The potential scale in Appendix A is based on the SHE. This scale is arbitrary, and by convention is taken to be zero at standard state as mentioned previously. For experimental work it is generally desirable to have a reference electrode in the system. The purpose of the reference electrode is to provide a known, stable potential against which other potentials can be measured. In principle, no current is passed through the reference electrode; therefore, it remains at its equilibrium potential—a potential that is known and well defined. In practice, a very, very small current is passed through the reference electrode to allow measurement of the potential; however, this current is not sufficient to move the electrode from its equilibrium potential. The desired characteristics for a reference electrode include the following:

• Reversible reactions
• Stable and well-defined potential
• Ion(s) that participates in the reference electrode reaction is present in the solution
• No liquid junctions that cause an offset in potential

The purpose of this section is to provide several examples of reference electrodes and to demonstrate calculation of the potential for some of these electrodes. The book by Ives and Janz should be consulted for more details.

Hydrogen Electrode

The hydrogen electrode (Figure 2.4) can be used in aqueous solutions over a wide range of pH values. It consists of a metal, such as platinum, on which hydrogen reacts rapidly and reversibly. The electrode is immersed in an aqueous solution, and hydrogen gas is bubbled around it. The hydrogen ion concentration (pH) is known in the aqueous solution that surrounds the electrode. The pressure above the solution is a combination of the hydrogen gas pressure and the water pressure. For example, the vapor pressure of water is 5 kPa at room temperature, so that a total pressure of 100 kPa corresponds to a hydrogen pressure of 95 kPa. Hydrogen is vented through a trap, which prevents air from diffusing into the cell. The reference electrode is connected to the point of interest through a capillary. The hydrogen electrode is appropriate for most aqueous solutions, but is not practical for many situations where hydrogen gas must be avoided. It is more difficult to use in unbuffered neutral solutions because of the challenge of maintaining a constant solution composition under such conditions. The hydrogen reactions under acidic and basic conditions are

and

The traditional hydrogen reference electrode is a normal hydrogen electrode (NHE), which has a hydrogen ion concentration in solution of one molar and a hydrogen gas pressure of 1 bar, and operates at a temperature of 25 °C. The NHE potential is very close to the SHE potential, but differs slightly due to non-ideal effects that are not present in the theoretical SHE.

Calomel Electrode

Another common reference electrode is the calomel electrode (Figure 2.5). Calomel refers to mercury(I) chloride, a sparingly soluble salt. This electrode is based on the reaction between Hg and Hg₂Cl₂:

For a saturated calomel electrode (SCE), the solution is often kept saturated by the addition of crystals of KCl to maintain a constant concentration of Cl⁻. Note that the standard potential given above corresponds to an ideal 1 molal solution, rather than the Cl⁻ concentration of a saturated solution. For the saturated solution, the potential is about 0.242 V (SHE). The electrode consisting of the Hg(ℓ), Hg₂Cl₂(s) and saturated KCl is connected to the electrolyte solution of interest through a porous frit; this porous frit is equivalent to a salt bridge or junction region. These electrodes will generally have a liquid junction (see Section 2.10), although the correction in potential is not accounted for in most measurements.

Calomel electrodes are best suited to electrolytes that contain Cl⁻ and, conversely, should not be used in situations where low levels of chloride contamination are not acceptable. Additional mercury salt electrodes such as Hg/Hg₂SO₄ (second in popularity to the calomel electrode) and Hg/HgO (alkaline solutions) are available for use with other electrolytes.

Silver–Silver Chloride

Another popular reference electrode is the Ag/AgCl electrode, which is based on the following reaction:

The Ag/AgCl electrode could consist of a simple silver wire upon which a silver chloride layer has been formed. Alternatively, a base metal such as Pt can be used for the deposition of both silver and the silver halide. These electrodes are small and compact and can be used in any orientation. They can be inserted directly into the electrolyte solution of interest with no significant contamination. Ag/AgCl electrodes can be formed by either electrolytic or thermal methods. Bromide and iodide electrodes can likewise be formed. The thermodynamic properties of these electrodes do depend slightly upon the method of preparation. Commercial Ag/AgCl electrodes have a controlled solution concentration, which further increases their stability and reproducibility.

A related reference electrode is the silver sulfate electrode, which is suitable for use in a lead–acid battery since it shares the sulfate ion with the acid.

ILLUSTRATION 2.7

Electrochemical experiments are frequently performed with a three-electrode experimental cell consisting of the anode, cathode and a reference electrode. Current flow, if present, occurs only between the anode and cathode; no current flows through the reference electrode. For our purposes here, let’s consider a laboratory-scale cell for the electrodeposition of copper from an acidic sulfate solution. The composition of the electrolyte is 0.25M CuSO₄ and 1.8M H₂SO₄. Assume that the cell does not have a separator, and that the electrolyte is saturated with oxygen. The pressure is 1 bar. The cell reactions are as follows:

and

A suitable reference electrode for this cell is the Hg/Hg₂SO₄ electrode, whose half-cell reaction is

The actual reference electrode solution is saturated K₂SO₄ (solubility ∼120 g·L⁻¹). Please determine the equilibrium potential for both the oxygen and copper electrodes versus the reference electrode. What is the equilibrium potential for the anode versus the cathode? Note: there is a liquid junction between the saturated K₂SO₄ solution of the reference electrode and the acidic sulfate electrolyte of the cell, which you may ignore.

SOLUTION:

First let’s find the potential between the oxygen electrode and the reference electrode. To do this, we will first find the potential of the oxygen electrode and the reference electrode relative to SHE. We then subtract the reference electrode potential from that of the oxygen electrode to get the desired value.

Oxygen versus SHE:

Reference versus SHE:

where  is the saturation value for the reference electrode. The water activity was assumed to be one. Using these values,

Similarly, for the copper electrode versus the reference electrode:

Copper versus SHE:

Since we already have the potential of the reference electrode versus SHE, we can calculate

The potential between the anode and cathode can be determined from these values:

which can be compared with the value calculated directly (with O₂ on the right):

Therefore, the reference electrode essentially splits the cell potential between the other electrodes into two pieces. This separation is a very valuable concept that will be used later.