The following post was submitted by Rajashri Lele in our Mumbai office.
If you commonly measure the pH of viscous solutions such as food samples, paint, cosmetics, or pastes, you probably find that it is hard to get a stable reading from the meter. The typical complaint for these applications is that the reading never stabilizes at any point for a reasonable length of time and the variations in the pH can be quite substantial.
Why is the meter reading unstable?
The reason usually is the electrode that you are using is unsuitable for measuring viscous samples (assuming that your meter is working perfectly well). It is not uncommon to find that one pH electrode is used for all kinds of samples in a laboratory. However, pH electrodes have their own specialties. In areas outside these specialities, the electrodes take time to stabilize, and even worse, can be totally unreliable.
There’s nothing really wrong with the electrode, except that it is probably being used in an application for which it is not suited. For electrodes to work properly, there must be sufficient flow (or a steady leak rate) of the internal solutions into the sample to enable the formation of a complete electrode chain (which in simple terms, enables pH to be measured). Most electrodes are designed to work with very fluid samples, such as water-based solutions. However, when these same electrodes are placed into viscous samples, this flow becomes insufficient to maintain the required electrode chain. This often leads to wild fluctuations in observed readings.
How can this problem be solved?
The solution is to use an electrode that will allow a higher flow rate of the internal solution without letting it affect the sample reading itself.
What are my options?
It depends largely on the type of samples and the temperature range that you have. Generally the Oakton® single-junction electrode is sufficient for most applications. However, for samples where the contamination of the reference junction is a problem (such as those containing sulphides, cyanides and bromides) use a double-junction electrode
Single junction: High viscosity solutions such as emulsions, suspensions, paints, varnishes, sauces, shampoos and varnishes where frequent cleaning of the reference junction is necessary.
Double junction: Reference contamination problems such as media containing sulphides, bromides, iodides and cyanides.
Use these electrodes with the Oakton® pH meters, famous for unique combination of design and exceptional value.