Selecting a Reference Electrode





When it's time to put together your electrochemical cell, what reference electrode do you reach for?  Here are some thoughts on selecting the right one.

My favorite.

My favorite reference electrode is the Ag/AgCl reference.electrode. Due to its simplicity, it is quite robust, and can be easily and effectively 'revived' if it should dry out. Moderately high temperatures ( 100 °C ) can be tolerated, as long as the AgCl does not completely dissolve and the construction materials are suitably chosen. I have used  KCl, NaCl, and LiCl filling solutions. It equilibrates fairly quickly when refilled.

Can you use a cell without a liquid junction? 

In some situations, it is possible to put the reference wire directly into the test solution and obtain a stable and well characterized reference potential. An advantage of this approach can be that no additional trace impurities are introduced into the system. The reversible hydrogen electrode (RHE) is often used as the reference electrode when studying single crystal electrode properties in H2SO4. It has the advantage that no foreign ions are introduced. Trace concentrations of ions (such as chloride)  might adsorb on the single crystal electrode and change its electrochemistry.

The RHE has the added advantage that it can be a low impedance reference electrode. This avoids problems with potentiostat stability.

Is chloride a problem?

In some cases even trace amounts of chloride can dramatically change the nature of the electrochemical reaction being studied. Any salt bridge or Vycor® plug which is used to electrically connect the inner filling solution with the main body of the cell may be permeable to the ions in the filling solution: The job of the salt bridge is to allow ionic conductivity! In cases where chloride must be rigorously excluded, the Hg/Hg2SO4 reference electrode (MSE) should be considered. This electrode is a favorite of those study of the common lead-acid battery!

What is the temperature range?

In the minutes of a NACE TEG349X committee meeting the maximum temperature for an SCE is given as 60°C. For a Ag/AgCl reference use up to 90°C is said to be OK.  These limits are in line with the temperature ranges that are given in the literature cited on the Calomel and Ag/AgCl pages of this web site.  Since they are aqueous solutions, use is very problematic at temperatures below the freezing point of the filling solution!

What is the solution's pH?

Using a Calomel or Ag/AgCl electrode is no problem in acid media, but you should be careful at basic pH if the electrode will be in the base for a long time. Sugar detection by HPLC is a common example -- the eluent is generally pH13 or so! After prolonged immersion, the pH of the internal filling solution will rise due to diffusion of OH- into the reference electrode. Soon the potential drifts since it is controlled by the reaction:

Ag2O + H2O + e- == 2Ag + 2OH-

In basic solutions, the Hg/HgO reference electrode is preferred. These electrodes are often made out of plastic rather than glass. Glass is slowly dissolved in basic solutions.





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