A standard copper/copper sulfate reference electrode
(also known as a half cell or reference cell) must be utilized in order to
obtain structure-to-soil potentials. The reference electrode must be maintained
in good working condition and must be placed in the soil in a vertical position
when conducting a test.
On those sti-Ps tanks that have a PP4 test station, a
reference electrode is permanently buried in the tank pit. Since it is
generally not possible to determine where the permanent reference electrode was
installed on these types of systems, it is also necessary to conduct
structure-to-soil potential measurements in the conventional manner (i.e. with
a portable reference electrode in the soil directly over the tank and at a
remote placement). A tank may not be passed on the basis of a structure-to-soil
potential obtained with a PP4 test station. Both the local and the remote
potential obtained in the conventional manner must indicate that adequate
cathodic protection has been provided regardless of what the PP4 test station
indicates.
Maintenance of the reference electrode is important for
accurate results and includes:
a. The
copper-sulfate solution inside the reference electrode should be clear. If the
solution appears cloudy, this may indicate that the solution has become
contaminated and the reference electrode should be compared with the known
standard as described in paragraph e below. Should it be necessary to replace
the solution, only distilled water and new copper-sulfate crystals should be
used. Excess copper-sulfate crystals must be present in order to assure a
saturated solution. Under average conditions, it is usually a good idea to
empty and replace the solution every 2 or 3 months.
b. The porous ceramic tip must be maintained
moist at all times. If the tip is allowed to dry out, it may lose its porosity
and a good low resistivity contact with the soil will not be possible. Periodic
replacement of the tip may be necessary.
c. The copper rod inside the reference
electrode should periodically be cleaned with non-metallic sandpaper. Do not
use black metal oxide sandpaper, steel wool or any other metallic abrasive as
this can cause the copper rod to become contaminated. If the copper rod becomes
contaminated, it is best to replace the reference electrode.
d. The copper-sulfate solution must be free
of contamination or errors will be introduced in the readings you observe. If
the reference electrode is submerged in water or placed in moist soils that are
contaminated, it is likely that the solution will become
contaminated.
e. The reference
electrode that is used in the field must be periodically calibrated. How often
the reference electrode needs to be calibrated depends upon several different
factors. Among the more important factors that should be considered are the
frequency of use and the exposure of the reference electrode to contaminants.
As a general rule, calibration should be checked once every week if the
reference electrode is used daily. If the reference electrode is only
periodically used, calibration should be checked prior to each use.
Calibration of the reference electrode is accomplished
by comparing it with another reference electrode that has never been used. The
unused reference electrode that is to act as the calibration standard should be
properly set up (ready for use) and must not have ever been used in the field
so that no chance of contamination exists.
To calibrate the field electrode:
1. Place the voltmeter on the 2 volt DC scale
(or lower) and connect the leads to the reference electrodes as shown in the
illustration below.
2. Place both
the field electrode and the standard electrode in a shallow nonmetallic
container that has one to two inches of tap water in the bottom of it. Do not
use distilled water. The reference electrodes must be placed vertically in the
container with the ceramic tip of each submerged in the water.
3. Observe the potential measurement
displayed on the voltmeter. If more than 10 mV potential exists between the two
reference electrodes, the field reference electrode should be properly cleaned
and refilled with new solution until the potential difference is 10 mV or less.
If you are unable to achieve a 10 mV or less potential difference after
cleaning/reconditioning, the field electrode must be replaced.
4. In order to lessen the chance of cross
contaminating the calibration electrode, you should leave the calibration
electrode in the water for the shortest time necessary to complete the test.
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