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