Selective Ion Sensitive Electrode and Method of Making It

The invention relates to a selective ion sensitive electrode, and more particularly, to such electrode which may be used to determine the concentration of a specific ion contained in a solution to be examined.

There have been proposed various selective ion sensitive electrodes having an ion sensitive assembly which is selectively sensitive to a specific ion for determining the concentration of the specific ion by comparing a potential developed across an interface between a solution to be examined and the ion sensitive assembly when the latter is immersed into the former with a suitable reference potential. FIG. 1 illustrates an example of conventional selective ion sensitive electrodes. Such electrode is so constructed that a supporter 3 holding an ion exchanger 2 which is selectively sensitive to a specific ion is joined to one end of a stem 1 made from material such as glass, polyvinyl chloride or the like with adhesives 4 such as silicon RTV rubber manufactured by SHINETSU Chemical Co., Ltd. A signal wire 5 formed with coaxial cable has its one end connected to the supporter 3 by passing through the stem 1 and through the medium of an electric conductive resin 6. In the selective ion sensitive electrode thus formed, when the side of ion exchanger 2 which acts as an ion sensitive assembly is immersed into a solution to be examined an interface potential corresponding to the concentration of a specific ion may be developed across an interface between the ion exchanger 2 and the solution to be examined, thereby enabling a determination of the concentration of the specific ion to be made by detecting the interface potential.

However, because such a conventional selective ion sensitive electrode is formed so that the signal wire 5 is connected to supporter 3 or ion exchanger 2 through conductive resin 6, there are disadvantages in that its manufacturing is difficult, the joining strength between signal wire 5 and supporter 3 is small and a mechanical failure often occurs. Therefore an inconvenience is experienced in that stable use over a long term of time can not be expected.

It is an object of the invention to provide a selective ion sensitive electrode in which an ion exchanger selectively sensitive to a specific ion is formed on a metal electrode member, an electrically conductive resin electrode or a chlorinated metal electrode to which a signal wire is connected.

In accordance with the invention, an ion exchanger is formed on a metal electrode member or the like. Easy manufacturing of the selective ion sensitive electrode and stable performance of an electric connection between the signal wire and the ion exchanger over a long period of time are achievable.

FIG. 1 is a cross section illustrating an example of conventional selective ion sensitive electrodes.

FIG. 2 is a cross section of a selective ion sensitive electrode according to an embodiment of the invention.

FIG. 3 is a cross section of a selective ion sensitive electrode according to another embodiment of the invention.

FIG. 4 is a cross section of a selective ion sensitive electrode according to a further embodiment of the invention.

FIG. 5 is a cross section of a selective ion sensitive electrode according to a still further embodiment of the invention.

FIGS. 6(A) and (B) are an outside perspective view and a half cutaway perspective view of a selective ion sensitive electrode according to a still further embodiment of the invention.

FIG. 7 is a cross section of a selective ion sensitive electrode according to a still further embodiment of the invention.

Referring now to FIG. 2, in which a selective ion sensitive electrode according to an embodiment of the invention is illustrated, the electrode has a disc-shaped metal electrode 12 fitted into one end of a stem 11 made from polyvinyl chloride. An ion exchanger 14 is formed so as to be substantially flush with the end plane of stem 11. This is achieved by applying or pouring a solution in which is dissolved an ion-exchange substance or the like which substance is selectively sensitive to a specific ion. This substance is dissolved in a volatile solvent which is applied to metal electrode member 12 over a layer of silane coupling agent 13. The solvent is then permitted to evaporate. Subsequently a signal wire 15 formed as part of a coaxial cable is directly connected to metal electrode member 12 (upper surface in the drawing) through stem 11.

The silane coupling agent 13, as is indicated by its molecular formula R'Si(OR).sub.3, includes more than one reaction radical (methoxy, silanol radical or the like) strongly joinable by chemically bonding an inorganic substance (glass, metal or the like) into a portion of its molecular formula and another reaction radical (vinyl, epoxy, methacryl, amino radical or the like) which chemically bonds with an organic substance (all kinds of synthetic resins). Consequently the silane coupling agent 13 acts as a mediator between inorganic and organic substances.

The effect of the silane coupling agent 13 differs depending upon a resin or an elastomer to be applied. By way of example, as to the silane coupling agent, NUC Silicones, manufactured by Japan Uniker Co., Ltd., A-186 [beta-(3,4 Epoxycyclohexyl)ethyltrimethoxysilane], A-187 (gamma-Glycidoxypropyltrimethoxysilane), and A-189 (gamma-Mercaptopropyltrimethoxysilane) are effective for polyvinyl chloride and in addition A-189, A-1100 (gamma-Aminopropyltrimethoxysilane) and A-1120 [N-beta-(aminoethyl) gamma-aminopropyl-trimethoxysilane] are suitable to nitrile.

On the other hand, in the selective ion sensitive electrode of the embodiment of the invention, the above-mentioned solution which forms ion exchanger 14, utilizes a blend of, for example tetrahydrofuran (THF), a supporting substance such as polyvinyl chloride (PVC) or dioctyl adipic acid (DOA) and an ion-exchange substance sensitive to a specific ion. When such solution is poured onto metal electrode member 12, THF, which is volatile, evaporates, whereby substances other than THF remain on the surface of metal electrode 12. Of the remaining substances, PVC may be chemically bonded with metal electrode member 12 by action of the silane coupling agent 13 as stated above. In addition, when a solution containing an ion-exchange substance is poured into polyvinyl chloride stem 11, a part of the inner wall thereof dissolves, thereby the stem 11 is bonded with the ion-exchanger 14 by merging of PVC supporting the ion-exchange substance after evaporation of the THF.

When the selective ion sensitive electrode of the embodiment thus formed is immersed in a solution to be examined, a potential corresponding to the concentration of a specific ion, as is the case with a conventional selective ion sensitive electrode illustrated in FIG. 1, may be developed across an interface between ion exchanger 14 and the solution to be examined, therefore permitting a determination of the specific ion concentration in the solution.

According to the electrode of the embodiment, the ion exchanger 14 is bonded with the PVC of stem 11 by action of THF in the solution and also is chemically bonded with the metal electrode member 12 by action of the silane coupling agent 13. Consequently a lowering of life of the electrode due to permeation of a solution to be examined between the stem 11 and the ion exchanger 14 can be avoided.

Furthermore, the signal wire 15 is directly connected to the metal electrode member 12, and not through a conductive resin, thus enabling the connection to be firm. As a result, it will be noted that with metal electrode member 12 and stem 11 firmly secured to ion exchanger 14, a selective ion sensitive electrode having a durable and compact construction and easy handling can be obtained.

In addition, compared with a conventional electrode, the one of the above embodiment has a smaller number of parts and therefore it is easier to manufacture. Thus these electrodes which are inexpensive and have uniform performance may be mass produced.

Still further, it should be understood that since ion exchanger 14 can be securely held on metal electrode member 12 its surface area can be increased. Thus a quick responding electrode is obtainable.

FIG. 3 illustrates a selective ion sensitive electrode of another embodiment according to the invention. The electrode has an electrode member 16 upon which is formed an ion exchanger 14. Member 16 is made from a thermoset conductive resin or a metal chloride. The electrode member 16 thus formed is fitted into one end of stem 11, electrode member 16 being held in an inner stem 17 made from glass or the same polyvinyl chloride as stem 11. This is a departure from the electrode shown in FIG. 2. Other members of the embodiment are constructed similarly to those of an electrode in FIG. 2. Accordingly, corresponding parts are designated by like reference numerals or characters without repeating their description. It is also to be noted that metal chlorides are generally easily obtainable by means of an electrochemical technique using a diluted hydrochloric acid solution.

It should be also understood that with the electrode of the embodiment thus formed the same function and effect as indicated in that shown in FIG. 2 can be obtained.

FIG. 4 illustrates a selective ion sensitive electrode of a further embodiment of the invention. The electrode, in which a coaxial cable is not used as a signal wire, has a signal wire 19 integral with a metal electrode 18 by machining the metal electrode 18 associated with ion exchanger 14. The whole member thus formed except a portion of ion exchanger 14 in contact with a solution to be examined is covered with an insulating member 20. A metal stem 21 is disposed around the outer periphery of insulating member 20 so as to provide a shielding effect and another insulating member 22 is disposed around the outer periphery of metal stem 21 except a portion in engagement with an electrode holder (not shown in the drawing). In this case the ion exchanger 14, which is similar to the electrode shown in FIG. 2, is formed on a metal electrode 18 over the silane coupling agent 13.

The selective ion sensitive electrode of the embodiment has a function and an effect similar to that shown in FIG. 2. Since signal wire 19 and metal stem 21 of the electrode may also serve as a connector (compared to a conventional electrode which incorporates a BNC connector), its manufacturing cost can be advantageously decreased. Consequently, it is also noted that when the electrode has not carried out its function, it is interchangeable without changing a connector and also can be easily interchanged when the concentration of a different ion is to be determined.

FIG. 5 illustrates a selective ion sensitive electrode of a still further embodiment of the invention. The electrode includes a conductive thin film 23 of 0.1 to 5.mu. thickness formed of corrosion resistant Au, Ag, Pt or an alloy thereof on the under and side surfaces (see FIG. 5) of metal electrode member 12 by an electrochemical technique, sputtering, CVD (Chemical Vapor Deposition) method, vacuum evaporation method or the like. An ion exchanger 14 is formed on the under surface of metal electrode member 12 which the conductive thin film 23 covers, over the layer of silane coupling agent 13. A signal wire 15 formed with a coaxial cable is directly connected to the upper surface of metal electrode member 12 within stem 11.

A corrosion resistant conductive thin film 23 is formed on metal electrode member 12 to solve the problem of limited life when a base metal such as Cu, Fe, Ni or the like or an alloy such as brass or the like is used to form metal electrode member 12. However, when a noble metal such as Au, Ag, Pt or the like having a good corrosion resistance is used for the entire electrode 12 cost is increased. Accordingly, in the electrode of the embodiment, a corrosion resistant conductive thin film 23 is formed on at least one surface of metal electrode member 12, in particular on at least the surface on which ion exchanger 14 is formed. An electrode member 12 can be formed with a high conductivity and low cost material such as Cu or the like and even when the thin film 23 is formed with a noble metal such as Au, Ag, Pt or the like the film thickness can be extremely small, therefore producing an inexpensive and durable electrode.

FIGS. 6(A) and (B) illustrate a selective ion sensitive electrode of a still further embodiment of the invention. In this electrode, which is a through-flow type, a rectangular opening is formed on a part of a hollow insulating stem 31 through which a solution to be examined flows and an electrode assembly 32 is disposed over the opening so as to determine the concentration of a specific ion contained in the solution to be examined while the solution flows through the stem 31. The electrode assembly 32 is formed with a conductive thin film 34 of 0.1 to 5.mu. thickness made from a noble metal such as Au, Ag, Pt or the like on the inside of a saddle-shaped metal electrode 33 and an ion exchanger 36 is formed on the film 34 over a layer of silane coupling agent 35. The electrode assembly 32 is secured to the area of stem 31 around the opening with insulating adhesives such as epoxy resin or the like and a signal wire 37 is connected to the outer surface of metal electrode 33.

The electrode assembly 32, which is provided at a portion of hollow insulating stem 31, may be provided around the periphery thereof. Specifically a ring-shaped electrode is employed as metal electrode 33 and an ion exchanger 36 may be formed over a conductive thin film 34 and the layer of silane coupling agent 35 on the inner periphery of the electrode.

In the electrode of the above embodiment which is formed as a through-flow type, the same function and effect as indicated for the electrode shown in FIG. 5 are obtainable.

FIG. 7 illustrates a selective ion sensitive electrode of a still further embodiment of the invention. This electrode differs from the one shown in FIG. 5 only in that a surface of metal electrode member 12A on which an ion exchanger 14 is formed is made rough to the extent that the roughness is 1 to 100.mu. and a conductive thin film 23 is formed on the rough surface. It will be understood that when the surface of metal electrode members 12A on which thin film 23 is formed is made rough the adhesion between thin film 23 and ion exchanger 14 can be improved. It should be also understood that in the electrode of a through-flow type shown in FIGS. 6(A) and (B), if the inner surface of metal electrode 33 is made rough to the extent that the roughness is 1 to 100.mu. the adhesion between ion exchanger 36 and thin film 34 can be improved.

In each of the above-mentioned embodiments, while a metal electrode member includes an ion exchanger which is formed over a layer of silane coupling agent, if the adhesion between the metal electrode and the ion exchanger is strong the silane coupling agent may be dispensed with.

It is also to be noted that in the electrode of the embodiments of FIGS. 5 to 7, while a conductive thin film is formed on a surface of the metal electrode member on which an ion exchanger is formed, it may be formed so as to cover the whole surface of the metal electrode member.