GB2132768A - Ion-selective electrode - Google Patents

Abstract

An ion-selective electrode has an outer case 40 with an aperture exposing an ion-selective membrane 31. An inner case 33, held rotationally fast with respect to the outer case, is restrained and compresses the membrane (through a washer 32) when a cap 44 is tightened. A tubular reference electrode 51 inside the inner case has a vent hole 56 so that reference solution can be syringed from the top into the tube 51, reliably wetting the membrane. A flange 52 on the top of the tube 51 ensures reproducible location of the tube with respect to the membrane 31. <IMAGE>

Description

SPECIFICATION lon-selective electrode This invention relates to an ion-selective electrode, which is especially suitable as a reference electrode, such as a surfactant-ion-selective reference electrode.

According to the present invention, an ionselective electrode comprises an external body having an aperture exposing an ion-selective membrane retaining an internal reference solution into which there dips a reference electrode for connection to an external measuring instrument, characterised in that the reference electrode is tubular and preferably has at least one generally radial hole.

Preferably the external body comprises an inner body tube and an outer case (with the aperture) extending upwardly as a sleeve for at least half the height of the inner body tube (in the normal orientation of the ion-selective electrode). Preferably the inner body tube and the outer case are held fast against mutual rotation, and preferably the membrane is held against the aperture by the inner body tube.

The outer case is preferably closed at its upper end by a cap which also restrains the inner body tube, whereby the membrane is held as aforesaid.

Preferably the tubular reference electrode is enlarged (e.g. flanged, flared or belled out) at its upper end to locate it reproducibly with respect to the membrane (for example by causing the 'bell' to bear on the inner body tube or on a member resting thereon).

Preferably the inner body tube is of a material whose contact angle with water is not less than that of polyvinyl chloride with water.

The invention will be described by way of example with reference to the accompanying drawings, in which Figure 1 shows the parts of an electrode according to the invention, and Figure 2 is a sectional view of the electrode (according to the invention) assembled from the parts of Fig. 1.

Known ion-selective membrane electrodes (not illustrated) consist of an ion-exchange membrane for separating an external solution to be tested from an internal reference solution, in which there dips a silver/silver halide reference electrode consisting of a halideplated stout silver wire. Electrical contact with an external measuring instrument is made through a number of compression spring contacts, each of which is at risk from splashes of the reference solution, which may corrode or otherwise impair the contacts.

The clamping of the membrane in the known electrodes is insecure and the membrane could be accidentally twisted and rucked on assembly. The housing for the membrane typically screws into a main body section, leaving a join below the typical depth of immersion of the electrode into the external solution, the join being sealed by expedients such as O-rings. As these O-rings suffer sliding friction every time the electrode is demounted or assembled, contamination of the internal reference solution by seepage in of the external solution-and thus loss of accuracy of the electrode-are constantly risked.

With carelessness, the reference electrode of known electrodes could wobble to scratch itself (removing the halide plating) inside the body parts or could drop or be pressed too far downwardly, damaging and perhaps even puncturing the membrane. The reference electrode of known electrodes cannot be placed in position until the reference solution has been poured in, constraining several steps in as-sembly of the electrode to be performed without ever holding the apparatus sideways or upside down. The pouring in of the reference solution may not displace all the air, i.e. a bubble may be left trapped, possibly blanking off some or all of the membrane, reducing the speed of response of the electrode.

Turning now to Fig. 1, the parts of an example of an electrode according to the invention include an ion-exchange membrane 31 for separating an external solution to be tested for presence of the ion from an internal reference solution. (The electrode is shown assembled in Fig. 2, with the same reference numerals, slightly simplified for clarity.) The membrane 31 in this case is selective to surfactant ions, but it will become apparent that the present electrode can have its membrane exchanged, and hence its whole function changed, quickly and easily.

A typical membrane 31 with satisfactory electrical and mechanical properties can be made by casting and drying thoroughly a film of polymer about 0.1 mm thick. Polyvinyl chloride is a suitable polymer if prepared with cationic end groups for anion-selective electrodes, and anionic end groups for cationselective electrodes. The electrode impedance and response time can be controlled by choosing correctly the molecular weight of the polymer. 40 parts by weight (typically) of this polymer are blended with 60 parts of a compatible solid polymeric plasticiser for example Elvaloy 742 (trade mark) made by E.l. Dupont de Nemours, and dissolved in pure dry tetrahydrofuran. An appropriate volume of this solution is poured into a meticulously cleaned glass dish with an optically flat bottom and set level in a dust-free enclosure while the solvent is allowed to evaporate.The film is cut loose from the edges of the dish and its drying is completed in a vacuum oven. This dried film then forms the membrane.

A soft neoprene sealing washer 32 is cemented with cyano-acrylate to a polyvinylchloride cylindrical inner body tube 33 of the same internal and external diameters. The inner body tube 33 has an upper shoulder 34 and an externally threaded upstand 35 for receiving a complementary internally threaded connector piece 36. Just below the shoulder 34, the inner body tube 33 has a short radially projecting silver locating pin 33a.

The connector piece 36 comprises a knurled polyvinylchloride internally threaded collar containing an upwardly directed coaxial connector 37. A compression spring 39 between the connector 37 and a candlestickshaped silver cup 38 urges the cup 38, in particular its depending stem 38a, downwardly, the cup 38 being restrained by a radial plate 36a integrally formed in the connector piece 36, and centrally in the aperture to allow the stem 38a to pass freely.

A hard high density polyethylene cylindrical outer case 40 (could alternatively be of PTFE, and preferably not of PVC) can receive the inner body tube 33 (without the connector piece 36) as a sliding internal fit. For assembly the membrane 31 is conveniently rested on the up-ended inner body tube 33, i.e. on the sealing washer 32, and the outer case 40 (also upside down) slide downwardly over it. The outer case 40 (in its normal attitude again) has a lower aperture 41, for exposing the membrane in use, formed in an integral, chamfered inwardly-directed flange at the base of the outer case 40. The upper end of the outer case 40 is externally threaded and also has an axially extending radial slot 42, cut out of its inner cylindrical surface, for receiving the locating pin 33a of the inner body tube.This ensures that the final clamping of the membrane 31 in place is a pure axial squeezing action; the inner body tube 33 cannot rotate relative to the outer case 40 and so ruck or twist the membrane 31.

A high density polyethylene knurled cap 44 screws onto the threading of the upper end of the outer case 40. The cap 44 is centrally apertured so as to be able just to slide over the upstand 35 and to engage the shoulder 34 of the inner body tube 33, whereby when the cap 44 is tightened the membrane 31 is clamped between the outer case 40 and the washer 32.

A silver/silver halide (e.g. bromide) reference electrode 50 consists of a standard thinwalled hollow silver tube 51 (external diameter 3 mm) plated with silver halide. The upper end of the tube 51 is belled out and soldered to a silver collar to form a stout flange 52. A radial air vent 56 is drilled rather higher than midway up the tube 51. A polytetrafluoroethylene sealing collar 53 is a tight fit about the upper end of the tube 51, and, once slid in place, need not be removed in normal use. The collar 53 just clears the vent 56 at its lower end, and at its upper end has a flange 54 matching the silver flange 52.

The tube 51 in practice is halide plated after the collar 53 is slid on. The collar 53 is a sealing fit at its toD (immediately below its flange 54) and at its bottom in the bore of the inner body tube 33 and its slightly waisted, otherwise it would be too stiff to be slid into and out of the inner body tube 33. The thickness of the flange 54 and the length of the silver tube 51 are so arranged that when the electrode 50 (with the collar 53) are pushed firmly home in the inner body tube 33, the lower end of the tube 51 hovers above the membrane 31 by a modest (and always constant) clearance. There is no possibility of the tube's hitting and damaging or puncturing the membrane. While a silver wire could be swaged or flanged to give a similar benefit, this operation is much easier on a tube.Moreover, a tube gives important benefits at the stage of pouring in the electrolyte, as will be described.

Also, in this position, the silver flange 52 engages the stem 38a, with compression of the spring 39, and abuts the plate 36a, ensuring consistent reliable electric contact.

In use, the electrode is assembled in this logical order from the relatively small number of parts into which it disassembles: Membrane 31, then Inner body tube 33, then Outer case 40, then Cap 44, then Electrode 50 including collar 53, then Electrolyte (to be described), then Connector piece 36.

Fig. 2 shows the electrode assembled. The electrolyte, an internal reference solution made up for the particular purpose in mind and compatible with the membrane 31, is inserted as the very last item before the (final) connector piece 36; up to that stage, the parts can be manipulated upside down or anyhow as convenient, whereas such latitude ceases at a much earlier stage in assembling the hitherto known electrodes. Moreover, even thereafter, the stem 38a seals the bore of the tube 51 well enough for occasional momentary inversions.

The electrolyte is inserted by using a hypodermic syringe whose needle is passed through the flange 52 into the bore of the silver tube 51, to below the air vent 56. The electrolyte thus is forced onto the membrane 31, displacing air, and runs up both the inside and outside of the tube 51; air in the annular region between the silver tube 51 and the inner body tube 33 escapes via the vent 56 up the bore in the silver tube 51 to the outside. A large solution/silver halide contact area is thus set up, leading to a more reproducible electrode performance. The volume of the electrolyte is such as to come up to about the level of the air vent 56.The needle of the syringe is cut to an appropriate length so that, on firmly resting the syringe body on the silver flange 52, one knows that the needle comes to below the air vent 56 but stops well short of the membrane 31, which is thus at no risk of being punctured.

The syringe is withdrawn, the flange 52 if accidentally wetted is wiped dry to prevent corrosion, and the connector cap 36 screwed on firmly, with the stem 38a engaging the flange 52 as described.

The electrode assembly can be demounted and re-assembled quickly, easily and reliably by relative unskilled operators, so that the membrane or the reference solution cna be inspected or changed with minimum fuss and quickly.

The height of the outer case 40 exceeds the typical depth of immersion of the electrode in a solution to be tested, and so that solution has no way of seeping into the reference solution (electrolyte), as would destroy the accuracy of the electrode, unless the membrane 31 or its sealing ring 32 is damaged (which would be immediately apparent due to loss of response).

Claims (8)

1. An ion-selective electrode comprising an external body having an aperture exposing an ion-selective membrane retaining an internal reference solution into which there dips a reference electrode for connection to an external measuring instrument, characterised in that the reference electrode is tubular and preferably has at least one generally radial hole.

2. An ion-selective electrode according to Claim 1, wherein the external body comprises an inner body tube and an outer case (with the aperture) extending upwardly as a sleeve for at least half the height of the inner body tube (in the normal orientation of the ionselective electrode).

3. An ion-selective electrode according to Claim 2, wherein the inner body tube and the outer case are held fast against mutual rotation.

4. An ion-selective electrode according to Claim 2 or 3, wherein the outer case is closed at its upper end by a cap which also restrains the inner body tube.

5. An ion-selective electrode according to Claim 2, 3 or 4, wherein the membrane is held against the aperture by the inner body tube.

6. An ion-selective electrode according to any of Claims 2 to 5, wherein the inner body tube is of a material whose contact angle with water is not less than that of polyvinyl chloride with water.

7. An ion-selective electrode according to any preceding claim, wherein the tubular reference electrode is enlarged at its upper end to locate it reproducibly with respect to the membrane.

8. An ion-selective electrode according to Claim 1, substantially as hereinbefore described with reference to and as shown in Figs. 1 and 2.