Reference Electrodes

In a potentiometric electrochemical cell one half-cell provides a known reference potential and the potential of the other half-cell indicates the analyte’s concentration. The ideal reference electrode provides a stable, known potential so that any change in Ecell is attributed to analyte’s effect on the potential of the indicator electrode. In addition, the ideal reference electrode should be easy to make and to use. Three common reference electrodes are portrayed here.

Although we rarely use the standard hydrogen electrode (SHE) for routine analytical work, it is the reference electrode used to establish standard-state potentials for other half-reactions. As shown here, the SHE consists of a Pt electrode immersed in a solution in which the activity of hydrogen ion is 1.00 and in which the fugacity of H2(g) is 1.00 . A conventional salt bridge connects the SHE to the indicator electrode’s half-cell. The standard-state potential for the reaction

H+(aq) + e– ↔ ½H2(g)

is, by definition, 0.00 V at all temperatures. Despite its importance as the fundamental reference electrode against which we measure all other potentials, the SHE is rarely used because it is difficult to prepare and inconvenient to use.

Figure11.10

A calomel electrode is based on the reduction of Hg2Cl2 to Hg

Hg2Cl2(s) + 2e ↔ 2Hg(l) + 2Cl(aq)

with the activity of Cl determining the electrode’s potential. In a saturated calomel electrode (SCE), shown below, the activity of Cl is determined by the solubility of KCl. The electrode consists of an inner tube packed with a paste of Hg, Hg2Cl2, and KCl, situated within a second tube containing a saturated solution of KCl. A small hole connects the two tubes and a porous wick serves as a salt bridge to the solution in which the SCE is immersed. A stopper in the outer tube provides an opening for adding addition saturated KCl. The potential of the SCE is +0.2444 V at 25 oC and +0.2376 V at 35 oC.

Figure11.11

Another common reference electrode is the silver/silver chloride electrode, which is based on the following redox couple between AgCl and Ag.

AgCl(s) + e ↔ Ag(s) + Cl(aq)

As is the case for the calomel electrode, the activity of Cl determines the potential of the Ag/AgCl electrode. A typical Ag/AgCl electrode (shown below) consists of a silver wire, the end of which is coated with a thin film of AgCl, immersed in a solution containing the desired concentration of KCl. A porous plug serves as the salt bridge. When prepared using a saturated solution of KCl, the potential of a Ag/AgCl electrode is +0.197 V at 25 oC. Another common Ag/AgCl electrode uses a solution of 3.5 M KCl and has a potential of +0.205 V at 25 oC.

Figure11.12

The standard state reduction potentials in most tables are reported relative to the standard hydrogen electrode’s potential of +0.00 V. Because we rarely use the SHE as a reference electrode, we need to be able to convert an indicator electrode’s potential to its equivalent value when using a different reference electrode. For example, the diagram below shows the relationship between the potential of an Fe3+/Fe2+ half-cell relative to a standard hydrogen electrode (blue),  a saturated silver/silver chloride electrode (red),  a saturated calomel electrode (green).

Figure11.13

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