Other standard electrodes

Other than the standard hydrogen electrode (SHE), two other standard or reference electrodes are widely used – the calomel electrode and the silver chloride electrode. Each of these electrodes, like the SHE, provides a constant potential that is insensitive to the electrolyte.


Silver chloride electrode

The silver chloride reference electrode consists of a AgCl coated silver wire that is dipped in a saturated KCl solution (also saturated with AgCl). The porous frit, which allows for the slow passage of ions, forms a liquid junction with the test solution. The electrode potential of 0.199 V versus SHE at rtp is given by the following half-cell reaction:

AgCl(s)+e^-\rightleftharpoons Ag(s)+Cl^-(sat'd)

Just like the constant bubbling of H2 in a H+ (1 M) electrolyte of the SHE half-cell provides the electrode with a constant potential,  the Ag wire, and the equilibrium between AgCl coated on the wire and the saturated AgCl internal solution, ensure that the AgCl electrode maintains a constant potential. Some literature quote the potential vs SHE as 0.22 V at rtp. This is the value when KCl is 1.0 M, whereas 0.199 V is measured when KCl is saturated.


The calomel electrode

The calomel reference electrode is composed of mercury in contact with a solution of saturated Hg2Cl2 (calomel), which in turn is in contact with saturated KCl. The electrode potential of 0.244 V versus SHE at rtp is given by the following half-cell reaction:

Hg_2Cl_2(s)+2e^-\rightleftharpoons 2Hg(l)+2Cl^-(sat'd)

Both AgCl and calomel reference electrodes are used for a wide range of electrochemical measurements. However, the toxicity of mercury in the calomel electrode poses health and environmental issues.

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