Recommended pH Probes in Highly Acidic Applications

One way to reduce the sheer volume of email from those you work with is to promise to blog them. Most take this as an idle threat, so unfortunately the emails keep flowing. Here’s a case where the threat is not idle, and here’s the post to prove it.

The original question came in from a process manufacturer to ModelingAndControl.com’s Greg McMillan and asked him for a recommended pH probe for low pH material (1-3pH). My hopefully trusty source, the pH entry in Wikipedia, puts that on a scale with gastric acid.

Greg contacted Dave Joseph, a senior industry manager in Emerson’s Rosemount Analytical Liquid business. Dave responded:

In my experience, measuring low pH values in the 1-3 range is not very difficult. Although there is a nonlinear effect called “acid error”, the primary source of error is junction potential due to the high concentration of H+. This manifests as a pH reading that ramps quickly into the ballpark but may take quite some time (100 secs or more) to get to the final value. It would be common for the reading to drop from 6 to 2.4 and then tick slowly down to 2.0, for instance. A good sensor for that kind of behavior is a more open junction like our PERpH-X design that allows the potential to stabilize quickly. It would also help cut the time necessary for calibration.

A clean ISFET [Ion-sensitive field effect transistor] sensor responds quickly regardless of the temperature, so the FET is an improvement for very low temperature processes (near 0°C) where high glass impedance causes slow response and noisy readings. In practice, most pH measurement issues have to do with the reference side of the sensor, which is subject to coating, plugging, poisoning, and junction potentials. pH applications can involve many different processes and conditions. Practically all of the troublesome measurements (high temperature, caustic (high pH), steam cleaning) for glass electrodes are even more problematic for ISFETs. In a low pH stream with no other concerns, an ISFET would be expected to function as well as a glass electrode, but with no specific advantages.

Greg’s follow up question was:

Are there any hydration requirements for an ISFET? My understanding is that a glass electrode depends upon a hydrated gel layer.

Dave responded:

The glass electrode does use a hydrated gel layer to produce a stable potential. An ISFET works more directly and does not need hydration to make the measurement. That means that an ISFET may recover from a dry environment faster than a glass electrode would. However, both types of electrodes require a reference with a silver/silver chloride solution of water, and the presence of water in the process is required for acceptable continuous measurement.

I thought there was some wisdom in the exchange that needed to be set free from the clutches of my email inbox. Then again, let’s see if Greg or Dave ever includes me on another email!

Posted Thursday, May 22nd, 2008 under Education, Measurement.

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