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RESOURCES > REF ELECTRODES > POTENTIALS > HgO
The potential of the Hg/HgO reference electrode can be calculated from the potential of the cell Pt, H2 (1atm) / NaOH (aq) / HgO (s) / Hg (liq) which is said (ref 1) to be 0.926V @25° and independent of the concentration of NaOH between 0.001 mol-kg-1 and 0.3 mol-kg-1. That does not mean that the Hg/HgO reference electrode potential is independent of [OH�], however. The value of 0.926V was mistakenly reported on the inside cover of Bard (ref 4)
The potential of the reference electrode half-cell NaOH (aq) / HgO (s) / Hg (liq) will depend on the activity of OH�, and hence, upon [NaOH]. E = 0.098 - (RT/F) ln ( aOH- / {aH2O}� ) |
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Electrochemistry The Bookstore Tell Us ! |
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For 0.1F NaOH, the mean ionic activity coefficient is 0.764 (ref 3). Using this coefficient and ignoring the activity of water gives 0.164V for the potential of the Hg/HgO electrode, in very good agreement with the value reported in this Web Site's table. Although the concentration is somewhat outside the range listed in (ref 1), calculation of the potential for 1F NaOH filling solution differs substantially from that in the table. Using an activity coefficient of 0.677 (ref 3) yields 0.108V substantially different from the value (0.140V) listed in the table. The activity of water may play an important part here. If you try to measure it yourself, be careful with the interpretation of the results. Liquid junction potentials can be large for concentrated NaOH. For example, the L-J potential between 1F NaOH and 3.5F KCl is about 10 mV, while against 0.1F KCl it is 45 mV (ref 5)
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