Diffusion potentials as source of error in electrochemical measurements in concrete

Condition assessment or controlling systems
for cathodic protection of reinforced concrete
structures are usually based on potential measurements.
In case of permanent monitoring systems,
reference electrodes are embedded in the concrete,
whereas for occasional investigations external reference
electrodes are normally used and placed on the
concrete surface. However, ionic concentration gradients
between the inner solution of the reference
electrode and the concrete pore solution give rise to
liquid junction potentials. In addition, internal gradients
in pH or chloride content, as well as permselective
properties of the cement paste lead to membrane
potentials. Although these phenomena are usually not
considered to be significant, they might be underestimated
in many cases and lead to misinterpretation of
measurement results. This is especially important with
regard to applications of potentiometric sensors such as
chloride or pH sensors, where accurate potential
readings are fundamental. This article presents the
theoretical background and summarises available
literature with regard to diffusion potentials in concrete.
Liquid junction potentials between various
solutions have been measured experimentally. Practical
consequences and possible attempts to minimise
measurement errors are discussed.