Avoiding the most common mistakes in pH measurement

If you’re reading this, then I’m sure you have already performed at least one pH measurement in your lifetime, since it is one of the most important parameters in analytical chemistry. I remember my first contact with a potentiometric pH meter and a pH electrode – and I can still remember how I felt back then.

I was young and completely unsure about how I should handle the instrument and the electrode. Was I doing everything correctly? Consequently I had many questions about the best practices.

Today, I am much more confident! Therefore, I would love to share with you some of the most common uncertainties and mistakes I see during my daily work when potentiometric pH measurements are performed. By the end of this article, I am certain that you will agree with me: pH measurement can be just as easy as it looks. I will cover the following topics (click on a link to go directly to each topic):

Is this the correct electrode for your application?

Troubleshooting already starts before you put the sensor into your sample solution. A wide variety of electrodes are available on the market, and it can be quite difficult to determine which electrode is the best for your application. Many different diaphragm types as well as glass membrane materials exist:

We’ve prepared a flyer for you to help find the perfect electrode for your application. Additionally, we have provided valuable information about maintenance and storage. You can download the flyer in several languages: English, German, French, or Spanish.

Flyer: Electrodes for pH measurements - Which electrode for which application?

What is most important when preparing the electrode for calibration or measurement?

Before starting your measurement, check the electrode for cracks or contaminations. Open the plug to ensure that the electrolyte can flow out (otherwise you may observe unstable results), and check the level of the electrolyte.

The electrolyte should always be filled up to the opening in order to ensure an outflow from the hydrostatic pressure. If the level of the sample is higher than the level of electrolyte within the sensor, then sample will enter the reference system of your electrode. This causes the reference potential to shift, and results are no longer reproducible.

https://metrohm.scene7.com/is/image/metrohm/electrode-immersion?ts=1649934841299&dpr=off

Make sure that you insert your sensor deep enough into the sample. At least the glass membrane and the diaphragm need to be covered, as shown in this example.

Calibration: When is it necessary, and what should be considered?

Calibrations must be performed on a regular basis. Depending on the number of measurements and the sample matrix, I recommend calibrating at least weekly. If used often, or if the sample matrix is contaminating the sensor, then you should calibrate daily or even more frequently. Of course you should always calibrate your sensor if you have received a new one, after maintenance, or after a longer storage period.

For calibration, consider the following points:

Always use fresh (not expired) buffers – the calibration can only be as good as the buffers used!
Perform at least a 2-point calibration.
Your sample pH should be within the calibration buffer pH value.
Always measure the temperature, as the pH value is temperature-dependent.
Most manufacturers already include buffer table templates with their instruments. Make sure that you select the correct one.

How should the pH electrode be stored?

The correct storage of the pH electrode can increase its lifetime significantly. Never store the pH electrode dry! The glass membrane builds up a hydration layer, which is necessary for proper pH measurement. If you store the electrode dry, this hydration layer will be destroyed. Even though the layer can be recovered by conditioning the sensor in deionized water, the sensor will become slower.

For electrodes filled with potassium chloride (c(KCl) = 3 mol/L) as reference electrolyte, we have developed a dedicated storage solution which keeps the glass membrane in top quality without impairing the performance of the diaphragm.

Need a dedicated pH electrode storage solution? Find it here!

The figure above shows how quickly the sensor responds when placed in a sample after a storage period. You can clearly see that storing the sensor in the dedicated solution leads to a much faster response time in comparison to storage in c(KCl) = 3 mol/L. This means even more productivity and less waiting.

All electrodes which are filled with a different reference electrolyte than c(KCl) = 3 mol/L are stored in their reference electrolyte.

How should the pH electrode be cleaned?

Between the measurements, the electrode must be rinsed well with deionized water. If the sample is sticky or contains proteins, use a suitable solvent to remove the contamination. From time to time, it is important to give the electrode a «special treat» and clean it with the pHit Kit, shown below. This set includes everything that is necessary to gently and efficiently clean the electrode.

Need a «special treat» for your pH electrode? Try our pHit Kit!

Very important: Never wipe the sensor off with a tissue! Similar to rubbing the surface of a balloon, you will charge the surface of the glass membrane. The built-up electrostatic energy will influence your measurement, which will get significantly longer. Additionally, you can scratch the sensitive glass membrane surface, thus destroying it.

To stir or not to stir?

Depending on the electrode type you are using, it is recommended to always stir constantly, at the same speed, during analysis. The following graph illustrates why:

The upper curve shows the measurement with an electrode having a fixed ground-joint diaphragm, and the lower curve utilized a very common electrode with a ceramic pin diaphragm.

Not only does the top electrode show less signal noise, the signal remains nearly unchanged once the stirrer is switched off. However, there is a significant signal drop for the ceramic pin diaphragm (bottom). Therefore, the stirring speed should be identical for all buffers and samples to minimize such effects.

Is my electrode still ok to use?

To get an idea about whether your electrode is still ok to use or not, it is generally enough to check the slope and the pH(0) after calibration. The slope should be between 95–103%, whereas the pH(0) should lie between pH 6.8–7.2. Further information can be gained if a pH electrode test is performed, which is implemented in some of Metrohm’s instruments, or a test according to Application Bulletin AB-188.

AB-188: pH measurement technique

If the electrode does not meet the specifications, clean it according to the instructions and perform the test again. If the sensor still does not pass, a replacement is inevitable.