The Difference between pH and ORP - Take Two

The writing of my book How Instrumentation Works has been the driving force behind most of the blog entries. I have been pleasantly surprised that a fair number of people have actually read the entries. After all, I have yet to publicize it beyond its nearly hidden status as just another menu item on the website.

I am especially gratified that no one called me on an entry I wrote about two months ago on the difference between pH and ORP. It contained a major misunderstanding. You can't read it because I yanked it. Why leave evidence behind of a mental blunder?

As I stated months ago I decided to write this book because I believed I knew the subject very well and I also believed I could write a book that most people—technical and non-technical alike—could read and understand. I would be willing to place a handsome bet that every author who has also written a book on any area of human knowledge has come to one conclusion. That is that he didn't know as much as he thought. It's a humbling realization but is one that is easily rectified by writing the book.

One concept that dominated my consciousness whether I was running, falling off to sleep or writing was the underlying principle behind pH and ORP measurement.

The measurement of pH is easy to understand. Since pH is proportional to the concentration of protons—aka hydrogen ions—it's easy to see how measuring pH simply requires one to measure the charge difference between a reference electrode—with a fixed concentration of protons—and the process—with a variable concentration of protons. Measuring this difference is no different than measuring the potential difference between two charged plates that are the foundation of every capacitor. Measurements between electrodes that have a voltage difference between them but no current are called potentiometric.

The principle of ORP (oxidation reduction potential), at least on the surface, seems totally different. In this case there are actual chemical reactions taking place at the process electrode. These are reactions that involve the swapping of electrons between reacting species and that we call oxidation-reduction reactions, aka "redox" reactions. It was the concept of real chemical reactions taking place that led me to declare in the post-that-shall-not-be-named that ORP measurements are amperometric because they are characterized by a current. I had assumed that, if there are redox reactions taking place at the process electrodes then there must be electrons flowing from the process to the reference electrode. Flowing electrons equals current.

Fortunately I came to my senses before the accuracy detectives picked up my trail. The "P" in ORP stands for potential. I can measure the voltage (or potential) of a battery by connecting the two terminals to the two leads of a voltmeter. A good voltmeter draws a negligibly small current. Though there are significant differences in operation, a battery operates on the exact same principle as an ORP sensor. In accordance with the analogy then, an ORP sensor gives a negligibly small current and is characterized by a voltage. It too is a potentiometric device.

There is not nearly enough space in a blog to go any further into the explanation. That's why I wrote the book. To anyone who actually read the faulty blog entry. Mea culpa. I'm sorry. Buy the book.