Electrode device with a solid state reference system

Details Electrode device with a solid state reference system Electrode device with a solid state reference system

2002-08-29

2004-10-19

Bell, Bruce F. (Department: 1746)

Chemistry: electrical and wave energy

Apparatus

Electrolytic

C204S419000, C204S435000

Reexamination Certificate

active

06805781

ABSTRACT:

The present invention concerns an electrode device comprising an ion selective material, a solid state, inner reference system of a bronze and a contact material, where the reference system mediates the electrochemical correspondence between the ion selective material and the contact material. More specifically, the invention concerns a planar, miniaturised electrode device with a solid state, inner reference system.
In many cases it is interesting to analyse samples for content or presence of different ions. For this purpose, electrode devices often include an ion selective membrane, an inner reference system and a contact material, which is connected with usual measuring equipment together with an external reference electrode. The inner reference system mediates the electrochemical correspondence between the ion selective membrane and the contact material, and it has the purpose of maintaining a stable inner electrochemical potential.
As inner reference system, conventional electrode devices comprise a metal dipped in into an electrolyte liquid. The equilibrium reaction between the metal ion of the reference system and the anion of the electrolyte liquid normally takes place via a solid metal salt of these ions, which salt is deposited on the metal. These types of reference systems containing a liquid are stable and reliable in use, but they have some disadvantages. They are relatively fragile, cannot be used at high temperatures, has a storage stability which is limited by evaporation of the electrolyte liquid, and they also limit the possible methods of manufacture due to the fluid electrolyte and the relatively large volume of electrolyte liquid which is required to achieve a sufficient stability for the system. If the volume of the electrolyte liquid is reduced substantially, the system becomes sensitive to redox active substances-and carbon dioxide. The conventional reference systems are actually unsuitable for miniaturisation. Attempts to reduce or avoid these disadvantages have lead to the development of electrode devices with a solid state, inner reference system.
Certain properties of the reference system are thought to have a stabilising effect on its potential. Thus, the ability of the reference system to take up and liberate electrons across a phase boundary Influences the stability of said reference system. The uptake and liberation of electrons typically take place between two phases in the system or between the system and the surroundings and result in a so-called exchange current. It also promotes the stability, if the reaction(s) implicating the uptake/liberation of the electrons are reversible and occur to such extent that the magnitude of the exchange current does not have a limiting effect.
In the literature, several solid state, inner reference systems are described. In one type of reference systems, the same metals and salts are used as in the conventional inner reference systems, but the salts are dissolved in hydrocolloids or other highly-viscous materials with water activity. As the conventional reference systems, these types of reference systems will, if miniaturised, be sensitive to redox active substances and to carbon dioxide present. Their use at high temperatures is also limited. Examples of such inner reference systems are described in e.g. U.S. Pat. Nos. 5,911,862, 5,552,032 and 5,041,792.
In another type of reference system, a redox active substance (e.g. a conventional redox pair or a redox polymer) is immobilised in the membrane or between the membrane and the contact material. The contact material often consists of a precious metal, but can also consist of graphite. The redox active groups may be incorporated in the membrane or constitute a layer between the contact material and the membrane. This type of reference system will typically be sensitive to redox active substances present. Examples of such inner reference systems are described in e.g. EP 498 572; U.S. Pat. Nos. 5,286,365; 5,326,452; 4,871,442; 4,981,570; 4,798,664; 4,816,118; 5,139,626; 5,192,417 and EP 927 884. Among these, typical examples of this type of solid state, inner reference system are EP 498 572, wherein a fortiophore is used for complex binding in the membrane of the ion present in the contact material, U.S. Pat. No. 5,286,365, wherein an electrode device is described in which a reference layer comprising a redox pair is placed between the contact material and the membrane, and U.S. Pat. No. 5,326,452 in which the electrode device has a reference system of iron oxide, and where iron oxide is also incorporated in the glass membrane. This electrode device can be prepared by thick film printing.
In a third type of reference system, a solid state reference system is used which possesses both electron conductivity and ion conductivity. This material may typically be a bronze or another metal oxide. In this case, the stable, inner potential is achieved by an exchange current between bronze/metal oxide and the membrane, and it can be further stabilised if two phases are present in the bronzelmetal oxide.
In GB 1 470 558 such electrode device for determining components in solid or fluid metals or alloys is described. It is especially used for determining sodium in such metals or alloys. The reference system of the electrode device comprises a solid state electrolyte of &bgr;-aluminium oxide containing the component which is to be determined, or it comprises the component which is to be determined in solid state, covered by the above-mentioned electrolyte for protection of the solid state reference system. The reference system should preferably have two phases. DE 25 38 739 (GB 1 521 964) and GB 1 602 564 disclose electrode devices, which are further developments of the above-mentioned electrode device. In DE 25 38 739 the reference system includes tungsten bronze or mercury amalgam, protected by the above-mentioned electrolyte. Both the reference system and the protection layer must contain the ion which is to be determined. The reference systems may have several different cations incorporated at the same time. Further, it is mentioned that the tungsten bronze may contain other transition metals including vanadium. These electrode devices are sensitive to the oxygen pressure above the reference system. In the electrode device disclosed in GB 1 602 564, the reference system is constituted by two aluminium oxide phases covered with a further layer for fixing the oxygen potential, in order to reduce this oxygen sensitivity. Metal/metal oxide mixtures are suitable for this layer, e.g. of copper, chromium or nickel. Neither the operational stability nor the storage stability are particularly good for the sodium tungsten bronze.
In WO83/03304 (U.S. Pat. No. 4,632,732) a H
30
-selective glass electrode device with a solid state, inner reference system is described preferably of biphased lithium vanadium bronze. It is mentioned that other lithium bronzes and sodium tungsten bronze may also be suitable. Such a lithium vanadium bronze as well as an electrode device including this must be prepared under anhydrous and oxygen-free conditions. Therefore, in practice, the electrode device will be complicated and expensive to prepare e.g. by thick film printing.
In U.S. Pat. No. 3,853,731 an ion selective glass electrode is described with a solid state reference system of a composite material of silver and silver halide. A paste of silver oxide and a silver salt of a halogen oxy acid fixed to the glass membrane is heated to obtain the composite material of silver and silver halide.
In U.S. Pat. No. 5,122,254 an Na
+
-selective electrode device is described with a solid state electrolyte membrane containing sodium, zirconium, silicon etc. The solid state reference system consists of e.g. sodium tungsten bronzes, sodium molybdenum bronzes or sodium alloys. These must be biphased compounds. The electrode device can be prepared by thick film printing.
DE 41 12 301 describes a reference electrode comprising an alkali compound in a transition metal oxide, e.g. nickel oxide or cobalt oxid

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