soil redox potential

  • Soil Redox PotentialLow cost, low maintenance, high accuracy redox electrodes
  • Ideal for the field, glasshouse or laboratory
  • The redox probes can be installed in the field for years at a time
  • Single point or multi point probes for flexible monitoring solutions
  • Connect the probes to our data loggers or connect to your existing system

 

related products

 

Paleo Terra produces sturdy probes to measure redox potential in soils, sediments and surface waters. The probes are built from fiberglass reinforced epoxy tubes and are stable in water and the soil environment over long periods of time. The redox sensing Pt is > 99.95% pure. The probe and cable connection are filled and sealed with epoxy to make a robust waterproof whole.

There are two categories or probes: passive and active probes. A passive probe is a traditional redox potential electrode that requires a high impedance data logger or mV/pH meter. An active probe has an inbuilt amplifier that provides the high impedance. Therefore, an active probe can potentially be connected to lower specification data loggers.

 

passive probe

A passive probe basically consists of a Pt sensor embedded in the fiberglass/epoxy probe and connected to a copper wire. The simplest standard probe is 8 mm in diameter, 30 cm long and has one Pt sensor placed at 2 cm above the pointed tip.

The regular size of the Pt is approximately 0.4 mm width by 12.5 mm length, yielding a surface area of 5 mm2. At an 8 mm diameter probe this forms a half ring around the probe.

The distance between the probe tip and a Pt sensor will often be as small as possible. A Pt sensor must be placed at least 2 cm above the pointed tip, or 1 cm above a flat tip. In some use cases, some extra probe length below a Pt sensor helps to prevent a probe from falling over, e.g. when a probe is used in a river to measure the redox potential only a few cm into the river bed.

At the top end of the probe, a minimal distance of 10 cm between the Pt sensor and the cable exit is recommended for a durable design. Reducing this distance down to 5 cm is possible, at the cost of a weaker cable connection.

In rare cases, if the probe hits a sharper object (e.g. a piece of gravel, a stone or some metal) under an unfortunate angle, the Pt can be pushed out of position. Although it will often still be possible to measure redox potential, and probes can often be repaired, a probe must be considered lost in some cases.

 

multiple sensors

Passive redox probes can be fitted with multiple Pt sensors, at intervals of multiples of 1 cm. On 8 mm diameter probes, a maximum of 10 sensors applies. Up to 18 sensors can be mounted on larger diameter probes. Every Pt sensor is connected to its own wire inside the cable, so a cable with enough cores must be chosen.

Soil pH Electrode

 

probe diameters and lengths

The redox probes come in many different lengths. Standard length is 30 cm, but lengths up to 250 cm are possible (max. 150 cm for 8 mm diameter). Probes up to 90 cm length can be shipped economically via the regular postal service, longer probes can only be shipped via courier services. It is often economic to limit the length of the actual redox probe, and use the probe with extension pieces.

The smallest diameter available (8 mm) is sturdy yet thin enough to push into many soils without the need to pre-auger. The 8 mm probes are most suitable when they can be installed by hand and are rigid up to about 40 cm. Longer 8-mm probes may bend, which can be unfavorable. Still, in many cases, longer 8 mm probes are perfectly suitable. 10-mm probes are more or less rigid up to 70 cm length. 12-mm probes have a thicker fiberglass outer wall and are therefore stronger than the 8- and 10-mm probes. 12-mm probes are recommended for difficult soils, where hammering may be necessary to install the probes. For long lengths in difficult soils, 16-mm diameter probes may be required.

Pre-augering may be necessary to limit the force exerted on a redox probe, but the pro’s and cons of augering must be carefully weighed. To establish a good contact between the soil and the redox sensing Pt, it is often best not to pre-auger, or at least pre-auger using a smaller diameter. In sandy soils, augering with casings that will be removed later on to let the sand collapse onto the probes may be appropriate.

 

reference electrode

Measuring redox potential means comparing the chemical potential at a redox sensor to a standard chemical potential. A referenece electrode provides such a standard chemical potential. Many types of reference electrodes are readily available, most often designed for measuring under laboratory circumstances. Paleo Terra has designed a reference electrode for use in the field, emphasizing a long lifetime in the field, easy servicing and waterproof wiring.

Redox Potential Reference

 

basic parts of a reference electrode

At the heart of a reference electrode is a solid metal | metal ion interface. The type of metal and the metal ion concentration (and temperature) determine the reference chemical potential. A counter ion that forms a very slightly soluble salt with the metal ions is added to the system, at a concentration much higher than the metal ion concentration. This high counter ion concentration in combination with the very low solubility of the metal ion – counter ion salt keeps the metal ion concentration virtually constant. This way, the reference system (solid metal | metal ion) can react and provide a small current to the measuring equipment, whilst its chemical potential remains stable.

Normally, the solution around the reference system must be kept separated from the solution under study, for several reasons:

  • counter ions inside the reference electrode should not flow away in order to keep their concentration stable
  • metal and counter ions inside the reference electrode may not be wanted in the solution under study
  • ions from the solution under study must be kept out of the reference electrode as these may interfere with the reference system

However, a very small electrical current must be able to flow between the reference electrode and a redox (or pH, or ion selective) electrode. The solution is found in allowing a minimal exchange of ions between the inside of the reference electrode and the outside solution. Usually a porous ceramic or glass frit is integrated in the tip of a reference electrode to achieve this.

 

the Ag|AgCl KCl reference electrode

The most common reference electrode consists of a silver (Ag) wire with a silverchloride (AgCl) coating. The wire is immersed in a potassiumchloride (KCl) solution. The KCl concentration varies between electrodes; common concentrations are 1M, 3M, 3.5M and saturated.

 

laboratory electrodes

Laboratory electrodes are of course designed to be accurate under laboratory circumstances. Several common design choices are logical for laboratory circumstances, but make lab reference electrodes less suitable for use in the field:

  • A porous ceramic or glass frit to limit the exchange of ions between the internal reference system and the outside world. These frits limit the flow of ions very effectively. Under lab circumstances, this is important because the volume of samples under study is often small. Ions leaking from the reference system have a relatively larger effect on smaller samples. Similarly, ions from samples do not migrate as easily into the reference electrode, enhancing the electrode service time. In the field however, a porous frit may become clogged relatively quickly with organic substances or clay particles, eventually completely blocking the flow of ions.
  • The wire connection to the reference electrode is often not waterproof to allow for pressure equalisation between the in- and outside of the electrode. When the ambient air pressure changes, equalisation helps to minimize fluid flow through the porous frit. In the lab, it is easy to keep the top end of an electrode dry, so the non-waterproof wire connection is not problematic. In the field however, it is not always as simple to keep the top end of an electrode dry, and a waterproof connection then is more important than pressure equalisation.
  • A filling hole is usually provided to refill the electrode and regenerate the reference system. Under lab circumstances, refilling (without first draining the remaining solution) is often adequate, as deviations of the reference potential can be detected in time. Under field circumstances however it may take weeks to months before a deviation is detected. It may then be necessary to first drain the (polluted) electrode solution before refilling. The filling hole in lab electrodes is often too small to allow for easy draining of the internal solution.
  • A dark coloured housing protects the internal AgCl from decomposition by light. Whilst effective, the dark housing also obscures the reference solution from simple visual checks.

The Paleo Terra reference electrode is designed for use in the field, emphasizing a long lifetime in the field, easy servicing and waterproof wiring. Good accuracy is of course taken into account as well, but did not get the highest priority while making design choices. The accuracy of a well maintained laboratory reference electrode is better than 1 mV. The Paleo Terra field reference electrode is +/- 5 mV accurate. For regular field redox measurements, this should be fine with respect to the total accuracy of redox potential measurements.

The internal electrode solution is separated from the outside world by a threaded plug instead of a porous frit. The threaded plug allows for easy cleaning in case the flowpath becomes clogged. Simply hold the electrode upside down, remove the plug, clean the threading and screw the plug back in place. Also, draining and refilling of the reference solution is easy when the plug is removed.

The replacement of a porous frit by a threaded plug comes with a disadvantage: an increased exchange of ions between the internal reference solution and the outside world. In the field, increased leakage of KCl to the soil / ground water will usually not pose a problem in terms of pollution of the sample under study. However, loss of ions from the reference system could be problematic. This is countered by using saturated KCl as filling solution, with an excess amount of solid KCl. As long as excess KCl is present, the reference system will be stable. Also, where most lab reference electrodes have a silver wire coated with a limited amount of AgCl at their heart, the Paleo Terra field reference electrode uses a silver wire surrounded by plenty AgCl inside a separate compartment. The transparent PMMA electrode bodyallows for easy visual inspection of the reference system and presence of solid KCl.

As for the redox probes, the connection between a field reference electrode and its cable is waterproof. A strong PUR sheathed coax cable is standard. The waterproof connection makes pressure equalisation on the cable side impossible. When ambient air pressure changes, pressure equalisation could increase the flow of solution in or out of the reference electrode. To minimize this, the electrode is designed to be easily filled without air bubbles. Also, the use of saturated KCl helps in this respect as less oxygen is soluble at higher salt concentrations.

 

cable options

Soil Redox Data LoggerA wire or cable is a fixed part of the redox probes to guarantee a waterproof design. The standard cable exit is from the top end of the probe like on most regular lab electrodes. Alternatively, the cable may exit the probe from the side, through the addition of a nylon top cap. The top cap allows to hammer on top of a probe. Also, the cable exit to the side may be advantageous if the probe and cable must be buried completely, but as shallow as possible.

Standard cable length is 3 meter, but any practical cable length is possible. When determining your required cable length, don’t forget to include some cable for routing into an enclosure if applicable.