Standard Potential of the Silver Half-Cell

Silver's high standard potential makes it a noble metal that resists oxidation by air or water; what tarnishes it is sulfide, not oxygen. The half-cell's real importance is practical: coat a silver wire with AgCl and immerse it in saturated KCl and you have the Ag/AgCl reference electrode, at +0.197 V against the hydrogen electrode, which is the reference inside virtually every pH probe, ORP probe and ion-selective electrode in use today.

It is stable, cheap, non-toxic and works over a wide temperature range — all the things a hydrogen electrode is not. The same chemistry underlies the Mohr and Volhard titrations for chloride, and the silver-halide photography that dominated imaging for a century: a photon reduces a few Ag⁺ ions to metallic silver, forming a latent image that development then amplifies by a factor of around 10⁹.