Dear Dr. Celia Fuentes Chust ,

in general, the input impedance of a pH meter is so high that the electrode should not be affected by the ignition or not of the instrument connected to it and therefore the life of the electrode is therefore not modified in keeping the electrode on or off. instrument.

The electronic part can be considered to be affected by the greatest electrical and thermal stresses upon switching on and off, while an instrument constantly switched on has a very slow but inexorable aging.

In the case of a pH meter, it is therefore necessary to rely on common sense based on the actual methods and frequency of use.

In the case of occasional use, it would be better to turn it off and turn it on only during the days of use, considering that cleaning, methods of use and conservation of the electrode over time are much more important.

My best regards, Pierluigi Traverso.

Dear Celia,

Your questions about turning the pH-meter on and off habits have been cogently answered by Pierluigi Traverso. Relying on common sense based on the actual frequency of use is indeed the best thing to recommend.

However, the understanding that the pH electrode has a half-life of around one year is not quite justified. If properly maintained, it can serve much longer. The decrease in the electrode efficiency (the slope of the potential vs. pH plot) due to aging is usually not too great. In any case, this decrease is accounted for in the calibration you perform regularly, usually every week, so that the performance of the system remains practically unchanged.

Best regards,

Rouvim Kadis

We turn off after use. Do not use daily. We have standard solution for calibration. We calibrate it before use.

Dear @celiafuentes

We turn off and unplug the ph meter after usage.

We always and only calibrate the ph meter before use.

We have used the same ph electrodes for years. There are some tricks for this:

If the electrode becomes difficult to stabilize and gives a slow signal, conditioning of the glass electrode is necessary. For this purpose, as the first process, 1 M HCl and 1 M NaOH are immersed in order and held for ten seconds each, and the process is repeated 10 times. If this is insufficient, it is immersed in 5% HF (only glass bulb) for 3 seconds, then washed with plenty of distilled water. Then it is kept in 0.1 M HCl for 5 hours. It is washed with distilled water again and kept in 3 M KCl until the next day. this will regenerate the electrode and improve the response. In addition, after pH measurements in organic matrix, immerse the electrode 0.1 M of nitric acid for 30 seconds and then wash it with distilled water and place it in a storage solution. this will break down organic materials.

Be sure to wash the salt bridge with plenty of water when it is not used for a long time. Make sure that the asymmetry voltage of the electrode does not exceed 15 mV and that the slope value does not fall below 95%. If these conditions are not met, change the calibration buffers first, if not, do the above steps. If this does not work, you will need to replace the probe.

You can ask further questions if you wish.

I hope that the comments are useful.

Lokman

Dear colleagues,

Although being outside the initial topic of the discussion, the steps described above for maintaining and restoring the pH electrode performance, which represent the experience of Dr. Lokman Liv’s laboratory, are worthy of notice. They all make sense, except perhaps for the cautions mentioned at the end: “Make sure that the asymmetry voltage of the electrode does not exceed 15 mV and that the slope value does not fall below 95%”.

I would like to note that the asymmetry potential, associated with a difference in physical and ion-exchange properties of two sides of the glass membrane, is not easy to measure with a common (commercial) glass electrode. But there is no need to do so. The point is that the influence of that potential and the slope of the electrode function are both accounted for in calibration (i.e. adjustment) of the pH-meter with standard buffer solutions.

If calibration is done regularly, you do not have to worry about these performance characteristics of the electrode. Only obvious indications of malfunction such as clearly erroneous result, slow response, and drift should cause concern. It must be kept in mind however that similar symptoms can be caused by poor performance of the reference electrode junction, not the glass electrode itself.

Best,

Rouvim Kadis

I would like to thank Dr. Rouvim Kadis for his valuable comments. I would like to express that asymmetry voltage and slope values ​​are measured by the device during calibration in almost all commercial-laboratory type pH meters. Therefore, there is no need to make an extra effort for this. In addition, these are very important parameters considering that the errors that may come from the calibration solutions also affect these performance characteristics. You can find statements similar to what I said in the guide below:

https://www.mt.com/mt_ext_files/Editorial/Generic/1/Guides_to_Electrochemical_Analysis_0x000248ff00025c9a00093c4a_files/guideph.pdf

Dear Dr. Lokman Liv,

The first thing we must do is clear up the confusion in terminology. From the very beginning of the development of glass electrode, the term “asymmetry potential” has meant a potential difference which develops across the glass membrane due to a slight distinction in physical and ion-exchange properties of two sides of the membrane. Specifically, the difference in the state of inner and outer hydrated surface layer (as a result of the exposure to sample solution) is responsible for the asymmetry potential. To measure this potential we must have a symmetrical cell with identical solutions and reference electrodes on each side of the membrane, which is not feasible with a sealed (commercial) glass electrode.

The quantity that the modern pH-meter displays during calibration, besides the slope, is also called the asymmetry potential, or asymmetry voltage, which is referred in this case to the pH electrode assembly. Actually, this includes the liquid junction potential contribution of the reference electrode, which often dominates, and already for this reason, it cannot characterize the performance of the glass electrode itself. The value of the asymmetry voltage is automatically derived by interpolation of the linear calibration function obtained with standard buffer solutions to the point of pH 7.0 where the output voltage of the system should theoretically be zero. After that, any observed asymmetry potential is compensated for by adjustment to zero with the instrument calibration control and thus, you can no longer worry about it.

From the above it follows that recommendations that the asymmetry voltage displayed during calibration of the pH-meter should not exceed 15 mV and the slope value not fall below 95%, which often can be found in guides on pH measurement, should not be taken too seriously if no other indications of malfunctioning are present.

Dear Dr. Rouvim Kadis

Your comments are really valuable. However, in pH meters, when calibration starts with pH 7 first, there is no calibration function and the device gives a direct asymmetry potential value. Although it fixes this value to ph 7, this may cause differences from the linear calibration equation at different pHs. In addition, we came across many pH meters that the pH and potential values we measured for pH 7 after calibration were not compatible and this showed that the fixation was not actually done meaningfully. Of course, as you said, these changes are not very important changes, but they are points that manufacturers should pay attention to.

But on the other hand, we can also understand that these parameters are important as follows:

Even if we do the calibration perfectly in terms of behavior, if there is an error due to the standard buffer solutions, we cannot make a correct decision without these parameters. For example, if the pH 7 standard solution deviates from this value in some way, this will be reflected in the asymmetry voltage and we cannot make a comment without reading this value. A similar situation is also valid for the slope value obtained after two or three-point calibration. Therefore, these values represent both the combined electrode performance and indirectly the performance of standard buffer solutions (such as deterioration from any factor). That's why I think the manufacturers have stated that these are important parameters.

Thank you very much for useful information 🙏

Thanks you very much for your complete answers! It is more detailed than I expected and I appreciate it very much :)