Semipermeable frits

In fact, the movement of ions in and out of a half cell (for example, across a semipermeable membrane or frit) gives rise to an additional form of potential, the so-called junction potential . Such potentials are discussed in Section 3.6.5. 

In reality, most real cells are never fully at equilibrium because the two half cells are located on either side of a semipermeable membrane, sinter or frit, such as a thin separator of rubber or terracotta (see Figure 3.14). Because ions transfer, and because the two half cells comprise different electrolytes (e.g. in terms of different concentration, etc.), the compositions of the two half cells change with time. 

Reduction of analyte occurs at the cathode (on the right-hand side of the cell). Once formed, however, the reduced form of the analyte couple diffuses across the cell - it may also be swept along by the stirred solution - and/or be re-oxidized again at the anode. Clearly, a single molecule of analyte could be oxidized and reduced many times, thus leading to an artificially high charge at the coulometer. For this reason, the two halves of the coulometry cell should be separated if possible, e.g. with a semipermeable membrane or frit, or we should ensure that the product of electron transfer should be a solid, i.e. it is immobilized as soon as it is formed. 

As seen in Figure 32-12, p. 379, a solution passing through two ceramic frits serves to provide the cross-flow field. The particles are retained on a semipermeable membrane. The field is a pressure field formed by the liquid flowing between the frits. According to J.C. Giddings Science, 260, 1456, 1993), "The sample particles are carried to the channel accumulation zone with the cross flow. Each species then diffuses into an equilibrium based zone of a specific... 

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