Redox probe cleaning, maintenance and calibration
Cleaning and polishing
- Piece of cotton cloth
- Soap, ethanol and/or mild acid (0.1M HCl)
- Distilled water. If you have no access to a lab with distilled water, use demineralized water as sold for refilling a steam iron or car battery
- Waterproof abrasive paper P1200 grit or higher
Redox probes do not normally need maintenance, but should be cleaned before use and whenever retrieved from the soil. Use soft cloth and general cleaning agents, no more aggressive than necessary: water, water and soap, ethanol or mild acid (e.g. 0.1M HCl). Rinse with distilled water after cleaning.
Probes that have been used in an oxidising environment may respond slowly to reducing environments. Probes that have been in contact with sulfides may be 'poisoned', meaning a thin layer of PtS may have formed. In both cases, polishing the Pt surface with abrasive paper P1200 grit or higher exposes a fresh Pt surface. Always wet polish as fiberglass dust is not healthy.
Calibration of a redox probe in practice boils down to calibration of a set consisting of a mV meter, a reference electrode and a redox probe. The reference electrode and redox probe are placed in a calibration solution and the resulting voltage is measured with the mV meter. If the mV meter reads the expected value for the calibration solution, the set calibrates ok and it is reasonable to assume all parts work. However, this is not 100% sure. It is possible the mV meter has an offset opposite to a reference electrode error.
mV meter (or data logger)
It is recommended to run a few simple tests on the mV meter before starting the actual calibration. Although thorough testing of the mV meter requires specialized equipment, the following simple tests will give a quick indication whether your mV meter is working fine.
- Short both inputs of the mV meter by connecting a short wire between the redox probe (+) and reference electrode (-) input. The mV meter should read 0 mV. As soon as the wire is removed again, the meter should display "random" values or show an out-of-range error
- Measure a 1.5V alkaline battery with the mV meter. A brand new cell should read around 1.6V, an old used cell normally reads between 0.9 and 1.4V. If you want a second mV meter to compare your meter with, get a cheap digital electricians multimeter. These cannot be used to measure redox potential, but will measure the voltage of a new or used battery just as well as your lab mV meter. The accuracy of a new multimeter is often very good with errors no larger than 1 or 2 mV.
- Measure the same battery the other way around (swap + and -). The mV meter should indicate the exact opposite value, maybe with a difference of 1 mV.
Wiring and connectors
Redox measurements can easily fail due to sloppy wiring. Make sure all wires and connectors are in good shape:
- Check wire insulation for damage
- Make sure all wire ends and connectors are clean and dry
- Take care that uninsulated parts of the wiring (e.g. crocodile clips) cannot leak current; air is a very good insulator, try to suspend uninsulated parts
- Corroded wire ends or connectors may cause an offset
Redox probe calibration
To calibrate a redox probe, connect a clean redox probe and reference electrode to a mV meter and place both the redox probe and reference electrode in a beaker filled with calibration solution. The mV meter should display a stable result within 10 seconds. The value obtained should lie within ± 10 mV from the expected value. This value depends on the calibration solution used, the type of reference electrode used and temperature (see tables below).
If the calibration is not ok, the error usually originates from the redox probe and/or from the reference electrode. In rarer cases, the mV meter or wiring may cause an error. Assuming the mV meter has passed the simple tests above and the wiring is fine, first try cleaning and/or polishing of the redox probe. If calibration still fails, check the reference electrode (see below).
Four common calibration solutions are Light's Solution, Zobell's Solution, Quinhydrone in pH 7.0 and Quinhydrone in pH 4.0. These solutions differ in toxicity and 'redox strength' (poise). Toxicity must be considered when handling and disposing of the solutions. Poise is important because an unsatisfactory redox probe may give accurate readings in a highly poised solution, whereas the same probe may fail calibration in a lightly poised solution. Many soil systems do not have a high poise, so it is important to calibrate a redox probe in a lightly poised solution.
- Light's Solution is made up from ferrous and ferric ammonium sulphate in sulphuric acid. Add 372 mg Fe(NH₄)₂(SO₄)₂ * 6 H₂O and 482 mg FeNH₄(SO₄)₂ * 12 H₂O to 100 mL 1 M H₂SO₄ while stirring. Light's Solution is very acidic (pH < 2) and highly poised. The high poise makes Light's Solution a moderate choice.
- Zobell's Solution is made from potassium ferri- and ferro- cyanide in pH 7 buffer. Add 528 mg K₄[Fe(CN)₆] * 3 H₂O and 412 mg K₃[Fe(CN)₆] * H₂O to 100 mL buffer pH 7 while stirring. Zobell's Solution is moderately toxic and hydrocyanic acid gas may form when mixed with acid. Zobell's solution is commercially availalbe but has a limited shelf life and must be stored away from light. It is commonly used for testing redox probes; sometimes in a 100:1 dilution to get a lightly poised solution.
- Quinhydrone Solutions are simply made by mixing around 1 gram quinhydrone in 100 ml pH 4 or pH 7 buffer solution. After 15 minutes of mixing, undissolved quinhydrone must remain. Quinhydrone is toxic. The solutions are lightly poised and must be used the same day.
- Finally, simple tap water must be mentioned. The calibration solutions above are desgined with a certain poise to get a calibration solution with a well defined redox potential. However, a good redox probe should also work fine in a very lightly poised environment. Simple tap water is a very lightly poised solution. As a result of the low poise, the redox potential of tap water is not as well defined as for the calibration solutions. Normal redox potentials for tap water lie between +200 and +450 mV, mostly between +350 and +450 mV. When the redox probe is calibrated versus a Ag|AgCl saturated KCl reference electrode, the mV meter would (almost) stabilize after a few minutes and display between 150 and 25s0 mV (or between 0 and 250 mV if your tap water is not as average). A reading in this range indicates the redox probe can work fine even in a low poise environment. Usually, the potential indicated keeps on increasing for a few hours or even days, due to the slow formation of a thin Pt-oxide layer on the redox probe surface.
|Calibration values for Zobell's Solution, quinhydrone in pH 7 and quinhydrone in pH 4|
|Reference electrode||Temperature||Zobell's Solution||Quinhydrone in pH 7||Quinhydrone in pH 4|
|Ag|AgCl 3M KCl||20°C||229 mV||87 mV||265 mV|
|Ag|AgCl 3M KCl||25°C||221 mV||82 mV||261 mV|
|Ag|AgCl sat'd KCl||20°C||239 mV||92 mV||268 mV|
|Ag|AgCl sat'd KCl||25°C||231 mV||86 mV||263 mV|
The easiest way to calibrate a reference electrode is to compare it with another reference electrode that is known to be good. In practice this means it is a good idea to get an extra reference electrode that is kept for calibrating other reference electrodes only.
- Fill a beaker with tap water and place both reference electrodes in the water.
- Connect one reference electrode to the regular mV meter reference input (-) and connect the other reference electrode to the input normally used for a redox probe (+).
- The difference between the references would ideally read 0 mV. In practice, ± 5 mV is fine for measuring soil redox.
- If the meter reading is stable, but too far from zero, most likely the KCl concentration of the internal solution has changed. Refill the reference electrode with fresh internal solution, leave the electrode overnight, and try again.
- If the meter reading is unstable, probably the liquid junction of one (or both) of the electrodes is blocked. The liquid junction can be a ceramic or glass porous frit, or the threading of a plug as for the Paleo Terra field reference electrodes. Try to clean the liquid junction; procedure depends on the type of junction and electrode construction. In case of the Paleo Terra field reference electrodes, it is also possible that the bottom cap is installed too tight, blocking all contact between internal solution and outside world. Try to loosen the cap a little.