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David C. Silverman |
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Calomel (Hg/Hg2Cl2) Reference Electrode The calomel or mercury/mercurous chloride electrode is a member of a group of electrodes in which a mercurous salt is in equilibrium with a soluble anion. Such anions have included bromide, iodide, phosphate, iodate, acetate, and hydroxide. But mercury type electrodes with chloride ion as the anion in the salt are the most commonly available. That electrode is the focus of this portion of the tutorial. The mercurous chloride paste is called calomel, hence the name of the electrode. An excellent early reference on this electrode is G. J. Hills and D. J. G. Ives, J. Chem. Soc., p. 319 (1951). This electrode had widespread use into the 1960’s but in more recent times has tended to be replaced by the silver/silver chloride reference electrode especially in corrosion measurements.The single junction calomel reference electrode is illustrated in this figure ) .
The electrode is not drawn to scale as a number of different sizes exist.
The electrode also can exist as a double junction reference electrode in
which a contained outer solution surrounds the electrode inner compartment and
filling solution or further
separates the electrode and filling solution from the environment.
This figure shows an example of a double junction in which the outer filling
solution is not part of the reference electrode per se
)
but completely surrounds that inner filling solution.
This outer solution is separated from the environment by means of an additional
frit (the second junction). The goal of the double junction is to increase the diffusion distance
between the environment and solution in direct contact with the electrode so
as to minimize contamination of either. The mercury-mercurous chloride reference electrode
tends to be used in this manner in corrosion studies. The protection of the mercury mercurous chloride
interface inside the tube from direct contact with the electrolyte tends to
decrease its contamination relative to some other reference electrodes.
The drawback is the additional junction potential and equipment.
The toxicity of mercury has decreased the popularity of this reference
electrode in more recent times.
The key characteristic is that the external connection is made to a wire (often platinum) that contacts the mercury which is adjacent to the calomel-mercury paste. This paste is separated by a junction that contacts the external chloride solution, usually potassium chloride. That junction enables equilibrium to be established between the calomel paste and the potassium chloride solution. The filling solution in turn is separated from the external environment by a junction through which a small leakage is allowed for electrical continuity. The potassium chloride solution is more commonly at saturation than at lower concentration because exchange current densities are hypothesized to be higher. The result is that the electrode process proceeds without polarization of the surface. In addition, the saturated solution helps to ensure that the electrode process is robust against outside interface. This form of the electrode is the "saturated calomel electrode" abbreviated as "SCE". Concentrations of 0.1N and 1N KCl have also been used. Potassium chloride is used as the filling solution because the transference numbers for the cation and anion are very similar. Since electrical neutrality requires both ions to be transferred through the plug, the desire is for them to diffuse at the same rate. Otherwise, an additional liquid-junction potential can develop, offsetting the measured potential. An additional weakness of this electrode lies in its limited temperature range. The literature suggests an upper limit of about 60°C though reproducible potentials have been reported as high as 100°C. for the saturated calomel electrode. Hydrolysis is one possible culprit for the temperature limitation. This limit is obviously compatible with direct immersion near room temperature or in situations in which an external reference electrode is used .
At 25°C, the saturated calomel electrode has a potential of 0.244V relative to the standard hydrogen electrode. This value is the average of 13 values reported in the literature between 1916 and 1956 and represents the measured values during that period. This value contains a possible junction potential error caused by the frit between the electrode and the environment. In addition, calculations were made to estimate the potential in the absence of the liquid junction. The resulting value is approximately 0.241V. Each is plus or minus approximately 1 millivolt. The response is a function of temperature. This figure shows the values of the potential between 0°C and 100°C. ) .
The equation that regressed the data is
(8)Note that the slope in equation (8) is slightly more negative than some values reported in the literature. Previous Page: Reference Electrode Thermodynamics |
David C. Silverman, Ph.D. - Primary Consultant |