TN 21 Maintaining Liquid Filled References

Portable, liquid filled, reference electrodes must be cleaned and refilled regularly.  The copper sulfate solution contains dissolved oxygen.  Oxygen gradually reacts with the copper to form copper oxide which shifts the potential of the reference.  The more copper oxide the greater the shift.  Potential shifts of up to 10 mV in a week’s time are possible.  This problem can be prevented by cleaning and refilling the reference electrode regularly (preferably weekly but at least monthly).  The newer gelled filled portable reference electrodes do not have this problem since the element and gel have minimal contact with the atmosphere.

Another problem occurs on reference electrodes that have a ceramic tip.  The insulating tip is porous so that the copper sulfate solution will leak through and allow conductance.  If the tip dries out, the holes can become partially plugged with copper sulfate salt which increases the electrical resistance through the reference.  The process is progressive, with the resistance increasing a bit more with each dry-down until, eventually, the tip becomes fully insulating (totally resistive).  Boiling the ceramic tip in distilled water for an hour or two will restore it.  A similar problem to this occurs when the ceramic tip becomes plugged with either dirt or oil.  When this happens, the ceramic tip should be replaced.

If you find these technical notes interesting, please share them with your colleagues. Encourage them to periodically visit our blog for new, and hopefully useful, technical notes.

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.

If you find these technical notes interesting, please share them with your colleagues. Encourage them to periodically visit our blog for new, and hopefully useful, technical notes.

TN 2 Element Selection – Copper/Copper Sulfate

Copper/copper sulfate elements are typically used in environments with less than 500 ppm chloride or other halides. They consist of high purity metallic copper immersed in a saturated copper sulfate (CuSO4) solution. Cu/CuSO4 elements can be used in portable, immersion or underground units.  Reference electrodes intended for long term service will contain a gelling agent and do not require any periodic maintenance.  Portable Cu/CuSO4 electrodes which contain a liquid rather than a gelled electrolyte will require periodic cleaning and electrolyte renewal.  Use of Cu/CuSO4 elements in electrolytes with higher halide levels or in electrolytes with any sulfides present will contaminate the element causing its reference potential to drift. Because of their widespread use, Cu/CuSO4 electrodes are the ones upon which many cathodic protection criteria are based.

If you find these technical notes interesting, please share them with your colleagues. Encourage them to periodically visit our blog for new, and hopefully useful, technical notes.