JPS6351503B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to electrochemical detection and measurement, and more particularly to potentiometric devices for the selective detection and measurement of ionic activity in solutions, and temperature compensated electrodes useful in such devices. be. Generally, ion-selective potentiometric electrodes are half-cells containing an ion-selective membrane, at least a portion of which is exposed to a solution containing the particular ionic species whose activity is to be detected and/or measured. Make physical contact. At least a portion of the backside of the membrane is electrically connected to a conductive lead that connects to the input of a high input impedance detector or electrometer.
In the electrode of the present invention, the membrane is electrically coupled to the conductor via an internal ionic solution containing a fixed concentration or activity of the ionic species of interest. The lead is typically connected to the internal solution via an Ag/AgCl or calomel reference electrode to provide a stable, well-defined internal contact potential. When the electrode is brought into contact with the sample system, the transfer of ionic charges to the opposite side of the membrane creates an electrical potential. The system can be completed by contacting the same sample solution with another half-cell, a potentiometric reference electrode that produces a fixed potential. 2
The sum of the potentials developed in the two half-cells can be measured by connecting the two half-cells in series with each other and also with an electrometer. The membranes of such ion-selective potentiometric electrodes are composed of either solid or liquid ion exchangers or neutral sequestrants, and have respective resistances for e.g. H ⁺ , Na ⁺ , K ^{+ ,} etc. Well-known selective glass membranes, crystalline membranes such as _LaF3 selective for ^F- , and liquid substances such as dodecyl phosphate, or antibiotics such as trinatin held in porous inert solid matrices. Matter is as diverse as. According to the well-known Nernst equation,
Potential E measured with an electrometer and activity level of target ion species
The relationship between As and As is usually log-linear over several orders of magnitude of ion activity (for example, for fluorine ions measured with a LaF electrode, from about 1M or more to about
10 ^-5 M or less). The slope of this log-linear relationship is represented by the Nernst coefficient and therefore varies with temperature. Theoretically, for a given electrode, the log-linear relationships for various temperatures intersect at a single point known as the equipotential point. Commercially available PH electrodes are designed so that the equipotential point is close to PH7, and the temperature compensation circuit of commercially available instruments should ideally be designed with this point in mind. However, real electrodes, whether ion-sensitive electrodes or reference electrodes, do not operate exactly as predicted by theory, and the various response curves do not intersect at a single point, but rather over a rather large area. They tend to intersect. The reason is that the Nernst equation for the battery potential E = k + RT / f log (As ⁺ trial) / (in As ⁺ ) (1) (In the formula, as is well known, k is a constant, and RT / f is the Nernst coefficient. (The value is normally 59.16 mv at 25℃), (As ⁺ sample) is the activity of the target ion species in the sample solution, (As ⁺ ) is the activity of the target ion species in the internal filling solution) It can be determined by examining.
These deviations from ideal behavior result from two sources: the time required to reach temperature equilibrium and the nonlinearity of the temperature/superpower characteristics of the electrodes. In equation (1), the second term RT/f log(As ⁺
)/(in As ⁺ ) is the potential due to the ion-selective membrane. The first term, k, is the sum of all other potential sources in the cell, including the external reference electrode potential, the internal reference electrode potential, the liquid electromotive force, and the thermal potential in the solution in contact with the reference element. be. Therefore, the theoretical potential/activity curves for ^different temperatures are: They intersect at true equipotential points. It is clear that these conditions are not met with conventional equipment. Regarding the problem of temperature equilibrium, if a pair of potentiometric electrodes making up a measuring cell are suddenly placed in a sample solution at a different temperature than before, all of the electrode components reach this new temperature. requires a considerable amount of time. This time is used to design the electrode,
Varies depending on environmental temperature and temperature difference. For typical commercially available assembled electrodes, e.g.
If the temperature is changed to ℃, it takes about 5℃ to reduce the internal temperature gradient to a few tenths of a ℃ or less.
A minute to 10 minutes is required. If the potential is measured during the time required to reach temperature equilibrium, the measured value will fluctuate. Difficulties arise when using a reference electrode for both an external reference electrode and an ion-selective electrode, since both require saturation with poorly soluble salts (eg Ag/AgCl). Due to the slow rate of dissolution and precipitation of salts with low solubility,
Such reference electrodes exhibit "temperature hysteresis" because reaching chemical equilibrium requires much more time than it takes to reach the new temperature. This slowness of chemical equilibrium with respect to temperature changes is the main factor causing electrode wander and thus the limiting process that determines the accuracy of measurements and the need for frequent re-standardization. If you wait long enough (sometimes hours or even days longer), current commercially available electrodes can stabilize both the chemical and temperature equilibria and provide reproducible and reliable data. However, there is little practical benefit to true time measurement or process control in such situations. US Patent No. 20 May 1969
In No. 3,445,363, Simon et al. disclose the promise of assembled electrodes in which the isotherms all intersect at the same point around the preferred PH7 and at zero cell potential. For this purpose, Simon et al. add to the internal electrolyte of the assembled electrode a solvent such as water, glycerin, etc., a mono- or polyacid acid such as p-nitrophenol, a mono- or polyacid base such as morpholine, and a buffer. teaches the addition of a buffer consisting of at least one source of ionic species that establishes the potential of an internal reference electrode in contact with the electrode. The internal reference electrode shown by Simon et al. is a conventional metal/metal salt electrode, such as an Ag/AgCl electrode or a Hg/Hg ₂ Cl ₂ electrode. The main object of the invention is therefore to provide a device sensitive to ions in solution and a method for making a potentiometric device, with temperature compensation in the electrode itself. Another object of the present invention is to provide a potentiometric method sensitive to ions in solution that is capable of stabilizing the thermal as well as chemical equilibrium of the electrode much more quickly than the types of electrodes currently in common use. The purpose is to provide electrodes. Yet another object of the present invention is to provide a potentiometric electrode that can be calibrated by the manufacturer to produce a substantially temperature-insensitive ion activity response over a wide range of activities and temperatures. and to provide such an electrode which can be used in a potentiometric measuring device without the need for a temperature compensation circuit, which has a rapid response and is stable at sub-millivolt levels for very long periods of time. It is an object of the present invention to provide a membrane electrode for potentiometric measurements that is simple and inexpensive to manufacture and is substantially temperature-insensitive. Other objects of the invention may be partly obvious;
Some will become clear below. Accordingly, the present invention comprises a process including the method of construction, combination, and arrangement of the components, as well as the various steps and the interrelationship and order of one or more of such steps. , all of which are exemplified in the disclosure below and the scope of application thereof is indicated in the claims. For a more complete understanding of the nature and objects of the invention, reference should be made to the following detailed description taken in conjunction with the drawings, in which FIG. 2 is a schematic diagram of a cross section of a chemical cell; FIG. 2 is a schematic diagram of a longitudinal section of the battery taken along line 2-2 of the drawing in FIG. 1; and FIG. FIG. 4 is an idealized graph of the voltage/time response to temperature changes in the membrane electrode of the present invention; FIG. 4 is a graph of the time/voltage response to substantial temperature changes in the battery of the present invention; FIG. FIG. 6 is a graph of the temperature coefficient curve versus concentration and ratio of a group of selected redox pairs; FIG. 6 is a graph of the temperature/voltage response of a balanced buffer and redox pair; FIG. FIG. 8 is a graph of PH/temperature showing the change in PH/temperature for a standard solution and the response to the standard solution for both an assembled electrode according to the invention and an assembled electrode according to the prior art, and FIG. 3 is a graph showing the temperature coefficient for a redox pair with a small exchange current when combined with The present invention generally comprises an outer envelope containing an internal electrolyte fill electrically coupled to a lead provided for connecting the electrode externally to, for example, an electrometer. Includes a novel potentiometric electrode for use with external sample solutions. The electrolyte preferably does not react with the sample solution and, in the present invention, contains a thermodynamically reversible redox couple with a high exchange current. The conductive wire is an electrically conductive material that is in direct physical contact with the electrolyte and is substantially chemically inert to the electrolyte. The term “potentiometric electrode” is used herein.
"electrode" is intended to cover all electrodes used in electrochemical measurements, in particular both ion-sensitive electrodes and reference electrodes. As used herein, the term “redox pair” is used herein.
couple) is intended to mean an electrolyte containing both oxidation and reduction stages due to different valence states of a given element or combination of elements, where the oxidation and reduction stages are thermodynamically It is reversible, i.e. each step can be converted with an infinitesimal change in potential or from one equilibrium value to another, and the inert metal in contact with the electrolyte changes the ratio of both steps present. It becomes a constant ambiguous potential depending on . Reference will now be made to the drawings, particularly in FIGS. 1 and 2, which illustrate an assembled potentiometric electrode 20 embodying the principles of the present invention. Assembly electrode 20
are typically made of a number of well-known high molecular weight polymers, glasses, etc., which are substantially chemically inert to the electrolyte into which they are intended, as discussed below.
It consists of a novel half-cell 21 containing a first envelope device in the form of a long hollow tubular container 22 made of a liquid-impermeable, substantially rigid, electrically insulating material. One end of the container 22 is sealed with a membrane, typically in the shape of a sphere 24, made of an ion-sensitive material. For purposes of illustration in the embodiment drawings of FIGS. 1 and 2, sphere 24 is considered to be a PH membrane; however, sphere 24 may be made of a wide variety of known materials with unique ionic sensitivities. You will see that you can. The preferred size, shape and strength of the spheres 24 are known in the art and will, of course, depend on the nature of the particular material from which the membrane is made. Membrane container 2
Sealing the ends of 2 will likewise depend on the nature of the material and the technique for performing the seal, all of which are well known in the prior art. As is also well known in the prior art, such potentiometric electrodes typically include an internal ionically conductive filler, ie, an electrolyte, that is electrically coupled to an external lead. For this purpose electrode 2
In the container 22, a phosphate buffer solution (e.g. 0.05M NaH ₂ PO ₄ and
The electrolyte material 26 includes a body of electrolyte material 26 made of a solution containing 0.05M Na ₂ HPO ₄ and selected redox pairs as described below. The electrolytic solution may of course be a true ionic solution, gel, sol, or the like. Electrolyte material 26 is in direct physical and electrical contact with at least one end of conductive wire 28. As is well known in the art, electrolyte 26 and conductor 2
8, the electrodes are a means of electrically coupling to the outside world while maintaining stable internal potentials generated at the various junctions between the various substances in contact with each other within the electrolytic structure. It's for a reason. The construction of the assembled electrode so far described, with the exception of the inclusion of the redox couple, of course, is quite typical of prior art ion-sensitive electrodes. However, unlike typical prefabricated PH electrodes of the prior art, the conductor 28 in contact with the fill fluid 26 is not a standard internal reference electrode, such as a typical Ag/AgCl electrode, but is instead a highly conductive electrode. , and is a substance that is chemically inert to the electrolyte 26. In a preferred embodiment, conductive wire 28 is simply a platinum wire. The electrolyte 26 of the present invention necessarily contains redox couples with relatively high exchange currents, ie, about 1.times.10.sup. ^-6 amp/ ^cm.sup.2 or even larger. As is well known, the exchange current is a kinetic property of a redox pair and is actually a measure of the reversibility of a reversible reaction of a redox pair. Representative redox pairs useful in the present invention are 2e ^- +I ^- ₃ 3I ^- e ^- +Fe ⁺⁺⁺ Fe ⁺⁺ e ^- +Fe(CN) ₆ ^-3 Fe(CN) ₆ ^-4 2e ^- +Br ₂ 2Br ^-. be. It has been observed that substantially inert conductive materials, such as platinum and similar noble metals, in contact with solutions containing redox couples in both oxidized and reduced forms produce fixed and reproducible potentials. Will. As the temperature of the solution changes, the equilibrium between the reduced and oxidized states in the redox pair moves rapidly to reach a new equilibrium, simultaneously generating a new fixed and reproducible potential. Since the substance of the conductor must not dissolve or precipitate at all, the change in the value of the potential occurs very quickly, ie in about a few seconds or even within a short time. Preferably, the redox solution has a very small temperature coefficient of redox potential, so as to overcome the problem of taking a long time to achieve temperature equilibrium. For example, suitable concentrations of I ^- and I ^- ₃ , such as I ^- with a concentration of 5.68 M and I ^- ₃ with a concentration of 3.6 x 10 ^-2 M,
An effective solution can be obtained using the iodide/triiodide pair. Using such a system, temperature changes in the components of the assembled electrodes containing the internal electrolyte, i.e. in the solution being measured, do not have a rapid effect on the potential of the filler and conductors on the cell potential. Therefore, the battery potential can be measured accurately and reproducibly even if temperature equilibrium has not been reached. An assembled electrode embodying the invention is shown in FIGS. 1 and 2 as a combination electrode made of two half-cells, ie, a complete cell. The half-cell 21 so far described can essentially be illustrated by the following example: Pt/electrolyte: I ₃ , I ⁻ /glass/sample solution. The remaining half-cell 29, which is the reference electrode, can be represented by the following example: Sample solution electrolyte: I ₃ , I ⁻ /Pt. The latter group is a summary description of a novel potentiometric reference electrode embodying the principles of the present invention, and which is preferably the same material used to make the container 22, typically is shown in FIGS. 1 and 2 as comprising a second outer packaging device in the form of a long hollow cylindrical container 30 made of a substantially rigid electrically insulating material impermeable to liquids. This will be recognized. The container 30 has a known flow restraining joint 3 shown as a fiber web at one end.
2, this could be a porous frit, a leaky seal, a porous polymer, etc., as is well known in the art. As is also well known, the junction 32 provides a free diffusion path for liquid flow between the interior and exterior of the container 30. Container 30 contains an ionically conductive fill liquid, or electrolyte 34, which contains selected thermodynamically reversible redox couples with the necessary exchange current to provide electrical conductivity. Typically, some arbitrary filler solution may be used to create the ion source, such as 3.5M KCl. This redox pair is preferably the same redox pair used in electrolyte 26, and since half-cell 29 is simply a reference electrode, no buffer is needed in the electrolyte. Finally, the half-cell 29 is shown in the form of a platinum wire 36 in physical contact with the electrolyte 34.
It contains a conductive wire that is chemically inert to the electrolyte 34. Although not necessary for the present invention, for convenience it may be preferable to combine the two half-cells 21 and 29 into a single structure. For this purpose, the first
As shown in FIG. 2, a third outer envelope 38 is provided around containers 22 and 30 and
Preferably, the seal is placed around the container 22 directly above the seal between the container 22 and the membrane 24. Electrolyte 4 is also included in the outer package 38.
0, typically 3.5M KCl compatible with electrolyte 34
Preferably, it is filled with a solution, from which electrolyte 34 can diffuse through junction 32. To complete the cell and provide an ionically conductive path between the half-cell 29 and the sample solution in contact with the outside of the bulb 24, one wall of the outer envelope 38, preferably in close contact with the membrane 24, is provided. , a second bridge, i.e., a flow-restricting junction 42 in the form of a small perforated plate, is provided. To explain the differences between the present invention and the prior art:
A typical example of a typical structure according to the prior art, manufactured by Gebruder Möller Glasbraseri, Zurich, Switzerland
Model 91-02 manufactured by Moeller Glasblaseri
(Model91-02) Combined PH electrode with buffer
A sample solution adjusted to 4.01 was tested in two separate portions of each solution. One solution was initially brought to a temperature of 80°C, and the other solution was initially kept at approximately 26.4°C. The Möller combination electrode was cycled between the two sample solutions, keeping the time interval between removing the Möller combination electrode from one sample solution and placing it into the other solution as short as possible. Orion Research, Cambridge, Massachusetts, USA
Electrometer with high input impedance, Orion Modell811 (Orion Modell811) PH manufactured by Research)
The assembled electrode was left in the given sample until the output was measured with reproducible readings. As can be seen in FIG. 3, the Meller assembled electrode required time intervals of approximately 1 to 2 minutes to obtain a substantially reproducible voltage (MV) at either temperature. A slight change in the successive temperature of the sample solution was observed, apparently due to heat transfer between the sample and the assembled electrode. An assembled electrode embodying the principles of the present invention was constructed with an internal electrolyte made of a phosphate buffer solution containing an iodide/triiodide redox couple and a PH-sensitive membrane. With this electrode, the same operation was performed in the same sample as with the Meller-assembled electrode. Initial temperature is 25℃
When the sample was cycled between temperatures of 1 and 80° C., a reproducible potential showing virtually no temperature dependence was reached within about 30 seconds, as shown in FIG. A device with a redox pair using chemically inert conductive wire as an internal electrolyte and bond for the present invention not only responds much more quickly to temperature changes than devices of the prior art; Also of substantial benefit to the present invention is the characteristic of the temperature coefficient of the pair, ie the change in the contact potential with the metal conductor due to the sensitivity of the equilibrium to the ambient temperature. For example, the temperature coefficients of representative redox pairs, such as iodide/triiodide, which necessarily have large exchange currents, can be adjusted for several different concentrations of the oxidation and reduction stages for various different ratios of triiodide to iodide. It was measured with The several curves shown in Figure 5 are plots of the scale of contact potential (MV) for a platinum wire magnified against temperature in degrees Celsius; This was obtained by experimenting with different values and ratios of iodide and triiodide.

[Table] Another curve E has a ratio of 2×10 ^-4 , an iodide concentration of 5.68M and a triiodide concentration of 3.67×
10 ^-2 M. This curve shows that the temperature coefficient is not completely a function of ratio, but is also dependent on concentration. Fifthly, for a redox pair, the slope, shape and symmetry of the temperature coefficient near the equipotential point of about 25°C can be selected according to the concentrations of the stages of the pair and their mutual ratio.
It is clear from the curves in the figure. Recalling that the internal electrolyte of the PH electrode, i.e., the half-cell 21, contains a mixture of redox couples and a buffer, which fixes the internal activity of the ion of interest, i.e., H ⁺ of a given sample, this Factors become important. As shown in FIG. 6, the temperature coefficient of a buffer solution, such as a phosphate buffer solution, used in the half cell 21 itself is determined by the horizontal axis where the temperature value increases from left to right and the potential, i.e.
If you draw a graph with a vertical axis in which the PH value increases from bottom to top, it is essentially a quadratic curve concave upward.
Curve A in FIG. 6 reproduces the temperature coefficient curve of a typical buffer solution. It is also observed that selected redox pairs exhibit downwardly concave temperature coefficient curves (indicated by B) with the same ordinate and abscissa such that the two temperature coefficients are substantially opposite to each other. Will. Therefore, the pH or Electrolytes 24 for other ion-sensitive electrodes can be prepared. Testing of such electrodes on sample solutions has impressively demonstrated the effect that the temperature coefficients of the redox couple and the buffer in the ion-sensitive electrode are substantially oppositely balanced. For example, a sample solution was prepared with National Bureau of Standards phosphate buffer and its published temperature coefficient is curve A in FIG. Standard Meller Model 91-02 according to prior art
The assembled PH electrode was used to measure the PH of this sample solution over a substantial number of temperatures from about 5°C to 90°C. The PH curve measured with the Meller electrode attached to the Model 811 Orion PH meter is shown in Figure 7B.
Shown below. Not only is it very obvious that there is no correlation between curve A and curve B, but it was also found that the reproducibility of curve B was very poor even after repeated repetitions. On the other hand, using the PH assembled electrode of the present invention in which the internal electrolyte 24 of the half-cell 21 is a mixture of a redox couple and a buffer solution of substantially opposite temperature coefficients, a highly reproducible good curve C
I got it. The coincidence of curves A and C shows the substantial difference between the effect of temperature on prior art electrodes and the substantial temperature independence achieved with electrodes embodying the invention. It shows. The nature of the compensation curve obtained by adding a desired redox pair to the internal electrolyte in the electrode is not necessarily limited to the curve obtained with a thermodynamically reversible redox pair with a large exchange current. Surprisingly, it is not necessarily electrochemically reversible. That is, useful reproducible △E/ △T
I discovered that it is also possible to create curves. This increases the number of redox pairs available for the present invention, thereby substantially increasing the range of potentials and slopes that can be used for compensation. However, such a redox pair with a low exchange current is only effective when combined with a thermodynamically reversible and chemically compatible redox pair with a high exchange current, and under the following conditions (the reason will be explained later). The redox pair with a large exchange current will be called the amplification pair, and the one with a small exchange current will be called the main pair). The two redox pairs must be intimately mixed in a common solution to a degree or greater. Two types of redox pairs are selected so that the main pair undergoes a reversible reaction with the amplification pair in a redox reaction. Can only be used when in . For example, the chlorine/chloride redox couple 2e ^- +Cl ₂ 2Cl ^- described below can be used as the primary pair, which is characterized by such a small exchange current that it cannot usually be measured reliably. . However, a suitable amplification pair meeting the above requirements is, for example, the well-known iodine/iodide redox couple 2e ⁻ +I ₂ 2I ⁻ below. These two react as follows: 2I ^- +Cl ₂ 2Cl ^- +I ₂ rapidly and reversibly. At equilibrium, as described above, the necessary condition is Σ(I ₂ +I ^- )<<<Σ(Cl ₂ +Cl ^- ), so the relative concentrations of Cl ₂ and Cl ^- do not substantially change. Although such amplification pairs are present at relatively low concentrations, the desired △
This results in an exchange current that allows an E/△T measurement to be made, but because the relative amounts of the principal pairs present are so large that the value of E is determined by the principal pair and not by the amplifying pair. It is decided. The effect of the primary pair cancels the effect of the amplifying pair on the measured potential. Prepare a solution containing 0.1 M HCl as a source of chloride ions, add 25 ppm _Cl2 as NaOCl, and add a small amount of amplification in the form of ^10-4 _MI2 ,
It showed two-pair system behavior. The solution was divided into two parts and each was placed in a beaker; the first part was kept at 25°C, and the temperature of the second part was varied. A platinum electrode was inserted into each beaker and connected to an electrometer as an input. The liquids in the two beakers were communicated with each other via a salt bridge in the form of a tube filled with the solution. The temperature of the second beaker was then gradually varied from a low temperature of about 5°C to a high temperature of about 75°C. The resulting potentials, shown graphically in Figure 7 as a function of temperature, are shown by the use of amplifying pairs, even from redox pairs whose exchange currents would normally be unsuitable for making such measurements. This shows that reproducible ΔE/ΔT can be obtained. Since considerable changes may be made in the apparatus described above without departing from the scope of the invention as encompassed herein, all material contained in the above description or shown in the drawings is given only by way of example. should be interpreted as such, and not in a restrictive sense.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of a cross section of a novel electrochemical cell based on the principles of the present invention, and FIG. 2 is a schematic diagram of a longitudinal section of the battery taken along line 2-2 in FIG. 3 is a graph idealizing the voltage/time response to temperature change of a typical membrane electrode according to the prior art, and FIG. 4 is a graph of the time/voltage response to temperature change of a battery according to the present invention,
Figure 5 is a graph of the temperature coefficient curve versus concentration and ratio of the redox pair, Figure 6 is the temperature/voltage response graph of the balanced buffer and redox pair, and Figure 7 is the pH of the standard solution. /Temperature graph and according to the present invention and prior art electrodes
8 is a graph of the temperature coefficient of a redox pair with a small exchange current when combined with an exchange current amplification pair, and 20 in FIGS. 1 and 2 is a graph of the potential difference of the present invention. Measurement electrodes, 21 and 29 are half-cells, 22 and 30 are containers, 26, 34 and 40 are electrolytes, 28 are conductive wires, 32 and 42 are junctions, 36 is a platinum wire, and 38 is an outer envelope.

Claims (1)

Claims: 1. A measurement electrode and a reference electrode, each having an inert electrode conductor and an exchange current of () 1×10 ⁻⁶ amp/cm ² or greater and () temperature independent. a filling solution comprising at least one redox couple having a ratio of reduced to oxidized forms of such magnitude as to provide an electrode potential, a potentiometric device characterized by rapid response and relative insensitivity to temperature changes electrode. 2. The potentiometric measuring device electrode according to claim 1, wherein the redox pair consists of a pair of I ^- and I ^- ₃ . 3. The potentiometric measuring device electrode according to claim 2, wherein 3 I ^- is present in a molar concentration and I ^- ₃ is present in a 1/100 molar concentration. 4. The potentiometric measuring device electrode according to claim 2, wherein the measuring electrode comprises an ion-selective PH membrane. 5. The potentiometric measuring device electrode according to claim 2, wherein the ratio of I ^- ₃ /( ^I- ) ³ is about ^10-4 to ^10-5 . 6. The internal fill liquid comprises at least one amplification couple in the form of a thermodynamically reversible redox couple that provides an exchange current of the order of 10 ^-6 amp/cm ² or more, and at least one amplification couple in the form of a thermodynamically reversible redox couple that provides an exchange current of the order of 10 -6 amp/cm 2 or more; It consists of a primary pair present at or above the concentration of the amplifying pair in a thermodynamically irreversible form with exchange currents of small magnitude, where the amplifying pair and the primary pair interact in a redox reaction. 2. A potentiometric device electrode as claimed in claim 1, which provides a substantially greater exchange current than that provided by the primary pair alone, but provides a voltage originally measured by the primary pair. 7. The electrode of claim 6, wherein the amplifying pair is an iodine/iodide pair and the dominant pair is a chlorine/chloride pair.