External Reference Electrodes

A complete potentiometric transducer includes an external reference electrode. This point is often ignored in the ISE literature, for it is assumed that some suitable device can be incorporated into the cell, and that calibration will overcome any problems associated with variability in this cell component. This approach is inadmissible for a chemical sensor, because the reference electrode must be included as part of the package. Too often, publication of an elegant sensor is marred by a test set-up which includes a large, conventional reference electrode. 

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The potential of die electrode is registered with respect to the external reference electrode. Hence, die cell potential (at 25°C and after introducing the definition of pH) follows the relation... 

A representative ISE is shown schematically in Fig. 1. The electrode consists of a membrane, an internal reference electrolyte of fixed activity, (ai)i , ai and an internal reference electrode. The ISE is immersed in sample solution that contains analyte of some activity, (ajXampie and into which an external reference electrode is also immersed. The potential measured by the pH/mV meter (Eoe,) is equal to the difference in potential between the internal (Eraf.int) and external (Eref.ext) reference electrodes, plus the membrane potential (E emb), plus the liquid junction potential... 

E,j) that exists at the junction between the external reference electrode and the sample solution. 

Without adequate silanization, an aqueous film can short-circuit the d-pipette by linking its two halves. In the absence of a surface aqueous layer the currents flow between each of two pipettes and the external reference electrode. In contrast, when the liquid film connects two orifices, the current flows directly from one barrel to the other. Two experiments allowing the distinction between these two cases were carried out using a well-characterized reaction of potassium transfer from water into DCE facilitated by dibenzo-18-crown-6 (DB18C6) [11]... 

It has become fairly common to adopt the manufacture of combinations of internal reference electrode and its inner electrolyte such that the (inner) potential at the glass electrode lead matches the (outer) potential at the external reference electrode if the glass electrode has been placed in an aqueous solution of pH 7. In fact, each pH glass electrode (single or combined) has its own iso-pH value or isotherm intersection point ideally it equals 0 mV at pH 7 0.5 according to a DIN standard, as is shown in Fig. 2.11 the asymmetry potential can be easily eliminated by calibration with a pH 7.00 0.02 (at 25° C) buffer solution. 

Whether the assumption about this linear relationship can be used for the zero shift as such is doubtful the situation becomes more reliable if the internal and external reference electrodes are equal so that E°mer and °uter cancel, hence eqn. 2.95 becomes En = (- 2.3026RT/F) pHinncr. Therefore, the zero shift can be eliminated instrumentally by setting the mechanical zero of the pH meter to pHjnncr (if previously known). With a non-combined glass electrode the external... 

The integrated planar silver chloride electrode uses a thin layer of 150 pm polymer that consists of a heat curing epoxy resin poly-hydroxy-ethylmethacrylate (PHEMA) to immobilize the KC1 electrolyte. The potential drift of the reference electrode reduced to 59 pV/h after a conditioning phase of several hours. However, this reference electrode was only used for P02 measurement, while an external reference electrode was used for pH measurement. 

After the pioneering work of Tao [111], a number of studies reported how to control the current flow via an external reference electrode in electrochemical STMJs andLAJs [93, 112-143]. 

The bulb is usually fabricated with common soda glass, i.e. glass containing a high concentration of sodium ions. Finally, a small reference electrode, such as an SCE, is positioned beside the bulb. For this reason, the pH electrode ought properly to be called a pH combination electrode, because it is combined with a reference electrode. If the pH electrode does not have an SCE, it is termed a glass electrode (GE). The operation of a glass electrode is identical to that of a combination pH electrode, except that an external reference electrode is required. 

Figure 15.1. Potentiometric measurement for pH. V), glass membrane V2, inner buffer solution V3, internal reference electrode relative to internal buffer V4, external reference electrode V5, diaphragm.

Figure 15.2. Design of the combined electrode. 1, Internal reference electrode, usually Ag AgCl 2, outer glass membrane 3, inner glass membrane 4, external reference electrode, usually Ag AgCl 5, diaphragm.

In the ISE, a2(i) is kept constant. If we express the potential of the ISE by Eise. that of the external reference electrode by Eref, and the liquid junction potential between the sample solution and the external reference electrode by p the emf of cell (5.37) is given by... 

Potentiometric measurements are based on the determination of a voltage difference between two electrodes plunged into a sample solution under null current conditions. Each of these electrodes constitutes a half-cell. The external reference electrode (ERE) is the electrochemical reference half-cell, which has a constant potential relative to that of the solution. The other electrode is the ion selective electrode (ISE) which is used for measurement (Fig. 18.1). The ISE is composed of an internal reference electrode (IRE) bathed in a reference solution that is physically separated from the sample by a membrane. The ion selective electrode can be represented in the following way ... 

Ion selective electrode / sample solution // external reference electrode... 

For studies in aqueous solutions, the external reference electrode is often an Ag/AgCl/KCl electrode. Electrical contact with the solution is achieved using a disc-like membrane made of porous fritted glass. Because ions have a tendency to migrate across the membrane, a small potential Ej is generated by this liquid junction. This phenomenon can be minimised by inserting a saturated KC1 solution as a salt bridge. 

The other group comprises silver-silver halide electrodes, mercury pools, metal-metal-ion electrodes, and others normally prepared In the solvent used for the compound being studied (and often, indeed, employed as internal "reference" electrodes). For such an electrode, the abbreviation alone signifies that the solvent was the same throughout the cell, while the symbol "(w)" for ("water") following the abbreviation signifies that the reference electrode was prepared with water and used as an external reference electrode. 

Fig. 6.10 (a) Classical arrangement of ISE in which Ag/AgCI electrode is used as internal reference, (b) Ag/AgCI electrode with liquid junction for use as external reference electrode... 

Because the electrical circuit is closed inside the sensor, no external reference electrode is necessary and the Severinghaus-type electrode can be used for measurement in either gaseous or liquid samples. It is important to remember, however, that the potential of the internal reference electrode must remain constant. In principle, it would be possible to use a liquid junction but it would add to the complexity of the design. Because the counterion resulting from the dissociation equilibrium is the only interfering ion, and because it is present in a very low concentration, it is possible to ascertain the constancy of the reference potential by careful choice of the internal electrolyte. Thus, for example, in the CO2 electrode the internal electrolyte is O.lMNaHCOs and 0.1 M NaCl and Ag/AgCl is used as an internal reference element. 

Positioning of the external reference electrode. The signal can be affected by local electric fields between indicator and external reference electrodes, an effect that is increased by solution flow, making it important to reduce the distance between the electrodes as much as possible. 

Combination electrode — is a combination of an - ion-selective electrode and an external -> reference electrode in a single unit, avoiding a separate holder for the external reference electrode, i.e., it usually contains one ion-selective membrane and two reference electrodes, one on either side of the membrane. Most commercial -> glass electrodes and -> ion-selective electrodes are combination electrodes. 

Isopotential point — In potentiometric measurements with the use of a -> ion-selective electrode (ISE) cell, the isopotential point is the potential difference between the internal and external reference electrodes which is independent of temperature. The isopotential point is governed by a particular activity of the ion being determined. Both ISE and the outer reference electrode must be specified. When an isothermal cell is used with identical reference electrodes, the isopotential point is defined by the activity of the sensed ions that gives zero net - membrane potential, e.g., sensed activity is the same in the inner and outer (test) solution. Calibration lines for different cell temperatures have different slopes, but they intersect at a common activity point. Cells with temperature gradients are not recommended. 

Figure 5.45. Schematic of a PEM fuel cell MEA with an external reference electrode [67]. (Reproduced by permission of ECS—The Electrochemical Society, from He W, Van Nguyen T. Edge effects on reference electrode measurements in PEM fuel cells.)...

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