This application claims priority under 35 U.S.C. §119 to German Patent Application No. 10214035.9 filed in Germany on Mar. 27, 2002, the entire content of which is hereby incorporated by reference.
1. Field of the Invention
The invention relates to a polymer electrolyte, to a half cell, and to uses of the half cell and of the polymer electrolyte.
2. Prior Art
Numerous half cells for electrochemical measurements, such as potentiometric or amperometric measurements, are known. In particular, such half cells can be embodied as reference electrodes, which are used in combination with potentiometric or amperometric sensors. In such reference electrodes, it is imporant that they output as constant a reference potential as possible.
In one type of reference electrodes, there is a fluid reference electrolyte, such as an aqueous potassium chloride solution, which can be brought into contact with a fluid measuring medium via a liquid connection (also known as a “liquid junction”). To avoid or reduce an unwanted mass transfer between the measuring medium and the reference electrolyte, the liquid junction is typically designed as a more or less porous diaphragm. One disadvantage of such liquid junctions, however, is that soiling or even plugging up of the pores can occur, which can lead to considerable faulty potentials and possibly interruptions.
In another type of reference electrodes, instead of the diaphragm, a single opening or a plurality of openings is provided, as a result of which the aforementioned problem of soiling can be largely avoided. However, this embodiment means that instead of the otherwise usual fluid or gel-like reference electrolyte, a non-flowable reference electrolyte is used, to prevent the reference electrolyte from flowing out. A polymer electrolyte in the form of a hydrogel, which for instance contains a saturated aqueous potassium chloride solution and preferably additionally suspended potassium chloride is especially well suited for this purpose.
In European Patent Disclosure EP 1124132 A1, which is hereby incorporated by reference in its entirety, a reference electrode of this generic type is described which contains a polymer electrolyte that contains a polymer on the basis of monomers selected from N-substituted acrylamides and/or methacrylates. For instance, the methacrylate is a methacrylate with at least two hydroxy groups, and in particular 2,3-dihydroxypropyl methacrylate.
An object of the invention is to furnish further polymer electrolytes which are especially well suited for electrochemical half cells and in particular for reference half cells. Other objects of the invention are to furnish an improved half cell and to disclose uses of the half cell and of the polymer electrolytes.
The polymer electrolytes of the invention contain a polymer which as a first monomer component contains at least one alkyl methacrylate. The aforementioned alkyl methacrylate has a substituted alkyl group with from three to seven carbon atoms and at least two substituents, and the aforementioned substituents are selected from the group comprising OR1 and NR2R3, and R1, R2 and R3 are selected from the group comprising hydrogen, methyl, and ethyl, on the condition that the substituted alkyl group contains the substituent OH at most once. By a suitable selection of the substituents in the aforementioned range, the properties of the polymer electrolyte, in particular its polarity and hence its resistance and stability to water or to polar or apolar solvents, can be adapted accordingly to the intended area of use. Moreover, the polymer electrolytes of the invention are distinguished by good resistance and stability to acids.
The half cell of the invention, which can be used as a component in potentiometric or amperometric sensors, contains one of the polymer electrolytes of the invention. The polymer electrolyte of the invention can also be used as a solid-phase electrolyte in a battery half cell.
Exemplary embodiments are directed to the substituted alkyl group of the alkyl methacrylate intended as the first monomer component. According to exemplary embodiments, the substituted alkyl group is a 3-amino-2-hydroxypropyl or a 2-amino-3-hydroxypropyl. Moreover, in accordance in exemplary embodiments, the substituted alkyl group may be a 3-diethylamino-2-hydroxypropyl, a 3-ethoxy-2-hydroxypropyl, a 3-methoxy-2-hydroxypropyl, or a 3- methylamino-2-hydroxypropyl. Furthermore, according to exemplary embodiments, the substituted alkyl group can be a —CH2—CH2OH—CH2—NR2R3 or a —CH2—CH2—NR2R3—CH2OH, in which R2 and R3, are as described above. Also, the substituted alkyl group can be a —CH2—CH2OH—CH2—OR4 or a —CH2—CH2—OR4—CH2OH, in which R4 is a methyl or ethyl. However, a mixture of alkyl methacrylates of the type named above can also be contemplated as the first monomer component.
According to exemplary emboidments, the polymer contains, as a further monomer component, a hydroxyalkyl methacrylate, which is preferably 2-hydroxyethyl methacrylate and/or 3-hydroxypropyl methacrylate. With this further monomer component, the polarity of the polymer electrolyte can advantageously be varied, and by a choice of the quantity ratio of the further monomer component to the first monomer component, the polarity is adjustable over a wide range.
Exemplary embodiments can be used in conjunction with a half cell with a glass housing. Because the polymer contains, as an additional monomer component, a silylated alkyl methacrylate, preferably 3-(trimethoxysilyl)propyl methacrylate, an adhesion of the polymer electrolyte to the glass housing is achieved, as a result of which a longer service life of the half cell and in particular better resistance to pressure and washing out is attained.
Exemplary embodiments are directed to a polymer electrolyte. This polymer electrolyte can additionally contain a concentrated aqueous solution of a salt or salt mixture, which is indicated for use in polar measuring media. The polymer electrolyte can contain a mixture of an organic solvent and an aqueous solution of a salt and is accordingly predominantly usable for less-polar measuring media. The organic solvent is selected from the group comprising glycerine, ethylene glycol, methanol, ethanol, N-propanol, isopropanol, acetone, and mixtures thereof. It is especially advantageous if the salt is additionally in the form of a suspension. As a result of the increased salt content, a longer resistance of the half cell to washing out of the salt as a result of the contact with the measuring medium is achieved. On the other hand, the progressive depletion of the salt caused by the washing out can be readily ascertained visually from the developing washing-out or depletion front, which forms the boundary between a region of the polymer electrolyte that is clouded by the salt suspension and a region of the polymer electrolyte that is more clear because the salt suspension is no longer present there. For instance, the aforementioned salt is selected from the group comprising potassium chloride, sodium chloride, lithium chloride, potassium nitrate, potassium perchlorate, sodium formiate, lithium acetate, lithium sulfate, ammonium chloride, methylammonium chloride, dimethylammonium chloride, trimethylammonium chloride, and mixtures thereof. This salt can, however, also be a further ionic organic halide, for instance. This salt can also form a redox system.
The half cell can have an open liquid junction between the polymer electrolyte and a surrounding medium—as a rule, a measuring medium or a fluid specimen. This embodiment is possible because the polymer electrolyte is essentially in solid form and accordingly cannot escape from the open liquid junction. By dispensing with a diaphragm, unwanted interfering potentials in the region of the liquid junction can be largely avoided.