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Copper-Cupric Sulfate (Cu/CuSO4•5H2O?)
The copper-sulfate reference electrode has the appearance of a
Class 2 reference electrode.
The actual classification may be more complicated because cupric sulfate has
the ability to form more than one type of phase. Thermodynamics strongly
suggests that the salt has 5 waters of hydration associated with it but under
saturation conditions, the number might be less. Hence the use of the question
mark in the title. The potential of this electrode is determined by the redox
reaction between a copper wire and a solution of saturated cupric sulfate
(copper(II) sulfate) in aqueous solution. A major application of this electrode
is to serve as a reference electrode for cathodic protection systems. The
electrode is illustrated in this figure
 .
The depiction is of a generic electrode as used in cathodic protection systems.
The question mark has been retained next to the 5 waters of hydration.
The response of the electrode when uncontaminated has often been stated to follow
the equations in which the copper(II) sulfate is in the form chalcocyanite or
CuSO4
(23)
or
(24)
where
(25)
Ksp in reactions (23) and (25) are for the solubility product
of CuSO4 forming Cu2+ and SO42-.
The question is if the above equations are the best depiction.
From a thermodynamic standpoint the most stable form of copper(II) sulfate
is as chalcanthite or CuSO4•5H2O in which the cupric
sulfate is hydrated with 5 waters of hydration. For this compound, the electrode
reactions become
(26)
or
(27)
where
(28)
Ksp in reactions (26) and (28) are for the solubility product of
CuSO4•5H2O forming
Cu2+, SO42-, and water.
The question is which of these two compounds forms the salt in the electrode in
equilibrium with copper. One way to examine this question is to compare the EMF
for reactions (25) and (28) calculated from thermodynamic data to the EMF reported
for this electrode as used in practice. The response of the electrode in
contact with a saturated copper(II) sulfate solution has been reported to
be approximately +0.316V (SHE) to +0.318V (SHE) versus the standard hydrogen
electrode at 25°C. The EMF of reactions (25) and (28) were estimated
from thermodynamic data for the compounds involved. At 25°C the estimated
EMF for the couple Cu/CuSO4 (equation 25) is 0.382V (SHE) and for
the couple Cu/CuSO4•5H2O is 0.302V (SHE), both
under saturated conditions for copper sulfate.
The potential of the Cu/Cu2+ couple is 0.338V (SHE). This potential
is more positive than the measured potential of the copper sulfate electrode.
The expectation would be that formation of the salt under saturated conditions
would decrease the EMF of the cell. To do so, the salt would have to be at
least somewhat sparingly soluble. The free energy change for salt dissolution
would have to be negative. CuSO4•5H2O is sparingly
soluble with a Ksp estimated to be about ~2.2x10-3
at 25°C. CuSO4 has a Ksp estimated to be
~9.5x102. It is not sparingly soluble. Thermodynamic analysis
suggests that much of the cupric sulfate in the copper sulfate reference electrode
is as chalcanthite or CuSO4•5H2O. A small amount
of chalcocyanite or CuSO4 might be present under saturation conditions
but characterization studies are needed for confirmation.
The influence of temperature on voltage of one example of this electrode has been reported in the
literature (F. Ansuini and J. Dimond, "Materials Performance", Vol. 33,
p. 14, 1994). The relationship converted to degrees centigrade is shown in
this figure  .
The slope of the line suggests a change in emf of about 9x10-4
Volts/°C. This same information can be estimated from thermodynamic data
for the two copper sulfate couples, equations (25) and (28) above. This figure
shows the result. The measured voltage vs. temperature slope does not
agree with the thermodynamic estimates.
One hypothesis for the discrepancy can be given by assuming that (1) the
thermodynamic estimates are reasonable, (2) that
CuSO4•5H2O and CuSO4
are the only salts present, and (3) enough time was allowed for temperature,
junction potential, and phase equilibrium were allowed to be established.
The possibility exists that temperature has a complex effect on the relative
distribution of these salts. If so, the response would be a complex function
of the amount of each type of cupric sulfate present at equilibrium. In addition,
thermodynamic data have been reported for the monohydrate
CuSO4•H2O and the trihydrate
CuSO4•3H2O though their existence is unclear.
If they can exist in the electrode,
their presence would further complicate the emf vs. temperature response of
this electrode. Of course, the possibility exists that the thermodynamic estimates,
the measurement, or both are in error. More detailed characterization
information seems to be needed to fully understand this electrode.
The porous plug in the electrode
provides a restricted diffusion path. This geometry means that a liquid junction
potential can exist between the electrode and the outside environment. The somewhat
simple theory provided in the section
Isothermal and Thermal
Liquid Junction Potentials - Theory is for univalent ions. Copper and
sulfate are divalent ions with different transport numbers (different
contributions to the conductivity). This characteristic suggests larger
liquid junction potentials than for a silver chloride reference electrode
with a plug of the same porosity. In addition, in practice the outside environment
tends to be
far different from that in the electrode interior, further affecting the
liquid junction potential.
Previous Page: Silver-Silver Sulfide (Ag/Ag2S)
Next Page: Mercury-Mercuric Oxide (Hg/HgO)
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