Hg/HgO Reference Electrode Potential


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)

2e + HgO(s) + H2O


Hg(liq) + 2OH

  E°= 0.098 (calc)

H2 + 2OH


2H2O + 2e 

  E°= 0.828 (ref 2)

HgO + H2


H2O + Hg

  E = 0.926 (ref 1)

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} )

The activity of water has been explicitly included here because at high base concentrations the activity of water may be substantially different than 1.0.

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

More about EISTo Ref Electrode Table

 How useful was this information? Give us your FEEDBACK !

[ More About Reference Electrodes ]  [ TOP

(1"Reference Electrodes, Theory and Practice", DJG Ives & GJ Janz, Academic Press, NY (1961)., p 335.
(2) (RT/F) * pKw = 0.05916 * 13.996 = 0.828 V (Red'n pot'l)  Note: rxn is written, above, as an oxidation
(3) "Handbook of Analytical Chemistry", L Meites, ed., McGraw Hill, NY (1963). Sect 1.
(4) "Electrochemical Methods" 2nd edition, AJ Bard and LR Faulkner, John Wiley & Sons, NY (2000). Inside back cover.
(5) Meites, loc. cit., Section 5.

Valid HTML 4.0!© Copyright 2001-2009 Research Solutions & Resources, LLC.
All rights reserved.