Evaluation of a Ag Reference Electrode for Use in Room Temperature Ionic Liquids

8/10/2019 Evaluation of a Ag Reference Electrode for Use in Room Temperature Ionic Liquids

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Evaluation of a AgjAg+ reference electrode for use in roomtemperature ionic liquids

G.A. Snook *, A.S. Best, A.G. Pandolfo, A.F. Hollenkamp

Commonwealth Scientific and Industrial Research Organisation (CSIRO) Energy Technology, Box 312, Clayton South, Vic. 3169, Australia

Received 19 June 2006; received in revised form 5 July 2006; accepted 5 July 2006Available online 4 August 2006

Abstract

Interest continues to grow in the use of room temperature ionic liquids (RTILs) as electrolytes in a range of electrochemical appli-cations, such as lithium batteries, supercapacitors and dye-sensitized solar cells. Underpinning this growth, investigations into the elec-trochemical behaviour of RTILs and RTIL-based systems rely on accurate and precise data on the potentials of redox processes. Whilemost researchers have continued the practice (developed with non-aqueous solvents) of reporting potentials relative to one of the metal-organic standards (such as ferrocene), little attention has been given to the development of a reliable reference electrode, based on anionic liquid. Such an electrode is always valuable, especially in situations where addition of a reference material is not possible.

We report a AgjAg+ reference electrode, incorporating a known concentration of silver trifluoromethanesulfonate (AgTf) in 1-butyl-1-methyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide (P14TFSI), which provides a stable and reproducible reference potential.Voltammetric monitoring of the redox potentials for ferrocene and cobalticinium hexafluorophosphate have shown that the electrodeAgjAg+(10 mM AgTf, P14TFSI) is stable to within a millivolt, over a period of around three weeks, when used in an argon atmosphereat room temperature. Higher concentrations of silver ion reveal close-to-Nernstian behaviour. All Ag jAg+ configurations were signifi-cantly more stable than a silver wire quasi-reference electrode, even when the latter was separated in a salt-bridge. Voltammetric datarecorded in a range of different ionic liquids, against the AgjAg+ (10 mM AgTf, P14TFSI) reference electrode, produced apparent junc-tion potentials of a few tens of mV. Changes in sign of the junction potential are usefully discussed in terms of the relative mobilities ofthe anions and cations present, while the magnitude can be discussed in the terms of a classic molten salt treatment.2006 Elsevier B.V. All rights reserved.

Keywords: Reference electrode; Quasi-reference electrode; Silver; Room temperature ionic liquid; Junction potential

1. Introduction

Room temperature ionic liquids (RTILs) continue toreceive much attention, due to their ability to be tailored

for an ever-expanding range of applications. One impor-tant research area is in the field of electrochemistry, wherethey are being considered as electrolytes in lithium batter-ies, supercapacitors and dye-sensitized solar cells. In theseapplications, accurate information is required on the posi-tion and the magnitude of the electrochemical window, thediffusion of charged species and the reversibility of charge-

transfer processes. This in turn requires stable referencingof the potential scale.

In probing the electrochemical properties of RTILs(e.g., with cyclic voltammetry), the experimental configura-

tion often reported includes a quasi-reference electrode.This practice goes back to electrochemistry in non-aqueoussolvents, where well-defined reference electrodes are diffi-cult to prepare and stabilize, and can introduce extrauncompensated resistance. From the literature on the elec-trochemistry of ionic liquids, little can be found on the useand understanding of reference electrodes. The majorityreport the use of quasi-reference electrodes, based on silver[110] or platinum wire [1114], which are immerseddirectly into the analyte solution. Less commonly, magne-sium [15] or aluminium wire [16] have been used in the

1388-2481/$ - see front matter 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.elecom.2006.07.004

* Corresponding author. Tel.: +61 395458863; fax: +61 395628919.E-mail address:[email protected](G.A. Snook).

www.elsevier.com/locate/elecom

Electrochemistry Communications 8 (2006) 14051411
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same role. The problems that can arise when using quasi-reference electrodes are often not appreciated and, weargue, are also likely to be more severe when the electrolyteis an RTIL.

As is generally known, the commonly used quasi-refer-ence electrode (based on either silver or platinum wire)

functions through the presence of various compounds(most likely oxides) on the metal surface. The exact identityof the redox couple is never known with any certainty. Asecond unknown is the amount of the oxidized compo-nent(s), which, clearly, is very small when compared withcommercial reference electrodes. The problem here is thatthe composition of the surface, and hence the potential ofthe reference electrode, is liable to change greatly in theevent of: (i) reaction with components of the surroundingsolution; (ii) dissolution in the electrolyte; (iii) polarization,due to lack of potentiostatic control.1 For these reasons,employing a quasi-reference electrode always carries theassumption that the potential(s) of the electrochemical pro-

cesses of interest will be recorded, at some stage, against atrue reference electrode (i.e., one based on a stable, well-defined redox couple) or that the potential of a suitablestandard compound (e.g., ferrocene) will be recorded underidentical test conditions[18].

In this study, we examine the behaviour of a Ag jAg+ ref-erence electrode comprising a silver wire immersed in a fixedconcentration of silver trifluoromethanesulfonate (AgTf)dissolved in a common pyrrolidinium ionic liquid, and sepa-rated by a porous junction from the test solution. In a recentbook, Katayama [19] briefly describes the usage of theAgjAg+ couple as a reference system in several ionic liquids.

While some details are presented on the construction of thereference electrodes, there is no discussion of the stabilityof this system. In a later work, Katayama and co-workers[20] report using a AgjAg+ reference electrode, withthe silversalt dissolved in 1-ethyl-3-methyl-imidazolium bis(trif-luoromethanesulfonyl)imide (EMImTFSI). Here a directmeasurement of the potential of the ferrocene oxidation pro-cess is reported, but there is no description of the character-istics (stability, etc.) of the electrode. More recently, Sahebet al. [21] described a AgjAg+ (imidazolium ionic liquid) ref-erence electrode in which the concentration of silver ion is setby addition of a solution of silver (I) nitrate in acetonitrile. Aproblem with such a system, noted by the authors, is that theelectrodes potential varies significantly with the volumefraction of acetonitrile, and some loss of the latter wouldbe likely after several days of use at ambient temperature,or indeed, after brief periods above ambient temperatures.

We compare a AgjAg+ reference electrode, constitutedin an ionic liquid, with a silver-wire quasi-reference elec-trode which is either immersed directly into the ionic liquidin the electrochemical cell, or separated from the latter bymeans of a salt-bridged compartment which contains the

ionic liquid. The performance of each reference system isevaluated in a range of RTILs by recording the voltammet-ric responses for ferrocene and cobalticinium hexafluoro-phosphate. Both compounds are widely used as internalstandards for referencing redox potentials [2,3,6,2224].In this paper, we argue that because many of the RTILs

commonly being used in electrochemical investigationsare excellent solvents for a range of metal compounds, agreater level of caution is required when employing anytype of quasi-reference electrode. Moreover, in some situa-tions, it will be prudent to utilize a true reference electrode,similar to those examined here. Such systems have provento be reassuringly stable and robust.

2. Experimental

All solution preparations and electrochemical measure-ments were performed in an argon-filled glove box. Allchemicals were used as received (unless indicated other-

wise). The ionic liquid 1-butyl-1-methyl-pyrrolidiniumbis(trifluoromethanesulfonyl)imide (P14TFSI) was eitherpurchased from Merck (high purity specification,


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Bis(cyclopentadienyl)cobalt(III) hexafluorophosphate

(cobalticinium hexafluorophosphate, or CcPF6) andbis(cyclopentadienyl)iron(II) (ferrocene, or Fc) were bothobtained in 98% purity from Aldrich, and were dissolvedat several different concentrations by stirring overnight at50 C.

All electrochemical experiments were undertaken with alAutolabIII potentiostat/galvanostat (Ecochemie, Nether-lands), at 23 1 C, with a 100 lm diameter Pt workingelectrode, a large area (wire-wound) Pt counter electrodeand one of the reference electrodes prepared above. Con-ductivity measurements were carried out at 25 C (exceptfor AmmoEng 100, 22C) with a HewlettPackard

4924A LCR Meter at frequencies from 20 Hz to 1 MHz.LabviewTM software on a PC was used to control temper-ature (via thermocouple and heated block), sweep the fre-quency of the meter and record the results. Samples wereprepared and kept under argon and the instrument was cal-ibrated against aqueous 0.1 M KCl. Viscosity was mea-sured at 25 C with an Anton Paar (AMVn) AutomatedMicroviscometer. The 2.5 mm diameter ball (7.85 g cm3)gave a constant of 0.114 mPa cm3 g1 with a 3 mm widecapillary and 20 dropping angle. Density of the ionicliquid was measured by accurately weighing the amountof liquid in the 1.45 cm3 capillary tube.

3. Results and discussion

3.1. Voltammetric studies of Fc and CcPF6in P14TFSI

A typical cyclic voltammogram for a solution of20 mM cobalticinium hexafluorophosphate and 5 mM fer-rocene in the ionic liquid P14TFSI is shown in Fig. 2a.Here a quasi-reference electrode (silver wire) is immerseddirectly in the ionic liquid. Responses were recorded atscan rates between 20 mV s1 and 1000 mV s1. Withthe distance between the working and reference electrodesset at10 mm, peak-to-peak separations were recorded in

the range of 60 to 80 mV for both redox couples, with the

values trending higher at the higher scan rates. This ismost likely due to the effects of a small amount of uncom-pensated solution resistance. According to the theory[22]that relates this resistance to the separation between thereference electrode and what is a relatively small(0.1 mm dia.) working electrode, the separation wouldhave to be reduced to below 0.5 mm before there wouldbe a significant decrease in uncompensated resistance.Given that physical shielding of the working electrodewould then be a serious additional problem, we decidedto leave the electrodes in their initial positions. Potentials

midway between the forward and reverse peaks were aver-aged (for scans at 20 mV s1 and 50 mV s1) to determinevalues of E0 0 [26] for the cobaltocenecobalticinium andthe ferroceneferricinium couples. Results are presentedin Table 1. Repeating the cyclic voltammetric measure-ments on this solution, on different days, produced arange of peak positions for which the standard deviationwas 100 mV (based on five runs).

The silver wire quasi-reference electrode was then placedin a compartment that was filled with P14TFSI (but noadded Ag+ salt) and immersed in the P14TFSI solution ofthe two metallocenes. From a set of repeat measurements(cyclic voltammograms) with this configuration, the varia-tion in peak potentials (Table 1) gave a standard deviationof 55 mV (based on four runs). In a study of the behaviourof CcPF6 and various derivatives of ferrocene in the ionicliquid 1-butyl-3-methyl-imidazolium hexafluorophosphate,Hultgren et al. [3] reported that separating a silver quasi-reference electrode (with a similar compartment-and-fritarrangement) reduced potential drift to within 5 mV overa 24 h period. These data show that while the behaviourof a system that employs a quasi-reference electrode mayappear to be stable, comparison of day-to-day potentialstability indicates a relatively large variability. The addi-tional variability associated with the directly immersed

electrode is probably because the silver wire is in contact

4.5mm 6.4mm

2.5mm

0.5mm Ag wireGlass Tubing

Teflon Cap

4.5mm

Ultrafine Porous

Glass Frit

Ultrafine PorousGlass Frit

Salt-bridge

b

a

Fig. 1. Diagram outlining the components and dimensions of: (a) theAgjAg+ reference electrode and (b) the reference electrode inserted into asalt-bridge compartment (optional).

Fig. 2. Cyclic voltammetric responses for 20 mM cobalticinium hexaflu-orophosphate (reduction) and 5 mM ferrocene (oxidation) in P14TFSIwith reference electrode: (a) Ag wire directly immersed [response ispositioned according to averaged data (Table 1)] and (b) AgjAg+ (10 mMAgTf, P14TFSI). Scan rate = 20 mV s

1.

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with not only the ions that constitute the ionic liquid, butalso the redox-active species (Cc+ and Fc).

Two variants of a AgjAg+ reference electrode were con-structed by filling salt-bridge compartments with either10 mM AgTf or 100 mM AgTf. Cyclic voltammogramswere then recorded with the same experimental arrange-

ment as used for Fig. 2a (viz., 20 mM CcPF6 and 5 mMFc in P14TFSI). A typical voltammogram, with the10 mM AgTf electrode in place, is shown in Fig. 2b. Peakpotential data (and interpolated values ofE0 0) for the tworedox processes, with each of the reference electrodes inplace, are summarized inTable 1. Repetition of these mea-surements on different days (with a given electrode, over aperiod of one month), showed variation in the peak poten-tials of 1 mV, up to the point when the electrode was threeweeks old. Beyond this, the potential shifted steadily, in adirection that indicated a progressive fall in the concentra-tion of silver (I). Given that the instrumentation sets

applied potential with an accuracy of 1 mV, the variationsprobably reflect a system-limit rather than one imposed bythe reference electrode. The AgjAg+ (P14TFSI) referenceelectrode therefore appears to satisfy the importantrequirement of stability.

Table 1 includes calculated potential differences (basedon the metallocene E0 0 data) for each of the silver-basedreference systems, relative to the AgjAg+(100 mM AgTf)electrode. Comparing the quasi-reference electrodes withthe AgjAg+ electrodes, we note that the latter registerpotentials some 800 mV more negative than the former.Similar shifts were measured directly by means of a digitalvoltmeter connected across two electrodes dipped inP14TFSI. These shifts in potential are consistent with thelikely differences in silver speciation for the two referencesystems.

In broad terms, the AgjAg+ (P14TFSI) reference elec-trodes register potentials similar to the standard potentialfor Ag+ (+0.799 V vs. NHE[27]) while the quasi-referenceelectrodes lie in the range occupied by the AgjAgX(X = halide, pseudohalide) redox couples (0 0.3 V vs.NHE). The former conclusion is based on our observationofE0 0(Fc, Fc+) in the region of0.4 V vs. AgjAg+(100 mMAgTf, P14TFSI) and the assumption that the generally sol-vent-independent standard potential of the ferrocene couple

(+0.400 V vs. NHE [28]) also applies in the ionic liquid

medium employed here. The non-coordinating nature ofthe anions present (triflate and TFSI) is also consistent withthis assessment.

For the quasi-reference electrode, the observed potentialis established by a concentration of silver that is defined bythe solubility of silver (I) species present on the surface of

the wire. Given that the ionic liquid used in this study con-tains traces of either bromide or iodide (from the prepara-tion), it is likely that AgBr or AgI will determine theposition on the potential scale. Presumably, the variabilitynoted here in the potential of the quasi-reference electrode,even when only P14TFSI from a single source was used, isdue to the very small amounts of silver species present,which are therefore susceptible to change from run torun, as well as the likelihood that other silver-containingspecies are present. A further indication of variabilitycomes from comparing our voltammetric data for thereduction of the cobalticinium cation (E0 0 =991 mV vs.

salt-bridge-separated Ag wire) with those of Hultgrenet al.[3]who reported a value of1145 mV, with the samequasi-reference system, but in the ionic liquid 1-butyl-3-methyl-imidazolium hexafluorophosphate. Their value is150 mV more negative, i.e., shifted towards the values thatwe obtained with AgjAg+ (P14TFSI) reference electrodes. Itis worth noting that Hultgren et al. used an alkyl chloridein the preparation of their imidazolium ionic liquid andthat the presence of trace levels of chloride (as opposedto bromide or iodide) would explain the direction of theshift in quasi-reference potential (based on the argumentproposed above). This argument is largely speculative,though, and a rigorous explanation of the potential shiftswould first require a complete analysis of all species present(halides, etc.) in the ionic liquids that were used.

It is also useful to compare our results with thoseobtained by Fukui et al. [20] who appear to be the firstgroup to have measured the ferroceneferricinium couplerelative to a AgjAg+ (ionic liquid electrolyte) referenceelectrode. In that case, the ionic liquid was EMImTFSIand the same AgTf salt (at 100 mM) was used. Fromthe voltammetry of ferrocene, they reported E0 0(Fc,Fc+) = 440 mV. This is within 5 mV of our reportedvalue (Table 1) and indicates a small dependence of theobserved reference electrode potential on changing from

P14TFSI to EMImTFSI.

Table 1Cyclic voltammetric dataa (in mV) for the reduction of 20 mM cobalticinium hexafluorophosphate and oxidation of 5 mM ferrocene in P 14TFSI at20 mV s1

Reference electrode E00 (Cc,Cc+) E00 (Fc,Fc+) DE00 (CcFc) Potential shiftb

Ag wire (immersed directly in electrolyte) 960c 373c 1333 818Ag wire (immersed in a salt-bridge filled with P14TFSI) 991

d 342d 1333 787AgjAg+ (10 mM AgTf, P14TFSI) 1723 390 1333 55

AgjAg+ (100 mM AgTf, P14TFSI) 1777 445 1332 0a Data reported as E00 = 1/2 [Ep(forward sweep)+ Ep(reverse sweep)].b Potential shift = [E00(Fc,Fc+) vs. reference electrode][E00(Fc,Fc+) vs. AgjAg+ (100 mM AgTf, P14TFSI)].c Average of five runs with standard deviation = 100 mV.d Average of four runs with standard deviation = 55 mV.

1408 G.A. Snook et al. / Electrochemistry Communications 8 (2006) 14051411


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The voltammetric data recorded with the AgjAg+ elec-trodes allow us to check the reversibility of this referencecouple in the pyrrolidinium ionic liquid by comparing theshift of the potential on changing the concentration ofAgTf from 10 mM to 100 mM. FromTable 1, the recordedshift is 55 mV, which compares well with the expected value

of 59 mV[26].

3.2. Voltammetric studies in RTILs other than P14TFSI

To assess the general utility of the P14TFSI-basedAgjAg+ reference electrode in studies of RTIL electro-chemistry, cyclic voltammograms were recorded for Fcand CcPF6in several different ionic liquids, for which vis-cosity and conductivity spanned a significant range. Wewere particularly interested in characterizing any junctionpotential[29,30]from contact between P14TFSI (in the ref-erence compartment) and the ionic liquid in the electro-chemical cell. Data describing the properties of the ionic

liquids and the results from the voltammetric studies arecollected inTable 2. Only the potential data for ferroceneare reported as the reduction process of cobalticiniumwas usually not completely resolved from the reductionlimit of the imidazolium ionic liquids.

Inspection ofTable 2reveals junction potentials (DEj) ofa few tens of millivolts. [Note, as implied earlier, we pro-ceed with the assumption that the formal potential of theFcFc+ couple does not change appreciably with a changein ionic liquid.] For the imidazolium ILs, there is no obvi-ous relationship between the observed DEj and either vis-cosity or conductivity, when compared with the

respective values for P14TFSI. To analyse this system,though, a description similar to that used for classic moltensalt electrochemistry is required. Laity [31]has outlined atreatment of molten salt liquid junctions in which DEj isdefined by the relative mobilities of the ions present.Importantly, when the junction features either a commoncation or anion, the governing equation becomes astraightforward relationship of the equivalent (molar)conductivities:

DEj RT

F

K13 K23z1K13 z2K23

ln

z1K13

z2K23

1

whereKis equivalent conductivity, zis ionic charge, species1 and 2 are the different ions (+ or ) and species 3 is theion common to both salts. From Table 2, this approachcan be taken with the junction between P14TFSI and

P14BOB. With values ofK(in S cm2 mol1) calculated fromdata inTable 2and Refs. [25,32], Eq.(1)gives a value of80 mV for DEj. The fact that this is much greater thanthe observed value is also a common finding in molten saltstudies[31]. Laity notes that the main assumption in deriv-ing Eq.(1) is that ion mobilities do not change with con-centration (i.e., where the two salts mix). In simple terms,the low value of DEj probably means, in this case, thatthe mobilities of the two anions are similar in the IL-mix-tures that form at the junction, while being clearly differentin the pure phases. The relative movement of ions forP14TFSIiP14BOB is illustrated schematically in Fig. 3a,

mainly to indicate that anion movement determines thesign ofDEj.

We extend the qualitative discussion to the Ammo-Eng100iP14TFSI junction, where DEj is quite large andhas changed sign (compared with the rest of the data).AmmoEng100 consists of a quaternary ammonium cationwith a long-chain alkyl group and methyl sulfate as theanion. Given the relatively large size of the cation,

Table 2Junction potentials (DEj) inferred from voltammetric data for ferrocene inseveral ionic liquids, together with data for viscosity and conductivity

Ionic liquid Viscositya

(mPa s)Conductivitya

(mS cm1)E0 0

(Fc,Fc+)b (mV)DEj(mV)

P14TFSI 85c 2.2c 390 0

EMImTf 39 9.9 416 26BMImBF4 87 4.1 418 28EMImEtSO4 125 4.0 432 42BdMImBF4 515 0.91 415 25P14BOB 5000 0.11 401 11AmmoEng 100 1665 0.042 338 +52

a 25 C except AmmoEng 100 at 22 C.b vs AgjAg+ (10 mM AgTf, P14TFSI).c

From Ref.[25].

C12-C18NMeR2+

MeSO4-

+ -

P14+

TFSI-

liquid junction

Reference

Compartment

Analyte

Compartment

P14+

BOB-

- +

P14+

TFSI-

Reference

Compartment

liquid junction

Analyte

Compartment

a

b

Fig. 3. Illustration of localized accumulation of charge (d+/) on eitherside of the liquid junction established between two different ionic liquids,leading to observed junction potentials (Table 2): (a) P14BOB (ana-lyte)jP14TFSI (reference) and (b) AmmoEng 100 (analyte)jP14TFSI

(reference). Length of arrow indicates relative mobility of ionic species.



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compared with the pyrrolidinium species, and assumingsimilar mobilities for the TFSI and MeSO4 anions, wesuggest that differential cation transport now dominatesthe localized accumulation of charge. A relative excess ofthe AmmoEng100 cation on the working solution side ofthe junction (Fig. 3b) gives the observed polarity. Without

a common ion, we are unable to utilize Eq.(1) to establishan upper estimate ofDEj. In summary, though, it is impor-tant to note that in each of the cases investigated (Table 2),stable performance of the AgjAg+ (10 mM AgTf, P14TFSI)reference electrode was maintained, thereby confirming theutility of this reference system for use in studies thatinvolve a range of different ionic liquids.

4. Concluding remarks

In this work, we have demonstrated that a AgjAg+ ref-erence electrode can be configured in a commonly availableionic liquid and that its performance, if maintained accord-

ing to the usual precautions for silver solutions, is stableover several weeks. A concentration of 10 mM Ag+ inthe reference element (added as the triflate) is suggested,as this is easily prepared (with the ionic liquid used here)and gives more stable potential behaviour than with higherconcentrations (the rate of degradation of silver (I) in theIL appears to be concentration dependent). Contaminationof the analyte solution by Ag+ is also less likely. The pres-ence of the triflate ion has no apparent influence on thebehaviour of the reference electrode, which is consistentwith the known non-coordinating properties of this ion(particularly with silver (I)). In terms of reproducibility of

the reference potential, we found that repeated prepara-tions of the AgjAg+ (10 mM AgTf, P14TFSI) electrodegave a variation in potential of 2 mV. Most of this vari-ance should be due to the errors associated with prepara-tion of the AgTf solution and selection and cleaning ofthe silver wire. Once prepared, individual examples are sta-ble, on day-to-day use, to within 1 mV, for up to aboutthree weeks. Further improvements on lifetime could beachieved by further deoxygenation and purification of theionic liquid, together with more rigorous protection ofthe reference electrode from light.

When used in voltammetric studies in a variety of otherionic liquids, the AgjAg+ (10 mM AgTf, P14TFSI) refer-ence electrode exhibits stable potential behaviour. Assum-ing that the potential of the FcFc+ couple does not varysignificantly between the different RTILs, the data indicatethe presence of junction potentials of a few tens of milli-volts. From a qualitative view, relative ionic mobilities pro-vide an explanation of the polarity of the junctions, while aquantitative treatment establishes an upper limiting valueof the potential for the junction between P14TFSI andP14BOB. More rigorous analysis will require informationon the transport properties of the mixed phases that format the boundary between two ionic liquids.

Finally, it seems clear that the AgjAg+ reference system

will also work well when constituted in one of a range of

commonly used ionic liquids. As noted earlier, Fukuiet al.[20]employ the AgjAg+ (100 mM AgTf, EMImTFSI)reference electrode in their studies with imidazolium andpyrrolidinium ionic liquids and report a potential for theoxidation of ferrocene that is within 5 mV of our value.In situations where a large number of experiments are to

be performed in the same ionic liquid, it will be sensibleto prepare a AgjAg+ reference electrode in the ionic liquidand evaluate its behaviour. As shown here, the latter isreadily accomplished by examining the voltammetricresponses for ferrocene and a cobalticinium compound.

Acknowledgements

We thank Dr. Thomas Ruether for synthesising samplesof the pyrrolidinium ionic liquids, and Drs. Noel Duffy andJunhua Huang for comments on the manuscript and help-ful discussions, and finally Dr. Junhua Huang for assis-tance with viscosity measurements.

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Evaluation of a Ag Reference Electrode for Use in Room Temperature Ionic Liquids