TUTORIAL ON REFERENCE ELECTRODES FOR CORROSION

David C. Silverman

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Mercury-Mercuric Oxide

The mercury-mercuric oxide electrode has a very reproducible standard state. Since mercurous oxide does not exist, this electrode has only a single valence of mercury. Precipitation reactions result only in mercuric oxide and mercury. This electrode is useful in alkaline solutions. The oxide is extremely sparingly soluble with a solubility product for the reaction Hg(OH)₂ → Hg²⁺ + 2OH^- being approximately 10^-24 at 298K. The electrode has been demonstrated in high temperature applications.

The standard potential for the mercury-mercuric oxide electrode has been reported in a technical report (S. Rondinini, et al. Pure & Applied Chem., Vol.66, p641, 1994) as both an acid potential and a base potential. The standard acid potential at 298K (25°C) is 0.927V for the reaction

HgO(s) + 2H⁺ + 2e^- → Hg(l) + H₂O

This potential was measured relative to the standard potential of the hydrogen electrode assumed to be zero at all temperatures. The standard base potential at 298K (25°C) is 0.099V for the reaction

HgO(s) + H₂O + 2e^- → Hg(l) + 2OH^-

The latter potential is the one usually reported. The two are related by the standard potential of the hydrogen electrode in basic solution where the activity of hydroxyl ion is assumed to be 1 and the activity of the hydrogen ion is equal to the water equililbrium constant. That potential is 0.828V. The reaction for the hydrogen electrode in basic solution is

2H₂O + 2e^- → H₂(g) + 2 OH^-

This figure shows the acid potentials as a function of temperature between 10°C and 90°C. The regression follows the equation       . This figure shows the base potential as a function of temperature between 10°C and 90°C. The regression follows the equation       . T is the temperature in degrees centigrade in these regressions. One of the problems with the actual measurement is the liquid junction potential between the electrode and environment caused by the large difference in transference numbers of sodium ion and hydroxide ion, NaOH being the base often used for these measurements.

Published information is sparse on the performance of this electrode as a function of temperature in environments of interest in corrosion. Some older work (B. Case and C. J. Bignold, in "High Temperature High Pressure Electrochemistry in Aqueous Solutions", NACE-4, 1973) has reported results for this electrode at more elevated temperatures. The electrode was found to be usable to 200°C. The potential of the cell Pt|H₂||NaOH(1Molar)|HgO|Hg was found to follow the curve similar to the one in this figure . That curve was corrected to a hydrogen pressure of 1 atmosphere. The plot is as the acid form of the reaction. The solubility constant increases enough over this potential range that possible mercury contamination can be a consideration. Appropriate isolation may be required because of the toxicity and chelating ability of mercury.

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David C. Silverman, Ph.D. - Primary Consultant
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