A reference electrode is indispensible to measure redox potential: the chemical potential at a redox sensor is compared to a standard chemical potential provided by a reference electrode. 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 strong waterproof wiring.
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, e.g. 1M, 3M, 3.5M or saturated.
Laboratory electrodes are of course designed to be accurate under laboratory circumstances. Several common design choices are good 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 electrode and the outside world. Pressure equalisation prevents an increased flow of filling solution through the porous frit as would be caused by changes in ambient air pressure. In the lab, it is easy to keep the top end of an electrode dry, so a leaky 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 system from simple visual checks.
Paleo Terra field reference electrode
The Paleo Terra reference electrode is designed for use in the field, emphasizing a long service life in the field (if serviced regularly!), easy servicing and strong waterproof wiring. 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 +/- 3 mV accurate. For regular field redox measurements, this should be fine compared to the total accuracy of redox potential measurements.
The picture above shows the Paleo Terra field reference electrode with the following parts from left to right:
- Strong PUR sheathed coax cable
- Cable is fixed waterproof in a cable gland
- Threaded connection between cable gland and polycarbonate electrode body
- Main electrode body with Ag|AgCl (saturated KCl) reference system cartridge
- Threaded connection acting as the liquid junction
- Sub electrode body with solid KCl
The internal electrode solution is separated from the outside world by a liquid junction formed by the accurately finished ends of the main and sub electrode body parts being pushed together in a threaded connection. This liquid junction resembles a classic ground-glass tapered sleeve and allows for easy cleaning in case the flowpath becomes clogged.
The choice for a saturated KCl reference system (as opposed to the regular choice for 3M or 3.5M KCl) results in a slightly reduced accuracy and a slightly 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 can be problematic in terms of stability of the electrode potential. This is countered by adding room for an excess amount of solid KCl. The standard sub electrode body holds around 1 gram of excess KCl; larger parts are available to allow for more solid KCl to be stored inside the electrode. As a rule of thumb, the Paleo Terra field reference electrodes leak around 5 mg of KCl per day. As long as excess KCl is present, so normally 3 - 6 months, the reference system will be stable. After this period, refreshment of the internal KCl solution and the addition of 1 gram of excess KCl to the internal solution will make the reference ready for the next few months.
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 electrode body allows for easy visual inspection of the reference system and presence of solid KCl.
The connection between a field reference electrode and its cable is waterproof as for the redox probes. A strong PUR sheathed coax cable is standard. The waterproof connection makes pressure equalisation on the cable side impossible. The absence of pressure equalisation on the cable side could increase the flow of solution in or out of the reference electrode on ambient air pressure variation, especially if air bubbles are present inside the electrode. Therefore 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.
As wet as possible
To minimize the risk of drying out of the reference electrode, it is best to install the reference electrode fully submerged, into the groundwater or in a nearby surface water. If this is not possible, try to install the reference electrode in a place as wet as possible.
Check / adjust the liquid junction
The transparent electrode body consists of two parts, joined by a threaded junction. The ends of the two parts being pushed together in the threaded connection form the liquid junction. When not in use, the connection can be covered with parafilm or clingfoil to minimize the leakage of KCl. The protective film must be removed before use. The two parts of the electrode body should be screwed together, but not too tight. The best way to check is by measuring the electrical resistance between two reference electrodes with an electricians multimeter. The resistance should not exceed 100 kΩ. Or measure the redox potential in a glass of tap water with your pH/mV meter. The liquid junction of the reference electrode used is good when values are not "jumping" up and down.
Distance from redox electrode(s)
In order to measure the potential between a reference and redox electrode, the electrical resistance of the soil (or water) between the electrodes must be small compared to the input resistance of the measurement device. Assuming an input resistance of at least 10 GΩ, and considering normal soil resistivity values of 25 - 2500 Ω-metre, any practical distance between reference and redox electrode is fine. Dry, sandy, poor soils may be the exception where it is best to place reference and redox electrode within short distance.
As explained before, the flow of ions from the reference electrode internal solution to the outside world is limited. As a consequence (not mentioned yet), a junction potential may develop if the flow of positive (K+) ions does not equal the flow of negative (Cl-) ions. The junction potential, usually a few mV at most, introduces an offset to the reference electrode potential. Measuring soil redox potential, this error of a few mV is usually negligible. However, in a special case, the junction potential may be much larger, tens of mV, perhaps more than 100 mV! If the reference electrode is immersed in a suspension with negatively charged (clay) particles, the flow of cations exceeds that of the anions, resulting in a large junction potential. To prevent this effect, do not install a reference electrode directly into a soil with charged particles (e.g. clay soils). It is recommended to install the reference electrode inside a dipwell into clean groundwater if possible. Apart from a small junction potential, installation into clean groundwater also often reduces the resistance between reference and redox electrodes, and enhances the reference electrode field life time as the electrode cannot dry.
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.
A Paleo Terra field reference electrode needs regular checking and maintenance. Preferably check the electrode once a month, but at least once every three months:
- clean the electrode with a piece of cloth to remove dirt, build-up of micro-organisms, etc.
- check the filling solution, it should be a clear and colourless liquid
- check for the presence of excess solid KCl, a white solid, in the filling solution
- check for the absence of air bubbles
- check the cable for any damage
If the filling solution is not longer clear and colourless and/or if the excess KCl is not white anymore, the internal filling solution and excess KCl need to be replaced. If air bubbles are present, the internal filling solution must be replenished. Once an air bubble is present, the internal fluid will drain from the electrode faster as ambient air pressure changes will make the air bubble want to expand and contract. Depending on the checking interval, it may be necessary to replenish the excess KCl as well. 1 gram of solid KCl (a layer of bout 1 cm inside the reference electrode) lasts for 3-6 months.
Keep the following at hand to service a Paleo Terra field reference electrode:
- piece of cloth or cleaning paper
- 15mm spanner
- pasteur pipette or syringe
- solid KCl >99% pure
- saturated KCl solution
Refilling the electrode
The reference electrode is filled with a saturated KCl solution, and about 1 gram of excess KCl. If only replenishing the internal solution to remove an air bubble, it is possible (but not preferable) to add some destilled water as long as plenty excess KCl is present. Many lab reference electrodes need KCl solution saturated with AgCl. This is not necessary for the Paleo Terra field reference, as plenty AgCl is present in the separate compartment inside the electrode.
To refill the reference electrode, proceed as follows:
- prepare a saturated KCl solution: add 40 gram solid KCl to 100 ml distilled water; stir or shake until almost all KCl has dissolved; at 20°C, 34 gram KCl can dissolve in 100 ml water, and 37,4 gram at 30°C
- clean the outside of the electrode body
- unscrew the top cable gland with reference element from the clear electrode body; use a 15mm spanner to hold the cable gland; be careful not to open the cable gland itself
- clean the outside of the reference element; if possible, keep the element wet during the remainder of this procedure
- if the KCl solution may be polluted (discoloration of the solution and/or presence of another solid than white KCl): dispose of the solution and rinse with fresh KCl solution
- if the threading at the liquid junction is polluted: unscrew, clean and re-assemble
- if solid KCl must be replenished: (re)fill with saturated KCl solution until the threading at the liquid junction is fully submerged, then add solid KCl; 1 gram lasts 3-6 months
- top up with saturated KCl solution
- Place the reference element into the solution and screw into place; some KCl solution will overflow. Fasten tight, but not overly tight. If not fastened tight enough, too much KCl will leak from the electrode. If fastened too tight, the electrical conductivity through the liquid junction will be too low, resulting in bad measurements. Ideally, if two reference electrodes are placed in the same bottle with KCl solution, the resistance between both electrodes as measured with an electricians multimeter would be between 20 and 100 kΩ.
Clean the electrode and refresh the internal filling solution as detailed above. Then either store the electrode in a bottle with saturated KCl or seal the threaded plug with e.g. laboratory film or a closely fitting protective cap.