Measuring redox potential
In most soils and natural waters, chemical species are present that can take part in redox reactions: reactions which involve the exchange of electrons between different chemical species. The reacting species all contribute to the redox potential (also called Oxidation Reduction Potential, ORP) of the system. To measure the redox potential, an inert electrode (i.e. the electrode itself does not react) is introduced into the system. A Pt electrode is commonly used, but e.g. Au, Ti and glassy carbon electrodes can also be used. The chemical potential at the redox electrode is compared to the chemical potential of a reference electrode in a voltage measurement.
A reference electrode consists of a chemical system at a fixed chemical potential. Several types of reference electrodes exist. The Standard Hydrogen Electrode (SHE) is chosen to be, by definition, the zero-point of chemical potentials. As the SHE is not convenient for daily use, other reference electrodes as the Saturated Calomel Electrode (SCE) and Ag|AgCl KCl electrode are normally used. The potential of the common Ag|AgCl 3M KCl reference electrode at 20°C equals +213 mV versus the SHE. When the voltage measurement of a redox electrode against the Ag|AgCl 3M KCl reference reads +60 mV, the redox potential equals +273 mV vs. the SHE.
The redox potential is sensed by the inert electrode via the electrons exchanged between the redox active species. Ideally, the reacting redox couples are in equilibrium and both the forward and reverse reactions are abundant. If so, the exchange current is relatively large, and the redox potential can be measured accurately. The FeII/FeIII redox couple, commonly present in soils, has a relatively large exchange current. The exchange current associated with e.g. oxygen is much smaller. In practice, most (if not all) natural systems are not in a state of equilibrium. Therefore, a redox potential measured in a natural system, is a mixed potential. The potential of the inert electrode is a response to all redox active species present, weighted by the exchange current of the redox couples.
Voltmeter input resistance
Thanks to the exchange currents at the inert redox electrode surface, it is possible to electrically measure a redox potential. In an ideal world, it would be possible to measure the chemical potential at the redox electrode without affecting the system being measured. However, measuring the voltage between a redox and reference electrode in practice means that electrons are moved to/from the redox electrode. This flow of electrons must be compensated for by the redox active species. If the chemistry can't keep up with the measurement demand of electrons, the redox potential of the system will change due to the measurement. Compare e.g. starting an automobile with the lights on. When the starter motor is actuated, the lights become dimmer as the terminal voltage of the battery drops. The amount of electrons needed by a voltmeter is specified by its input resistance. A higher input resistance means less electrons needed.
The ISO standard 11271:2002 Soil quality - Determination of redox potential - Field method states that one needs a millivoltmeter with an input resistance not less than 10 GΩ. Laboratory mV/pH meters should easily meet this specification. Several dataloggers meet this specification as well, but many general (milli)volt dataloggers don't. General purpose oscilloscopes, development boards (e.g. Arduino) and electricians multimeters do not have a high enough input resistance. Modern digital multimeters (DMMs) may be an exception to this rule as the input resistance of some models is surprisingly high.
Focussing on voltmeter input resistance alone is not enough. Current (i.e. electrons) can also leak through cable isolation, connectors, etcetera, lowering the measurement overall input resistance. General purpose PVC insulated multicore wiring has a relatively low insulation resistance. The resistance of a five meter length may be as low as 5 GΩ. Single core thick sheathed PVC wiring of the same length reaches 60 GΩ insulation resistance. PE or PP isolated wiring performs much better, with an insulation resistance of over 1 TΩ for five metre of cable. PE insulation is usually found around the inner core of coax cables and in specialized multicore cabling.
The range of redox potentials found in the natural environment is limited by the stabilty of water. At a very high redox potential, water would decompose into oxygen and protons. Likewise, at a very low redox potential, hydrogen would form. As the decomposition reactions of water involve protons, the redox potential at which water decomposes varies with pH. At pH around 7 to 8, the redox potential may vary between -400 and +800 mV versus the SHE. At pH 4 (acidic), redox potentials may rise to +900 mV; at pH 9 (basic) redox potentials may be as low as -600mV.
As redox potential in the natural environment are usually mixed potentials, it is not very straightforward to relate a certain redox potential to the presence or absence of certain chemical species. Different indicative values are found in the literature, depending on measurement methods, pH and other natural circumstances. However, very roughly, measured redox potentials can be related to a main electron acceptor in the soil:
|Redox potential||Main electron acceptor|
|+400 to +700 mV||oxygen|
|-50 to +100 mV||iron|
|< -200 mV||carbon dioxide|
Unfortunately, the common use of the mV as unit for redox potential measurements suggests an accuracy of 1 mV to be realistic. Perhaps in carefully designed laboratory experiments, but not so in the field. Measurement errors come from many sources, and keeping all of them < 1 mV in the field would be unrealistically expensive. Keeping a reference electrode within 5 mV accurate in the field will already require regular (weekly to monthly) maintenance. Millivoltmeter or datalogger accuracy will typically be around +/- 2 mV. More important is maintaining the very high input resistance (and wiring insulation resistance!). Moisture is the ever present enemy to electronics in the field, condensation on cooling during the night could easily lower the input resistance by a few orders of magnitude. Rodents and visitors often keep attacking your cables, threatening the insulation resistance.
On a more positive note, realistic measurement accuracy and data interpretation seem to go well together. An overall measurement accuracy of +/- 10 mV should be possible to attain, and allows for interpretation on a scale of 10 to 50 mV intervals.
The following is written with the terminology of Campbell Scientific (CS) dataloggers in mind. Being general purpose dataloggers, wiring doesn't follow one prescribed pattern, so users must understand the choices they need to make. The wiring of equipment specifically designed to measure redox potential usually follows a prescribed pattern. Still, it may be useful to read through the following as background information.
Ground, Analog Ground and Earth Ground
Grounding and earthing are sometimes difficult to understand, mostly because similar terminology is used for four different matters:
- Return of power supply current. As current always flows in a loop, every power supply comes with a return path.
- Zero-point of voltage measurements. A potential cannot be measured by itself, but is always compared to a known potential.
- Weak electrical signals as redox potentials can be affected by electrostatic and electromagnetic interference. Shielding can protect signals from interference.
- Earthing as a safety measure prevents dangerous voltages on parts of equipment that can be touched, and may help to keep lightning induced currents away from sensitive parts. (A direct lightning strike contains so much energy that it would still destroy your equipment.)
Sometimes, it is possible to combine all four functions into one ground wire. More often, as on CS loggers, different types of ground wiring are available. They may, in the end, all connect together, but not necessarily.
- G - Ground - serves as return path for power supplies. It can handle the amount of current usually needed to power sensors.
- AG - Analog Ground - provides the zero for Single Ended voltage measurements. It is separated from G for more stability.
- Earth Ground is intended to connect to the physical earth. Often, as in CS loggers, it is connected to the other ground connections through a high value resistor. Wiring to the physical earth should be relatively thick to allow for easy flow of current (low impedance). Both effective interference shielding and some protection against lightning induced currents require a low impedance path to the physical earth.
Single ended or differential measurements?
To measure the voltage between a redox electrode and a reference electrode, both Single Ended (SE) and Differential (DIFF) voltage measurements are available on CS loggers. To measure SE, a redox electrode is connected to a SE input. The logger measures the SE input versus AG and a refererence electrode must be connected to AG. To measure DIFF, a redox electrode is connected to the high (H) input of a DIFF channel, and a reference electrode to the corresponding low (L) input. Usually, twice as much SE inputs are available on a datalogger as DIFF inputs, so more redox potentials can be measured with one logger. On the contrary, DIFF measurements are usually more accurate than SE, although the difference is often not significant compared to the overall accuracy of soil redox measurements.
As twice as much SE inputs are usually available as DIFF inputs, it seems obvious to always measure redox potential using SE inputs. However, the need to connect the reference electrode to AG comes at a cost: no other connection between datalogger ground and the physical earth is allowed. When using DIFF measurements however, the same precautions are recommended, although not strictly necessary. So in the end, there may not be much difference between using SE or DIFF inputs apart from the number of inputs on one datalogger.
One or more reference electrodes?
One reference electrode can provide a chemical reference potential to more than one redox electrode, as long as electrically conductive paths exist between the reference and redox electrodes. One field plot with tens of redox electrodes can be served by one reference electrode. However, if an experiment uses isolated plastic columns or containers, one reference electrode per column or container is needed. Alternatively, it is possible to use one reference electrode and connect the separate compartements via salt bridges. In practice though, salt bridges are not always easy to use: junction potentials and algae growing on the agar are common problems.
Relying on one reference electrode alone may not be a good idea. If this one reference electrode would fail, all measurements of an experiment would be useless. Therefore consider to install at least two reference electrodes. If the experiment is divided over several isolated containers, consider a second reference electrode in each container. Each second reference electrode may be connected as if it were a redox electrode. As long as the potential between both reference electrodes reads around 0 mV, it is likely that both reference electrodes are still fine. When the potential difference between both references becomes larger, it must be investigated which reference electrode is failing. Even if the primary reference electrode has failed, it is still possible to correct measurements using the potential of the second reference electrode.
For SE measuring, connect the reference electrode to AG. Connect each redox electrode to a SE input. If an extra reference is installed to check the working of the first reference electrode, connect the second reference to a SE input (as if it were a redox electrode). If the experiment is divided over several isolated containers, at least one reference electrode per container must be installed and connected to AG (so several reference electrodes will be connected together to AG). Each second reference electrode in an isolated container must be connected to a separate SE input.
Using SE inputs, with the reference electrode connected to AG, it is essential that no other electrical connection between the datalogger and the physical earth exists. Any additional connection would likely present a different potential to the earth than the reference electrode potential, and would cause a ground loop current through the reference electrode. Sooner or later, this ground loop current would exhaust the reference electrode chemistry, and thus change the reference electrode potential. See under Isolating the system from the physical earth for the practical implications.
For DIFF measuring, the reference electrode is connected to all Low inputs of the DIFF channels used. Use jumper wires between L inputs. If the experiment is divided over several isolated containers, each reference electrode is connected to the L inputs of the DIFF channels used for redoxes in that same container. If a second reference electrode is present to check on the first reference electrode, connect them to one DIFF channel, the first reference to the L input, the second reference to the H input.
When measuring via DIFF channels, it is still recommend to avoid other electrical connections between the datalogger system and the physical earth. As the reference electrode is connected to an input with high resistance, the risk of a ground loop current destroying the reference electrode is now absent. However, the redox measurements may still be affected via another mechanism. Each time an electrical connection between logger and earth is made or broken, the potential of the logger system versus the earth changes. This includes the potential of the redox probes cables. The capacitance of the cables causes a (very small) current to flow into the cables. As the datalogger inputs are high resistance, most of this current will flow through the Pt sensor, affecting the redox equilibrium at the Pt.
The magnitude of this effect will change with cable type, cable length, soil type, chemistry of the redox active species, etc. The frequency of the potential change is also important, conductivity sensors for example may send out an alternating current purposely. Redox potential changes of a few mV easily occur, so observing the precautionary principle it is recommend to also isolate the logger system from the physical earth when measuring DIFF. Another approach may be to first install the redox sensors, measure them for a while, and then check the effect of connecting other sensors or equipment on the redox readings.
Isolating the system from the physical earth
- Run the datalogger off batteries, without any (charger) connection to mains power. Mains power supply is always connected to the physical earth somewhere.
- Do not connect a computer or other equipment which is in turn connected to the earth. It is safe to connect a wireless modem powered from the same batteries as the datalogger, or to connect a laptop computer running off its internal batteries.
- Only connect other sensors to the datalogger if these have no electrical connection to the physical earth. A temperature sensor fully encapsulated in plastic can safely be connected. A submerged conductivity sensor is obviously not electrically isolated from the physical earth and should not be used.
- Isolate metal parts of the datalogger itself (and of any connected devices) from the earth. This includes not connecting the earth ground, even though contrary to CS recommendations. CS advises the datalogger to be grounded to earth for maximum input protection and safety. However, to properly measure redox potential a decrease in input protection cannot be avoided. To isolate the datalogger from the earth it is best placed inside a plastic box. Then place the plastic box in an outer metal enclosure. Make sure to properly earthen the metal enclosure, to restore (to a certain extent) input protection and safety. Connect the redox probe cable shields to the metal enclosure where the cables enter the outer metal enclosure. Use a ground rail or EMI cable glands. Note that not grounding the datalogger directly to the earth still reduces the overall safety of the system, e.g. in case of lightning. Take extra care if a thunderstorm is nearby. Do not get near the datalogger system: sensor wiring may provide a path for lightning induced currents into the datalogger system.
A multiplexer is a device to connect sensors one after another to the same datalogger input. CS has the AM16/32 multiplexer. Apart from the obvious advantage of attaching more sensors to one datalogger, the CS multiplexer comes with another advantage for redox measurements. The AM16/32B uses relays to connect sensors one after another. As the resistance of an open relay is much higher than the logger input resistance, the time-average input resistance of the system is also much higher. Especially if the exchange current from redox active species at the redox electrode is relatively low, e.g. in poorer, dryer and/or more sandy soils, the quality of the redox measurements may improve.
Datalogger system maintenance
Dataloggers are designed to be left alone and do their job for months or perhaps years in a row. Still, many things can go wrong in the field; moisture, empty batteries and rodents are usually the main risks. When installation is ready, a first return visit could be scheduled after one week, the second check-up no longer than one month later, and then a regular schedule based on importance of the experiment, distance to the field site, possibility to check the system remotely, etc.
Moisture often causes damage, not only by water leaking into the enclosure, but also because of condensation. Warmer air inside the enclosure cools down during the night with condensation as a result. Include silica gel in the datalogger enclosure to absorb moisture and prevent condensation. If a system is remotely located and data transmitted, consider adding an RH sensor (e.g. the CS210) inside the enclosure. Enclosre and cable glands may be IP67/IP68 rated, it is still difficult to install everything fully waterproof. A logger inside a plastic inner box, in turn inside a metal outer enclosure, as recommended for safety grounding, also helps to keep water away from the logger. Make sure not to pinch wires when securing the lid.
Redox probe installation
The redox electrodes are strong, still install them with care. Too much force on the probes may cause the probes to bend and possibly break. As the outer wall of the probes is made out of fiberglass reinforced epoxy, a damaged probe may be very sharp!
Push or hammer
In soft soils as peat, unripened clay and rice paddies, the probes can often be pushed into the soil by hand. More difficult soils may require hammering, this is only possible on probes fitted with a nylon top cap. Preferably use a nylon hammer. If a probe gets stuck on a stone, stop further hammering. If pre-augering, try to use an auger with a diameter slightly smaller than the probe, to ensure good contact between the Pt sensors and the soil. Probes have been installed in sandy soils by first augering through a tube, then installing the probe inside the tube, and finally removing the tube to make the sand collapse onto the probe.
Field installed cabling is often attacked by rodents. Good protection is possible using conduit. Standard plastic and metal conduit is available in many sizes; polyethylene tubing as used for drinking water may be a strong and still flexible alternative. If for any reason conduit will not be used in the field, at least use probes with robust PUR multicore cables.
Redox signals are weak electrical signals. Therefore, electromagnetic interference (EMI) may have a significant impact on the measurements. Some EMI is almost always present, but e.g. power stations, electric motors and welding equipment cause a lot of EMI. Cable shields help to protect signals from EMI, but only if the shields are connected properly. Inside the probes, the shields are not connected. They should be connected to the earth on the datalogger side. If using an earthed metal (outer) enclosure, connect the shields to the enclosure, preferably using EMI cable glands. The metal enclosure and cable shields then make up a continuous EMI shield.
Retrieving a probe can be difficult. Pulling from the cable often results in damage. 8mm probes with igus CF2.01.04 cable, without nylon top cap, are your best bet if pulling from the cable will be the only option. Ask for extra strong embedded cables when ordering. 16mm probes can be made with a thin strong rope to pull the probe.