= Solution
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.
\b[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. \b[Mercury is also electrochemically suitable] in a mercury/mercurous-sulfate reference system, although its toxicity and handling requirements make silver attractive where practical.
\b[Sodium is unsuitable] because it reacts with water rather than maintaining a quiet metal/ion reference equilibrium. \b[Calcium and strontium are likewise too reactive] in water. \b[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.
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