GB1597628A - Electrode apparatus - Google Patents

Description

(54) ELECTRODE APPARATUS (71) We, SIRA INSTITUTE LI MITED, a British Company, of South Hill, Chislehurst, Kent BR7 5EH, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to electrode apparatus.

In determinations of pH or ionic activity by potentiometry, the potential of the pH or ion selective electrode is measured with respect to a reference electrode immersed in the sample solution. Ideally the reference electrode should maintain a stable potential with reference to the sample solution.

One of the factors causing instability in reference electrode potential is the diffusion of water and ions across the liquid junction formed between the electrode filling solution and sample. This liquid junction usually consists of a porous plug or ground glass joint permitting a slow flow of filling solution into the sample.

Furthermore, it is also desirable to reduce the flow of filling solution into the sample to prevent contamination of the sample and prevent loss of the filling solution to the minimum desirable.

Because large concentration gradients exist across the junction, significant quantities of ions and water can enter the reference electrode by diffusion even though there is a net outward flow of liquid. The net ingress of water causes dilution of the filling solution which in turn produces a change of reference electrode potential. The ingress of ions such as bromide, iodide or sulphide can poison the internal element of the reference electrode causing premature failure.

Conventional electrode apparatus comprises a container for the electrode filling solution, the electrode filling solution itself, a reference element, and a liquid passage to the exterior in which the liquid junction is provided, the liquid passage normally being restricted for example by a porous plug or ground glass joint.

The present invention provides electrode apparatus for immersion in a sample solution comprising a container for containing a liquid solution, a reference element, and a liquid passage to the exterior to electrically interconnect the liquid solution and the sample solution, the liquid passage being provided by a length of tube between 10 cm and 10 m in length and of not more than 0.7 mm inner diameter which diameter is smaller than the cross-section of the container.

In the accompanying drawings, Figure 1 shows a conventional reference electrode, and, Figure 2 shows a part of a conventional reference electrode, and the remaining Figures illustrate preferred arrangements of the invention by way of example only in which: Figure 3 shows a reference electrode according to the invention utilising a long tube, Figure 4 illustrates a second embodiment of the invention also utilising a long tube.

Referring to Figure 1 there is shown a reference electrode 10 comprising a container 11 in the form of a cylindrical tube typically 10 to 15 cm long and 1 to 1.5 cm in diameter of suitable glass or plastics material. A cap 12 at the upper end of the container 11 forms a junction between a lead 13 and a platinum wire 14 which depends downwardly from the cap 12 into the container 11. The platinum wire 14 is surrounded by a narrow tube 16 and at the lower end the platinum wire 14 is in contact with a reference element in the form of mercury 17 calomel potassium chloride paste 18. A packing 19 is situated at the lower end of the tube 16 for retaining the mercury 17 and paste 18 within the tube 16.

The above describes a conventional calomel reference element but other conventional reference elements such as silver/silver chloride or thallium amalgam/thallous chloride elements may be used.

The container 11 incorporates a filling hole 21 adjacent its upper end and at the lower end there is an aperture 22 which is closed by a porous plug 23 to form a liquid junction as will be clear later. In use the electrode is filled with a filling solution 24 which in the preferred embodiment is a solution of potassium chloride of a concentration between 3M and saturated (preferably 3.5M).

In use the reference electrode 10 is inserted into a sample solution so that the porous plug 23 forms a liquid junction between the sample solution and the filling solution 24.

Figure 2 illustrates an alternative conventional reference electrode 10 in which in place of the plug 23 there is provided a ground glass cone 26 having a liquid passage 27 therethrough, the lower end of the passage 27 passing through the ground conical face 28, being surrounded by a sleeve 29. In this way, limited liquid passage is allowed between the interior of the container 11 and its exterior.

Referring now to the remaining Figures, it will be understood that similar reference numerals are used throughout to illustrate the same parts. In Figure 3, the filling solution 24 of 3.5M potassium chloride is replaced by a mixture of (preferably 3.5M) potassium chloride and polyvinyl pyrrolidone or some other water soluble polymer.

The effect of this is to increase the viscosity of the liquid filling solution 24 considerably to render its outward passage from the container 11 more restricted. In this embodiment and in the embodiment of Figures 4 also the quantity of polyvinyl pyrrolidone added may be 250g/l which would produce a viscosity of approximately 12000 centistokes. However, other quantities may be added as desired to produce a viscosity in a range of 1 cSt to 20,000 cSt.

To further reduce the liquid passage between the outside and interior of the container 11, the aperture 22 of Figure 1 is replaced by a plug 31 through which passes in liquid tight fashion a small bore tube 32 of, for example, 0.3 to 0.7 mm diameter.

The open lower end 35 of the small bore tube 22 is arranged below the plug 31 so as to be in the sample solution. The remaining length of the tube 32 within the container 11 is wrapped around a suitable former 33 in coiled fashion and the opposite end 34 of the tube is arranged to be below the level 36 of the surface of the filling solution 34. In practice the total length of the tube 32 which may be incorporated in the reference electrode 10 may be as small as 10 cm and as great as 10 m which means that the concentration gradient between the exterior and interior of container 11 is spread over this length thereby reducing the concentration gradient and hence the diffusion of ions and water between the interior and exterior of the container 11.The length of the tube 32 is limited by the maximum resistance of the reference electrode permissable with a given type of pH meter such as an upper limit of 100 KQ. However with currently available amplifiers much higher reference electrode resistances and hence longer tube lengths would be possible. A 10 cm tube 0.23 mm diameter bore gives a resistance of about 80 KQ. Even this comparatively short tube gives a diffusion path length some 20 times greater than that of conventional electrodes.

In Figure 4 a somewhat similar arrangements is described to that in Figure 3 except that the tube 32 is coiled around the exterior of the container 11 so that the necessity for any aperture 22 is overcome, the upper end of the tube 32 passing through an aperture 36 adjacent the cap 12 of the container 11 and above the level 34 of the filling solution 24 and then downwardly into the filling solution 24. The lower end 30 of the tube 32 may be disposed anywhere convenient outside the container 11 as desired. For some applications it is advantageous for the end of the tube to extend some distance beyond the outer container. The whole unit depicted in Figures 3 and 4 can then be enclosed in a thermostatically controlled box, with the tube passing through a wall of the box and dipping into the sample.This arrangement would help to maintain stability of potential in applications where the ambient temperature varies widely.

The invention is not restricted to the details in the foregoing example.

WHAT WE CLAIM IS: 1. Electrode apparatus for immersion in a sample solution comprising a container for containing a liquid solution, a reference element, and a liquid passage to the exterior to electrically interconnct the liquid solution and the sample solution, the liquid passage being provided by a length of tube between 10 cm and 10 m in length and of not more than 0.7 mm inner diameter which diameter is smaller than the cross-section of the container.

2. Electrode apparatus as claimed in claim 1 in which the tube is of 0.3 to 0.7 mm inner diameter.

3. Electrode apparatus as claimed in claim 1 in which the tube is approximately 0.23 mm inner diameter.

4. Electrode apparatus as claimed in claims 1, 2 or 3 in which the container is in the form of a cylinder 10 to 15 cm long and 1 to 1.5 cm diameter.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (12)

**WARNING** start of CLMS field may overlap end of DESC **. The above describes a conventional calomel reference element but other conventional reference elements such as silver/silver chloride or thallium amalgam/thallous chloride elements may be used. The container 11 incorporates a filling hole 21 adjacent its upper end and at the lower end there is an aperture 22 which is closed by a porous plug 23 to form a liquid junction as will be clear later. In use the electrode is filled with a filling solution 24 which in the preferred embodiment is a solution of potassium chloride of a concentration between 3M and saturated (preferably 3.5M). In use the reference electrode 10 is inserted into a sample solution so that the porous plug 23 forms a liquid junction between the sample solution and the filling solution 24. Figure 2 illustrates an alternative conventional reference electrode 10 in which in place of the plug 23 there is provided a ground glass cone 26 having a liquid passage 27 therethrough, the lower end of the passage 27 passing through the ground conical face 28, being surrounded by a sleeve 29. In this way, limited liquid passage is allowed between the interior of the container 11 and its exterior. Referring now to the remaining Figures, it will be understood that similar reference numerals are used throughout to illustrate the same parts. In Figure 3, the filling solution 24 of 3.5M potassium chloride is replaced by a mixture of (preferably 3.5M) potassium chloride and polyvinyl pyrrolidone or some other water soluble polymer. The effect of this is to increase the viscosity of the liquid filling solution 24 considerably to render its outward passage from the container 11 more restricted. In this embodiment and in the embodiment of Figures 4 also the quantity of polyvinyl pyrrolidone added may be 250g/l which would produce a viscosity of approximately 12000 centistokes. However, other quantities may be added as desired to produce a viscosity in a range of 1 cSt to 20,000 cSt. To further reduce the liquid passage between the outside and interior of the container 11, the aperture 22 of Figure 1 is replaced by a plug 31 through which passes in liquid tight fashion a small bore tube 32 of, for example, 0.3 to 0.7 mm diameter. The open lower end 35 of the small bore tube 22 is arranged below the plug 31 so as to be in the sample solution. The remaining length of the tube 32 within the container 11 is wrapped around a suitable former 33 in coiled fashion and the opposite end 34 of the tube is arranged to be below the level 36 of the surface of the filling solution 34. In practice the total length of the tube 32 which may be incorporated in the reference electrode 10 may be as small as 10 cm and as great as 10 m which means that the concentration gradient between the exterior and interior of container 11 is spread over this length thereby reducing the concentration gradient and hence the diffusion of ions and water between the interior and exterior of the container 11.The length of the tube 32 is limited by the maximum resistance of the reference electrode permissable with a given type of pH meter such as an upper limit of 100 KQ. However with currently available amplifiers much higher reference electrode resistances and hence longer tube lengths would be possible. A 10 cm tube 0.23 mm diameter bore gives a resistance of about 80 KQ. Even this comparatively short tube gives a diffusion path length some 20 times greater than that of conventional electrodes. In Figure 4 a somewhat similar arrangements is described to that in Figure 3 except that the tube 32 is coiled around the exterior of the container 11 so that the necessity for any aperture 22 is overcome, the upper end of the tube 32 passing through an aperture 36 adjacent the cap 12 of the container 11 and above the level 34 of the filling solution 24 and then downwardly into the filling solution 24. The lower end 30 of the tube 32 may be disposed anywhere convenient outside the container 11 as desired. For some applications it is advantageous for the end of the tube to extend some distance beyond the outer container. The whole unit depicted in Figures 3 and 4 can then be enclosed in a thermostatically controlled box, with the tube passing through a wall of the box and dipping into the sample.This arrangement would help to maintain stability of potential in applications where the ambient temperature varies widely. The invention is not restricted to the details in the foregoing example. WHAT WE CLAIM IS:

1. Electrode apparatus for immersion in a sample solution comprising a container for containing a liquid solution, a reference element, and a liquid passage to the exterior to electrically interconnct the liquid solution and the sample solution, the liquid passage being provided by a length of tube between 10 cm and 10 m in length and of not more than 0.7 mm inner diameter which diameter is smaller than the cross-section of the container.

2. Electrode apparatus as claimed in claim 1 in which the tube is of 0.3 to 0.7 mm inner diameter.

3. Electrode apparatus as claimed in claim 1 in which the tube is approximately 0.23 mm inner diameter.

4. Electrode apparatus as claimed in claims 1, 2 or 3 in which the container is in the form of a cylinder 10 to 15 cm long and 1 to 1.5 cm diameter.

5. Electrode apparatus as claimed in any

of claims 1 to 4 in which the tube is coiled.

6. Electrode apparatus as claimed in claim 5 in which the tube is coiled around a former within the container.

7. Electrode apparatus as claimed in claim 5 in which the tube is coiled around the exterior of the container.

8. Electrode apparatus as claimed in any of claims 1 to 7 including said liquid solution which includes a water soluble polymer.

9. Electrode apparatus as claimed in claim 8 in which the water soluble polymer is polyvinyl pyrrolidone.

10. Electrode apparatus as claimed in any of claims 1 to 9 including said liquid solution which has a viscosity of 1cSt to 20,000 cSt.

11. Electrode apparatus as claimed in claim 10 in which the viscosity is of the order of 12,000 cSt.

12. Electrode apparatus as claimed in claim 1 substantially as hereinbefore described with reference to Figures 3 and 4 of the accompanying drawings.