WO2007020410A1

WO2007020410A1 - Detection of ammonia by electrodes comprising glassy carbon or boron-doped diamond

Info

Publication number: WO2007020410A1
Application number: PCT/GB2006/003022
Authority: WO
Inventors: Richard Guy Compton; Craig Edward Banks; Xiaobo Ji
Original assignee: Isis Innovation Limited
Priority date: 2005-08-12
Filing date: 2006-08-14
Publication date: 2007-02-22

Abstract

The invention provides methods for the electrochemical detection of ammonia which involve the use of a working electrode comprising one or both of boron-doped diamond and glassy carbon. In certain aspects, an electrolyte solution is used which comprises a solvent having a vapor pressure lower than that of water. Electrochemical cells and electrodes for the detection of ammonia are also provided.

Description

DETECTION OF AMMONIA BY ELECTRODES COMPRISING GLASSY CARBON OR

BORON-DOPED DIAMOND

Field of the Invention

This invention relates to the electrochemical detection of ammonia.

Background to the Invention

The use of ammonia sensors is widespread, primarily because of the high toxicity and abundance of processes in which ammonia is used or generated. For example, the detection of ammonia is important in fields such as environmental protection, clinical diagnosis, industrial processes, food processing, power plants, diagnostics, water treatment, refrigeration and fertiliser manufacture.

Sensors for ammonia are typically based on a gas diffusion cell approach, in which ammonia gas diffuses through a gas-permeable, hydrophobic membrane into a recipient solution in which ammonia is detected by electrochemical methodologies (e.g. using an NH₄ ⁺ selective electrode) or colorimetric pH indicators. Electrochemical techniques commonly utilise metal electrodes such as gold, silver, platinum and copper electrodes. Conventional techniques are often hampered by large background currents, which limit their analytical efficiency. Furthermore, the electrolyte solutions used in these sensors are generally water-based. The vapour pressure of water is such that the sensors will inevitably dry out over time, thereby limiting their lifetime and reliability. The volatility of the electrolyte solution can be reduced by using high concentrations of sulphuric acid, but this places significant voltammetric constraints, due to conversion of ammonia into the electroinactive ammonium ion. There remains a need for improved electrochemical techniques and sensors for the detection of ammonia.

Boron-doped diamond and glassy carbon electrodes are known. Boron-doped diamond possesses semimetal electronic properties, making them useful for electrochemical measurements. Thin film electrodes can be grown using hot filament- or microwave- assisted chemical vapour deposition (CVD) and doped to levels as high as 10000 ppm B/C. Boron-doped diamond thin films typically possess a rough, polycrystalline morphology with grain boundaries at the surface and a small-volume fraction of non- diamond carbon impurity. Consequently, the electrical conductivity of the film surface and the bulk is influenced by the boron doping level, the grain boundaries and the level of impurity.

Summary of the Invention

The present invention is based in part on a realisation that, by using an electrolyte comprising a solvent having a vapour pressure lower than that of water, the limitations of water-based ammonia sensors can be overcome. For example, the vapour pressure of propylene carbonate is significantly lower than that of water, thus allowing for an electrochemical sensor which is less likely to dry out than conventional, water-based ammonia sensors. Furthermore, it has been discovered that a desirable, analytically useful response to ammonia can be attained in such solvents when sensing substrates comprising glassy carbon and/or boron-doped diamond are used.

Accordingly, in a first aspect of the present invention there is provided a method of detecting ammonia in a sample, which comprises the steps of contacting the sample with working and counter electrodes in the presence of an electrolyte solution, and determining the electrochemical response of the working electrode to the sample, wherein the working electrode comprises glassy carbon or boron-doped diamond, and wherein the electrolyte solution comprises a solvent which has a vapour pressure lower than that of water.

A second aspect of the present invention is an electrochemical cell comprising a working electrode, a counter electrode and an electrolyte solution, wherein the working electrode comprises glassy carbon or boron-doped diamond, and wherein the electrolyte solution comprises a solvent which has a vapour pressure lower than that of water.

Another aspect of the invention is an electrode for an electrochemical cell, which comprises a substrate and, supported thereon, one or more particles comprising glassy carbon or boron-doped diamond. An electrochemical cell comprising such an electrode is also provided.

It has also been discovered that, by oxidising ammonia at a boron-doped diamond electrode, an improved voltammetric or amperometric response can be obtained relative to conventional electrode substrates. This improved level of response may be attained across a range of electrolyte systems, including but not limited to electrolyte solutions of the type described above. Thus, in a further aspect the present invention provides a method of detecting ammonia in a sample, which comprises the steps of contacting the sample with working and counter electrodes in the presence of an electrolyte, and determining the electrical response of the working electrode to the sample, wherein the working electrode comprises boron-doped diamond.

A further aspect of the present invention is the use of a boron-doped diamond electrode for the detection of ammonia.

Brief Description of the Drawings

Fig. 1 is a schematic representation of a cell of the invention, incorporating a gas diffusion membrane.

Fig. 2 shows various cyclic voltammograms for a solution of propylene carbonate solution and 0.1 M tetra-n-butylammonium perchlorate (TBAP) after bubbling 10 vol% ammonia for 45 seconds. Glassy carbon (GC), boron-doped diamond (BDD), edge plane pyrolytic graphite (EPPG) and basal plane pyrolytic graphite (BPPG) working electrodes were used. The dashed line indicates the response of the electrodes in the absence of ammonia.

Fig. 3 shows the accessible potential windows of GC, BDD, EPPG and BPPG electrodes recorded in propylene carbonate.

Fig. 4 shows the cyclic voltammetric responses obtained from exposing a solution of propylene carbonate and 0.1 M TBAP to 10 vol% ammonia for 30, 45, 60, 75, 90, 105 and 120 s in the presence of a GC electrode of 3 mm diameter.

Fig. 5 shows the cyclic voltammetric response of a GC-BPPG electrode in a solution of propylene carbonate and 0.1 M TBAP through which 10 vol% ammonia has been bubbled for 45 s. The dashed line indicates the response of the bare BPPG electrode.

Fig. 6 shows the amperometric response obtained using the GC-BPPG electrode. Fig. 7 is a cyclic voltammogram, showing the response of boron-doped diamond (BDD), glassy carbon (GC), edge plane pyrolytic graphite (EPPG), basal plane pyrolytic graphite (BPPG) and highly ordered pyrolytic graphite (HOPG) electrodes to a 0.1 M KCI solution in which 0.1 % ammonia had been bubbled for 4 minutes. The dashed line shows the response of the electrode in the absence of ammonia.

Fig. 8 shows the cyclic voltammetric responses to 0.1 M KCI solutions in which 0.1 % ammonia had been bubbled for 4, 8, 12, 16, 20 and 30 minutes, using a (3 mm diameter) BDD electrode.

Fig. 9 shows the response to increasing the scan rate at a BDD electrode in a 0.1 M KCI solution in which 0.1 % ammonia had been bubbled for 30 minutes. The scan rates were 50, 100, 200, 300, 500 and 800 mVs^"1.

Fig. 10 shows the amperometric response of a cell comprising particulate BDD.

Description of Various Embodiments

The invention involves the use of a working electrode comprising glassy carbon and/or boron-doped diamond (BDD) materials. Glassy carbon electrode materials are known in the art, and are typically in solid form. A working electrode comprising boron-doped diamond may be, for example, in the form of a thin film electrode. Boron-doped diamond electrodes are commercially available and may be produced by processes such as hot filament- or microwave-assisted CVD.

In a particular embodiment, the working electrode comprises a substrate and, supported thereon, one or more glassy carbon or boron-doped diamond particles. The particles may be, for example, microparticles or nanoparticles, but it will be appreciated that other (e.g. larger) particles may be used. The term "microparticles" as used herein refers to particles having a size of the order of microns (μm), an example being particles having a diameter of from about 1 to about 100 μm, more particularly from about 10 to about 50 μm, more particularly still from about 10 to about 20 μm. The term "nanoparticles" as used herein refers to one to more particles having a diameter of the order of nanometres (nm), an example being particles having a diameter of from about 10 to about 900 nm, more particularly from about 10 to about 500 nm, more particularly from about 25 to about 200 nm. The particles may be irregular or substantially spherical in shape. The substrate may comprise graphite (e.g. edge or basal plane pyrolytic graphite) or a metal. The particles can be attached to the substrate by abrasive or other suitable (e.g. adhesive) means. The particles are typically immobilised on the substrate. In one embodiment, the particulate is printed onto the substrate, for example by pad or screen printing. The substrate is preferably gas-permeable, such that a gaseous sample can diffuse therethrough. The substrate may also be hydrophobic. A preferred substrate material is polytetrafluorethylene (PTFE).

The counter electrode may be any suitable electrode, for example, a platinum or graphite electrode.

An electrochemical cell of the invention also comprises an electrolyte, which is typically present in solution. The electrolytic species may comprise KCI or TBAP, preferably 0.1 M KCI or 0.1 M TBAP. Other electrolytes may be used in the invention and will be apparent to the skilled person. In particular aspects, the invention involves the use of an electrolyte solution comprising a solvent having a vapour pressure lower than that of water. According to Lide, CRC Handbook of Chemistry and Physics, 1999-2000, the vapour pressure of water is 17.5 mm Hg at 20 ⁰C. The solvent preferably has a vapour pressure (at 20 ⁰C) of less than 10 mm Hg, more preferably less than 1 mm Hg, more preferably less than 0.1 mm Hg. A preferred solvent is propylene carbonate, which has a vapour pressure of approximately 0.03 mm Hg at 20 ⁰C. Methods for measuring vapour pressure will be apparent to those skilled in the art.

The sample may be in liquid or gaseous form. Typically, the sample is gaseous and is bubbled into the electrolyte where it is contacted with the electrodes. The cell may be a Clark cell-type gas sensor. In one embodiment, the sample is passed through a gas- permeable membrane prior to being contacted with the electrodes.

The electrochemical response of the working electrode may be determined using any suitable technique known in the art. This typically involves applying a potential across the working and counter electrodes, and determining the response of the working electrode to the sample. A potential may be applied across the electrodes using a potentiostat, and the response of the cell to the sample determined.

Various electrochemical techniques, for example voltammetry (e.g. cyclic voltammetry) and amperometry, are encompassed by the present invention. For determination of the voltammetric response, the applied potential is varied relative to a reference electrode; in this way, a cyclic voltammogram may be obtained. Alternatively, the amperometric response of the cell can be determined by applying a fixed potential across the electrodes, optionally controlled relative to a reference electrode. The reference electrode may be, for example, a saturated calomel electrode (SCE) or a silver electrode.

A particular embodiment of an electrochemical sensor of the invention is illustrated in Fig 1. Fig 1A depicts a plastic casing 1 comprising a gas-permeable membrane 2. By way of illustration, the diameter and height of the casing may be approximately 3 cm and 1.5 cm respectively. As shown in Fig. 1B, the casing houses an electrochemical cell 3, comprising working, reference and counter electrodes (4, 5 and 6 respectively), separated by wetting filters 7. In this particular embodiment, the counter electrode is a graphite electrode, whilst the reference electrode comprises silver wire. Electrolyte solution 8 may comprise, for example, 0.1 M KCI. It will be apparent to the skilled person that other electrodes and electrolytes may also be suitable for use in this arrangement. In use, gaseous sample 9 is passed over the membrane, where it diffuses into the electrolyte solution. A varied or fixed potential is applied across the working and counter electrodes and the electrochemical response of the cell to the sample determined.

The following Examples illustrate the invention.

Materials & Methods

All chemicals used were of analytical grade and used as received without any further purification. Ammonia was obtained from BOC Gases. Solutions were prepared with deionised water of resistivity not less than 18.2 M Ohm cm (Vivendi). Propylene carbonate (99.7%, Aldrich) and tetra-n-butylammonium perchlorate (TBAP, Fluka) were purchased at the highest grade available and used directly without further purification. Glassy carbon spheres (having a diameter of 10-20 μm, 99 %, Aldrich) were used as purchased. The size of the spheres was confirmed by SEM, using a Jeol 6500F instrument.

The working electrodes used were GC, BDD (3 mm diameter, Windsor Scientific Ltd.), BPPG (Le Carbone Ltd.), EPPG (Le Carbone Ltd.) and HOPG. The BPPG and EPPG electrodes were made by machining discs of pyrolytic graphite to a 4.9 mm diameter, with the disc face parallel with the edge plane, or basal plane as required. The GC and BDD electrodes were polished using diamond lapping compounds (Kemet), while the EPPG electrode was polished using alumina lapping compounds of decreasing size (0.1- 5 mm) on soft lapping pads. The BPPG electrode was prepared by polishing the surface using carborundum paper and then pressing cellotape on the polished surface. The cellotape was then removed, taking with it surface graphite layers. The electrode was then cleaned in acetone to remove any adhesive. The HOPG electrode was constructed according to Bowler et al, Anal.Chem. 2005, 77, 1916.

A GC-BPPG electrode was prepared as follows. A BPPG electrode was first prepared as described above. GC spheres were then abrasively attached to the BPPG electrode by gentling rubbing the BPPG electrode on a fine quality piece of filter paper on which the GC spheres were placed.

The counter electrode was a bright platinum wire with a large surface area. A silver wire quasi-reference electrode or saturated calomel electrode was used to complete the circuit.

The electrochemical cell used was a septum sealed three-necked flask which was held under a nitrogen atmosphere, ensuring a fixed amount of ammonia was maintained throughout the experiments. Ammonia was introduced into the cell directly from a cylinder (10 vol%, BOC) by bubbling for various periods of time. Nitrogen comprised the remaining part of all the gas mixtures. All solutions were vigorously degassed with oxygen-free nitrogen (BOC) until oxygen was electrochemically undetectable.

In all experiments, the solution was purged first with oxygen-free nitrogen gas to remove any oxygen present (BOC Gases) for at least 30 minutes. Ammonia was introduced into the solution directly from a cylinder containing 0.1 % ammonia (1000 ppm), and the solution saturated for various periods of time.

All experiments were carried out at a temperature of 295 ± 3 K. Electrochemical experiments were performed using an Autolab type Il potentiostat (Eco-Chemie) controlled by General Purpose Electrochemical Systems v.4.7 software. Example 1 : Amperometric detection of ammonia in propylene carbonate using GC and BDD working electrodes

The electrochemical oxidation of ammonia was detected by bubbling ammonia gas (10 vol%) into 25 ml propylene carbonate containing 0.1 M TBAP for 45 seconds. The amperometric responses at GC and BDD working electrodes were compared with those at EPPG and BPPG working electrodes, using cyclic voltammetry. The scan rate was 10O mV s^"1.

The observed voltammetric responses are depicted in Fig. 2. The dashed lines represent the voltammetric response in the absence of ammonia. The GC and BDD electrodes resulted in large oxidation waves at ca. 1.6 V and ca. 2.1 V (vs. Ag wire) respectively. The GC electrode provided the analytically most useful response. The EPPG and BPPG electrodes produced no significant oxidation waves.

Fig. 3 shows the accessible potential windows for all the electrode substrates studied; the potential window is defined as the range of potentials between which the current is less than 1.6 mA cm^'2. The GC and BDD electrodes had wide potential windows of 5.9 V and 6.3 V respectively, while the EPPG and BPPG electrodes had smaller potential windows of 2.9 V and 3.7 V volts respectively. The GC and BDD electrodes have the largest anodic range, extending up to ca. + 2.7 V and ca. + 3.0 V respectively before the onset of solvent breakdown. The BPPG and the EPPG electrodes had lower anodic limits of ca. + 1.9 V and + 1.6 V (vs. Ag wire) respectively.

In summary, the GC and BDD electrodes produced clear, well defined signatures. The GC electrode also exhibited the lowest oxidation potential for ammonia.

Example 2: Cyclic voltammetric detection of ammonia in propylene carbonate using a GC working electrode

A solution of propylene carbonate and 0.1 M TBAP was exposed to 10 vol% ammonia for 30, 45, 60, 75, 90, 105 and 120 s in the presence of a GC electrode of 3 mm diameter. The solution volume was 25 ml. Scans were run at 100 mV s^"1.

The voltammetric response is shown in Fig. 4. It is clear that the response generally increases with bubbling time. A plot of the oxidation peak current versus bubbling time (see the insert of Fig. 4) shows an approximately linear relationship, suggesting that the GC electrode is especially suitable for use as an electrode substrate in electrochemical ammonia sensors.

Example 3: Detection of ammonia in propylene carbonate using a GC-BPPG working electrode

Ammonia gas (10 vol%) was then bubbled through a solution of propylene carbonate and 0.1 M TBAP for 45 s, and the voltammetric response of the GC-BPPG electrode tested.

Fig. 5 illustrates the voltammetric responses at the modified (solid line) and bare BPPG (dashed line) electrodes, respectively. It is clearly seen that there is no oxidation wave observed on the bare BPPG electrode (dashed line in Fig. 5), however, the modified GC- BPPG electrode exhibits a single oxidation corresponding to the electrochemical oxidation of ammonia, suggesting that the electrochemistry dominates at the glassy carbon spheres.

The amperometric response of the GC-BPPG electrode to ammonia was also determined using chronoamperometry. The potential was held at + 2.5 V (vs. Ag wire). The system was allowed to stabilise, after which 10 % ammonia gas was bubbled into the voltammetric cell. The response is shown in Fig. 6, where a rapid jump in current is observed due to the ammonia diffusing to the electrode and being rapidly oxidized.

These results indicate that glassy carbon catalyses the electrochemical oxidation of ammonia, making it an ideal electrode material for amperometric electrochemical ammonia sensors based on propylene carbonate.

Example 4: Voltammetric detection of ammonia in 0.1 M KCI using a BDD working electrode

The cyclic voltammetric response of a BDD electrode to a 0.1 M KCI solution in which 0.1 % ammonia had been bubbled for 4 minutes was determined (at a scan rate of 100 mVs^'1). Fig. 7 shows that the BDD electrode exhibited the analytically most useful oxidation wave at ca. + 1.1 V (vs. SCE). In this solvent system, the GC and BPPG electrodes exhibit a voltammetric wave corresponding to the electrochemical oxidation of ammonia that is ill-defined with large background currents which will inevitably limit their analytical efficiency. In comparison with the BPPG and GC electrodes, a relatively low background current is observed at the BDD electrode for the oxidation of ammonia, thereby facilitating low detection limits and high sensitivities. For clarity, the voltammogram also show the response in the absence of ammonia (shown as a dashed line).

Fig. 8 illustrates the voltammetric response of the BDD electrode to a 0.1 M KCL solution in which 0.1 % ammonia gas had been bubbled from 4 up to 30 minutes. The scan rate was 100 mVs^"1. A linear response of peak height versus bubbling time is also shown in the accompanying insert.

The observed high solubility of ammonia in aqueous solution is well known when ammonia acts as a base, acquiring hydrogen ions from water to yield ammonium and hydroxide ions. To ensure that the response in Figs. 7 and 8 arose purely from the electrochemical oxidation of ammonia (and not the oxidation of hydroxide ions), hydroxide solutions were prepared over the range 1 mM to 1 M and the voltammetric response explored. The voltammogram was swept from 0 V up to 1.4 V (which is just before the onset of solvent decomposition) but no voltammetric waves were observed.

Next, a 0.1 M KCI solution was saturated with 0.1 % ammonia for 30 minutes and the response over range of scan rates determined explored. The scan rates were 50, 100, 200, 300, 500 and 800 mV s^"1. The observed voltammetric response is depicted in Fig. 9, where a plot of peak height (IH) versus square root of scan rate produced a linear response (I_H / A = 2.03 x 10^"4 V^1/2s^"1/2) indicating that the voltammetric response arises from diffusion rather than adsorption. Tafel analysis of voltammograms, plotted as potential vs. log current, produced a value of 142 mV per decade (at a scan rate of 100 mVs^"1), suggesting an electrochemically irreversible process. The oxidation of ammonia in 0.1 M sodium hydroxide (pH 13), 0.1 M borate buffer (pH 10) and 0.1 M ammonium chloride (pH 4.5) was explored and found to produce identical responses to that shown in Fig. 7.

Example 5: Amperometric detection of ammonia in 0.1 M KCI using a BDD working electrode A miniature amperometric electrochemical sensor was produced, based on a gas diffusion cell approach using a BDD electrode. The sensor is shown schematically in Fig. 1 and has a three-electrode arrangement with a graphite counter electrode and silver wire acting as a quasi-reference electrode. Each of the electrodes was separated by wetting filters, and with the overall size of sensor was approximately 3 cm in diameter and 1.5 cm in height. The supporting electrolyte was 0.1 M KCI.

The resulting electrochemical sensor was connected to a potentiostat and operated amperometrically with the working electrode held at a potential of + 1.08 V (vs. Ag wire). After ca. 66 s, 0.1 % ammonia gas was flowed over the gas-permeable membrane of the sensor for ca. 5 s. Fig. 10 shows a rapid jump in current attributable to ammonia diffusing through the membrane to the electrode, where it was rapidly oxidised. The ammonia gas was then stopped at ca. 71 s, after which the current was observed to slowly decrease as ammonia was continuously oxidised. Nitrogen gas was introduced (at ca. 82 s) to help accelerate the current back to a stable baseline, so that it reached the same magnitude as before the 0.1 % ammonia was introduced. At ca. 102 s, 0.1 % ammonia gas was reintroduced above the gas-permeable membrane for ca. 5 s, which again resulted in a large current increase. The current was again observed to decrease as all the saturated ammonia was electrochemically oxidised at the BDD surface, and then returned to the current level that was observed before the ammonia was introduced, indicates that the sensor can be used numerous times. The observed sharp rises in current when ammonia is introduced above the membrane could be used to trigger an alarm to indicate the escape of ammonia gas, for example in industry settings.

Claims

1. A method of detecting ammonia in a sample, which comprises the steps of contacting the sample with working and counter electrodes in the presence of an electrolyte solution, and determining the electrochemical response of the working electrode to the sample, wherein the working electrode comprises glassy carbon or boron-doped diamond, and wherein the electrolyte solution comprises a solvent which has a vapour pressure lower than that of water.

2. A method according to claim 1 , wherein the working electrode comprises a substrate and, supported thereon, one or more particles comprising glassy carbon or boron-doped diamond.

3. A method according to claim 2, wherein the particles are microparticles or nanoparticles.

4. A method according to claim 2 or claim 3, wherein the particles are substantially spherical or irregular in shape.

5. A method according to any of claims 2 to 4, wherein the substrate comprises graphite (e.g. edge plane pyrolytic graphite or basal plane pyrolytic graphite) or a metal.

6. A method according to any of claims 2 to 5, wherein the substrate is gas- permeable.

7. A method according to any of claims 2 to 6, wherein the substrate is hydrophobic.

8. A method according to any preceding claim, wherein the working electrode comprises glassy carbon.

9. A method according to any preceding claim, wherein the working electrode comprises boron-doped diamond.

10. A method according to any preceding claim, wherein the solvent has a vapour pressure of less than 17.5 mm Hg at 20 ⁰C.

11. A method according to any preceding claim, wherein the solvent has a vapour pressure of less than 1 mm Hg at 20 ⁰C.

12. A method according to any preceding claim, wherein the solvent is propylene carbonate.

13. A method of detecting ammonia in a sample, which comprises the steps of contacting the sample with working and counter electrodes in the presence of an electrolyte, and determining the electrochemical response of the working electrode to the sample, wherein the working electrode comprises boron-doped diamond.

14. A method according to claim 13, wherein the working electrode comprises particulate boron-doped diamond immobilised on a substrate.

15. A method according to claim 14, wherein the substrate is gas-permeable.

16. A method according to claim 13 or claim 14, wherein the substrate is hydrophobic.

17. A method according to any preceding claim, which comprises applying a potential across the electrodes and determining the electrochemical response of the working electrode to the sample.

18. A method according to any preceding claim, which comprises determining the voltammetric response.

19. A method according to any of claims 1 to 17, which comprises determining the amperometric response.

20. A method according to any preceding claim, wherein the sample is a liquid.

21. A method according to claim 20, wherein the sample is an aqueous sample.

22. A method according to any of claims 1 to 19, wherein the sample is gaseous.

23. A method according to claim 22, wherein the sample is passed through a gas- permeable membrane prior to being contacted with the electrodes.

24. An electrochemical cell comprising a working electrode, a counter electrode and an electrolyte solution, wherein the working electrode comprises glassy carbon or boron- doped diamond, and wherein the electrolyte solution comprises a solvent which has a vapour pressure lower than that of water.

25. A cell according to claim 24, wherein the working electrode comprises a substrate and, supported thereon, one or more particles comprising glassy carbon or boron-doped diamond.

26. A cell according to claim 25, wherein the particles are microparticles.

27. A cell according to claim 25 or claim 26, wherein the particles are substantially spherical or irregular in shape.

28. A cell according to any of claims 24 to 27, wherein the substrate comprises graphite (e.g. edge plane pyrolytic graphite or basal plane pyrolytic graphite) or a metal.

29. A cell according to any of claims 24 to 28, wherein the working electrode comprises glassy carbon.

30. A cell according to any of claims 24 to 29, wherein the working electrode comprises boron-doped diamond.

31. A cell according to any of claims 24 to 30, wherein the solvent has a vapour pressure of less than 17.5 mm Hg at 20 ⁰C.

32. A cell according to any of claims 24 to 31, wherein the solvent has a vapour pressure of less than 1 mm Hg at 20 ⁰C.

33. A cell according to any of claims 24 to 32, wherein the solvent is propylene carbonate.

34. A cell according to any of claims 24 to 33, for the detection of ammonia.

35. Use of a cell of any of claims 24 to 34, for the detection of ammonia.

36. An electrode for use in an electrochemical cell, which comprises a substrate and, supported thereon, one or more particles comprising glassy carbon or boron-doped diamond.

37. An electrode according to claim 36, wherein the particles are microparticles or nanoparticles.

38. An electrode according to claim 36 or claim 37, wherein the particles are substantially spherical or irregular in shape.

39. An electrode according to any of claims 36 to 38, wherein the substrate comprises graphite (e.g. edge plane pyrolytic or basal plane pyrolytic graphite) or a metal.

40. An electrode according to any of claims 36 to 39, wherein the electrode comprises glassy carbon.

41. An electrode according to any of claims 36 to 40, wherein the electrode comprises boron-doped diamond.

42. An electrode according to any of claims 36 to 40, wherein the one or more particles are immobilised on the substrate.

43. An electrode according to any of claims 36 to 41 , wherein the substrate is gas- permeable.

44. An electrode according to any of claims 36 to 43, wherein the substrate is hydrophobic.

45. Use of an electrode of any of claims 36 to 44, for the detection of ammonia.

46. An electrochemical cell comprising a plurality of electrodes and an electrolyte, wherein at least one of the electrodes is an electrode of any of claims 36 to 44.

47. A cell according to claim 46, for the detection of ammonia.

48. Use of a cell according to claim 47, for the detection of ammonia.

49. Use of an electrode comprising boron-doped diamond for the detection of ammonia.