Argentum Solutions, Inc.
Sterling guidance on corrosion and materials degradation
| Potential-pH Diagrams |
 |
|
| Intelligent Tools |
 |
|
 |
|
 |
|
| Corrosion Calculator |
 |
|
| Corrosion Economics Estimator |
 |
|
|
David C. Silverman |
|
Use of mercury-Mercurous sulfate reference electrodes has been suggested for those situations in which chloride contamination from the electrode could be an issue. This figure shows an illustration of this electrode ) .
The porous plug at the bottom forms the junction between the electrode and
environment. One application for this electrode has been as an external
reference electrode for electrochemical corrosion studies in sulfuric acid environments
in which chloride is not present and chloride contamination could interfere with
the electrochemical results and subsequent corrosion prediction.
The electrode has some disadvantages. It contains mercury. The mercurous sulfate salt can hydrolyze because of its high solubility in aqueous environments. This high solubility forces the filling solution to be saturated in a sulfate salt, the most common of which is potassium sulfate. The concentration is high leading to salting out near the frit. Crystals are always present. On the plus side, the high solubility also leads to low polarizability and excellent reproducibility of potential. The electrode has usually been used at temperatures near room temperature. This figure )
shows the standard potential (EHg/Hg2SO4)
vs temperature between 0°C and 60°C. The temperature coefficient
is -8.1x10-4V/°C. This potential has to be corrected for
the concentration of sulfate ion to obtain the electrode output. When the
salt is saturated potassium sulfate, the electrode potential relative to the
saturated
calomel electrode is about 0.400 volts. One or two millivolt
variation between electrodes is not uncommon. One additional issue is that
the junction
potential is higher for this electrode than for one containing
potassium chloride. The reason is that the difference in transference
numbers between the sulfate and potassium ions is greater. In addition,
two potassium ions have to accompany each sulfate ion for charge neutrality
to be maintained.
Previous Page: Isothermal and Thermal Liquid Junction Potentials - Calculation |
David C. Silverman, Ph.D. - Primary Consultant |