Differential capacitance and frequency dispersion on solid and liquid mercury in liquid ammonia

The behaviour of solid and liquid mercury electrodes in 0·1 M ammonium nitrate solutions of liquid ammonia is investigated. The differential-capacitance/potential curves for the solid (at −40°C) and the liquid (at −38°C) are U-shaped and have a minimum of 11·2 μF/cm² at 0·45 V (vs Cd/satd. Cd(NO₃)₂ in liquid NH₃) for the solid of and 10·25 μF/cm² at 0·4 V for the liquid. The curves have no anodic hump. The values for the solid are about 10% higher than that for the liquid but this is still within the error of the surface area measurement. The frequency dispersion of the interfacial impedance was measured between 400 and 20,000 Hz at 0·60, 0·40, 0·10, and −0·25 V. Assuming de Levie's model of parallel grooves of depth l and aperture 2 β to be applicable, the parameter l/sin β was calculated from the data. Values obtained for the liquid mercury electrodes are 5·5–6·8 × 10⁻² cm and are attributed to shielding and solution creepage. For the solid electrodes l/sin β is 12–23 × 10⁻²cm. Although these numbers are too large, the increase in l/sin β by a factor of 3 when going from a liquid to a solid surface is parallel to an increase in roughness factor to about 3 expected for this case. This suggests that l/sin β may be a useful way to measure surface roughness and/or surface roughness changes.