TN 19 Junction Potentials

Junction potentials are a common error source encountered when making potential measurements in or through concrete.  Current is carried through an electrolyte by means of ions which physically move through the electrolyte.  In a potential field, anions move in one direction and cations in the opposite direction.  If the mobilities of the ions are unequal, a balancing potential builds up due to separation of the charges.  This potential, termed a junction potential, becomes incorporated into the measurement as an error.  In concrete, it is quite common to have areas of different electrolyte compositions.  For example, sodium chloride (NaCl) is often spread on the surface for deicing; sodium and chloride ions have very different ionic mobilities.  Another example is carbonation of concrete, the reaction of the material with atmospheric carbon dioxide, which proceeds inward from an exposed surface and causes a change in both the chemical composition and pH of concrete.  Each of these can contribute to a junction potential error in concrete measurements.

A junction potential can also form within a silver-silver chloride reference electrode if sodium chloride is used for the filling solution.  The different ionic mobilities will cause the potential to build up across the membrane or frit separating the filling solution from the external environment.   Potassium chloride should be used for the filling solution for silver-silver chloride reference electrodes since the mobility of potassium and chloride ions is similar thus minimizing any junction potential across the membrane.

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