Reference electrode internal

The pressed disc (or pellet) type of crystalline membrane electrode is illustrated by silver sulphide, in which substance silver ions can migrate. The pellet is sealed into the base of a plastic container as in the case of the lanthanum fluoride electrode, and contact is made by means of a silver wire with its lower end embedded in the pellet this wire establishes equilibrium with silver ions in the pellet and thus functions as an internal reference electrode. Placed in a solution containing silver ions the electrode acquires a potential which is dictated by the activity of the silver ions in the test solution. Placed in a solution containing sulphide ions, the electrode acquires a potential which is governed by the silver ion activity in the solution, and this is itself dictated by the activity of the sulphide ions in the test solution and the solubility product of silver sulphide — i.e. it is an electrode of the second kind (Section 15.1). 

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... 

One barrel-tip contains the organic membrane phase and an internal reference electrode the other constitutes a second reference electrode. A four-barrel configuration with a 1-pm tip in which three barrels are liquid membrane electrodes for Na, Ca and and the fourth is a reference electrode has been reported Some representative applications of ion-selective electrodes for intracellular measurements are shown in Table 3. 

HTLC High-temperature liquid (2) Internal reference electrode... 

Accordingly, the standard potential of the ISE depends on the specific choice of the internal reference electrode (e.g. Ag-AgCl) and also that of the fixed A" concentration of the filling solution, so that simply... 

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. 

An inner filling solution and internal reference electrode are used in macro ISEs due to a very good stability of the potential at the inner membrane-solution interface in such a setup (see Fig. 4.4). However, the presence of a solution inside a sensor could be a serious limitation for development of microelectrodes and may be undesired for a variety of other reasons, including ionic fluxes in the membrane and limited temperature range of sensor operation. There are several requirements for such an inner contact. First of all, a reversible change of electricity carriers ions-electrons must take place at the membrane-substrate interface. The potential of the electrochemical reaction, ensuring this transfer, has to be constant, stable, and must not depend on the sample composition. At last, the substrate must not influence the membrane analytical performance. 

Ion-selective electrodes are systems containing a membrane consisting basically either of a layer of solid electrolyte or of an electrolyte solution whose solvent is immiscible with water. The membrane is in contact with an aqueous electrolyte solution on both sides (or sometimes only on one). The ion-selective electrode frequently contains an internal reference electrode, sometimes only a metallic contact, or, for an ion-selective field-effect transistor (ISFET), an insulating and a semiconducting layer. In order to understand what takes place at the boundary between the membrane and the other phases with which it is in contact, various types of electric potential or of potential difference formed in these membrane systems must first be defined. 

The actual ISE system consists of internal reference electrode 2, internal solution of the ion-selective electrode 2 and the membrane. This system is immersed in andysed solution (analyte) 1, in which reference electrode 1 is also immersed, usually connected through a liquid bridge. The electromotive force of cell (3.1.1) is... 

Macroelectrodes with solid membranes contain homogeneous [142] or heterogeneous [25] membranes. The construction of an ISE of this type with an internal reference electrode is shown in fig. 4.1. For good functioning of an ISE it is necessary that the membrane be completely sealed in the electrode body, with no cracks leading to short-circuiting between the external and internal solutions. Cements based on Teflon, PVC or epoxy resin are used (170). 

The temperature coefficient of the ISE potential has received relatively little attention. As follows from (3.1.7), the constant term (the ISE standard potential), the determinand and interferent activity coefficients and the selectivity coefficient, liquid-junction potentials and, of course, also the RTIZfF coefficient, depend on the temperature [118]. When the internal reference electrode and the reference electrode in the test solution are identical, the interferent activity sufficiently low and the liquid-j unction potentials negligible, then the constant term depends on the determinand activity in the electrode internal solution alone and thus the temperature coefficient of the measured EMV depends only on the temperature coefficient of the determinand activity coefficient and on the/ 77z,F coefficient. Measuring instruments are usually... 

Thus, the system comprising membrane, solution 2 of constant composition (internal filling solution), and electrode 2 (internal reference electrode) constitutes an ion selective electrode. The electrically neutral carrier antibiotics of the valinomycin group and related lipid-soluble compounds can serve as the active components of highly selective liquid... 

S. Mine, J. Jastrzebska, Rocz. Chem. 1954, 28, 519-520. In reporting the E20 value for copper in various methanol-water mixtures, these authors did not specify whether percent methanol referred to weight, volume, or mole per cent. Although the potential values are claimed to be referenced to aqueous SHE, the actual internal reference electrode used was not specified and no mention was made of a correction for the liquid junction potential. Since the actual liquid junction potential for aqueous reference electrodes in methanol-water mixtures was not made until a later date [ j —0.152 V for 80% methanol (w/w) M. Alfenaar, C. L. deLigny, Reel. Trav. Chim. Pays-Bas... 

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.

Figure 2.17 Cyclic voltammogram of a MeCN solution of [Cun(16)](CF>SC)2b. Supporting electrolyte 0.1 M [Bu4N]C104 scan rate 0.1 V/s internal reference electrode Fc + /Fc. Diagram adapted from Ref. 20.

Ion-Selective Electrodes [22a] Ion-selective electrodes (ISEs) are usually electrochemical half-cells, consisting of an ion-selective membrane, an internal filling solution, and an internal reference electrode (Eq. 5.37) ... 

Fig. 5.36 Various types of ion-selective electrodes (a) internal reference electrode (b) silver wire for direct contact to the membrane ...

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 ... 

Internal reference electrode 11 internal solution / membrane... 

The potential difference between the internal reference electrode and internal surface of the membrane is constant. Its value is fixed by the design of the electrode (i.e. the nature of the internal reference electrode and internal solution). However,... 

When the concentration of H+ is different on either side of the membrane, a potential difference is generated, which is related to the activity of H+ ions in solution, i.e. pH. The latter is determined using an electronic millivoltmeter, the pH meter, which monitors the potential difference between the glass electrode and an internal reference electrode of Ag/AgCl (currently preferred to the mercurous chloride (Hg) electrode for environmental purposes). After calibration, the instrument will directly yield the pH of a solution. 

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. 

When the glass membrane is exposed to water, a hydrated layer, approximately 50-100 nm thick, is formed at its interface. In addition to water, the chemical composition of the glass in this layer is the same as that in dry bulk. The concentration of the anionic binding sites is estimated between 3 and 10 M. The membrane is usually blown into a bulb of a typical thickness of the wall 50-200 jitm. The optimum thickness of the wall is a compromise between mechanical stability and the electrical resistance. The latter is typically on the order of 10MQ. The interior of this bulb is sealed and contains the internal reference electrode. Thus, the glass membrane is bathed on both sides by solution and a similar hydrated layer develops on the inside of the glass bulb as well (Fig. 6.14). 

Ion-selective membranes can be used in two basic configurations. If the solution is placed on either side of the membrane, the arrangement (e.g., Fig. 6.16a) is symmetrical. It is found in conventional ion-selective electrodes in which the internal contact is realized by the solution in which the internal reference electrode is immersed. In the nonsymmetrical arrangement (Fig. 6.16b), one side of the membrane is contacted by the sample (usually aqueous), and the other side is interfaced with some solid material. Examples of this type are coated wire electrodes and Ion-Sensitive Field-Effect Transistors (ISFETs). 

The potential profile through the membrane that is placed between the sample and the internal reference solution was shown in Fig. 6.3. The composition of the internal solution can be optimized with respect to the membrane and the sample solution. In the interest of symmetry, it is advisable to use the same solvent inside the electrode as is in the sample. This solution also contains the analyte ion in the concentration, which is usually in the middle of the dynamic range of the response of the membrane. The ohmic contact with the internal reference electrode is provided by adding a salt that contains the appropriate ion that forms a fast reversible couple with the solid conductor. In recent designs, gel-forming polymers have been added into the internal compartment. They do not significantly alter the electrochemistry, but add mechanical stability and convenience of handling. 

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. 

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