What is a storage solution (KCL) used for?
With the exception of organic plant cultivation, growers cannot do without regular and accurate measurement of pH. In this article, we will advise you on how to extend the life of a pH metre probe many times over and how to ensure accurate results from every measurement in the long term. Did you know that it is very important that your pH probe does not dry out? If it does not remain moist, you will not be able to calibrate your pH metre and the measurement of pH values will be distorted. Read on to find out why and how to use maintenance or storage solutions!
In general, all manufacturers provide only a 6-month warranty with regard to probe life, because there is no way to verify that the customer uses a storage solution. That makes it even more important to use a storage solution so that your probe serves you for as long as possible even after the period guaranteed by the manufacturer has expired.
The function of an electrolyte-based storage solution is to keep the hydrated membrane sensitive to pH and the so-called porous membrane hydrated. Simply put, it ensures that your probe does not dry out.
With a new probe for pH metres, you will usually find that there is a small cap on its tip, the so-called filling cap, containing a storage solution. The filling cap should be kept for future use. If you notice small salt crystals around the seal of this cap, this is not a fault. Chemically, it is completely non-toxic potassium chloride. The cap should be tightened by hand and any excess salt can be rinsed off with tap water.
In the picture you can see an example of a pH filling “cap”, usually supplied with all pH sensors.
You now know that the glass membrane on the pH sensor has a thin gel layer that must remain hydrated for the probe to stay functional.
If you did let this gel layer dry out, the probe’s response to changes in pH would slow down significantly, or there might be no response at all, which of course we do not want. Long-term loss of hydration is often irreversible. If the drying was only brief, you can test the probe’s response in pH buffer solutions. The probe should respond to a 3 pH change, for example from pH 4 to pH 7, in less than 30 seconds.
The response of a pH probe can sometimes be improved by alternating exposure to a strong base and acids. The procedure is as follows:
- Immerse the probe for 10 minutes in 0.1 – 1M NaOH.
- Then immerse the probe for 10 minutes in 0.1 – 1M HCl.
- After regeneration, place the probe in a storage solution for another 15 minutes.
But why not save yourself the trouble and work by using a storage solution straight away, right? For inspiration, we at least present a trio of proven storage solutions that our customers buy most often:
- Growth Technology KCL storage solution, 300 ml
- Jungle Indabox 3M - KCL storage solution
- Milwaukee KCL storage solution, 230 ml
When considering how you will store a pH probe, it is necessary to assess each part of the electrode assembly: the measuring electrode, the reference electrode or the combined electrode. The storage period must also be taken into account. There is a difference between long-term storage for weeks or months and short storage intervals between individual measurements.
Measuring electrodes can be stored dry for a long time. However, before use, the measuring electrode must be hydrated for at least 48 hours in normal tap water or a slightly acidic solution so that an outer gel layer forms on the pH-sensitive membrane. This is where cleaning solutions also come into play, as they are ideal for this purpose. Nevertheless, to have the probe ready for immediate use, most manufacturers supply their measuring electrodes hydrated (see above).
Storage of reference electrodes
For wet storage, which is in no way incompatible with long-term storage, nothing beats a quality storage solution. Never store reference electrodes dry; always store them wet only, because in dry conditions the reference electrolyte slowly penetrates the membrane and crystallises on the outside of the electrode. Salt crystals do not in themselves cause a problem, but the reference electrode could dry out completely, which would result in a substantial increase in membrane resistance. Even if the reference electrode is then refilled with the correct electrolyte (storage solution - KCl), the high membrane resistance may not disappear immediately, which leads to major measurement errors or makes measurement impossible altogether. When storing reference electrodes, it is therefore strongly recommended to keep them hydrated using a storage solution. However, storage in tap water or distilled water should be avoided. Any penetration of these liquids through the membrane will significantly increase the membrane potential, dilute the electrolyte concentration and significantly distort subsequent pH measurement.
Storage of combined electrodes
The combined electrode consists of a measuring electrode and a reference electrode, connected into one sensor. Storage conditions must therefore be suitable for both the measuring and reference electrode. It has been found that the optimal storage liquid is 3M KCl electrolyte, such as the Growth Technology KCL storage solution. If the sensor is refillable, the filling opening must be closed during storage.
Everything explained above in the case of storing the reference electrode also applies to storing the combined electrode. But beware, gel-filled combined pH sensors are an exception. These electrodes have no filling opening and their membrane must be prevented from drying out at all costs! Therefore, gel-filled combined electrodes must be stored moist in a storage solution, which also applies to polymer pH sensors.
For completeness, let us add that when it comes to separate EC probes, they should always be stored clean and dry. If you would like to ask us anything, we are here for you at the well-known e-mail address info@higarden.eu.
We also recommend these products:
- Measuring sets
- Replacement probes for pH and EC metres
- Aqua Master Tools P160 PRO, pH and EC/TDS/PPM metre and thermometer
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