Different between usage of Ag and Au electrodes?

I'd say cost mostly.

Chemically Au is more inert than Ag, but the coin metals are all great conductors (Au>Ag>Cu).

Both Au and Ag have low half cell reactions (as does copper) which is a benefit for measuring pH as the potential to moving coulombs of electrons and charged ions like any battery or rust pit. Coin metals have very stable bases of measurement.

Ask your science teacher about cleaning tarnished silver with aluminum foil in a warm solution of baking soda.

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