Potassium chloride solution for redox and pH combination electrodes
For refilling and for the storage of standard redox and pH combination electrodes (3 molar) or special pH combination electrodes for low ion water (4 molar - only if the instructions or the manufacturer expressly prescribe this concentration!).
Not for refilling gel electrodes!
Gel electrodes cannot be maintained, and are therefore for aquaristic purposes (continuous measurement!) actually hardly suitable. If they can no longer be calibrated or if they only reach the measured value extremely slowly, their service life has come to an end and they must be replaced with a new specimen.
You can try to extend the service life of these electrodes by regularly placing them in potassium chloride solution. With already rather sluggish electrodes, however, this measure must be repeated at increasingly shorter intervals in order to achieve a usable response.
It should be mentioned that a particularly fast response is usually not necessary for continuous measurements, as the pH-controlled regulation of the CO2 content is in any case a rather slow matter and therefore a few minutes more or less do not matter.
All other electrodes can be strongly supported in their function by refilling or replacing the potassium chloride electrolyte.
Normally and in the overwhelming majority of all cases, 3 molar solutions are used, but for low ionic media special electrodes with 4 molar or saturated electrolytes can also be found on the market.
| 3 molar | 22.37 g potassium chloride (KCl) / 100 ml solution |
| 4 molar | 29.82 g potassium chloride (KCl) / 100 ml solution |
| Saturated (at 20 °C: 4.61 molal) | 34.40 g potassium chloride (KCl) / 100 g water |
We use exclusively potassium chloride in p.a. quality for the production of KCl solutions.
Cleaning
Electrodes in continuous operation in the aquarium tend over time to develop a biological “lawn” (biofilm). A bacterial layer forms, which makes the measurement sluggish and falsified; in the worst case, the electrodes even become overgrown with algae.
For the removal of protein-containing deposits, cleaning solutions have been developed which are modelled on our stomach contents. They contain pepsin and 0.1 molar hydrochloric acid (pH ~ 1) and remove proteins well that arise from the decomposition of protein-containing substances in the aquarium. However, many bacteria and algae survive even this treatment relatively unscathed.
According to our experience, there is a favourable and effective alternative which, when used for a short time, does not damage the electrode at all, and which has been shown to kill and detach bacteria and algae.
The electrode is placed for 15 min in a 6 % hydrogen peroxide solution. It is quite sufficient to repeat this treatment every 4 - 6 weeks.
As an ultimo ratio for electrodes which hardly react at all, there remains the regeneration attempt with diluted hydrofluoric acid or ammonium bifluoride solution. However, this treatment should be left to professionals, not only because incorrect or careless application can do more harm than good to the pH electrode, but because burns with hydrofluoric acid (even diluted) are among the worst injuries that can be sustained when using chemicals! The tissue is destroyed by precipitation of the vital calcium. Such wounds are very deep and extremely lengthy in treatment and healing. As a first antidote when working with hydrofluoric acid, at least a container with a solution of a calcium salt should be kept ready (works by formation of sparingly soluble CaF = fluorspar)! Salts with organic anions such as calcium gluconate, which are well absorbed by the tissue, are best suited for this purpose.
Further information
Read a detailed, more in-depth article by Klaus Haber on the principles of pH measurement and the construction, function and maintenance of electrodes.
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13 February 2025 00:00
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