WO2011151637A1

WO2011151637A1 - Electrochemical method for detecting hydrogen sulfide

Info

Publication number: WO2011151637A1
Application number: PCT/GB2011/051017
Authority: WO
Inventors: Richard Guy Compton; Aoife Maria O'mahony
Original assignee: Isis Innovation Limited
Priority date: 2010-06-01
Filing date: 2011-05-27
Publication date: 2011-12-08
Also published as: GB201009136D0

Abstract

A method for detecting hydrogen sulfide in a sample comprises the steps of contacting the sample with working and counter electrodes in the presence of an ionic liquid, eg. 1 - ethyl-3-methyl-imidazolium bis(trifluoromethylulfonyl)imide [C₂mim][NTf₂], and a mediator and determining the electrochemical response of the working electrode to the sample, wherein the mediator comprises a compound of formula I or formula II: wherein: X and Y each independently represent O or NR₄; R₁ represents a halide, or a C₁C₂₀ substituted or unsubstituted alkyl group; R₂, R₃ and R₄ each independently represent H, a halide, or a C₁C₂₀ substituted or unsubstituted alkyl group. In a preferred embodiment, the mediator is 3,5-tert-butyl-o-benzoquinone (CAS Registry No. 3383-21-9).

Description

ELECTROCHEMICAL METHOD FOR DETECTING HYDROGEN SULFIDE

[0001 ] This invention relates to the electrochemical detection of analytes, in particular hydrogen sulfide.

BACKGROUND

[0002] Hydrogen sulfide gas (H₂S) is toxic and has a detrimental effect on living organisms. H₂S often results from the anaerobic breakdown of organic matter by bacteria in environments such as swamps, sewers and oil and gas wells. The toxicity of H₂S means that its presence needs to be appropriately handled. For example, petroleum refineries need to be able to safely remove, contain and dispose of any H₂S that may be present in the oil source.

[0003] There is therefore a need to be able to detect and quantify the amount of H₂S that is present. Numerous sensors have been developed to detect H₂S. Amperometric sensors are used commercially and many conventional sensors for H₂S detection employ aqueous electrolyte, typically a H₂S0₄/H₂0 mixture (Silvester, D. S. and Compton, R. G., Z Phys. Chem., 2006, 220, 1247-1274).

[0004] The electrochemistry of H₂S has been studied at the platinum electrode in various aqueous solvents such as sulphuric acid, (Najdeker, E. and Bishop, E., Journal of

Electroanalytical Chemisty, 1973, 41 , 79-87; Ramasubramanian, N., J. Electroanal.

Chem., 1975, 64, 21 -37. 6, 7); acetonitrile containing different supporting electrolytes,

(Evans, J.F.; Blount, H., Electroanalytical chemistry and interfacial electrochemistry, 1975, 59, 169-175); and phosphoric acid (Chin, D. T. and Howard, P. D., J. Electrochem. Soc, 1986, 133(12), 2447-2450). The oxidative features have been predominately examined and observed at high positive potentials where the H₂S is strongly adsorbed onto the electrode surface.

[0005] The voltammetry of H₂S has also been studied at the platinum microelectrode in room temperature ionic liquids (RTILs), solvents composed entirely of ions which exist in the liquid state at 298 K. These solvents often possess properties such as low volatility, high thermal stability, and intrinsic conductivity (see Silvester, D. S. and Compton, R. G., Z Phys. Chem., 2006, 220, 1247-1274; Buzzeo, M. C; Evans, R. G. and Compton, R. G., Chem Phys Chem, 2004, 5, 1 106-1 120; Hapiot, P. and Lagrost, C, Chem. Rev., 2008, 108, 2238-2264). O' Mahony et al. have examined the oxidation of H₂S in several RTILs and strong adsorption onto the electrode surface was observed at potentials in excess of 1 .5 V vs. Pt, as well as high solubility of H₂S in the RTILs (O' Mahony, A. M.; Dickinson, E. J. F.; Aldous, L; Hardacre, C. and Compton, R. G., J. Phys. Chem. C, 2009, 1 13, 10997- 1 1002.). The reduction of H₂S in several RTILs was also examined, with the direct reduction of the gas at the Pt electrode observed at potentials in excess of -1 .5 V vs. Pt (O' Mahony, A.; Silvester, D. S.; Aldous, L; Hardacre, C. and Compton, R. G., J. Phys. Chem. C, 2008, 1 12, 7725-7730.).

[0006] These methods, however, suffer from a number of limitations. Adsorption of H₂S on the electrode surface can block electroactive sites from further electrochemical activity. Furthermore, the potential at which the H₂S is detected is quite high, and allows the possibility of electrode poisoning from other electroactive species as well as increasing scope for interferences by other species capable of oxidation.

[0007] The mediated detection of H₂S in aqueous solution has also been investigated. For example, numerous investigations have been published by Lawrence and co-workers, including Lawrence, N. S. et al., Electroanalysis, 2001 , 13, 432-436; Lawrence, N. S. et ai, Electroanalysis, 2001 , 13, 143-148; Lawrence, N. S et al, Mikrochim. Acta, 2001 , 137, 105-1 10; Thompson, M. et al, Sens. Actuators, B, 2002, 87, 33-40; Lawrence, N. S et al, Electrochim. Acta, 2006, 52, 499-503; and Robinson, K. and Lawrence, N. S.,

Electroanalysis, 2006, 18, 677-683. In each of these cases, mediation of H₂S detection takes place in the following manner:

2h/T + H₂S→2M + 2l-r + S (2)

where the mediator (M) is oxidised and a voltammetric signal is observed (eqn. 1 ). Upon the addition of H₂S the oxidised mediator (M+) is reduced (eqn. 2), which then allows it to be reoxidised and thus increases the observed voltammetric signal. Higher concentrations of H₂S augment this effect and the voltammetric signal increases with concentration.

[0008] These sensors have a limited lifetime, as the aqueous electrolytes used are subject to evaporation, especially at high temperatures and pressures. Also, compounds other than H₂S (i.e. interfering species) can reduce the mediator, thereby decreasing the selectivity of these sensors. There is therefore a need for an improved technique for detection of hydrogen sulfide.

[0009] WO 2008/1 10830 describes an electrochemical sensor comprising a

microelectrode array that has an ionic liquid medium that extends between the working and counter electrodes. The sensor is shown to be useful for the detection of carbon dioxide, which can be detected by direct reduction of the carbon dioxide at the surface of the working electrode. The inventors of WO 2008/1 10830 also believe that the sensor they disclose can be used to detect sulfur oxides, phosphorous oxides and oxygen. BRIEF SUMMARY OF THE DISCLOSURE

[0010] The present invention is based at least in part upon the discovery that hydrogen sulfide can be detected electrochemically, e.g. by electrochemical oxidation, with a sensor that avoids or minimizes the limitations described above. In particular, we have developed a sensitive electroanalytical method for the detection of hydrogen sulfide by use of a mediator in an ionic liquid.

[0011 ] In one aspect the invention provides a method of detecting hydrogen sulfide in a sample, which comprises the steps of contacting the sample with working and counter electrodes in the presence of an ionic liquid and a mediator and determining the electrochemical response of the working electrode to the sample, wherein the mediator comprises a compound of formula I or II:

wherein:

X and Y each independently represent O or NR₄;

Ri represents halide, or a C C₂o substituted or unsubstituted alkyl group;

R₂, R₃ and R₄ each independently represent H, halide, or a Ci-C₂₀ substituted or unsubstituted alkyl group.

[0012] In an embodiment the mediator is a compound of formula I. In another embodiment the mediator is a compound of formula II.

[0013] In an embodiment at least one of X and Y is NR₄, for example both X and Y are NR₄. In another embodiment, at least one of X and Y is O, for example both X and Y are O.

[0014] In an embodiment at least one of Ri and R₂ is an alkyl group, for example both and R₂ can be alkyl groups, e.g. and / or R₂ can be tertiary butyl groups.

[0015] In an embodiment the mediator is a compound of formula I and is attached to carbon 3 of the 6-carbon ring and / or R₂ is attached to carbon 5 of the 6-carbon ring.

[0016] In an embodiment R₃ is H or a halide, e.g. R₃ is H. In another embodiment R₃ is an alkyl group. [0017] In an embodiment R₄ is H or an alkyl group, e.g. R₄ is H. In an other embodiment R₄ is a halide.

[0018] In an embodiment the mediator is 3,5-tert-butyl-o-benzoquinone.

[0019] In an embodiment the ionic liquid is a room temperature ionic liquid (RTIL).

[0020] In an embodiment the ionic liquid comprises a cation selected from a

dialkylimidazolium, N-alkylpyridinium, tetraalkylammonium or tetraalkylphosphonium cation. For example the ionic liquid can comprise a 1 -alkyl-3-methylimidazolium ([C_nmim]) cation, where C_n represent an alkyl group and n is typically an integer from 1 to 10, e.g. an integer from 2 to 4.

[0021 ] In an embodiment the ionic liquid comprises an anion selected from

hexafluorophosphate [PF₆]; tetrafluoroborate [BF₄]; trifluoromethylsulfonate [CF₃S0₃]; bis[(trifluoromethyl) sulfonyl]amide [(CF₃S0₂)2N] ; bis[(trifluoromethyl) sulfonyl]imide [NTf₂]; trifluoroethanoate [CF₃C0₂]; acetate [CH₃C0₂]; nitrate, and halide. For example, the ionic liquid can comprise an anion selected from [PF₆], [BF₄] and [NTf₂].

[0022] In an embodiment the ionic liquid comprises at least one of 1 -Ethyl-3- methylimidazolium bis(trifluoromethylsulfonyl)imide ([C₂mim][NTf₂]), 1 -Butyl-3- methylimidazolium tetrafluoroborate ([C₄mim][BF₄], or 1 -Butyl-3-methylimidazolium hexafluorophosphate ([C₄mim][PF₆]).

[0023] In an embodiment the mediator is present in the ionic liquid at a concentration of not more than 500 mM, optionally at a concentration of not more than 100 mM, further optionally at a concentration of not more than 50 mM, still further optionally at a concentration of not more than 30 mM. In another embodiment the mediator is present at a concentration of not less than 2 mM, optionally at a concentration of not less than 5 mM, further optionally at a concentration of not less than 10 mM, still further optionally at a concentration of not less than 20 mM. As the skilled person would appreciate, the numeric values for the upper and lower concentration ranges can be combined to generate many disclosed concentration ranges, e.g. 2 to 500 mM, 5 - 500mM, 10 - 500 mM, 20 - 500 mM; and concentration ranges with equivalent lower limits, but with upper concentration limits of 100 mM, 50 mM and 30 mM respectively.

[0024] In an embodiment the mediator is present at a concentration of about 20 mM. In another embodiment the ionic liquid is saturated with the mediator.

[0025] In an embodiment the working electrode is a platinum electrode.

[0026] In an embodiment the counter electrode is a platinum electrode, e.g. a platinum wire electrode. [0027] In an embodiment the mediator forms an adduct with hydrogen sulfide, and said adduct can be electrochemically oxidised or reduced. Suitably, the mediator reversibly forms an adduct with hydrogen sulfide. Suitably, the mediator reacts with hydrogen sulfide via reversible proton uptake.

[0028] In an embodiment the method comprises applying a potential across the electrodes and determining the electrochemical response of the working electrode to the sample. For example, the electrochemical response can be a voltammetric response. In a related example, the response of the working electrode is determined using cyclic voltammetry.

[0029] In another aspect the invention provides the use of a mediator of formula I or formula II for the electrochemical detection of hydrogen sulfide, wherein the mediator is dissolved in an ionic liquid. The mediator and ionic liquid of this aspect can be further defined according to any of the embodiments of any other aspect of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which:

Figure 1 is a reaction scheme illustrating the reactions between 3,5-tert-butyl-o- benzoquinone and H₂S in [C₂mim][NTf₂].

Figure 2 shows the cyclic voltammetry response for the oxidation of (a) 20 mM DMPD in [C₄mim][BF₄] (solid line) + H₂S 100% (dotted line), (b) 20 mM catechol in [C₄mim][BF₄] (solid line) + H₂S 100% (dotted line), (c) 10 mM Ferrocene in [C₂mim][NTf₂] (solid line) + H₂S 1 % (dotted line), (d) excess Potassium Ferrocyanide in [C₄mpyrr][NTf₂] (solid line) + H₂S 1 % (dotted line), and (e) PVF in [C₆mim][PF₆] (solid line) + H₂S 1 % (dotted line), at Pt electrode (10 μηι diameter) at a scan rate 100 mV s-1 and temperature of 298 K.

Figure 3 shows the cyclic voltammetry response for the reduction of (a) 20 mM, (b) 2 mM, (c) 5 mM (d) 10 mM 3,5-tert-butyl-o-benzoquinone (solid line) and 10-15 mins H₂S 1 % flow (dotted) at Pt electrode (10 μηι diameter) at a scan rate 100 mV s^"1 and temperature of 298 K.

Figure 4 shows the cyclic voltammetry response for the reduction of (a) 20 mM, (b) 2 mM, (c) 5 mM (d) 10 mM 3,5-tert-butyl-o-benzoquinone (solid line) and 60 mins N₂ flow (dotted) after H₂S exposure at Pt electrode (10 μηι diameter) at a scan rate 100 mV s^"1 and temperature of 298 K.

Figure 5 shows the (a) cyclic voltammetry response for the reduction and oxidation of

2 mM 3,5-tert-butylo- benzoquinone (dotted) and 6-42 mins H₂S 1 % exposure(solid curve); (b) cyclic voltammetry response for the reduction and oxidation of 2 mM 3,5-tert-butyl-o- benzoquinone (dotted curve) and 42-78 mins H₂S 1 % exposure (solid curve); (c) cyclic voltammetry response for the reduction and oxidation of 2 mM 3,5-tert-butyl-o- benzoquinone after 78 mins H₂S 1 % exposure(solid) for 0, 15 and 60 mins N₂ purging; (d) a plot of time/min against current/nA for the reduction of 3,5-tert-butyl-o-benzoquinone over the time period of from 6 to 78 mins (■), for the oxidation of 1 ,2-hydroxyquinone species after reduction (o) and for the oxidation of 1 ,2-hydroxyquinone species without reduction ( Δ ).

Figure 6 shows the cyclic voltammetry response for the reduction and oxidation of 2 mM 3,5-tert-butyl-obenzoquinone in [C₂mim][NTf₂] (dotted) and the reduction and oxidation of added H₂S 200 ppm for 140 mins flow at Pt electrode (10 μηι diameter) at a scan rate 100 mV s^"1 and temperature of 298 K.

DETAILED DESCRIPTION

[0031] The term "alkyl" or "alkyl group" as used herein refers to an optionally substituted straight or branched chain alkyl moiety having from one to twenty carbon atoms, for example 1 -10, 1 -8 or 1 -6 carbon atoms. This term refers to groups such as methyl, ethyl, propyl, isopropyl, butyl, tert-butyl, pentyl, hexyl and the like. The group may be substituted with one or more, e.g. 2, substituents, the substituents being the same or different and selected from halogen, hydroxy, amino, nitro, cyano, carboxy, amido and the like.

[0032] The term "halide" as used herein refers to F, CI, Br or I.

[0033] The term "ionic liquid" as used herein refers to a salt in which the ions are poorly coordinated, which results in these substances being liquid below 100 °C. An ionic liquid may be a room temperature ionic liquid {"RT!L"), i.e. a solvent composed substantially (or entirely) of ions which exist in the liquid state at 298 K (25°C).

[0034] Hydrogen sulfide may be detected by electrochemistry with methods that use an ionic liquid and a mediator that is a compound of formula I, as defined above. These methods provide a number of advantages. For example, the methods and uses of the invention provide improved reuseability and an extended lifetime, e.g. due to negligible evaporation of the ionic liquid. The methods and uses of the invention also provide a decrease in signal potential, which can, e.g. improve the specificity of the method or use.

[0035] The sample will typically be in gaseous form, as hydrogen sulfide is a gas of limited solubility in typical solvents, e.g. water. It may, however, be that the sample is present in another state, e.g. solid or liquid, with analysis performed on the hydrogen sulfide gas emitted from the sample, for example after diffusion through a membrane.

[0036] The compound may be detected using an electrochemical sensor containing a working electrode with which the compound is contacted. Typically, electrochemical sensors are based upon the configuration of an electrochemical cell, comprising a working electrode, a counter electrode and an electrolyte, for example. The sensor may further comprise a reference electrode. Suitable sensor designs are well known in the art.

[0037] The working electrode may be any suitable electrode known in the art, for example a metallic or carbon electrode. Examples of metallic electrodes include gold, silver and platinum electrodes. Examples of carbon electrodes include an edge plane pyrolytic graphite electrode, a basal plane pyrolytic graphite electrode, a glassy carbon electrode, a boron doped diamond electrode, a highly ordered pyrolytic graphite electrode, carbon powder and carbon nanotubes. The working electrode may be a microelectrode or a macroelectrode. The counter electrode may be any suitable electrode, for example, a platinum or graphite electrode.

[0038] Any suitable ionic liquid may be employed in the methods and uses of the present invention. As the skilled person will appreciate, suitable ionic liquids could be chosen on the basis of parameters such as: compatibility with the operational temperature range; and / or viscosity (e.g. not more than 50 cP); and / or conductivity (e.g. at least 5 mS cm^'"1). For example, suitable ionic liquids are described in WO 2008/1 10830, in particular on page 6, line 12 to page 8, line 14, the contents of which are incorporated herein by reference. The ionic liquid medium may comprise one or a mixture of two or more ionic liquids, e.g. the ionic liquid may comprise a mixture of three or four ionic liquids.

[0039] The cation of the ionic liquid can be organic. Suitable ionic liquids include those comprising an organic cation selected from a di, tri and tetraaSkySimidazoSium, e.g. a 1 - a!ky!-3-meihyiimidazoiium ([dmim]}, aikyipyridinium, e.g. an N-aikyipyridinium, dialkylpyrroSidinium, dia!ky!piperidinium, tetraalkylammonium, ietraaSkyiphosphonium or triaSkySsuifonium cations. Preferred ionic liquids comprise an organic cation selected from a dialkylimidazolium, N-a!ky!pyridinium, tetraalkylammonium or tetraaikyiphosphonium cations. Ionic liquids comprising a [C_nmim] cation are particularly preferred for use in the methods of the present invention. The alkyl group may be any suitable alky! group, with C, to do aikyls being preferred, more preferably from G to C₆, especially from C₂ to C_A. Particularly preferred cations include 1 -ethyS-3-methySimidazoiium {[C₂mim]) and 1 -butyl-3- m eth ! i m idazo Hum ([C,< m i m]} .

[0040] The ionic liquid may comprise any suitable anion. The anions may be either organic or inorganic. Examples of suitable anions include hexafluorophosphate [PF₆]; tetrafluoroborate [BF₄]; iriiluoromethylsulfonaie [CF₃S0₃]; bis[(trif!uoromeihyl)

suifonyl]amide [(CF₃S0₂)₂N]; bis[(trifiuoromethyi) sulfonyijimide [NTf₂]; trifiuoroethanoate [CF₃C0₂]: acetate [CH₃CQ₂]; nitrate, and haiides, including fluoride [F], chloride [CI], and bromide [Br]. Preferred anions for use in the ionic liquid include [PF_e], [BF₄] and [NTf₂].

[0041 ] The anion mainly determines the solubility with water. For example, for the same 1 -buty!-3-me†hy!imidazo!ium cation, the BF ^", CF₃S0₃ ^~, GF₃C0₂ ^~, N0₃ ^", and haiide salts display a complete miscibility with water at 25°C. The PF_S ^~, SbF₆ ^~, NTf₂ ^", BR₄ anions show a very low miscibility with water. However, for the PF₆ ^~ based melt, the shorter symmetric substituted 1 ,3-dimethy!imidazo!ium PF₆ salt becomes water-soluble. For a series of 1 -aikyi-3-methyiimidazolium cations, decreasing the alkyl chain length from octyl to butyl decreases the hydrophobicily and the viscosity of the ionic liquid, whereas densities and surface tension values increase. Salts based on the 1 ,3-dialkylimidazolium cation are generally preferred as they generally interact weakly with the anions and are more thermally stable than other quaternary ammonium cations.

[0042] Preferred ionic liquids for use in the methods of the invention include 1 -Ethyl-3- methylimidazolium bis(trifluoromethylsulfonyl)imide ([C₂mim][NTf₂]), 1 -Butyl-3- methylimidazolium tetrafluoroborate ([C₄mim][BF₄], and 1 -Butyl-3-methylimidazolium hexafluorophosphate ([C₄mim][PF₆]).

[0043] The mediators employed in the methods and uses of the present invention are the substituted o-benzoquinones or substituted o-benzodiimines of formula I and substituted p-benzoquinones or substituted p-benzodiimines of formula II as previously defined. Where any ofRi R₂, R₃ and R₄ are alkyl groups, they may be a straight or branched chain alkyl group. For example, any one or more of R_{1 ;} R₂, R₃ and R₄ may independently represent a C₄ - C_20, e.g. C₄ - Ci_0, C₄ - C₈ or C₄ - C₆, branched alkyl group. In an embodiment is a tertiary butyl group. In another embodiment R₂ is a tertiary butyl group. For example, both and R₂ and optionally R₃ can be tertiary butyl groups. Ri can be attached to carbon 3 of the benzoquinone or benzodiimine ring. R₂ can be attached to carbon 5 of the benzoquinone or benzodiimine ring. A preferred compound of formula I is 3,5-tert-butyl-o-benzoquinone.

[0044] The substituted obenzoquinones or substituted o-benzodiimines of formula I act as a mediator in a different manner to that of other mediators described in equations 1 and 2, as is clear when equations (1 ) and (2) are compared with the reaction scheme of figure 1 , with a key difference being that the mediators of the present invention form an adduct when they react with H₂S, as opposed to the oxidation of H₂S to sulfur indicated by equation (2). The para- compounds of formula II are believed to act as a mediator in a similar manner to the ortho- compounds of formula I. [0045] The compounds of formula I and/or formula II, when dissolved in an ionic liquid, are believed to form adducts with H₂S and undergo electrochemical reactions. Figure 1 illustrates a typical reaction scheme between H₂S and a mediator of use in the methods of the present invention, namely the compound 3,5-tert-butyl-o-benzoquinone (a compound of formula I), when the compound is dissolved in the ionic liquid [C₂mim][NTf₂]. The mediator 3,5-tert-butyl-o-benzoquinone (a), reacts with H₂S in the RTIL, e.g.

[C₂mim][NTf₂], to form the di-substituted 1 ,2-hydroxybenzene species (b) shown in Step 1 . This species can be electrochemically reduced or oxidised. On forming, the di-substituted 1 ,2-hydroxybenzene species reacts with the parent mediator and is oxidised back to a thiol-substituted benzoquinone species (c) shown in Step 2. The parent species itself is reduced to a di-substituted 1 ,2-hydroxybenzene species (d). The newly formed

benzoquinone species is then free again to react with H₂S to form 2,4-thiol-3,5-tertbutyl-o- benzoquinone (e) shown in Step 3.

[0046] 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.

[0047] 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.

[0048] In one embodiment, the current is measured using linear sweep or cyclic voltammetry. In another embodiment, said current is measured using square wave voltammetry. In an alternative embodiment, the current is measured using a pulsed voltammetry technique, e.g. differential pulse voltammetry.

[0049] The following examples illustrate the invention.

EXPERIMENTAL

Chemical Reagents

[0050] 1 -Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C₂mim][NTf₂]) was prepared by standard literature procedures.21 , 22 1 -Butyl-3-methylimidazolium tetrafluoroborate ([C₄mim][BF₄] and 1 -Butyl-3-methylimidazolium tetrafluoroborate

([C₄mim][PF6]) were kindly donated by Merck KGaA. N,N-dimethyl-1 ,4-phenylenediamine (DMPD), Aldrich, >97%), Catechol (Aldrich, 99+%), ferrocene (Aldrich, 98%), potassium ferrocyanide (BDH), polyVinylFerrocene (Polysciences Inc.), dichloromethane (DCM, Fisher Scientific, HPLC grade), 3,5-tert-butyl-o-benzoquinone (Aldrich, 99+%), tetrabutylammonium perchlorate (TBAP, Fluka, Puriss electrochemical grade, >99.99%) and acetonitrile (Fischer Scientific, dried and distilled, >99.99%) were used as received without further purification. Hydrogen sulfide gas (99.99% pure, 0.01 % nitrogen fill) was purchased from CK Gas Products Ltd., Hampshire, U.K. Hydrogen sulfide gas (20ppm nitrogen fill) and nitrogen gas (oxygen free) was purchased from BOC.

Instrumental

[0051 ] Electrochemical experiments were performed using a computer controlled μ- Autolab potentiostat (Eco-Chemie, Netherlands). A conventional two-electrode system was used, typically with a platinum electrode (10 μηι diameter) as the working electrode, and a 0.3 mm diameter platinum wire as a quasi-reference electrode. The platinum microdisc working electrode was polished on soft lapping pads (Kemet Ltd., U.K.) using alumina powder (Buehler, IL) of size 5.0, 1 .0 and 0.3 μηι. The electrode diameter was calibrated electrochemically by analyzing the steady-state voltammetry of a 2 mM solution of ferrocene in acetonitrile containing 0.1 M TBAP, with a diffusion coefficient for ferrocene of 2.3 x 10^"5 cm² s^"1 at 293 K.

[0052] The electrodes were housed in a glass cell "T-cell" designed for investigating microsamples of ionic liquids under a controlled atmosphere, according to the design of Schroder, U. et al., New J. Chem., 2000, 24, 1009-1015 and Silvester, D. S. et al., J. Phys. Chem. B, 2007, 1 1 1 , 5000-5007. RTILs are sensitive to water, the presence of which can alter the viscosity of the ionic liquid and reduce the electrochemical window, therefore the samples are purged under vacuum before voltammetry is carried out. The working electrode was modified with a section of disposable micropipette tip to create a small cavity above the disc into which a drop (20 μί) of ionic liquid was placed.

[0053] DMPD was directly dissolved in [C₄mim][BF₄] at concentration 20 mM. For addition of catechol and ferrocene to the RTIL, a drop (20 μί) of 20 mM catechol or ferrocene in acetonitrile was added to the RTIL in the cavity and purged under vacuum to remove acetonitrile. Potasssium ferrocyanide was also directly dissolved in excess in [C₄mpyrr][NTf₂]. PVF was immobilised onto the electrode surface via the solvent evaporation method. The polymer was dissolved in dichloromethane (DCM) to a concentration of 20 mM and then a known amount of solution was dropped onto the working electrode surface. The electrode was then set up in the T-cell and purged under vacuum until the DCM had evaporated and left a film of PVF on the electrode surface. 20 μΙ_ [C₄mim][PF₆] was dropped onto the electrode and the PVF-modified electrode was examined using cyclic voltammetry. 3,5-tert-butyl-o-benzoquinone was directly dissolved in [C₂mim][NTf₂] at concentration 20 mM. Smaller concentrations of the species were achieved by diluting the solution with pure [C₂mim][NTf₂].

[0054] Prior to voltammetric scanning, the RTIL solution was purged under vacuum (Edwards High Vacuum Pump, Model ES 50) for ca. 90 minutes, which served to remove trace atmospheric moisture naturally present in the RTIL. All experiments were performed inside a fume cupboard, in a thermostated box (previously described by Evans et al.)28 which also functioned as a Faraday cage. The temperature was maintained at 298 (± 1 .0) K. When the baseline showed no presence of impurities, hydrogen sulfide, H₂S, gas was introduced (via PTFE tubing) through one arm of the cell. The gas was allowed to diffuse through the sample for varying periods of time according to the specifics of an experiment. An outlet line (made of PTFE) led from the other end of the cell into a fume cupboard.

[0055] Experiments diluting hydrogen sulfide gas (1 .01 % H₂S nitrogen fill) to a concenatration of 200ppm were carried out using a gas mixer which consists of two volume gauges, one for hydrogen sulfide, and another for N₂ (oxygen free). Each gas passes through the volume gauge at a specific rate and into a mixing tube where they are mixed to the desired concentration. The mixture is then flowed through the T-cell.

Microdisc Chronoamperometric Procedures

[0056] Chronoamperometric transients were achieved using a sample time of 0.01 s. After preequilibration for 20 seconds, the potential was stepped from a postion of zero current to a chosen potential after the reductive peak, and the current was measured for 5 s. The software package Origin 7.0 (Microcal Software Inc.) was used to fit the

experimental data. The equations proposed by Shoup and Szabo (Shoup, D. and Szabo, A., J. Electroanal. Chem., 1982, 140, 237-245), listed as equations (3) to (5) below, were imported into the non-linear curve fitting function, and the computer was instructed to perform 100 iterations on the data.

I = -AnFDcr (T) (3) f(T) = 0.7854 + 0.8863?"^" + 0.2146exp(-0.7823f ² ) (4)

ADt

τ = 2 (5) where n is the number of electrons transferred, F is the Faraday constant, D is the diffusion coefficient, c is the initial concentration of parent species, rd is the radius of the disc electrode, and t is the time. The equations used in this approximation are sufficient to give D and c within an error of 0.6 %.

[0057] The value for the radius (previously calibrated) was fixed, and a value for the diffusion coefficient and the product of the number of electrons and concentration was obtained after optimization of the experimental data.

COMPARATIVE EXAMPLE 1

[0058] Comparative example 1 does not fall within the scope of the invention, but it is included here to indicate how other mediators, e.g. compounds known to act as mediators in electrochemistry of H₂S in aqueous solution, perform in the electrochemical detection of H₂S when the mediator is dissolved in an ionic liquid.

[0059] Figure 2 (a)-(e) shows the attempted mediated detection of H₂S using the mediators N,N-dimethyl-1 ,4-phenylenediamine (DMPD), catechol, ferrocene, potassium ferrocyanide and polyvinylferrocene (PVF) respectively, in a range of room temperature ionic liquids (RTILs). Each of these species have previously been utilized as mediators in aqueous systems, particularly DMPD and ferrocene or ferrocene-based species for the mediated detection of H₂S.

[0060] Figure 2 (a) shows the attempted mediation of H₂S 100% using 20 mM DMPD in the ionic liquid [C₄mim][BF₄]. The mediator was dissolved directly in the RTIL. The voltammetry of DMPD in RTILs has previously been observed and yielded two reversible oxidative signals for the oxidation of DMPD to DMPD⁺ and DMPD+ to DMPD²⁺. In Figure 1 (a), the solid line denotes the presence of DMPD only in the RTIL. A cyclic

voltammogram was taken of DMPD in the potential range of 0.0 to 2.0 V. On the forward scan two oxidative peaks were observed at potentials 0.4 V and 1 .0 V vs. Pt, denoting the oxidation of DMPD to DMPD⁺ and DMPD⁺ to DMPD²⁺ respectively. The scan was reversed at 2.0 V and the reduction of DMPD²⁺ and DMPD⁺ was observed at 1 .0 V and 0.38 V vs. Pt respectively. H₂S was flowed through the T-cell for ca. 40 minutes. A cyclic voltammogram was then taken in the potential range of 0.0 to 1 .5 V. As before, on the forward scan, two signals for the oxidation of DMPD and DMPD⁺ were observed at 0.3 and 1 .0 V vs. Pt respectively. While the current increased a little (+0.12 nA for DMPD oxidation and +0.24 nA for DMPD⁺ oxidation), the magnitude of this increase was not significant enough to suggest that any useful mediation was taking place. Rather this increase is likely due to the change in viscosity of the RTIL due to the presence of the highly RTIL- soluble H₂S gas.

[0061 ] Three other mediating species, catechol, ferrocene and potassium ferrocyanide were also dissolved in the RTILs [C₄mim][BF₄], [C₂mim][NTf₂], and [C₄mim][PF₆] respectively using solvent evaporation outlined in Section 2, and mediated H₂S detection was attempted using each. Figure 2 (b), (c) and (d) show the voltammetry of each of these species in RTILs denoted by the solid lines. The oxidation of 20 mM catechol in

[C₄mim][BF₄] was observed in Figure 2 (b) in the potential range of 0.0 to 2.0 V. On the forward scan, the electrochemically irreversible oxidation of catechol to orthoquinone was observed at potential 1 .1 V vs. Pt. The adsorption of the orthoquinone species onto the Pt electrode surface was observed as a prewave at the less positive potential of 0.7 V vs. Pt. H₂S 100% was flowed through the T-cell for 40 minutes and a cyclic voltammogram was taken in the potential range of 0.0 to 1 .5 V. A shift was observed in the electrochemical signals on the forward scan for the oxidation of catechol to orthoquinone at 1 .2 V vs. Pt, and the adsorption of the orthoquinone species onto the electrode surface at 0.9 V vs Pt. This is due to the use of a quasi-reference electrode in the system. A slight increase in the oxidation of catechol at 1 .2 V vs. Pt was observed, but as with DMPD, the increase was not significant enough to constitute mediation, rather a change in the viscosity of the RTIL due to the presence of H₂S.

[0062] Similar behaviour was observed for attempted mediation using ferrocene and potassium ferrocyanide whereby the current in the oxidative signals increased slightly with the presence of H₂S, but not significantly, and can be attributed to changes in viscosity.

[0063] The attempted mediation of H₂S detection was also carried out using PVF as a mediator and is shown in Figure 2 (e). PVF was immobilised directly onto the electrode surface using solvent evaporation methods outlined by O'Mahony (O' Mahony, A. M.; et al., J. Electrochem. Soc, 2010, 157, F49-F53). Figure 1 (e) shows the oxidation of PVF in [C₄mim][PF₆] in the potential range of -0.3 V to 0.95 V. The forward scan shows the oxidation of PVF to PVF+ at 0.4 V vs. Ag, and the reverse scan shows the reduction of

PVF⁺ to PVF at 0.3 V vs Ag. H₂S gas (1 % H₂S with 99% N₂ fill) was flowed through the cell for ca. 40 minutes and a cyclic voltammogran was taken in the potential range of -0.3 V to 0.95 V. The oxidation peak of PVF to PVF⁺ is observed at potential 0.55 V vs Ag and the reduction of PVF⁺ to PVF at 0.45 V vs. Ag. A shift in the potential of the electrochemical signals was observed as before due to the use of a quasi-reference electrode. No increase was observed in the oxidative and reductive signals on addition of H₂S to the T- cell showing that no mediation has taken place. Unlike the systems observed in Figures 2 (a)-(d) even a slight increase in the electrochemical signals was not observed for the voltammetric signals of PVF due to change in viscosity of the RTIL. This is likely because PVF was immobilized directly onto the electrode surface and not free to diffuse throughout the entire RTIL, therefore changes in viscosity would not influence diffusion of PVF in this system.

[0064] Of these five mediators tested, none were likely to be practically useful for the mediated detection of H₂S in RTILs, despite some having previously been so in aqueous solution.

EXAMPLE 2

[0065] Example 2 investigated the reduction of H₂S in [C₂mim][NTf₂] using 3,5-tert-butyl- o-benzoquinone as a mediator over relatively short periods of time.

[0066] Figure 3 shows the cyclic voltammograms for the reduction of different concentrations of the mediator 3,5-tert-butyl-o-benzoquinone. Figure 3(a) shows the reduction of 20 mM mediator in [C₂mim][NTf₂] in the potential range of 0.0 V to -1 .1 V, denoted by the solid line. A signal is observed at -0.9 V vs. Pt on the forward scan. This is attributed to the direct two electron reduction of the mediator outlined in Figure 1 . The cyclic voltammogran is reversed at potential -1 .1 V and an oxidative signal is observed at - 0.85 V vs. Pt which is attributed to the re-oxidation of the reduced species. A potential step was carried out on the reductive wave of the mediator from 0.0 V (corresponding to no faradaic current) to -0.95 V vs. Pt and the current was measured for 5 s. The experimental data was theoretically fitted to the Shoup and Szabo expression (Shoup, D. and Szabo, A., J. Electroanal. Chem., 1982, 140, 237-245). The diffusion coefficient obtained for the mediator in [C2mim][NTf2] was calculated to be 1 .4 ^χ 10-1 1 m² s^"1. The chronoamperometric fitting also gave the nc value (no. of electrons ^χ concentration) which was 36 mM. This indicates a 2-electron transfer since the concentration of mediator dropped onto the electrode was ca. 20 mM, so c = 18 mM and n = 2. The reductive signal for the mediator is oberved in Figure 3(b)-(d) also for concentrations 2mM, 5mM and 10mM mediator respectively at potential -0.75 V vs. Pt, denoted by the solid lines.

[0067] H₂S gas (1 % H₂S with 99% N₂ fill) was flowed into each of these systems for 10- 15 minutes. Figure 3(a) shows cyclic voltammetry after the addition of H₂S denoted by the dotted line in the potential range of 0.0 V to -1 .3 V. On the forward scan, a reductive signal is observed at potential -0.53 V vs. Pt (current -0.13 nA), and at a more negative potential a further signal is observed at -1 .0 V vs. Pt (current -0.59 nA). The first signal is attributed to the direct reduction of the mediator only. The shift in potential from -0.9 V to - 0.7 V vs. Pt is attributed to the effect of H₂S on the quasi-reference electrode. It has been confirmed, using the lUPAC-recommended Fc/Fc+ redox couple as an internal reference (see Rogers, E. I. et al., J. Phys. Chem. C, 2008, 1 12, 2729-2735.), that this signal is due to the reduction of the mediator only. The decrease in the mediator only signal is attributed to the consumption of the mediator with H₂S over time. [0068] The next reductive signal at a potential of -1 .0 V vs. Pt is assigned to the reduction of the product formed as a result of the reaction between the mediator and H₂S, which is outlined in Step 2 of Figure 1 . Figure 3(b)-(d) also shows cyclic voltammograms for the addition of H₂S after 10-15 minutes exposure for 2, 5 and 10 mM mediator respectively, denoted by the dashed lines. In each case, a potential shift and current decrease of the signal of the mediator is observed. The signal for the product of the reaction is not as clear for these smaller concentrations of 3,5-tert-butyl-o-benzoquinone as with 20 mM concentration. This is likely due to lower currents of the signal and relatively higher currents of the direct reduction of H₂S at the Pt electrode, the onset of which is observed at ca. -1 .5 V vs. Pt.

[0069] After 10-15 minutes exposure to H₂S, the t-cell was purged with nitrogen gas for approximately 60 minutes. Figure 4(a) shows the initial cyclic voltammogram taken of the mediator only in the potential range of 0.0 V to -1 .1 V denoted by the solid line. It also shows the cyclic voltammogram taken after 60 minutes N₂ purging in the potential range of 0.0 V to -1 .3 V denoted by the dashed line. This signal shows an increase in the current of the mediator signal from -0.13 nA (after 10 minutes exposure to H₂S) to 1 .29 nA (after 60 minutes N₂ purging), which is a near 100% recovery of the initial signal of the mediator before any exposure to H₂S. It should be noted that the reaction times quoted in this work are subject only to the system examined and the need to equilibrate the gas in a relatively large volume of electrolyte. In a practical sensor, a thin layer of electrolyte would diminish this reaction time substantially. The recovery of the initial current of the mediator signal is one of the most promising aspects of the use of this mediator for the detection of H₂S since a sensor incorporating this system can be used, treated, and re-used, making it very attractive as a practical sensor. In Figure 4(b)-(d) similar behaviour is observed whereby the current of the reductive signal of the mediator is recovered after exposure to H₂S through N₂ purging (dashed line), to the initial current of the mediator before exposure to H₂S (solid line) for three concentrations of mediator species (2, 5 and 10 mM).

[0070] Therefore we attribute Step 1 of Figure 1 to be reversible within the timescales presented in these experiments. This system represents a viable sensor for H₂S detection since a clear change in voltammetry is observed and the sensor can be treated for re-use.

EXAMPLE 3

[0071 ] Example 3 investigated the oxidative and reductive features of H₂S in

[C₂mim][NTf₂] using 3,5-tert-butyl-o-benzoquinone as a mediator over relatively long periods of time, e.g. when compared to the time periods used in Example 2.

[0072] Figure 1 outlines the reaction between H₂S and the mediator 3,5-tert-butyl-o- benzoquinone. Step 1 shows the formation of a substituted 1 ,2-dihydroxybenzene species which reduces at a potential more negative to that of the mediator. The mediator itself is partly consumed in the reaction. It is noteworthy that the partial consumption of the mediating species is not a limitation to this system since it can be overcome in a practical sensor by using a mediatorsatu rated solution in contact with the electrode. To further examine Step 1 of Figure 1 , cyclic voltammetry was carried out to observe the oxidative signal of the product assumed to be a substituted 1 ,2-dihydroxybenzene. Figure 5(a) shows the voltammetry of 2 mM 3,5-tert-butylo-benzoquinone and added H₂S gas [C2mim][NTf2] in the potential range of -1 .5 V to +1 .5 V. A cyclic voltammogram was taken on 2 mM of the mediator in [C₂mim][NTf₂] from 0.0 V to -1 .5 V, then reversed and scanned to 2.0 V, and finally reversed again and scanned back to 0.0 V. Figure 5(a) shows this scan denoted by the dotted line. On the negative sweep the reductive signal for the mediator only is observed at potential -0.75 V vs. Pt at current -0.185 nA. On the positive sweep (from 0.0 V to 2.0 V) no voltammetric signals are observed.

[0073] H₂S gas (1 % H₂S gas with 99% N₂ fill) is introduced into the system and cyclic voltammograms are taken at regular time intervals in the potential range of -1 .3 V to +1 .3 V in a similar manner to that of the mediator only. Figure 5(a) shows the decrease of the mediator signal on the negative sweep from 6 - 42 mins, denoted by the solid lines. The reductive signal of the mediator has shifted potential to -0.4 V vs. Pt and decreases to -0.1 nA over 42 mins. On the reverse sweep, the onset of a new signal is observed at potential 1 .0 V vs. Pt. This is attributed to the oxidation of a 1 ,2-dihydroxybenzene species formed when the H₂S and the mediator react as outlined in Step 1 of Figure 1 . Over time this signal increases as the mediator signal decreases.

[0074] This reaction was observed over a long period of time. Figure 5(b) shows cyclic voltammograms of the mediator and added H₂S from 42 to 78 mins taken at regular intervals in the potential range of -1 .3 V to +1 .4 V. On the forward scan the complete consumption of the mediator signal is observed over time at -0.4 V vs. Pt. This is due to the chemical reaction of 3,5-tert-butyl-o-benzoquinone with H₂S gas. At a more negative potential, the onset of a signal at -0.6 V vs. Pt is observed. This is attributed to the reduction of a species formed due to the reaction of the parent mediator with the 1 ,2- dihydroxybenzene species outlined in Figure 1 Step 2. The parent species and the 1 ,2- dihyroxybenzene species undergo reduction and oxidation respectively when they react together to form 3,5-tert-butyl-4-thiol-o-benzoquinone (Figure 1 (c)) and 3,5-tert-butyl-1 ,2- dihydroxybenzene (Figure 1 (d)). The former species is available for further reaction with H₂S to form the final product. The voltammetric signal observed at -0.6 V vs. Pt is attributed to the reduction of this final product. [0075] This chemical reaction is irreversible. After 78 mins exposure to H₂S, N₂ gas was pumped through the T-cell to purge it of H₂S. Figure 5(c) shows cyclic voltammograms of the mediator/H₂S system (after 78 mins H₂S exposure) for N₂ purging of 0, 1 5 and 60 mins. These cyclic voltammograms were taken in the range of -1 .2 V to +1 .5 V. The forward scan shows a reduction peak at -0.6 V vs. Pt which is attributed to the reduction of the final reaction product outlined in Figure 1 . With 60 mins of N₂ purging the signal remains unchanged indicating an irreversible chemical reaction. The oxidative scan shows little change in the oxidative peak of the 1 ,2-dihydroxybenzene species at 1 .2 V vs. Pt, again indicating an irreversible chemical reaction. The slight decrease in this signal over 0 to 60 mins N₂ purging is attributed to the removal of excess H₂S from the system, the direct oxidation of which proceeds directly after the oxidation of 1 ,2-dihydroxybenzene, and the presence of which alters the viscosity of the RTIL slightly.

[0076] Figure 5(d) shows the change in the modulus of the current of the reductive mediator signal (■) and the change in the current of the oxidative 1 ,2-dihydroxybenzene species (o) over time. Initially (0-20 mins) each signal changes until the rate of change slows. This change is attributed to Step 1 of the mechanism outlined in Figure 1 which is chemically reversible. After 20 mins, the rate of change of each signal increases again until the reductive signal reaches a minimum, and the oxidative signal reaches a maximum. This is attributed to the further reaction of the 1 ,2-dihydroxybenzene species with the parent mediator until the parent is completely consumed leaving only the final product shown in Step 3 of Figure 1 .

[0077] Also shown in Figure 5(d) is a third signal (Δ) which shows the change in the oxidative signal over time in the potential range of 0.0 V to 1 .5 V without ever scanning reductively. This signal increases much more slowly than that of the oxidative signal after reduction (o). This shows that while the mediator and H₂S do react to form the 1 ,2- dihydroxybenzene species regardless of voltammetric cycling, however voltammetric cycling can activate the electrode towards the oxidation of this species.

[0078] 3,5-tert-butyl-o-benzoquinone has been presented as a suitable mediator for H₂S detection in RTILs. The reductive voltammetry changes, due to consumption of the mediator and formation of an adduct. The decrease of the reductive signal and the onset of an oxidative signal are both viable markers for the presence of H₂S. Also, within certain time limits, this chemical reaction is reversible whereby a recovery of the initial mediator signal is observed after purging the cell with N₂ over a period of time, allowing re-use of the sensor. Therefore this species is a suitable mediator for detection of H₂S as it incorporates the benefit of a re-usable sensor and all the attractive properties of RTILs. EXAMPLE 4

[0079] Example 4 investigated the mediated detection of low concentrations of H₂S in RTILs.

[0080] Figure 6 shows the mediated detection of H₂S gas (200 ppm H₂S with N₂ fill) in [C₂mim][NTf₂] using the mediator 3,5-tert-butyl-o-benzoquinone (2 mM). Cyclic

voltammetry was carried out on 2 mM 3,5-tert-butyl-o-benzoquinone in [C₂mim][NTf₂] only (dotted line). The two-electron reduction of the mediator was observed at -0.73 V vs. Pt on the forward scan. The scan was reversed at -1 .5 V and an oxidative sweep was carried out to potential 1 .5 V. No oxidative voltammetric features were observed. The scan was reversed again at 1 .5 V and cycled to 0.0 V. H₂S 1 % was mixed with N₂ to a concentration of 200 ppm and flowed through the system. Cyclic voltammograms were taken at regular time intervals in the potential range of -1 .2 V to 1 .2 V. The scan was cycled from 0.0 V to -1 .2 V, and the reduction of the mediator was observed at -0.59 V vs. Pt, the current of which decreased slightly over time suggesting that H₂S was reacting with the mediator to form the 1 ,2-hydroxybenzene species outlined in Figure 2(b). The scan was reversed at -1 .2 V and scanned to 1 .2 V. Initially, no oxidative signal for the 1 ,2- hydroxybenzene species. After 140 mins of H₂S flow however, a signal was observed on the oxidative scan at 1 .0 V vs. Pt shown in Figure 6 (solid line). This is attributed to the oxidation of the 1 ,2-hydroxybenzene species formed by chemical reaction of H₂S and 3,5- tert-butyl-o-benzoquinone. As mentioned in Example 2, the reaction times quoted in this work reflect the cell used which contains a relatively large volume of electrolyte: faster response times are expected with a real sensor.

[0081 ] Therefore, at very low concentrations of H₂S, chemical reaction with the mediator occurs to decrease the reductive mediator signal and cause the onset of an oxidative signal not present before the addition of H₂S.

[0082] Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of them mean "including but not limited to", and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0083] Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

[0084] The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

Claims

1 . A method of detecting hydrogen sulfide in a sample, which comprises the steps of contacting the sample with working and counter electrodes in the presence of an ionic liquid and a mediator and determining the electrochemical response of the working electrode to the sample, wherein the mediator comprises a compound of formula I or formula II:

wherein:

X and Y each independently represent O or NR₄;

Represents a halide, or a Ci-C₂₀ substituted or unsubstituted alkyl group;

R₂, R₃ and R₄ each independently represent H, a halide, or a C C₂o substituted or unsubstituted alkyl group.

The method of claim 1 , wherein X and Y are both NR₄.

The method of claim 1 , wherein X and Y are both O.

The method of any preceding claim, wherein at least one of and R₂ is an alkyl group, optionally wherein both and R₂ are alkyl groups.

The method of any preceding claim, wherein and / or R₂ are tertiary butyl groups.

The method of any preceding claim, wherein the mediator is a compound of formula I, and Ri is attached to carbon 3 of the 6-carbon ring and / or wherein R₂ is attached to carbon 5 of the 6-carbon ring.

The method of any preceding claim, wherein R₃ is H or a halide, optionally H.

The method of any preceding claim, wherein R₄ (if present) is H or an alkyl group.

The method of claim 1 , wherein the mediator is 3,5-tert-butyl-o-benzoquinone.

The method of any proceeding claim, wherein the ionic liquid is a room

temperature ionic liquid (RTIL).

1 1 . The method of any preceding claim, wherein the ionic liquid comprises a cation selected from a diaikyiimidazoiium, N-aikyipyridinium, tetraalkylammonium or tetraa!ky!phosphonium cation.

12. The method of any preceding claim, wherein the ionic liquid comprises a 1 -alkyl-3- methyiimidazolium ([C_nmim]} cation, where n is an integer from 1 to 10, optionaJiy an integer from 2 to 4.

13. The method of any proceeding claim, wherein the ionic liquid comprises an anion selected from hexaf!uorophosphate [PF_e]; tetrafluoroborate [BF ];

trifiuoromethyisu!fonate [CF₃SO_?,]; bis[{irifiuoromethy!) su!fonyijamide [{CF₃S0₂)₂N]; bis[{trifluoromeihyi) sulfonyljimide [NTf₂]; trifluoroefhanoate [CF₃C0₂]; acetate

[CH₃C0₂]; nitrate, and ha!sde.

14. The method of any proceeding claim, wherein the ionic liquid comprises an anion selected from hexafluorophosphaie [PF_e], tetrafluoroborate [BF₄] and

bis[{trifiuoromeihyi) suifonyijimide [NTf₂],

15. The method of any of claims 1 to 1 1 , wherein the ionic liquid comprises at least one of 1 -Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C₂mim][NTf₂]), 1 -Butyl-3-methylimidazolium tetrafluoroborate ([C mim][BF₄], or 1 -Butyl-3- methylimidazolium hexafluorophosphaie ([C₄mim][PF₆]).

16. The method of any preceding claim, wherein the mediator is present in the ionic liquid at a concentration of not more than 500 mM, optionally at a concentration of not more than 100 mM, further optionally at a concentration of not more than 50 mM, still further optionally at a concentration of not more than 30 mM.

17. The method of any preceding claim, wherein the mediator is present at a

concentration of not less than 2 mM, optionally at a concentration of not less than 5 mM, further optionally at a concentration of not less than 10 mM, still further optionally at a concentration of not less than 20 mM.

18. The method of any of claims 1 to 15, wherein the mediator is present at a

concentration of about 20 mM.

19. The method of any of claims 1 to 15, wherein the ionic liquid is saturated with the mediator.

20. The method of any preceding claim, wherein the working electrode is a platinum electrode.

21 . The method of any preceding claim, wherein the counter electrode is a platinum electrode, optionally a platinum wire electrode.

22. The method of any proceeding claim, wherein the mediator forms an adduct with hydrogen sulfide, wherein said adduct can be electrochemically oxidised or reduced.

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

24. The method of claim 22, wherein the electrochemical response is a voltammetric response.

25. The method of claim 23 or claim 24, wherein the response of the working electrode is determined using cyclic voltammetry.

26. Use of a mediator as defined in claim 1 for the electrochemical detection of

hydrogen sulfide, wherein the mediator is dissolved in an ionic liquid.

27. The use of claim 26, wherein the mediator is as defined in any of claims 2 to 9.

28. The use of claim 26 or 27, wherein the ionic liquid is as defined in any of claims 10 to 15.

29. The use of any of claims 26 to 28, wherein the mediator is dissolved in the ionic liquid at a concentration as defined in any of claims 16 to 19.