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Basic Chemistry and Basic Corrosion Theory 2:15
CP 1 – Cathodic Protection Tester Course Manual
©
NACE International, 2000
02/01/05
necessary to throw the reversing polarity switch. The meter will now read
to the right, but the position of the switch will indicate that it is a negative
reading.
Reference Electrodes (Half-Cells)
General
Reference electrodes, or half-cells, are important devices that permit
measuring the potential of a metal surface exposed to an electrolyte. An
example is a structure-to-soil potential measurement.
Structure-to-soil potentials are measured in reference to an electrode. What
is often referred to as a structure-to-electrolyte potential is actually the
potential measured between the structure and a reference electrode. The
electrolyte itself has no potential value against which the potential of a
structure can be measured independently of the potential of the reference
electrode used. Therefore, before discussing how to measure potentials
along a structure, we must discuss reference cells.
There are several potential benchmarks in common use, but all of them are
related to a basic standard. In this standard one-half of the cell generating
the potential to be measured is represented by a platinized electrode over
which hydrogen gas is bubbled while immersed in a solution having a
definite concentration of hydrogen ions. If it is arbitrarily agreed that the
potential of the platinized electrode covered with hydrogen in its standard
solution is zero on a scale of potentials, then the potentials of all the other
metals in their appropriate solutions can be described in terms of this
reference.
The standard hydrogen electrode half-cell is awkward to use in most
circumstances in which potential measurements are to be made. Instead,
other combinations of metal electrodes in solution with a specific
concentration of ions are used. The reference cell must be stable and
capable of producing reproducible data.
Copper-Copper Sulfate Electrode
Copper sulfate reference electrodes (CSE) are the most commonly used
reference electrode for measuring potentials of underground structures and
also for those exposed to fresh water. The electrode is composed of a
copper rod, immersed in a saturated solution of copper sulfate, held in a