It is a common misconception that there is little diagnostic data that we can interpret about a pH electrode. If you have adopted smart digital technology there are many data points you can use in evaluating a sensor’s performance. These data points let us know how successful calibrations are, optimize performance in the application, or even tell us when it is time to replace the sensor. One particularly beneficial data point that we sometimes overlooked is glass impedance. Before we understand exactly how to interpret glass impedance, it is important to understand the makeup of a pH electrode.

Combination pH Electrode Basic Principle 

A combination pH electrode has three main components: a pH measuring electrode, a reference electrode, and a temperature element. A glass pH measuring electrode is sensitive to hydrogen activity in the measured solution and develops electrical potential that is directly related to this hydrogen activity. The reference electrode provides a constant potential to which the pH measuring electrode is compared. The comparison between the changing potential on the pH measuring electrode and the potential on the reference electrode is what provides the pH voltage. Using the Nernst equation, the transmitter then converts the pH voltage to a pH value on the 0 – 14 scale. The pH voltage will also change with temperature, so the temperature element is present to compensate for this.

What is Glass Impedance?

The glass impedance of a combination electrode is the measurement of the electrical resistance of the pH measuring electrode when the potential is applied to it. This is typically high and measured in MΩ’s (Megaohms). All pH sensors have a glass impedance range that depends specifically on the glass recipe. Sensors that use different glass recipes will have different glass impedance ranges.

Value of Monitoring pH Glass Impedance

The importance of pH glass impedance is not necessarily the MΩ value but what it tells you. The key to diagnosing glass impedance is to look for a drop low or a spike high in MΩ value. Should you see it go in one of these directions, it is telling you something about the electrode condition.

The most common glass impedance spike is that the electrode is not submerged in the process and is now drying. Inversely, buildup or a crack in the glass itself often causes a significant drop in glass impedance . When looking at glass impedance in this context, the value that it provides is the necessary guidance regarding maintenance of the electrode. Based on the MΩ value, one can determine if the electrode needs re-hydration after being left dry, cleaned of buildup on the electrode, or ultimately replaced if cracked. We have discussed proper techniques for both cleaning and re-hydration of pH electrodes in previous blog posts.

Summary

As we transition toward an environment where data is provided instantaneously and continuously, receiving and interpreting an increased number of diagnostic points is crucial to enhancing pH electrode utilization. Glass impedance is useful to identify an electrode issue quickly and give insight into many different maintenance issues. Making quick decisions based on that data can reduce downtime, enhance efficiency, and minimize sensor replacement costs.