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A reference electrode should provide a stable, reproducible potential from a well-defined reversible equilibrium, with fixed relevant activities. It should pass negligible measuring current, resist contamination and drift, and avoid changing the sulfate sample through incompatible ions or reactions.

Silver is suitable when coupled to a sparingly soluble silver salt and a controlled filling electrolyte; a silver sulfate reference electrode avoids introducing chloride into a sulfate system. Mercury is also electrochemically suitable in a mercury/mercurous-sulfate reference system, although its toxicity and handling requirements make silver attractive where practical.

Sodium is unsuitable because it reacts with water rather than maintaining a quiet metal/ion reference equilibrium. Calcium and strontium are likewise too reactive in water. Magnesium is not a dependable reference simply because it sometimes reacts slowly: corrosion, oxide/hydroxide films and mixed potentials make its open-circuit potential dependent on surface history and solution conditions. A relatively inert-looking surface is not equivalent to a reproducible reversible electrode.

Even a suitable reference is not just an arbitrary piece of metal immersed in the sample. The metal, equilibrium salt, controlled electrolyte and liquid junction together define the reference system.