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Quinhydrone is a sparingly soluble 1:1 addition compound of hydroquinone (H2Q) and quinone (Q). When dissolved in water, equal concentrations of H2Q and Q result. A platinum electrode dipped in this solution responds to the reversible half-reaction: Q + 2H⁺ + 2e⁻ <=> H2Q, E_rp = +0.46 V vs SCE. A sample of unknown pH was saturated with quinhydrone and the platinum electrode showed a potential of +0.22 V vs SCE. What is the pH of the sample? [Given: 2.303RT/F = 0.06 V]
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Correct answer: 4
Solution
Because [Q] = [H2Q], the Nernst equation simplifies to E = E_rp - 0.06 * pH. Setting E = +0.22 V and E_rp = +0.46 V: pH = (0.46 - 0.22)/0.06 = 0.24/0.06 = 4.
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