Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
  • G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
  • G01N27/28—Electrolytic cell components
  • G01N27/30—Electrodes, e.g. test electrodes; Half-cells
  • G01N27/301—Reference electrodes

Definitions

• the present invention relates to a reference electrode for the electrochemical measurement of a solution, consisting of a housing made of an inert, electrically insulating material, which contains an electrochemical reference system in its interior, and at least one electrically conductive electrode in the interior of the housing, which is connected to the electrochemical reference system is in contact and which can be connected to a measuring instrument through an electrically conductive feedthrough through the housing, and from a liquid junction via which the electrochemical reference system is in ion-conducting contact with the solution to be measured.
• the reference electrode is a necessary part of the measuring equipment for potentiometric determination of the activities of substances in solution, e.g. that of protons (pH) or of other ions.
• Such measurements are widely used in the laboratory, in the process control industry, in environmental protection, in medicine and in other fields. The reliability of these measurements is of the utmost importance.
• the behavior of the reference electrode has a significant influence on the displayed measured value.
• a reference electrode must therefore meet the following requirements:
• reference electrodes A general description of the operation of reference electrodes and of various embodiments can be found e.g. in "Reference Electrodes, Theory and Practice", by D.J.G. Ives and G.J. Janz, Academic Press, New York and London, 1961.
• a practically usable reference electrode consists of the electrochemical reference system and the "liquid junction", via which the reference system is in electrical contact with the solution to be measured by ion conduction.
• So-called electrodes of the second type such as, for example, the Ag / AgCl, the Hg / Hg2S04 or the Hg / Hg2Cl2 electrode, are most widespread as an electrochemical reference system.
• redox systems in which both the oxidized and the reduced component are in solution, or in which the reduced component is a metal and forms the electron-conducting phase of the reference system, while the oxidized component is a suitable compound of the same metal in Solution have been proposed for use.
• the liquid phase of the electrochemical reference system contains a dissolved salt in high or saturated concentration. This salt is mostly KC1.
• the liquid junction which creates the ion-conducting connection between the reference system and the solution to be measured, generally consists of a porous stopper, e.g. a glass frit.
• the KC1 solution either flows slowly from the reference solution into the solution to be measured, or the KC1 by diffusion, or components of the solution to be measured diffuse into the reference solution.
• Another possibility for realizing the liquid junction is to use ion exchange membranes. J.E. Leonard describes e.g. in European Patent Application number 86 306847.4, the use of a mixture of anion and cation exchangers, or, of zwitterion exchangers.
• Electrodes of the second type are expensive, sensitive and difficult to miniaturize.
• the therefore proposed reference electrodes with redox pairs which are very simple and robust in design and can also be easily miniaturized, also have decisive disadvantages: on the one hand, the resting potential setting of many redox pairs on a metal electrode is slow or not stable. On the other hand, the concentrations of the components of the redox couple change as a result of diffusion out through the liquid junction, and this also changes Reference electrode potential over time.
• the realization of the liquid junction by means of ion exchange membranes offers at least a partial solution, since the redox pairing can be chosen so that its components can diffuse out only very slowly or not at all through the ion exchanger of the liquid junction and thus the concentrations or the reference electrode potential , remain constant.
• the liquid junctions with ion exchangers, or with mixtures of cation and anion exchangers, which have been proposed up to now do not offer a satisfactory solution either. If only one anion or cation exchanger is used, the reference electrode may depend to an unacceptable extent on the composition of the solution to be measured.
• the use of a mixture of anion and cation exchangers theoretically solves this problem.
• practice shows that the mixing ratio is critical and not easy to control.
• the invention has for its object to provide a reference electrode which has the advantage over the known reference electrodes that it fulfills all the requirements mentioned at the beginning of this description.
• FIG. 1 schematically shows a complete measuring equipment equipped with a first embodiment of the reference electrode according to the invention
• FIGS 2, 3 and 4 show other versions of the reference electrode according to the invention.
• Figure 5 shows a complete, with a further embodiment of the reference electrode according to the invention.
• FIG. 1 shows the complete measuring equipment for the potentiometric determination of the activity of a species in solution, which is equipped with an embodiment of the reference electrode according to the invention.
• the housing (1) of the reference electrode consists of a solid, inert plastic.
• the electrochemical reference system is an electrode of the Ag / AgCl type.
• the metallic conductor (2) which establishes the electrical connection between the electrochemical reference system (3) (Ag) and (4) (AgCl), and the corresponding pole of the measuring instrument (9), is preferably also made of silver.
• an ion-selective electrode (8) such as a pH glass electrode or an ion-selective ISFET, is connected to the other pole of the measuring instrument.
• the inside of the housing is filled with the reference solution (5), which contains KCl in saturation concentration in addition to the components of the electrochemical reference system.
• the liquid junction which is immersed in the solution (10) to be measured, consists of two separate, adjacent openings (6) and (7), one of which is filled with a cation-conducting polymer and the other with an anion-conducting polymer. It is particularly important to note that the cross sections and depths of the openings (6) and (7) are not necessarily the same size. You can also have any shape.
• the openings can optionally be provided with a porous plug made of the same or a different material as the housing. The cation and anion conducting polymers then fill the pores of these porous plugs.
• the two openings (6) and (7) can be placed anywhere in the housing (1), provided that they are both completely immersed in the solution to be measured. However, it has proven to be particularly advantageous if the two openings are adjacent, whereby adjacent is to be understood as meaning that the distance between the openings is of the same order of magnitude as the dimensions of the smaller opening, or even less.
• An essential advantage of the embodiment of the liquid junction with two openings according to the invention is that it is not necessary to mix cation- and anion-conducting polymers or to have to use zwitterion exchangers. This avoids harmful side reactions of the active cation and anion centers in the polymers.
• the equivalence of the cation and anion flows necessary for the independence of the reference electrode potential from the composition of the solution to be measured is achieved by adapting the ratio of the cross sections of the openings (6) and (7) or the thicknesses of the polymer layers. These ratios depend on the ion conductor properties of the polymers used, ie the concentration and mobility of the ions in the polymers.
• ion-specific membranes instead of the ion exchange polymers previously proposed.
• Such membranes are described, for example, in "Medical and Biological Applications of Electrochemical Devices", Edited by J. Koryta, John Wiley & Sons, Chichester, New York, Brisbane, Toronto, 1980. These membranes are produced simply by mixing a carrier polymer, for example PVC, with so-called ionophores and subsequent polymerization. Numerous examples of ionophores are given in the cited book, for example valinomycin for K + and Aliquat 336 for. Cl ⁇ .
• the use of ion-specific membranes significantly reduces the sensitivity of the liquid junction to contaminants that come from the solution to be measured and thus increases the stability of the potential of the reference electrode.
• ion-specific membranes Another advantage of using ion-specific membranes is as follows: From the general electrochemical theory of the potential that develops on a liquid junction, it is known that this potential goes to zero and thus becomes independent of the composition of the solution to be measured if the cation and anion flows through the liquid junction are equivalent. In aqueous solution this is only approximately the case for a few ion pairs. A good example of this is KCl, which is why it is most often used in liquid junction. In the embodiment of the liquid junction with two openings according to the invention, this equivalence can be produced by adapting the cross sections or the thickness of the polymer layers, regardless of the chemical nature of the diffusing ions.
• FIG. 2 shows another embodiment of the reference electrode according to the invention.
• the housing (1) made of inert material in turn has a liquid junction with two openings (6) and (7), which is designed as in the previous example. In this case, however, the electrochemical reference system is a redox system.
• the reference solution (5) contains the dissolved components of the redox system and a salt, the ions of which, like * in the previous example, can diffuse separately through the openings (6) and (7) provided with the cation- and anion-conducting polymers.
• the - redox system can be chosen arbitrarily, but preferably in such a way that its dissolved components diffuse outwards very slowly or not at all through the liquid junction, which is achieved, for example, by the choice of compounds from large molecules.
• the quinone-hydroquinone redox system may be mentioned here as an example.
• the electrode (11), which establishes the contact between the solution (5) and the measuring instrument via the metallic conductor (2), consists in this case of an inert metal, such as platinum, gold, palladium, silver, etc.
• the metallic Conductor (2) can consist of the same metal, or else of a different metal, which is compatible with the material of the electrode (11) and the housing (1).
• a system in which the reduced component is a metal can also be selected as the redox system.
• the electrode (11) consists of precisely this metal and the solution contains a corresponding connection of the metal.
• Any metal with one of its compounds, e.g. also in complex form, e.g. Au, Ag, Cu, Ni, Cd, etc. can be used.
• FIG. 3 Another embodiment of the reference electrode according to the invention with a redox system is described in FIG. 3.
• the peculiarity of this embodiment is that in the housing (1) which contains the reference solution (5) contains with the redox system and two electrodes (11) and (12) are attached by the liquid junction with the two openings (6) and (7), which are filled with the ion-conducting polymers, with the solution to be measured , between which a symmetrical alternating current of any shape but small amplitude is applied.
• the two electrodes (11) and (12) consist of the same metal.
• the two electrodes consist of an inert metal such as Pt, Au, Ag, Pd etc.
• the electrodes (11) and (12) from this metal.
• This embodiment with two electrodes provides particularly stable potentials and rapid potential setting.
• the housing (1) is then preferably made of silicon single crystal by means of the methods known from microelectronics, as shown in FIG. 4.
• the housing can be closed, for example, by means of a glass plate by "anodic bonding".
• the electrical lead, or the electrodes (11) and (12) can be produced by doping the silicon at the relevant points.
• the places in the interior of the housing that have become conductive either serve directly as electrodes for the electrochemical reference system, or they are coated with a suitable metal, for example by electroplating.
• the two openings (6) and (7) for the liquid junction can then be provided in the glass cover or in the silicon housing.
• Such a reference electrode can also be integrated with an ion-specific ISFET on the same silicon substrate and is particularly suitable for medical applications.
• the housing (1) of the reference electrode consists of two separate chambers, the second chamber forming part of the liquid junction.
• the first chamber contains the electrochemical reference system, which, for example, in the embodiment shown in FIG.
• the liquid junction now comprises the two openings (6) and (7), of.

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Cited By (2)

Citations (2)

• 1987
  • 1987-09-25 CH CH370887A patent/CH673158A5/de not_active IP Right Cessation
• 1988
  • 1988-09-26 WO PCT/CH1988/000171 patent/WO1989003032A1/de not_active Application Discontinuation
  • 1988-09-26 EP EP19880908123 patent/EP0333806A1/fr not_active Ceased

Patent Citations (2)

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