Which reference electrode should I use? Part I - Ag/AgCl
Cormet has three alternative reference electrodes for high temperature high pressure applications. All of them can be installed in an autoclave or a flow through cell. The differences are related to the operation environment.
My preferred choice for a reference electrode is a Ag/AgCl electrode. Its signal depends only on the temperature and the concentration of the internal KCl solution. The Ag/AgCl electrode signal transition is simple because you can check the effects of the operation temperature and the [KCl] in a table included with Cormet’s delivery.
The typical damaging mechanism is related to the dilution of the KCl solution mostly due to the pressure transitions and diffusion. We try to reduce the effect of diffusion by selecting the internal KCl solution concentration (or ionic strength) to be similar with the one of the testing environment.
Another mechanism is typical for all the reference electrodes with a capillary: a bubble that cuts the contact between the Ag/AgCl rod and the testing environment. We try to prevent the bubble formation by degassing of the KCl solution before the electrode assembly.
The sulfur species are the weakness of the Ag/AgCl electrode. If sulfur species get in the contact with AgCl, more stable AgSx compounds will form. As a result, the measured potential will be some kind of mixed potential without much value for an electrochemist.
The Ag/AgCl electrode must be given a service occasionally. The maintenance process includes a few steps, but after a couple of services, Operator can service an electrode in a couple of hours.
The quality assurance test is performed in ambient environment by measuring the Ag/AgCl electrode potential against regular glass reference electrode. Obviously, the glass reference electrode must be in good shape and the measurement device must have a high input impedance. Many portable DVM’s fail to measure the potential correctly.
The photo on the right shows a new Ag/AgCl reference electrode ready to be shipped for a Customer.
The quality assurance test in high temperature high pressure environment is somewhat more difficult. I have used a method where I compare the reference electrode reading to the autoclave body electrode potential.
According to experimental know-how, the electrode potential of stainless steel is on H2/H+ equilibrium line (“hydrogen line”) in oxygen free (preferably some hydrogen containing) high temperature water. Based on this assumption, the stainless steel specimen (or cell body) electrode potential measured against Ag/AgCl reference electrode is
ESS vs. Ag/AgCl = EH2/H+ - ESHE
EH2/H+ = potential of “hydrogen line” in high temperature water: 0 – 2.303 * RT/nF *pHT
ESHE = reference electrode potential vs. SHE at the operation temperature (e.g. -39 mV at 300°C for 0.1 M KCl),
As an example of the application of equation above, the potential reading ESS vs.Ag/AgCl of a 0.1 M KCl filled Ag/AgCl reference electrode at 300°C should be about -600 mV. I often measure ESS vs Ag/AgCl about -520 mV - -550 mV. If some hydrogen were added in the autoclave, one would certainly measure the ESS vs Ag/AgCl = -600 mV.