= Solution
Use a silver wire or strip in contact with excess solid silver sulfate, immersed in a sulfate filling solution of known, stable activity. A porous frit connects the filling solution ionically to the sample while limiting bulk mixing. The reference housing should maintain the salt/electrolyte composition rather than letting the sample directly determine it.
Connect the unknown working electrode to the positive input and the silver reference to the negative input of a high-input-impedance voltmeter. The frit or sulfate-compatible <salt bridge> completes the ionic circuit. At negligible measuring current,
$$
V_{\rm meas}\simeq E_{\rm working}-E_{\rm reference}+E_{\rm junction}.
$$
A stable, small junction contribution is desirable. The reference electrode should not carry the counterelectrode current in a driven experiment: a separate counterelectrode is used when current is passed.
\Image[/past-exam-of-the-mphil-in-micro-and-nanotechnology-enterprise-course-of-the-university-of-cambridge/2023/paper-1-additional-reference.png]
{title=A sulfate-compatible silver reference measures a potential difference with negligible current}
{description=Original circuit schematic. Silver, excess silver sulfate and controlled sulfate electrolyte define the reference compartment; the frit provides ionic contact. The voltmeter polarity fixes the sign of the measured working-minus-reference potential.}
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