Knowledge Base
Frequently Asked Questions
Here is a list of some frequently asked questions by our clients. If you don’t find the answer you seek, please feel free to contact us.
Soil Redox probes
Materials:
- Piece of cotton cloth
- Water and soap, ethanol and/or mild acid (0.1M HCl)
- Distilled water
- Waterproof abrasive paper: P1200 grit or higher
Maintenance of the Redox electrode(s) – Pt ring(s) – of the SWAP soil Redox probes is relatively simple. They should always be cleaned before use and whenever retrieved from the soil. Use soft cloth and general cleaning agents: water and soap, ethanol and/or mild acid (e.g. 0.1M HCl). Do not use chemicals that are more aggressive than necessary. Rinse with distilled water after cleaning.
Probes applied in oxidising environments may respond slowly when subsequently used in reducing environments. Probes may be ‘poised’ when they have been in contact with sulfides: a thin layer of PtS may have formed that influences the Redox measurements. Polishing the Pt surface with abrasive paper P1200 grit or higher may solve these problems, since this generates a fresh Pt surface.
Caution: always wet polish as contact with fiberglass dust from the probes is undesirable.
The SWAP soil Redox probes are delivered uncalibrated. It is possible to establish an external calibration by measuring 1 (offset) or more (calibration line) Redox/ORP test solutions with specified Redox/ORP values. This can be done with any suitable Redox test solution (e.g., Zobell’s Solution, Quinhydrone Solutions and/or tap water). There are also ORP test solutions on the market that contain/consist of sulphuric acid with acidities as low as pH < 1. We do not recommend these test solutions since they can dissolve the glass fibre epoxy housing of the SWAP soil Redox probes.
External calibration can be performed as follows:
Materials:
- SWAP soil Redox probe
- readout unit or datalogger
- Redox calibration / test solutions
External calibration
- Connect the SWAP soil Redox probe (clean the probe before calibration) to the readout unit or datalogger.
- Place the probe in a beaker filled with the Redox calibration / test solution. Make sure that the reference electrode, the Redox electrode (Pt ring) and temperature sensor of the SWAP soil Redox probe are fully immersed in the solution. Register/save/store the measured Redox potential and temperature (when the signal is stable).
- Repeat step 2 for every Redox calibration / test solution. Rinse (with demi water) and dry your probe before inserting into a new Redox calibration / test solution.
- For calibration / test solutions with a high ‘Redox strength’ (poise) the mV reading should be stable within 30 seconds. When a calibration / test solution with a low poise is used – like tap water – it might take an hour before the mV reading is stable. In general, Redox species in tap water react slowly with Pt.
- Determine the offset when 1 calibration / test solution is used or the slope, intercept and correlation coefficient of the calibration line when at least 2 calibration / test solutions are used
Note: the mV reading depends on the calibration / test solution used, the type of reference electrode used and temperature.
Till May 2025 SWAP soil Redox probes had integrated 3.0 M KCl gel Ag|AgCl reference electrodes. After this date, this has been changed to 4.76 M KCl (@ 25°C).
Uncontrolled quality
When the calibration data show deviating values from the quality criteria, the error can, for example, be caused by:
- A contaminated or expired calibration / test solution.
Check the expiry date of the calibration / test solution. Use for every calibration fresh solution to avoid contamination. - A fouled Redox electrode (Pt ring).Go to FAQs – How can I clean my SWAP soil Redox probe?
- Problems with the reference electrode. Go to FAQs – How can I maintain my SWAP reference electrode?
- Problems with the readout unit or datalogger. Check the wiring of the readout unit or datalogger.
Redox measurements are influenced by temperature, but in general cannot be compensated for this effect. Redox measurements reflect all oxidation and reduction reactions taking place at the surface of the Redox electrode. Since many chemical species can be involved in these reactions – each with their own temperature dependency – it is not possible to define a single temperature compensation equation.
However, if a sample consists of a single or dominant (high poise) redox species, temperature compensation can be applied. For example, Redox test (calibration) solutions often consist of a chemical species with a high poise. The temperature influence of Consort Redox test solution 358mV, for example, is established at approximately -1.2 mV per increase of 1 °C (between 5 and 40 °C). Also Redox measurements in chlorinated water can be corrected for the influence of temperature.
Feel free to contact us if you are interested in methods to establish temperature correction factors for Redox measurements performed with our SWAP soil Redox probes.
Reference electrode
SWAP instruments offers both integrated (e.g., in SWAP soil redox probes) and stand-alone reference electrodes. These are all saturated (4.76 M @ 25°C) KCl Ag|AgCl double junction reference electrodes. Although, SWAP reference electrodes are designed for long-term monitoring, the liquid junction can get clogged (e.g., with soil particles) and the KCl concentration of the internal solution can change during use. Therefore, regular maintenance is strongly advised (preferably monthly).
Use the following materials for maintenance:
- Piece of cotton cloth and/or tissue that doesn´t release fibers
- Water and soap
- Distilled water (in laboratory wash bottle)
- SWAP Electrolyte-100sat (4.76 M KCl @ 25°C)
- SWAP KCl (≥99.9%)
- SWAP tools (syringe, pipette tip, spoon and cocktail stick) for refilling the reference electrode
- Clean the SWAP probe containing an integrated reference electrode or the SWAP stand-alone reference electrode with water and soap using a piece of cotton cloth and or tissue. Rinse the probe with distilled water. Dry with a piece of cotton cloth and/or tissue.
- Open the reference electrode – preferably in a clean workspace to avoid contamination of the inside of the reference electrode.
- Unscrew the external sealing cap of the reference electrode. Remove the internal solution (and undissolved KCl) from the main chamber by rinsing it with distilled water.
- Unscrew the internal sealing cap of the small chamber of the reference electrode. Remove the internal solution (and undissolved KCl) from the small chamber by rinsing it with distilled water.
- Remove the (adhering) distilled water from the small and main chamber of the reference electrode by holding it in one hand and shaking gently.
- Dry the main chamber of the reference electrode using a piece of cotton cloth or tissue.
- Hold the SWAP probe or stand-alone reference electrode with the reference electrode upward and the cable top downward.
- Flush and fill the small and main chamber of the reference electrode completely with SWAP Electrolyte-100sat (4.76 M KCl @25°C) using the SWAP syringe. Make sure the syringe is clean.
- Remove the air from the small chamber by moving the blunt side of the cocktail stick approximately 20 times in and out the small chamber. Leave the cocktail stick in the small chamber.
- Add ~0.3 g KCl (≥99.9%) to the small and main chamber using the supplied spoon (~2 level scoops). Remove the cocktail stick from the small chamber.
- Remove the air from the small chamber by moving the sharp side of the cocktail stick approximately 20 times in and out the small chamber.
- Screw the internal sealing cap on the small chamber (hand tight).
- Screw the external sealing cap on the main chamber (hand tight).
- Remove remnants of Electrolyte-100sat and/or KCl powder from the outside of the probe or stand-alone reference electrode using distilled water.
Recalibration and testing
After maintenance of an integrated reference electrode (e.g., in a SWAP soil Redox probe), recalibrate the probe.
Go to FAQs – How can I calibrate my SWAP redox soil probe?
After maintenance of a stand-alone SWAP reference electrode, it can be tested versus another reference electrode that is known to function properly.
Go to FAQs – How can I test my SWAP (stand-alone) reference electrode?
SWAP instruments offers both integrated (e.g., in SWAP soil redox probes) and stand-alone reference electrodes. Both reference electrodes have a microporous membrane (frit). This is the white ring in the sharp black point of the reference electrode.
The frit in the reference electrode is a porous barrier that separates the reference electrode´s internal electrolyte from the sample (e.g., soil, sediment, solution, etc.) being measured. The frit ensures that the electrolyte concentration inside the reference electrode remains effectively constant, which is essential for maintaining its known potential.
Without a frit, the internal solution (electrolyte) could mix freely with the sample, introducing unwanted ions that may interfere with the experiment. By limiting leakage, the frit protects both the reference electrode and the sample. Reduced leakage ensures long-term stability and accurate potentiometric readings.
A dried out, chipped (damaged) or clogged frit can increase the electrode impedance, which can result in noisy and (slowly) drifting signals or unreliable measurements. Proper care includes always keeping the frit wet with electrolyte and replacing it if damaged.
SWAP instruments offers spare reference electrode membranes, called MPPE-5.
Use the following materials for replacing the microporous membrane (white ring):
- Piece of cotton cloth and/or tissue that doesn´t release fibers
- Water and soap
- Distilled water (in laboratory wash bottle)
- SWAP MPPE-5
- Clean the SWAP probe containing an integrated reference electrode or the SWAP stand-alone reference electrode with water and soap using a piece of cotton cloth and or tissue. Rinse the probe with distilled water. Dry with a piece of cotton cloth and/or tissue.
- Open the reference electrode – preferably in a clean workspace to avoid contamination of the inside of the reference electrode.
- Unscrew the external sealing cap of the reference electrode. Remove the old white ring and place it with a new white ring. Screw the external sealing cap back on.
- Since the reference electrode has been opened, it is best to refill it again with electrolyte. Go to FAQs –
How can I maintain my SWAP reference electrode? Refilling with saturated KCl (4.76 M KCl @25°C)!
or
How can I maintain my SWAP reference electrode? Refilling with 3 M KCl gel!
To determine if your SWAP (stand-alone) reference electrode (still) functions according to specifications, it can be tested (calibrated) against another reference electrode that is known to be good.To ascertain this, this other reference electrode should only be used for calibration purposes. The reference electrode should be the same type of reference electrode (Ag|AgCl) with the same internal solution as the SWAP (stand-alone) reference electrode.
The test procedure is as follows:
Materials:
- Beaker with tap water
- SWAP (stand-alone) reference electrode
- Reference electrode for calibration purposes
- mV meter
Testing:
- Clean the SWAP (stand-alone) reference electrode with water and soap using a piece of cotton cloth. Rinse the probe with distilled water. Dry with a piece of cotton cloth.
- Connect the SWAP (stand-alone) reference electrode to one input of the mV meter and the other reference electrode to the other input of the mV meter.
- The readout, difference between the two reference electrodes, should ideally be 0 mV. In practise, ± 5 mV is good for soil redox measurements.
- If the readout is > 5 mV, this can be caused by a) the KCl concentration of the internal solution has changed, b) the liquid junction is blocked, c) the reference electrode for calibration purposes or the mV meter doesn’t work properly.
Ad a) Refill the reference electrode with fresh internal solution and repeat the test procedure.
Go to FAQs – How can I maintain my SWAP reference electrode?
Ad b) Clean the liquid junction of the SWAP (stand-alone) reference electrode according to 1) and repeat the test procedure.
Ad c and d) Make sure that the reference electrode for calibration purposes and/or mV meter work properly.
ORP data
When deviating ORP values are observed while performing oxidation-reduction potential (ORP) measurements in, for example, soils and (waterlogged) sediments, this can have one of the following causes:
- Electromagnetic noise
- Interference with other electrodes, sensors or probes
- Reference electrode that is not working properly.
- Scaling/fouling of the platinum ring(s)
Ad 1) Electromagnetic noise could be observed when a data logger with soil Redox probes is powered by the mains/power grit (see Figure 1A). To avoid electromagnetic noise, it is best to power the data logger with a battery. Electric equipment (e.g., air conditioning) in the surrounding of the data logger can also cause noise in the ORP data.
Ad 2) Other electrodes, sensors or probes that are connected to the same data logger can cause interference (see Figure 1B). If possible, don´t measure or switch on all probes at the same time.
Ad 3) The reference electrode has a semi-permeable membrane (frit). This is the white ring in the SWAP instruments reference electrode. It ensures that the electrolyte concentration inside the reference electrode remains effectively constant, which is essential for maintaining its known potential. If the frit is dried out, damaged or blocked it can cause noisy signals or unreliable measurements (see Figure 1C). Please replace the membrane (item: MPPE-5). Noisy signals or unreliable measurements can also be caused by air in the reference electrode (see Figure 1C). This can be solved by refilling the reference electrode with KCl electrolyte (see the instructions in the manual and/or instructional video).
Ad 4) The platinum ring(s) in the soil Redox probes can get scaled or fouled. This can cause unreliable data, often characterized by a flat signal (see Figure 1D). The platinum rings should have a shiny appearance. Please remove scaling or (bio)fouling following the instructions in the manual and/or instructional video.
Figure 1. Examples of causes of deviating ORP data when performing soil Redox measurements. It´s about the patterns and not the shown values on both axis.
– – – – – – – theoretical value of an ORP test solution
Archive: Reference electrode
SWAP instruments offers both integrated (e.g., in SWAP soil redox probes) and stand-alone reference electrodes. Till June 2025 these were all Ag|AgCl 3.0 M KCl gel double junction reference electrodes. Although, SWAP reference electrodes are designed for long-term monitoring, the liquid junction can get clogged (e.g. with soil particles) and the KCl concentration of the internal solution (gel) can change during use. Therefore, regular maintenance is strongly advised.
Maintenance can be performed as follows:
Materials:
- Piece of cotton cloth
- Water and soap
- Distilled water (in laboratory wash bottle)
- 3.0 M KCl gel
- SWAP tools for emptying and refilling the reference electrode
Maintenance:
- Clean the SWAP probe containing an integrated reference electrode or the SWAP stand-alone reference electrode with water and soap using a piece of cotton cloth. Rinse the probe with distilled water. Dry with a piece of cotton cloth.
- Open the reference electrode – preferably in a clean workspace to avoid contamination of the inside of the reference electrode.
- Unscrew the external sealing cap of the reference electrode. Remove the internal solution (gel) from the main chamber with the SWAP syringe.
- Unscrew the internal sealing cap of the small chamber of the reference electrode. Remove the internal solution (gel) from the small chamber with the SWAP syringe.
- Hold the SWAP probe or stand-alone reference electrode with the reference electrode upward and the cable top downward.
- Flush and fill the small chamber completely with 3.0 M KCl gel using the SWAP syringe (make sure the syringe is clean and doesn’t contain old internal solution (gel)). Screw the internal sealing cap on the small chamber (hand tight).
- FLush and fill the main chamber completely with 3.0 M KCl gel using the SWAP syringe (make sure the syringe is clean and doesn’t contain old internal solution (gel)). Screw the external sealing cap on the main chamber (hand tight).
- Remove remnants of the gel from the outside of the probe or stand-alone reference electrode using distilled water.
Recalibration and testing
After maintenance of an integrated reference electrode (e.g., in a SWAP soil Redox probe), recalibrate the probe.
Go to FAQs – How can I calibrate my SWAP redox soil probe?
After maintenance of a stand-alone SWAP reference electrode, it can be tested versus another reference electrode that is known to function properly.
Go to FAQs – How can I test my SWAP (stand-alone) reference electrode?
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