Interior electrode of a polarographic electrode

In order to simplify manufacture of the interior electrode of a polarographic electrode with an electrically insulating shell containing a wire lead and carrying a reactive surface at its front end, which surface is formed by the free cross-sectional areas of wires arranged in the shell and in electrical contact with the wire lead, the wires and the site where they are connected to the wire lead are embedded in a plastic compound filling the front end of the shell.

BACKGROUND OF THE INVENTION 
This invention relates to an interior electrode of a polarographic 
electrode with an electrically insulating shell containing a wire lead and 
carrying a reactive surface at its front end, which surface is formed by 
the free cross-sectional areas of wires arranged in the shell the wires 
being in electrical contact with the wire lead, and to a method of 
manufacturing such an interior electrode. 
To facilitate measuring, the wires of such an interior electrode are 
extremely thin, with diameters of about 0.01 mm, and comparatively long. 
They must be smooth and, above all, free of bends and breaks when they are 
sealed into the electrode, and they must be in good electrical contact 
with the wire lead. Because of these requirements and the thinness of the 
wires, such electrodes are difficult to produce. 
DESCRIPTION OF THE PRIOR ART 
An interior electrode of the above type is disclosed in AT-PS 359 750, 
which is made in the following way. First of all, the wire lead and a 
round glass rod are firmly attached to each other, forming a semispherical 
transition from the thinner wire to the thicker glass rod. At the 
transition region forming a supporting area the thin wires are connected 
with the wire lead so as to be electrically conductive, and are bent over 
the semispherical supporting area and bonded to the glass rod by means of 
an adhesive compound. Then the assembly consisting of wire lead and glass 
rod with attached wires is placed into a glass shell, whose inner diameter 
corresponds to that of the glass rod with the affixed electrode wires. The 
end of the glass pipe is sealed together with the glass rod, such that the 
thin electrode wires are completely sheathed by the glass. In this context 
it is important that the dimensions of the individual components be 
perfectly matched in order to compensate for the expansion and shrinkage 
occurring during processing and to prevent them from doing damage to the 
thin wires. Finally, the electrode tip is inspected with the microscope 
for the area where the sealed-in wires are distributed most uniformly, and 
the electrode is cut off at this place. The disadvantage of this type of 
interior electrode is the extremely complex manufacturing process with its 
high percentage of rejects and high cost arising therefrom. 
U.S. Pat. No. 3,700,580 shows an electrochemical sensor in which the 
interior electrode is configured as a wire which is connected to a wire 
lead, wire and connecting site being potted in epoxide resin. A similar 
variant is known from GB-PS 2,114,304. 
SUMMARY OF THE INVENTION 
It is an object of the invention to propose an interior electrode of the 
above type, which may be manufactured in a simpler manner without 
impairing measuring sensitivity. 
This object is achieved in the invention by conventionally embedding the 
wires and the site where they are connected with the wire lead in a 
plastic compound filling the front end of the shell, and by providing that 
the shell, relative to the inner diameter at the front end, have a reduced 
cross-section between the connecting site and the reactive surface, where 
the wires touch the walls of the shell and are directed essentially 
parallel to the axis of the shell. 
In the invention a method of manufacturing such an interior electrode is 
characterised in that the wire lead including the wire ends sticking out 
therefrom is pulled into the shell by the free end of the wire lead, such 
that the wire ends touch the walls of the shell in the region of reduced 
cross-section and are thus given a direction essentially parallel to the 
axis of the shell, and further that the front end of the shell is filled 
in a known manner with a plastic compound, and that its surface is 
finished (e.g., by grinding) after the plastic compound has cured, in 
order to expose the cross-sectional areas of the wire ends. This will 
ensure in a simple manner that the thin electrode wires are embedded in 
the potting compound practically without any voids, and that they are not 
caught in the transitional region between shell material and plastic 
compound. 
Basically, the above casting method is suited for the manufacture of all 
electrodes operating according to the amperometric principle, including, 
among others, electrodes for measuring O.sub.2, glucose, lactate, etc. 
The electrode of the invention is particularly advantageous in applications 
where the substance to be determined (for instance, oxygen) is consumed 
and only small sample volumes are available. 
An advantageous connection between the wire lead and the wires ending in 
the reactive surface is obtained by providing that two wires form the free 
ends of a piece of wire and the wire lead have a flat portion at the site 
where it connects to the piece of wire, which flat portion embraces the 
piece of wire in hook-wise fashion. Of course, the wire lead and the thin 
wires could also be joined by soldering or with the use of an adhesive. 
A further advantage is gained by providing the shell of the interior 
electrode with a step upon which the flat portion of the wire lead rests. 
When the thin wires are inserted into the shell, the wire lead may be 
threaded until a stop is reached, without having to be adjusted 
accurately. 
In the invention the electrode shell may be made of glass, ceramics or a 
plastic material, and the plastic compound may consist of epoxide resin 
(as is known in the art), polyester or liquid polymethyl methacrylate, and 
the wires of precious metal, precious metal alloy or carbon fibers. 
Suitable shell-building materials thus do not only include glass and 
ceramics, but the insulating shell may also be configured as a plastic 
component made by mechanical processes or injection-moulding, all 
materials with satisfactory bonding to the potting compound being 
acceptable. Preferred wire materials are gold, platinum, silver, iridium 
and carbon fibers; using a plastic potting material will also permit the 
use of wire materials that cannot be sealed into glass. 
Desirable properties of the potting material are low shrinkage upon curing, 
good adhesion to both wires and shell material, good insulation, low water 
absorption, low gas permeability and good machinability. Another advantage 
of an interior electrode as specified by the invention is its low 
manufacturing cost.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The interior electrode 1 presented in FIG. 1 has an electrically insulating 
shell 2 containing a wire lead 3. The reactive surface 4 of electrode 1 is 
formed by the free cross-sectional areas of the wires 5, which are in 
electrical contact with the wire lead 3. The wires 5 and the site where 
they are connected to the wire lead 3 are embedded in a plastic compound 
filling the front end of the shell 2. 
The inner diameter D of shell 2 has a reduced cross-section 7, where the 
wires 5 touch the walls of the shell upon insertion of the wire lead 3 and 
are directed towards the reactive surface 4. 
Connecting the wires as shown in FIG. 1 and, using details, in FIGS. 3 and 
4, is performed in several steps. At first a number wire leads 3 of 
conventional wire, e.g. made of copper, i.e., polished, zinc-plated or 
silver-plated, or of silver, with a diameter of 0.5 mm approx., are cut 
and cold-formed on one end, producing flat portions 9 of a width of 1 mm 
approx. The wire leads 3 with their flat portions 9 are placed side by 
side, leaving given spaces to allow a thin wire, for instance made of 
platinum, with a diameter of 5 to 50 micrometers, to be placed across the 
flat portions 9 (FIG. 3). The the flat portions 9 are bent over in 
hook-wise fashion, such that the platinum wire is embraced and held. The 
platinum wire is cut between each pair of wire leads, such that each wire 
lead 3 grips a piece of wire 8, whose free ends sticking out at a right 
angle constitute the wires 5. Electrical conductivity of the connecting 
site between wire lead 3 and wire piece 8 may be increased with a drop of 
conductive adhesive (arrow 8') in FIG. 4. 
Then the wire lead 3 is pulled into the shell 2 by its free end, until its 
flat portion 9 rests against a step 10 of the shell 2, the wire ends or 
wires 5 being directed towards the reactive surface 4 by the reduced 
cross-section 7. The front end of the shell 2 is now filled with a plastic 
compound 6. After the curing process, protruding parts of the compound are 
removed mechanically, for instance by grinding, and the surface is 
polished. 
FIG. 2 shows a polarographic electrode with the interior electrode 1 
inserted, which latter is held by ribs 12 formed integral with the 
electrode body 11. Between the electrode body 11 and the interior 
electrode 1 the wire lead 3 is surrounded by a flexible sleeve 13 and is 
electrically insulated. The interior 15 of the polarographic electrode, 
which is formed by a housing 14 and filled with an electrolyte, has a 
reference electrode 16 projecting into it, which is configured as a silver 
pin held by the electrode body 11. Expansions, for instance of the air in 
the electrolyte chamber, which are due to temperature and design, are 
compensated by the flexible sleeve 13. 
The housing 14 locks onto the electrode body via two locking positions, and 
is provided with a soft cap 18 at the entrance point of the interior 
electrode 1, a membrane 19 selective to the test substance forming part of 
the cap. The ribs 12 formed integral with the electrode body 11 lock with 
recesses 23 of the housing 14, preventing the latter from turning and thus 
relieving the membrane 19 from any additional load. The cap 18 serves as a 
membrane support in addition to sealing off a measuring chamber. 
When the electrode is inserted into a measuring chamber V, the housing 14 
is pushed into the second locking position 17', thus stretching the 
membrane 19 and pulling it against the interior electrode 1; in this way 
the working life of the electrode is determined by its actual use in the 
measuring chamber, and, as a consequence, considerably extended. The 
slight overpressure caused by inserting the electrode again is compensated 
by the flexible sleeve 13. 
At the end facing away from the interior electrode 1, the polarographic 
electrode has a plug base 20 with an integrated plug pin 21, electrical 
contact with the wire lead 3 on the one hand and with the reference 
electrode 16 on the other hand being established via connections using a 
conductive adhesive 22.