The Hg/HgO electrode is usually utilized as a reference electrode in alkaline solution such as for development of an alkaline hydrogen electrode. The reference electrode provides a suitable reference point but is available from few commercial vendors and suffers from inadequate documentation on potential in varying electrolytes. A new numerical method uses activity, activity coefficients, and a few correlated empirical equations to determine the potential values in both dilute and concentrated sodium hydroxide solutions at temperatures of 0-90 degrees C and at concentrations of 0.100-12.8mol kg(H2O)(-1). The computed potentials of the Hg/HgO electrodes versus a normal hydrogen electrode (NHE) at 25 degrees C and 1 atm are 0.1634 V for 0.100m, 0.1077Vfor 1.00m, and 0.0976 V for 1.45m NaOH solutions. The Hg/HgO reduction potential further changes to -0.0751 V versus NHE and hydrogen evolution potential changes to -0.9916 V versus NHE in a solution of 30.0 wt.% NaOH at 80 degrees C. The calculated values are compared with the measured data at 25 and 75 degrees C. The experimental data agree well with the numerical values computed from the theoretical and empirical equations. (c) 2006 Elsevier B.V. All rights reserved.