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hydrogen electrode[′hī·drə·jən i′lek‚trōd]
a platinum plate coated with an electrodeposit of platinum black that is immersed in an acid solution having a specific concentration of hydrogen ions H+ and bathed in a flow of gaseous hydrogen. The potential of the hydrogen electrode is produced by the reversible reaction . An equilibrium is established between the hydrogen adsorbed by the platinum black and the hydrogen ions in the solution. The potential on the electrode E is determined from the Nernst equation
where T is the absolute temperature (°K), aH+ is the active concentration of hydrogen ions (g-ion/l), p is the hydrogen pressure [kgf/cm2 (atm)], and E° is the normal (or standard) potential of the hydrogen electrode when p = 1 kgf/cm2 (1 atm) and aH+ = 1. For any given temperature, E° is conventionally considered to be zero. The potentials of all other electrodes are calculated with respect to the hydrogen electrode (the so-called hydrogen potential scale). When working with a hydrogen electrode, the hydrogen must be carefully purified from admixtures. Sulfur and arsenic compounds are especially hazardous as is oxygen, which reacts with the hydrogen on the surface of the platinum to form water that disturbs the equilibrium . This electrode is employed as a reference electrode.