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.

All SWAP soil Redox probes (with SDI-12 output) are precalibrated with Redox test solutions. You can recalibrate your SWAP soil Redox probe with any suitable Redox test solution (e.g., Zobell’s Solution, Quinhydrone Solutions and 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.

Recalibration can be performed as follows:

Materials:

  • SWAP soil Redox probe (with SDI-12 output)
  • readout unit (e.g., SWI unit or SWAP datalogger)
  • Redox calibration / test solutions

Recalibration:

  1. Connect the SWAP soil Redox probe (clean the probe before recalibration) to the readout unit.
  2. 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 the measured Redox potential and temperature (when the signal is stable).
  3. Repeat step 2 for every Redox calibration / test solution. Rinse (with demiwater) and dry your probe before inserting into a new Redox calibration / test solution.
  4. 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 half an hour before the mV reading is stable. In general, Redox species in tap water react slowly  with Pt.
  5. Determine the off-set 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. Replace the existing calibration settings with the new calibration data if the quality criteria are met. For more details, consult the soil Redox probe manual.

Note: the mV reading depends on the calibration / test solution used, the type of reference electrode used and temperature.

SWAP soil Redox probes have integrated Ag|AgCl 3.0 M KCl gel reference electrodes.

Uncontrolled quality

When the calibration data show deviating values from the quality criteria, the error can, for example, be caused by:

  1. 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.
  2. A fouled Redox electrode (Pt ring).Go to FAQs – How can I clean my SWAP soil Redox probe?
  3. Problems with the reference electrode.Go to FAQs – How can I maintain my SWAP reference electrode?
  4. Problems with the readout unit.Check the wiring of the readout unit.

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 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:

  1. 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.
  2. Open the reference electrode – preferably in a clean workspace to avoid contamination of the inside of the reference electrode.
  3. Unscrew the external sealing cap of the reference electrode. Remove the internal solution (gel) from the main chamber with the SWAP syringe.
  4. 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.
  5. Hold the SWAP probe or stand-alone reference electrode with the reference electrode upward and the cable top downward.
  6. 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).
  7. 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).
  8. 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?

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 (3 M KCl gel) 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:

  1. 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.
  2. 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.
  3. The readout, difference between the two reference electrodes, should ideally be 0 mV. In practise, ± 5 mV is good for soil redox measurements.
  4. 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.

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