A 1.00 mL sample of tap water is combined with

A 1.00 mL sample of tap water is combined with 2.00 mL of buffer (maintains a pH of 4.00) and diluted in a volumetric flask to a final volume of 10.00 mL. The electrode reads +3.00 mV vs the Saturated Calomel Electrode (SCE) reference. Another 1.00 mL sample of tap water is spiked with 2.00 mL of 4.00×10^-5 M fluoride standard, plus 2.00 mL of the pH 4.00 buffer, and diluted to 10.00 mL. The diluted solution reads -0.021 V vs SCE. Assume Nernstian response

Hg₂Cl₂(s) + 2e^–  → 2Hg(l) + 2Cl^–   (sat’d KCl)     E^o = +0.242 V