GB2373053A

GB2373053A - Measuring electrode, particularly pH sensing electrode

Info

Publication number: GB2373053A
Application number: GB0105116A
Authority: GB
Inventors: John Topping; Christopher Neil Mcleod
Original assignee: Oxford Brookes University
Current assignee: Oxford Brookes University
Priority date: 2001-03-01
Filing date: 2001-03-01
Publication date: 2002-09-11
Also published as: AU2002236044A1; AU2002236044A8; GB0105116D0; WO2002071047A2; WO2002071047A3

Abstract

The electrode comprises an elongated insulated conductor of a first material with a coating of a second conductive material sputtered or evaporated onto an exposed end face of the first material, the first material may be copper, aluminium, nickel or silver and the second material may be antimony, bismuth, arsenic, tantalum or niobium. A pH sensing electrode is formed by cleaving an insulated copper conductor to provide an exposed clean end surface, and sputtering or evaporating an antimony coating thereonto. The pH sensing is achieved by measuring the voltage difference between the electrode and a silver reference electrode immersed in the same material. The electrode finds application, for example, as a catheter for measuring the pH value of bodily fluids.

Description

Measuring Electrode

This invention relates to a measuring electrode that finds application, for example, in the pH sensing of an electrolyte.

US-A-4119498 discloses an electrode for the measurement of liquids, the electrode having a metal sensing element contained in a holder, and is intended primarily for the determination of pH but can also be used for measuring the partial pressure of oxygen or carbon dioxide in the liquid. Glass electrodes are extensively used in such applications but have disadvantages, for example they can be difficult to handle and can be difficult to make, especially where small accurate dimensions are required. Metal electrodes may comprise iridium, palladium, antimony, or platinum, and being robust are suitable for the measuring of the pH in soil, or in bodily fluids, for example blood. The preferred electrode of this publication comprises monocrystalline antimony with only one plane crystal face exposed to the liquid. Typically, the purity of the antimony has to be at least 99.95%.

US-A-4561963 (corresponding EP-A-0171959) discloses an antimony and graphite hydrogen ion electrode in which the electrical conductor is formed of a graphite core an end of which is coated with a mixture of antimony and antimony oxide. The core and end coating are subsequently covered within an impermeable non-conductive plastic sheath which is preferably secured to the coating by an epoxy, so as to leave an end surface of the antimony/antimony oxide coating exposed to the electrolyte. The electrode is constructed by tightly wrapping untreated graphite threads around a rod of antimony oxide prior to enclosure within the plastic sheath.

US-A-3742594 discloses an antimony electrode in which short fine rods of antimony are soldered to one end of a fine silver wire which is then coated with a plastic sheath for insulation and stiffening purposes, leaving the tip face of the antimony exposed. The sheath is preferably resin.

There are problems associated with such electrodes, however, in that they can be difficult to manufacture, and usually have to be manufactured individually by hand, which is both

time consuming and expensive. Furthermore, where a graphite or carbon interface is required, this can lead to instability of the electrode.

It is one object of the present invention to provide a measuring electrode that overcomes, or at least alleviates difficulties associated with known electrodes.

In accordance with one aspect of the present invention, there is provided a measuring electrode comprising an elongate insulated conductor of a first conductive materia, onto an exposed (clean) end face of which has been sputtered or evaporated a coating of a second conductive material.

Advantageously the first material is selected from the group consisting of copper, aluminium, nickel and silver.

Advantageously the second material is selected from the group consisting of antimony, bismuth, arsenic, tantalum and niobium.

The measuring electrode is advantageously associated, in operation, with a reference electrode and a suitable measuring circuit, in order to provide an output signal that is characteristic of an electrolyte in which the measuring electrode is immersed.

In accordance with a further aspect of the present invention there is provided a method of manufacturing a measuring electrode, wherein an elongate insulated conductor of a first conductive material is cleaved to expose an end face thereof, and wherein a coating of a second conductive material is sputtered or evaporated onto the exposed end face of the first material before substantially any oxidation thereof has occurred.

The method of the invention is advantageously arranged to produce an electrode in accordance with the said first aspect of the invention.

Although sputtering is the preferred method of depositing the second conductive material, and will be referred to hereinafter by way of example, it is to be understood that the coating may alternatively be produced by evaporation.

Contrary to expectations, it has been found that the sputtering of the second material, for example antimony, onto the first material, for example, copper, has not been found to be subject to any insignificant instability, and the invention thus allows manufacture of a measuring electrode by means of a batch process.

Thus, in accordance with the present invention, a plurality of insulated conductors can be cleaved to so as to expose a clean surface, the conductors can be inserted into a vacuum chamber, which can then be evacuated, for example to 104 torr. The chamber can then be filled with an inert gas, preferably argon to a pressure of 10-2 torr, and the antimony can then be sputtered onto the plurality of exposed conductor surfaces. This processing can be carried out in a comparatively short time scale prior to there being any significant oxidation of the cleaved conductor surfaces.

Whereas traditionally, it has been believed that a very pure, for example electronics grade (99. 999%) monocrystalline antimony was preferred, it has been discovered that a less pure form of antimony may be used, and, furthermore, that it need not be monocrystalline, i. e. polycrystalline antimony can be used successfully.

The material to be sputtered onto the insulated conductor will be chosen in accordance with the application of the electrode. For example, where the pH level of a human stomach is required to be measured, the electrode in the form of a catheter preferably has a sputtered tip of antimony, thus giving an expected pH of < 7 (acidic). Such an electrode can also conveniently be used to measure the partial pressure of oxygen or other gases in the blood.

It will be appreciated that the batch production that is now feasible with the present invention significantly reduces the cost of the electrodes produced, which may consequently be made disposable with all of the advantages associated therewith.

Furthermore, the sputtering of the second material onto the conductor material inherently ensures proper securement, and does so without the need for any external means, such as an enclosing plastic sheath.

Although an electrode in accordance with the present invention is particularly convenient for use as a catheter in the monitoring of human beings, it will be appreciated that the electrode may also find other applications, such as in process monitoring, ensuring that food stuffs steps are properly mixed, or for checking the pH level in a reaction vessel involved in the preparation of food, for example. As a further example the electrode may be used to test the pH level of ground water.

The size of the conductor can be selected to suit, and may have an diameter from one or more microns up to several millimetres. The smaller gauge conductor may be of particular advantage in checking the pH level of plants, for example, whilst a larger diameter would provide a more robust device. Typically, the conductor may be made of 26 gauge wire.

An example of a measuring electrode, its method of manufacture and its use, each in accordance with the present invention, will now be described, by way of example, with reference to the accompanying circuit diagram.

The electrode is provided in the form of a catheter for measuring the pH of the oesophagus.

The electrode comprises a 26 gauge copper conductor within an insulating sheath whose end has been cleaved and onto which has been deposited a layer of antimony by sputtering.

It will be appreciated that antimony has been specifically medically approved for use within the human body. The electrode is made pH-sensitive by being introduced as a catheter into the oesophagus in association with a silver reference electrode, with both electrodes being immersed in the same material.

The output signal from the antimony pH electrode is applied to the non-inverting input of a high input impedence amplifier 2. The signal from the reference electrode, substantially zero volts, is connected to the OV of the amplifier circuit 2. Typically the signal from the pH electrode is small with respect to that of the reference electrode, being for example around 300 mV. The high input impedance of the amplifier 2 ensures that virtually no current is drawn from the sensing electrode. The output from the amplifier 2 is thus effectively the signal from the pH sensing electrode. In an application such as measuring the pH in the human oesophagus, which will be expected to be acidic, thus giving a

pH < 7, the output signal from the amplifier 2 will be negative. This signal is supplied as one input to a second amplifier 4 where it is inverted to provide a positive output. An offset adjusting signal is applied to the other input of the inverting amplifier 4, and allows for the manual removal, by adjustment of the variable 1 megohm resistor R3, of offset errors introduced by any imperfection in the amplifiers 2 and 4. The offset adjustment can conveniently be made by physically electrically short circuiting the sensing electrode to the reference electrode and adjusting the control until exactly OV appears at the output.

The circuit also comprises a resistor RI in the line interconnecting the amplifiers 2 and 4, and a resistor R2 that is connected as a feedback loop of the amplifier 4. Each of the resistors RI and R2 may be 10 kilohm, but if a larger output signal is required from the electrical circuit, then conveniently the resistor R2 can be replaced by of one a higher value, it being understood that the amplification is the ratio ofR2toRl.

The amplifiers 2 and 4 may be any suitable high input impedance amplifiers, for example LF347 types.

Claims

1. A measuring electrode comprising an elongate insulated conductor of a first conductive material, onto an exposed end face of which has been sputtered or evaporated a coating of a second conductive material.

2. An electrode according to claim 1 wherein the first material is selected from the group consisting of copper, aluminium, nickel and silver.

3. An electrode according to claim 1 or claim 2, wherein the second material is selected from the group consisting of antimony, bismuth, arsenic, tantalum and niobium.

4. A measuring electrode substantially as hereinbefore described with reference to the accompanying drawing.

5. An electrode assembly comprising a measuring electrode according to any one of the preceding claims, a reference electrode, and a measuring circuit arranged to receive an input from each of said electrodes and to output a signal that is characteristic of an electrolyte in which the measuring electrode, in operation, is immersed.

6. A method of manufacturing a measuring electrode, wherein an elongate insulated conductor of a first conductive material is cleaved to expose an end face thereof, and wherein a coating of a second conductive material is sputtered or evaporated onto the exposed end face of the first material before substantially any oxidation thereof has occurred.

7. A method according to claim 6, wherein the first material is selected from the group consisting of copper, aluminium, nickel and silver.

8. A method according to claim 6 or claim 7, wherein the second material is selected from the group consisting of antimony, bismuth, arsenic, tantalum and niobium.

9. A method of manufacturing a measuring electrode substantially as hereinbefore described with reference to the accompanying drawing.