Does anyone know how a pH probe works?

It's the most common, the oldest and the simplest measurement made in all of electrochemical measurements. And yet, I have found that almost no one understands how the humble pH probe works. I don't mean the measuring circuit that comes after the bulb that sticks in the water. I mean the bulb itself. How does the pH of the water affect that little glass envelope so that the rest of the probe can make a measurement? It turns out that even authors who write articles and books on the subject don't always have a grasp.

One website that is solely devoted to pH measurement asserts that "when the probe is immersed in an acidic environment the glass membrane is permeated by hydrogen ions." This is totally wrong since glass is a very solid barrier and doesn't get "permeated" by anything we would ever measure. Some claim that there is a gel layer that is applied to the glass to attract hydrogen ions. If you clean the bulb too strongly you'll remove the gel and destroy the probe. There is a gel layer but it forms all by itself and, if you scrub it off, it will reform in a day or two.

The mental hurdle we all face is that hydrogen ions (the little particles that comprise pH) don't go through membranes and they don't react with anything. This is totally different from most electrochemical analyzers such as those that measure chlorine or oxygen. Chlorine really does permeate a membrane and then reacts with a piece of metal, called the cathode, to generate a current. This we understand. But tiny, little hydrogen ions (aka "protons"). They just sit there! I am not writing this blog entry simply to give a tutorial on how pH probes work. I write this because, in our industry, there is a lot of misconception about how instrumentation works. I have to admit that, when I purchased Aquametrix over 3 years ago, I googled "pH sensor." And I have a PhD in chemistry.

The confusion doesn't end with pH probes. The ORP sensor is almost identical to a pH sensor. The only difference is the process electrode, which is a band of platinum or gold. A pH electrode is selective to hydrogen ions but a platinum band is not selective to anything. How can a neutral solution of chemicals dissolved in water create a voltage difference?And why does it take as long as an hour for an ORP probe to reach a final reading but only 1 minute in calibration solution. Electrochemical sensors are relatively simple devices but the confusion that surrounds their operation is startling. I hope to clear up much of this confusion in future posts.

Before I close—here, in a nutshell, is how a pH electrode works. The glass in a pH electrode attracts hydrogen ions. What goes on in the glass requires greater explanation than I will provide but, for now, let's just say that all those positively charged hydrogen ions create a sizable positive charge around the electrode. The reference electrode is in neutral pH so, between the two electrodes, there is a charge imbalance It is that charge imbalance that we measure. We aren't measuring a current like we do with a chlorine analyzer. We are measuring a voltage. And for that all you need is a voltmeter. So why is pH 7 solution, with it's zero charge, not pH 0? Where did "7" come from? Coming up to blog near you. Mark