GB1597493A - Ion selective electrodes - Google Patents

Description

(54) ION SELECTIVE ELECTRODES (71) We, NATIONAL RESEARCH DEVELOPMENT CORPORATION, a British Corporation established by Statute, of Kingsgate House, 66 - 74 Victoria Street, London, S.W.1, 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 ion selective electrodes which are an improvement on some of the electrodes described in Patent Specification No. 1,521,964 having inventors John Vincent Dobson and Thomas Dickinson.

In the above mentioned application the measurement of the concentration of certain ions in solutions is described using an electrode which includes a tungsten bronze containing cations of the type whose concentration is to be measured.

The general formula for the tungsten bronze is either: Ax W03 where A represents one of the following: Na, K, Rb, Li, Co, Tl, Ag, Cu, NO, Ga , Cs, NH4 and In ions, and x varies in the range 0 < x S 1; or Ax My W03 where A has the above meaning, M represents K or Li or NH4 ions, and x and y each vary in the range 0 < x or y S 1, but x plus y is not greater than 1.

According to a first aspect of the present invention there is provided an ion-sensitive electrode for determining the concentration of predetermined cations in an electrolyte, comprising particles of one of the following: a tungsten bronze, a titanium bronze, a vanadium bronze, and a molybdenum bronze, the bronze including cations of the type whose concentration is to be determined, and the particles being held together with a binding material; and means for making electrical contact with the bound particles.

According to a second aspect of the invention there is provided a method of measuring the concentration of cations of a predetermined type in a solution, comprising contacting the solution with a reference electrode and a ion-selective electrode, and measuring the electrical potential between the electrodes, the ion-selective electrode comprising particles of one of the following: a tungsten bronze, a titanium bronze, a vanadium bronze and a molybdenum bronze, the bronze including cations of the predetermined type, and the particles being held together with a binding material.

Where the bronze is a tungsten bronze it may have either of the above quoted formulae, and in particular the tungsten bronze may be Nax W03, or Lix W03, or Kx W03.

Preferably the crystal structure of the tungsten bronze is Tetragonal I.

The binding material is preferably an epoxy resin but other materials which can be used include slicone rubber. The binding material should be of low conductivity and be chemically and electrically inert with respect to the electrolytes in which the electrode is to be used.

Preferably the matrix formed by the particules and the binding material is made up in the form of a pellet which is then held in contact with, or cemented using a conductive adhesive to a conductive member which is itself electrically connected to a copper wire. The pellet may be prepared by crushing the bronze to a powder and pressing a mixture of the powder and epoxy resin in a die. After pressing the pellet is heated in an oven until the resin has set.

Such pellets should have an electrical resistance in the range 50 to 5,000 ohms if they are to function satisfactorily.

Certain embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure I shows a first electrode according to the invention employing a tungsten bronze pellet clamped to a brass disc, Figure 2 shows a second electrode according to the invention employing a partly encapsulated tungsten bronze pellet, and Figure 3 shows a third electrode according to the invention employing a partly encapsulated tungsten bronze pellet attached to a flexible conductor.

Examples of methods of preparing tungsten bronzes for embodiments of the invention are first described.

Sodium tungsten bronzes were prepared according to the method described by M.E.Straumanis in the Journal of the American Chemical Society, Vol. 71, 1949, page 679, in which sodium tungstate (Na2WO4), tungstic oxide (WO3) and tungsten (W) were used in proportion according to the equation: 3xNa2WO4 + (6 - 4x)W03 + xW = 6NaxWO3 The ingredients were mixed intimately and heated for several hours in an inert atmosphere.

Lithium tungsten bronzes were prepared according to the equation: xLiOH + 6 - x W03 + xW = LiXW03 6 and potassium tungsten bronzes according to the equation: xK2CO3 + 6 - xW03 + xW = KxWO3 + xCO2 2 6 6 2 Reagent grade Na2WO4, W03 and W, and LiOH.H2O were used and the K2CO3.1aH2O was of analytical reagent standard.

For each bronze the ingredients were dried and weighed out in powder form, and placed together in a 25ml glass screw top jar. The total weight of ingredients for each synthesis was 10g. The powders were mixed together by tumbling the mixing jar for two hours.

The bronzes were made in silica boats. Each boat was about 8cm. long, made from a half section cut from a silica tube of 1.5cm. diameter. Two boats, each filled with mixed powder, were placed in a silica tube, forming part of an electrical furnace, which could be heated to 950"C. Commercial grade argon was passed through the silica tube before and during the synthesis, and during cooling. A typical synthesis took 8 hours at 850"C. Cooling took about 30 minutes. Table I gives a summary of the bronzes which were made.

TABLE I Value of x Temp. during Time of synthesis synthesis ( C) Na tungsten bronzes 0.3 850-880 8 hours 0.35 1 it 0.4 800 0.45 850-880 0.5 0.6 D 0.75 800 K tungsten bronzes 0.5 850-880 Li tungsten bronzes 0.27 n 0.38 After synthesis, the bronzes were crushed in an agate mortar. Each sample was purified by placing in a 250ml. pyrex (Registered Trade Mark) flask with a glass stopper, and shaking with 0.1M aqueous alkali (NaOH for sodium bronzes, KOH for K bronzes, and Li OH for Li bronzes - all of analytical reagent standard). The bronze, being almost insoluble, settled to the bottom. The supernatant was removed by suction, using a specially adapted wash bottle. Washing with alkali was repeated once.The action of alkali was to remove tungsten oxides by converting them to soluble tungstates. To complete the purification, the samples were washed twice in a similar manner with 0.5M HC1 and finally washed six times with distilled water, the final rinse being with triple distilled water. They were then separated on a Buchner funnel, dried at 1200C, and stored in glass jars.

Some of the sodium bronzes were graded, using Endecott sieves. Bronzes with x = 0.4, 0.6 and 0.75 were sieved to produce particles of maximum diameter = 106 .

Each embodiment of an ion sensitive electrode to be described with reference to the figures employs a pellet of tungsten bronze and examples of the preparation of these pellets are now given.

A pellet, 5mm in diameter and about 2mm thick, was used in each electrode.

Pellets were made by pressing a mixture of tungsten bronze and epoxy resin in a steel die of about 2 tons p.s.i. After pressing, pellets were heated in an oven to 100"C, until the resin had set. Once set, pellets were hard and durable. They could be worked with a saw, file, or abrasives and, moreover, their surfaces could be polished. The resin used was Araldite (Registered Trade Mark) MY750, with hardener HT956.

The consistency of the bronze/resin "mix" was important. Not enough resin, and the pellet was liable to disintegrate; too much, and it would not be electrically conducting. The optimum "mix" was made by first placing a few drops of resin-hardener mixture in an agate mortar. Small amounts of tungsten bronze powder were added, say a spatula end full at a time, and each addition was mixed thoroughly with the resin. Mixing was easy at first, but became much harder as more bronze was added. When the mix was very stiff, a trial pellet was pressed. If the pellet stuck to the press, the mix still contained too high a proportion of resin, and more bronze was added. The electrical resistance of the trial pellet was then measured, using a device described by J.V.Dobson, R.E.Firman and H.R. Thirsk, in the Journal of Physics E, Scientific Instruments, 1973, Vol. 6, page 22. If it fell within the range 50-500Q, the mixture was used to press a batch of pellets. Pellets with resistances in the above range were found to be mechanically durable. The consistency of mixture which gave rise to pellets with such resistances was fairly powdery and flaky.

In Figure 1 where a first type of electrode is shown, a tungsten bronze pellet 10, made as described above, is clamped to a brass disc 11 by a PTFE end piece 12 which screws on to the threaded end of a PTFE body member 13 and sandwiches the pellet and disc between two O-rings 14 and 15. A copper connecting wire 16 passes through a central bore in the member 13 and is soldered, at one end, to the disc 11.

In another type of electrode shown in Figure 2 the tungsten bronze pellet 10 is fixed to a thin copper disc 17 using conducting adhesive 18. The disc 17 is soldered to a copper wire 19 which passes through a central bore in a PTFE body member 20. The pellet 10 and the disc 17 are encapsulated in an end piece 21 moulded from epoxy resin. The mould used for making the end piece 21 is made from PTFE and has a threaded portion corresponding to the threaded portion of the member 20.

In Figure 3 a third type of electrode is constructed with the tungsten bronze pellet 10 again fixed to the copper disc 17 by means of conducting adhesive 18. A PVC insulated copper wire 23 is soldered to the disc 17 and the pellet 10 and disc 17 are encapsulated in moulded epoxy resin 24.

In making the electrodes of Figures 1, 2 and 3 the resin used may be Araldite MY750 and two different hardeners can be used: HT950 and HT972. HT956 is a "general purpose" hardener. The manufacturers state that castings made from MY750/HT972 have good resistance to high temperatures, and that they can be held at 1500C for long periods without loss of strength. Detailed information about the preparation and properties of Araldite resins is contained in Araldite Instruction sheets C39d and ClOc, published by Ciba-Geigy Ltd., Duxford, Cambridge, England.

The resin MY750 is a colourless, viscous liquid before curing and hardener HT972 is in the form of white flakes, which are melted at 100"C before mixing with the resin in the ratio 27:100 (hardener: resin) by weight. The mix, once poured into the mould takes about thirty minutes to set at 1000C.

The greatest problem encountered during casting the resin is the difficulty of avoiding the inclusion of air bubbles. It has been found by experience that the best way to minimise bubbles was to heat the resin to about 60"C before adding the hardener. This causes the resin to become runny, and any bubbles introduced during stirring usually come out on standing for a minute or so. The mix should be poured slowly into the mould.

Two types of conducting adhesive may be used to fix the pellet 10 to the disc 18, but both have similar properties. One is Johnson Matthey Thermosetting Silver Preparation. The other is known as "Hysol" (Registered Trade Mark). They each consist of two component adhesives, each component of which is mixed with silver powder. They are used in a similar manner to epoxy adhesives such as Araldite.

The tungsten bronze pellets of Figures 2 and 3 are attached to the internal conductor in a similar fashion. First, a copper disc, 5mm diameter and 0.089mm thickness, is cut from a piece of copper foil. One face is soldered to a copper wire and the other face is cleaned with emery paper to remove oxide layers and abrade the surface, so as to allow the conducting adhesive to stick. One face of the tungsten bronze pellet is then abraded using wet carborundum powder on glass, and the pellet is allowed to dry. The conducting adhesive is made up, and the pellet is stuck to the copper disc, the assembly being heated in an oven to 100"C until the adhesive has hardened.When the adhesive is hard, any pieces of adhesive which adhere to the edge of the pellet are removed, and the cylindrical edge of the pellet is roughened with emery paper to enable the resin to key more effectively with the tungsten bronze when the resin encapsulation is cast.

The pellet assembly is then inserted into the mould, and enough resin is poured in to cover the bronze pellet completely. When the resin has set, the excess is removed by cutting it away with a hacksaw, to within a millimetre or so of the pellet surface. The remaining resin is removed with a file, and the electrode is finished by abrading with wet carborundum powder on glass.

To measure the concentration of ions in a solution an electrode according to the invention made from tungsten bronze containing ions of the type whose concentration is to be measured are immersed in the solution; for example to measure the concentration of sodium ions an electrode having a pellet made from sodium tungsten bronze is used. A reference electrode is also immersed in the solution and the potential between the two electrodes is measured in the usual way, using a digital voltmeter, for example, to provide an indication of the required ion concentration. To this end a previous calibration of ion concentration against potential may be used.

WHAT WE CLAIM IS: 1. An ion-sensitive electrode for determining the concentration of predetermined cations in an electrolyte, comprising particles of one of the following: a tungsten bronze, a titanium bronze, a vanadium bronze, and a molybdenum bronze, the bronze including

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

Claims (11)

**WARNING** start of CLMS field may overlap end of DESC **. rise to pellets with such resistances was fairly powdery and flaky. In Figure 1 where a first type of electrode is shown, a tungsten bronze pellet 10, made as described above, is clamped to a brass disc 11 by a PTFE end piece 12 which screws on to the threaded end of a PTFE body member 13 and sandwiches the pellet and disc between two O-rings 14 and 15. A copper connecting wire 16 passes through a central bore in the member 13 and is soldered, at one end, to the disc 11. In another type of electrode shown in Figure 2 the tungsten bronze pellet 10 is fixed to a thin copper disc 17 using conducting adhesive 18. The disc 17 is soldered to a copper wire 19 which passes through a central bore in a PTFE body member 20. The pellet 10 and the disc 17 are encapsulated in an end piece 21 moulded from epoxy resin. The mould used for making the end piece 21 is made from PTFE and has a threaded portion corresponding to the threaded portion of the member 20. In Figure 3 a third type of electrode is constructed with the tungsten bronze pellet 10 again fixed to the copper disc 17 by means of conducting adhesive 18. A PVC insulated copper wire 23 is soldered to the disc 17 and the pellet 10 and disc 17 are encapsulated in moulded epoxy resin 24. In making the electrodes of Figures 1, 2 and 3 the resin used may be Araldite MY750 and two different hardeners can be used: HT950 and HT972. HT956 is a "general purpose" hardener. The manufacturers state that castings made from MY750/HT972 have good resistance to high temperatures, and that they can be held at 1500C for long periods without loss of strength. Detailed information about the preparation and properties of Araldite resins is contained in Araldite Instruction sheets C39d and ClOc, published by Ciba-Geigy Ltd., Duxford, Cambridge, England. The resin MY750 is a colourless, viscous liquid before curing and hardener HT972 is in the form of white flakes, which are melted at 100"C before mixing with the resin in the ratio 27:100 (hardener: resin) by weight. The mix, once poured into the mould takes about thirty minutes to set at 1000C. The greatest problem encountered during casting the resin is the difficulty of avoiding the inclusion of air bubbles. It has been found by experience that the best way to minimise bubbles was to heat the resin to about 60"C before adding the hardener. This causes the resin to become runny, and any bubbles introduced during stirring usually come out on standing for a minute or so. The mix should be poured slowly into the mould. Two types of conducting adhesive may be used to fix the pellet 10 to the disc 18, but both have similar properties. One is Johnson Matthey Thermosetting Silver Preparation. The other is known as "Hysol" (Registered Trade Mark). They each consist of two component adhesives, each component of which is mixed with silver powder. They are used in a similar manner to epoxy adhesives such as Araldite. The tungsten bronze pellets of Figures 2 and 3 are attached to the internal conductor in a similar fashion. First, a copper disc, 5mm diameter and 0.089mm thickness, is cut from a piece of copper foil. One face is soldered to a copper wire and the other face is cleaned with emery paper to remove oxide layers and abrade the surface, so as to allow the conducting adhesive to stick. One face of the tungsten bronze pellet is then abraded using wet carborundum powder on glass, and the pellet is allowed to dry. The conducting adhesive is made up, and the pellet is stuck to the copper disc, the assembly being heated in an oven to 100"C until the adhesive has hardened.When the adhesive is hard, any pieces of adhesive which adhere to the edge of the pellet are removed, and the cylindrical edge of the pellet is roughened with emery paper to enable the resin to key more effectively with the tungsten bronze when the resin encapsulation is cast. The pellet assembly is then inserted into the mould, and enough resin is poured in to cover the bronze pellet completely. When the resin has set, the excess is removed by cutting it away with a hacksaw, to within a millimetre or so of the pellet surface. The remaining resin is removed with a file, and the electrode is finished by abrading with wet carborundum powder on glass. To measure the concentration of ions in a solution an electrode according to the invention made from tungsten bronze containing ions of the type whose concentration is to be measured are immersed in the solution; for example to measure the concentration of sodium ions an electrode having a pellet made from sodium tungsten bronze is used. A reference electrode is also immersed in the solution and the potential between the two electrodes is measured in the usual way, using a digital voltmeter, for example, to provide an indication of the required ion concentration. To this end a previous calibration of ion concentration against potential may be used. WHAT WE CLAIM IS:

1. An ion-sensitive electrode for determining the concentration of predetermined cations in an electrolyte, comprising particles of one of the following: a tungsten bronze, a titanium bronze, a vanadium bronze, and a molybdenum bronze, the bronze including

cations of the type whose concentration is to be determined, and the particles being held together with a binding material; and means for making electrical contact with the bound particles.

2. An ion-sensitive electrode according to Claim 1 comprising a tungsten bronze having the formula: Ax WO3 where A represents one of the following: Na, K, Rb, Li, Co, Tl, Ag, Cu, NO, Ga, Cs, NH4 and In ions, and x varies in the range O < x s1.

3. An ion-sensitive electrode according to Claim 1 comprising a tungsten bronze having the formula: Ax My W03 where A represents one of the following: Na, K, Rb, Li, Co, Tl, Ag, Cu, NO, Ga, Cs, NH4 and In ions.

M represents K or Li or NH4 ions, and x and y each vary in the range 0 < x or y S 1, but x plus y is not greater than 1.

4. An ion-sensitive electrode according to Claim 1 comprising a tungsten bronze chosen from the following:- Nax WO3, LiX WO3 and Kx W03, where x varies in the range O < x 1.

5. An ion-sensitive electrode according to any preceding claim comprising a tungsten bronze with crystal structure which is Tetragonal I.

6. An ion-sensitive electrode according to any preceding claim wherein the binding material is chosen from the following: an epoxy resin and a silicon rubber.

7. An ion-sensitive electrode according to any preceding claim wherein the particles and the binding material form a body whose resistance between a face positioned in the electrode to contact an electrolyte and the means for making electrical contact is in the range 50 to 5,000 ohms.

8. A method of measuring the concentration of cations of a predetermined type in a solution, comprising contacting the solution with a reference electrode and an ion-selective electrode, and measuring the electrical potential between the electrodes, the ion-selective electrode comprising particles of one of the following: a tungsten bronze, a titanium bronze, a vanadium bronze and a molybdenum bronze, the bronze including cations of the predetermined type, and the particles being held together with a binding material.

9. An ion-sensitive electrode substantially as hereinbefore described and as shown in Figure 1 of the accompanying drawings.

10. An ion-sensitive electrode substantially as hereinbefore described and as shown in Figure 2 of the accompanying drawings.

11. An ion-sensitive electrode substantially as hereinbefore described and as shown in Figure 3 of the accompanying drawings.