Reference electrode leak rate

In common industrial and laboratory continuous analyzers, the requirements on the magnitude of the cell are not as rigorous as with chromatographic, CFA and FIA detectors and thus the construction of the detector can be quite simple. The principal requirements are a sufficiently high and reproducible flow-rate in the vicinity of the ISE membrane, good electrical connection between the ISE and the reference electrode by the flowing liquid and, when a differential circuit is not used, placement of the reference electrode after the ISE to avoid possible interference from solution leaking from the reference electrode. 

Selection of the type of RE is determined in large part by the composition of the fill solution. The fill solution can have two major effects on the electrochemical measurement. Along with the metal electrode, it controls the value of the RE potential. In addition it is generally concentrated (i.e., on the order to 0.5 to 5 M salt). Thus it can act as a source for ionic contamination of the test solution. Because the most popular reference electrodes contain chloride ion, a known aggressive species, this contamination source must be considered carefully. The rate at which the fill solution contaminates the solution (and vice versa) is strongly dependent on the leak rate of the frit. If the frit (or valve) prevented all movement of ionic species, then the RE would not function as a complete electrical circuit, which is required to allow a potential to be measured. Thus frits are generally designed to leak at rates on the order of lpL/h. 

The reference electrode normally has a slow leak into the solution at a rate of a few microlitre per hour. It is therefore advisable to ensure the filling solution (normally 3 mol/L KCl) is clean, either by using a high grade salt for its preparation, or by piuification. Double-junction reference electrodes exist and these are useful for instance if silver or sulphide are to be determined due to the presence of ionic silver in the silver reference electrode cartridge. 

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