HE hydrogen electrode

The hydrogen electrode is made by bubbling dihydrogen into a solution with a known pH, on a platinum electrode (or platinised platinum ), corresponding to the following half-cell  

Therefore, the half-cell involves the following HVH2 couple  

In equilibrium, the Nernst law can be used to express the half-cell s potential with the following equation  

Platinised platinum is used to make the reaction very fast. In practice, the gaseous dihydrogen, the dihydrogen dissolved in the solution and the hydrogen adsorbed at the electrode surface must also reach a state in which they are in equilibrium. In experimental conditions this is not an easy situation to manage, and therefore it is tricky to use such an electrode as reference. 

The silver chloride electrode is made up of a silver wire coated with silver chloride, immersed in an aqueous solution of potassium chloride. Usually a solution is used with a high concentration of chloride ions (from 1 to 3 mol L ), yet with levels below saturation in KCI. This is done to prevent silver chlorocomplexes (AgCl2 and AgCl3 ) from forming, as this would consume AgCI and chloride ions, and in turn fix a different potential value from the value originally expected. 

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