Choosing the correct reference electrode?

Some of the properties of an ideal reference electrode are as follows

1. it should be reversible and obey the Nernst equation with respect to some species in the electrolyte

2. its potential should be stable with time

3. it does not exhibit hysteresis after passing small currents

In 3-electrode electrochemical experiments, the reference electrode holds a vitally important job as a stable "reference point" for the potential of the working electrode (the electrode of interest). It allows for the accurate measurement of the current-voltage behavior of the working electrode. For this reason, the selection of the appropriate reference electrode is paramount.

Aqueous Environments

There are a number of suitable candidates for use in aqueous solutions.

1. Calomel Reference Electrode

The calomel reference electrode had been used extensively for electrochemical measurements as it is fairly simple to make and is robust. However, it has fallen out of favor because of its use of mercury and mercury compounds.

2. Silver/Silver Chloride Reference Electrode

The Silver/Silver Chloride reference electrode has superseded the calomel reference electrode in many applications because of its reduced toxicity and ease of fabrication. One disadvantage is that silver ions are present in the solution surrounding the reference wire which could potentially contaminate the experimental setup. A double junction reference electrode is a possible solution to avoid this issue.

Non-Aqueous Environments

The most commonly used reference electrode for non-aqueous environments is the silver ion reference electrode. Preparation of this electrode involves the addition of a small amount of a soluble silver salt (typically silver nitrate) to a solution of the electrolyte being used in the main body of the electrochemical cell.

Pseudo-Reference Electrodes

In some situations a regular reference electrode cannot be employed a metal wire is often used as a pseudo-reference electrode. In halide containing solutions a silver wire will develop a fairly stable potential. Platinum wires are also commonly used. One situation you need to be careful of is that the electrolyte environment around the wire does not change drastically as this can affect the stability of the reference electrode.