Voltammetry of chromium(VI) at the liquid|liquid interface

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Electrochemistry Communications 7 (2005) 976–982

Voltammetry of chromium(VI) at the liquid|liquid interface

Aoife M. O�Mahony a, Micheal D. Scanlon a, Alfonso Berduque a, Valerio Beni a,Damien W.M. Arrigan a,*, Enrico Faggi b, Andrea Bencini b

a Tyndall National Institute, Lee Maltings, University College, Cork, Irelandb Dipartimento di Chimica, Universita di Firenze, Via della Lastruccia 3, 50019 Sesto Fiorentino, Firenze, Italy

Received 16 June 2005; accepted 30 June 2005Available online 8 August 2005

Abstract

The voltammetry of hexavalent chromium (ammonium dichromate) at the interface between two immiscible electrolyte solutionsis reported. Detection of Cr(VI) by ion transfer voltammetry is possible by use of an organic phase ionophore, which facilitates thetransfer of Cr(VI) from the aqueous into the organic phase. The ionophore was the penta protonated form of polyamine macrocycle2,5,8,11,14-pentaaza[15]-16,29-phenanthrolinophane (NeoTT). Cyclic voltammetry showed an increase of the peak current onincreasing the concentration of Cr(VI). Square wave voltammetry with background subtraction was employed for low level concen-tration detection. The lowest concentration detected was 0.25 parts per million of Cr(VI).� 2005 Elsevier B.V. All rights reserved.

Keywords: Liquid|liquid interface; ITIES; Hexavalent chromium; Facilitated ion transfer; Ammonium dichromate

1. Introduction

Speciation is one of the major challenges in analyticalchemistry, particularly metal speciation [1], whereinanalysis of species containing chromium is of great envi-ronmental relevance due to the different possible oxida-tion states in which this element can exist [2] and to thedramatically different effects of such oxidation states onhealth and the environment. In solution, chromium mayexist as Cr(III) and Cr(VI) [3] and these two oxidationstates have a contrasting impact in environment andhealth. Hexavalent chromium is quite toxic due to itshigh oxidation potential and its relatively small size,which enables it to penetrate biological cell membranes.It is limited in groundwater by the World Health Orga-nisation (WHO) provisional guideline value of 50 partsper billion (lg kg�1, ppb) [2,4]. However, trivalent

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doi:10.1016/j.elecom.2005.06.011

* Corresponding author. Tel.: +353 21 4904079; fax: +353 214270271.

E-mail address: [email protected] (D.W.M. Arrigan).

chromium is relatively harmless and plays an essentialrole in biological processes, and it is about 100–1000times less toxic than the hexavalent state [5]. The acutetoxic effects of chromium include immediate cardiovas-cular shock, with later effects on the kidney, liver andblood-forming organs [6,7]. The main sources of chro-mium pollution in ground water are plating industries,cooling towers, timber treatment, leather tanning, woodpreservation and steel manufacturing [2,8]. Moreover,chromium particles in air play an important role in theoxidation of sulphur dioxide and formation of acidicaerosols involved in global acid rain [9].

Two basic electrode systems are generally used forstripping voltammetric measurements of Cr(VI), themercury-film electrode (MFE) and hanging mercurydrop electrode (HMDE). However, because of the toxic-ity of mercury, alternative electrode materials aresought, particularly to meet the growing demands foron-site environmental monitoring of trace chromium[10]. Thus, mercury electrodes have been replaced by so-lid electrodes [2]. Direct electrochemical reduction of

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A.M. O’Mahony et al. / Electrochemistry Communications 7 (2005) 976–982 977

Cr(VI) has been studied on gold and carbon electrodes[2]. Moreover, bismuth film-electrodes have been inves-tigated as possible alternatives. Bismuth is an environ-mentally friendly element, with very low toxicity, anda widespread pharmaceutical use [11]. A sensitiveadsorptive stripping protocol was used to detect Cr(VI)via its reduction and subsequent complexation withdiethylenetriammine pentaacetic acid (DTPA) at a bis-muth film electrode [10].

An electrochemical detection strategy which hashardly been investigated [12] for Cr(VI) voltammetry ision transfer across the interface between two immiscibleelectrolyte solutions (ITIES) [13–17]. The ITIES may beused as the basis of voltammetric detection methods be-cause the transfer of charged analyte species across it re-sults in current generation [18,19]. Although there aremany examples of cationic species determination byion transfer and facilitated ion transfer, the voltammetryof anions has been hardly addressed [20–22]. Amongstpossible methods for facilitating the transport of anionicCr(VI) across the ITIES, complexation with a supramo-lecular complexation agent and interaction with organ-ic-phase electrolyte materials (as recently described forthe transfer of silver(I) cations across the ITIES [23])may be considered. Supramolecular chemistry of anio-nic species has emerged as an important area of interestdue to the presence of such species in both organic (bio-logical) and inorganic environments [24]. The use ofnon-covalent interactions with positive charged centers,including the use of polyammonium receptors, and ofcoordinative interactions with metal ions, constitutetwo major approaches for strong and selective anionbinding [24]. Among carriers used for Cr(VI) transportin membrane extraction processes, quaternary ammo-nium salts have been most extensively studied [25–27].

The aim of this work was to investigate if Cr(VI)transfer across the ITIES was possible using either or-ganic phase electrolyte-facilitated or macrocyclic iono-phore-facilitated processes. This study is a preliminarystudy toward an electroanalytical methodology basedon voltammetry at the ITIES for the determination ofCr(VI).

Fig. 1. Chemical structure of the ionophore employed in this work,the penta-protonated form of 2,5,8,11,14-pentaaza[15]-16,29-phenanthrolinophane (NeoTT) [31].

2. Experimental

2.1. Reagents

Reagents used were of maximum purity commer-cially available. Lithium chloride anhydrous (LiCl), tet-radodecylammonium tetrakis-(4-chlorophenyl)borate(herein called ETH 500), bis (triphenylphosphoranylid-ene) ammonium chloride (BTPPACl) and potassiumtetrakis(4-chlorophenyl-borate) (KTPBCl), hydrochlo-ric acid 1.040 N, ammonium chloride (99.99%), sulphu-ric acid 0.5 N, tetraethylammonium chloride (TEACl)

and the organic solvents used (1,2-dichloroethane,DCE; and 1,6-dichlorohexane, DCH) were purchasedfrom Sigma–Aldrich. Ammonium dichromate atomicspectroscopy standard solution ((NH4)2Cr2O7,1000 ppm of chromium) was purchased from BDH Lab-oratory Supplies (England); due to the low concentra-tions and the acidic nature of the aqueous phaseelectrolytes employed, the dominant Cr(VI) species as-sumed present is the monoatomic HCrO�

4 [2]. Solutionsof LiCl 10 mM, LiCl 10 mM +HCl 1 mM, or HCl 0.1 Min de-ionised water were used as aqueous phases. Theorganic phase contained either bis(triphenylphosphora-nylidine) ammonium tetrakis(4-chlorophenyl-borate),BTPPATPBCl, at 10 mM, or ETH 500 (purchased fromFluka) 10 mM as organic phase electrolytes dissolved inDCE or DCH. Solutions of hexavalent chromium wereobtained by dilution of the stock solution of ammoniumdichromate in the aqueous phase electrolyte. DCH waspurified as described by Katano et al. [28]; DCE wasused as received. The organic phase electrolyte,BTPPATPBCl, was prepared by metathesis ofBTPPACl and KTPBCl [29,30].

The ionophore used, the polyamine macrocycle2,5,8,11,14-pentaaza[15]-16,29-phenanthrolinophane(NeoTT), was synthesised at the Department of Chemis-try, University of Florence, Italy [31,32]. This compoundconsists of a pentaamine linking the 2,9-phenanthrolinepositions. Fig. 1 shows the chemical structure of NeoTT.In its penta-protonated form, the acidic protons arelocalised on the aliphatic amine groups [31]. NeoTTwas prepared as the hydrogen bromide, NeoTT.5HBr(Mw = 798.1 g mol�1), soluble in water. In order tomake NeoTT soluble in the organic phase and suitablefor use as an organic phase ionophopre, the Br� anionswere substituted by tetrakis(4-chlorophenyl-borate)anions (TPBCl�), by metathesis, as follows: 5 mol ofKTPBCl (in acetone) were added to 1 mol ofNeoTT.5HBr (in water). After acetone evaporation(overnight), a yellow jelly-powder was collected andplaced in a desiccator (overnight) to remove water.

978 A.M. O’Mahony et al. / Electrochemistry Communications 7 (2005) 976–982

The resultant compound was NeoTT.5TPBCl (as con-firmed by NMR spectrometry), hereafter referred to asNeoTT.

2.2. Apparatus

All cyclic voltammetry (CV) and square wave voltam-metry (SWV) experiments were performed using CHInstruments electrochemical workstations (CHI 660B,CHI 620A or CHI 620B) (from IJ Cambria Scientific,UK). The electrochemical cell used was a customisedfour-electrode cell, where the interfacial potential differ-ence was applied between two Ag|AgCl reference elec-trodes (prepared by potentiostatic oxidation of silverwires in a solution of KCl 3 M) and the current wasmeasured by two platinum mesh counter electrodes.The geometric area of the interface was 1.005 cm2. Aseries of cell configurations was used in this work, asshown in Scheme 1. Different aqueous and organicphases were used to investigate the transfer of hexava-lent chromium across the interface. According to the

Ag (s) AgCl (s) BTPPATin DCE (

Cell 1:BTPPA+Cl- 10 mM in LiCl 10 mM (Luggin capillary)

Ag (s) AgCl (s) ETH DCE

A 10 mM in LiCl 10 mM (Luggin capillary)

Ag (s) AgCl (s)BTPPAT+ X mMDCE (or

BTPPA+Cl- 10 mM in LiCl 10 mM (Luggin capillary)

Cell 2:

Cell 3:

Ag (s) AgCl (s)BTPPAT+ X mMDCE (or


Ag (s) AgCl (s)BTPPAT+ X mMDCH (or


Ag (s) AgCl (s)BTPPAT+ X mMDCH (or


Cell 4:

Cell 5:

Cell 6:

Scheme 1. Cell configurations used, where a is ammonium dichromate, ofconcentration of NeoTT ionophore used.

interface polarisation convention adopted in the experi-mental setup, the transfer of anionic species from theaqueous phase into the organic phase occurs on the neg-ative-going potential direction, producing a negativecurrent. In the CV experiments the forward scan wasthat going from more positive to more negative poten-tial and the SWV experiments were performed by sweep-ing the potential from more positive to more negativepotential.

3. Results and discussion

3.1. Cyclic voltammetry

Initial experiments were focused toward establishingwhether electrolyte-facilitated transfer of Cr(VI) waspossible. Because of the low concentrations of Cr(VI)and the acidic nature of the aqueous phase electrolytesemployed in this work, the dominant Cr(VI) speciespresent is assumed to be the monoatomic HCrO�

4

Ag (s)AgCl (s)PBCl 10 mM org)

aj mM in

Ag (s)AgCl (s)500 10 mM in (org) LiCl 10 mM and

H2SO4 1 mM

aj mM in

Ag (s)AgCl (s)PBCl 10 mM

NeoTT in g)

LiCl 10 mM

aj mM in

LiCl 10 mM and H2SO4 1 mM


NeoTT in g)

aj mM in


NeoTT in g)

LiCl 10 mM and HCl 1 mM

aj mM in


NeoTT in g)

HCl 0.1 M

aj mM in

LiCl 10 mM and HCl 1 mM

j concentration; A is tetradodecylammonium chloride and X is the


species [2]. A quaternary ammonium salt organic phaseelectrolyte (cell 2 in Scheme 1) and a non-quaternaryammonium salt organic phase electrolyte (cell 1 inScheme 1) were examined. No additional complexingagent or ionophore was added. Cyclic voltammetry(CV) showed that Cr(VI) (as the salt ammonium dichro-mate) did not transfer across the ITIES under these con-ditions, as no transfer peaks were observed. In theabsence of an ionophore, Cr(VI) did not transfer, be-cause it is too hydrophilic to transfer within the avail-able potential window. Therefore, the ionophore(NeoTT) was introduced into the organic phase to lowerthe Gibbs free energy of transfer for Cr(VI) ions. NeoTTis able to form complexes with anionic species such assulfate [24]. Although the presence of aromatic groupsdoes not favour binding due to repulsion between theanion and the electronic p cloud of the aromatic rings,relatively large amine chains (such as the pentaaminein NeoTT) can form complexes in which the anion is lo-cated far from the aromatic structure [24].

However, CV of the blank solution (cell 3, Scheme 1,no Cr(VI) in the aqueous phase) containing NeoTT ion-ophore in the organic phase, produced a peak-shapedvoltammetric response. These peaks may be due to thecomplexation of the aqueous phase chloride ions inthe NeoTT structure. NeoTT itself is not believed totransfer across the ITIES, as when a hydrophilic versionof NeoTT (i.e., NeoTT.5HBr) was present in the aque-ous phase (not shown) no transfer peaks were recorded.Thus, NeoTT (NeoTT.5TPBCl, in the organic phase)should not transfer across the interface. Fig. 2 (curve1) shows an example of the voltammetric response ofthe blank containing NeoTT 0.3 mM in the organicphase (cell 3, Scheme 1). Curves 2–6 in Fig. 2 showthe voltammetric response at different sweep rates fol-lowing the addition of Cr(VI) to the aqueous phase.

-25

-20

-15

-10

-5

0

5

10

15

20

25

0.1 0.2 0.3 0.4 0.5

Potential /

Cu

rren

t / µ

A

y = 106.78x - 1.9778

R2 = 0.934

y = -76.813x + 1.1842

R2 = 0.9549

-20

-15

-10

-5

0

5

10

15

20

0 0.03 0.06 0.09 0.12 0.1

(Scan Rate)1/2 / (Vs-1)1/2

Cu

rren

t /µ

A

Fig. 2. Cyclic voltammetry of Cr(VI) at the liquid|liquid interface using celCr(VI) 58 lM facilitated ion transfer at different scan rates: 7.5 (2); 10 (3), 15current vs. the square root of the scan rate. Forward (d) and reverse (}) pe

As can be seen from Fig. 3, the addition of Cr(VI) re-sults in the disappearance of the signal previously re-corded in the absence of Cr(VI) (curve 1) and in theappearance of a new peak signal at higher potential(curves 2–6). This second signal is probably due to thetransfer of Cr(VI) and to formation of the complexNeoTT=HCrO�

4 . The fact that the signal recorded inthe presence of Cr(VI) is at higher potential than thoseattributed to the ionophore-Cl� complex, and the disap-pearance of this peak following the addition of theCr(VI) (even when Cl� is at ca. 350-fold excess) are clearindication of the selectivity of the NeoTT for Cr(VI)over chloride. The peak currents recorded in the pres-ence of Cr(VI) had a linear dependence on the squareroot of the potential sweep rate, in accord with theRandles-Sevcik equation

ip ¼ ð2.69� 105Þn3=2ACD1=2m1=2 ð1Þwhere ip (A) is the peak current, n is the charge ofthe ionic species, A (cm2) is the area of the interface,C (mol cm�3) is the concentration of Cr(VI), D(cm�2 s�1) is the diffusion coefficient of Cr(VI) and m(V s�1) is the sweep rate. This linear dependence impliesthat the transfer process was diffusion-controlled. Eq.(1) was used to calculate the diffusion coefficient ofCr(VI), 1.3 · 10�5 cm2 s�1, in good agreement with a lit-erature value 1.6 · 10�5 cm2 s�1 [2].

Furthermore, the addition of further aliquots ofammonium dichromate to the aqueous phase resultedin an increase in peak current with Cr(VI) concentration(Fig. 3, curves 2–6 and inset). Thus, Cr(VI) interactswith the NeoTT receptor in such a manner that it canbe quantified. The inset in Fig. 3 shows the peak currentincrease with concentration measured from the forward(negative-going) potential sweep (the transfer of Cr(VI)from aqueous to organic phase). Replacing ammonium

0.6 0.7 0.8 0.9 1

V vs Ag|AgCl

1 2

5

43

5 0.18

l 3. Voltammogram of blank at 5 mV s�1 (1) and voltammograms of(4) and 25 mV s�1 (5). Concentration of NeoTT: 0.3 mM. Inset. Peakak currents.

-10

-5

0

5

10

15

20

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Potential / V vs Ag|AgCl

Cu

rren

t / µ

A

1

54

32

6

y = -21.104x - 1.2399

R2 = 0.9963

-4

-3

-2

-1

0

0 0.025 0.05 0.075 0.1

Concentration / mM

Pea

k cu

rren

t / µ

A

Fig. 3. Cyclic voltammetry of Cr(VI) at the liquid|liquid interface using cell 3. Voltammogram of blank (1) and voltammograms of differentconcentrations of Cr(VI): 5 (2); 15 (3), 25 (4) 50 (5) and 100 lM (6). Concentration of NeoTT: 0.3 mM. Inset. Forward peak current (j) vs. theconcentration of Cr(VI). All scan rates: 5 mV s�1.


dichromate with ammonium chloride in cell 3 showedno change of the peak current response observed fromthe CV of the blank (i.e., before addition of the ammo-nium chloride). Thus, it can be concluded that the peakcurrent increase (Figs. 2 and 3) is due to Cr(VI) iontransfers and not to the ammonium ions. These latterexperiments were necessary in order to rule out the pos-sibility of cation transfer, as it is know that NeoTT caninteract with and coordinate also with cations such ascopper [31] and cadmium [32].

The use of different organic solvents (DCE andDCH) in the organic phase and of different aqueousphase electrolyte phases (LiCl 10 mM and HCl 1 mM;and HCl 0.1 M) was also evaluated. No significantchanges were observed on using mixtures of LiCl andHCl or HCl alone in the aqueous phase electrolyte (cell6). The use of 1,6-dichlorohexane as organic phase elec-

-60

-50

-40

-30

-20

-10

0

10

20

30

40

0 0.2 0.4

Potential / V vs A

Cu

rren

t / µ

A

Fig. 4. Square wave voltammetry analysis of Cr(VI) (HCrO�4 ) transfer acro

1 mM as aqueous phase electrolyte). SWV of blank: forward current segment

difference current segment of 0.335 mM Cr(VI) (4).

trolyte did lead to a larger potential window probablydue to its lower dielectric constant, compared to thatof DCE.

3.2. Square wave voltammetry

Square wave voltammetry (SWV) is an electrochemi-cal technique commonly used for quantitative purposesdue to the better detection limits achievable [33]. Cur-rent measurements in SWV are taken twice per cycle:the forward current sample (if) and the reverse current

sample (ir) [33]. A difference current Di results from thesubtraction if � ir. The SWV experiment can thus berepresented as three square wave voltammograms show-ing difference, forward and reverse currents versus theapplied potential [33].

0.6 0.8 1 1.2

g|AgCl

1

2

3

4

ss the ITIES. Cell 5 (DCH organic solvent, and LiCl 10 mM and HCl(2), reverse current segment (1) and difference current segment (3); and


The aim of these studies was to determine the limit ofdetection and the linear range of concentration depen-dence for Cr(VI) detection by voltammetry at the ITIES.As seen in Fig. 4, the characteristics of the difference,forward and reverse voltammograms for the detectionof Cr(VI) by NeoTT-facilitated ion transfer are quitecomplex in comparison to SWV at conventional elec-trodes. Voltammogram 2 in Fig. 4(a) is the forward cur-

rent sample of the blank solution, using cell 5 (theconcentration of NeoTT in the organic phase was0.3 mM) and a negative sweep from 1.05 to 0.05 V. Re-sults showed a negative peak response at around 0.64 V,possibly due to the transfer of chloride (as already dis-cussed for the CV experiments). However, voltammo-gram 1, which is the reverse current sample, also yieldsa negative peak at 0.64 V, of lower intensity than thepeak in voltammogram 2. Therefore, the subtractionif � ir leads to a current Di (voltammogram 3) that doesnot improve the peak definition but leads to a less-well

0

10

20

30

40

50

60

0 0.5 1 1.5

Concent

Pea

k C

urr

ent

/ µA

0

5

10

15

20

25

Pea

k C

urr

ent

/ µA

-60

-50

-40

-30

-20

-10

0

10

20

30

0 0.2 0.4

Potential /

Cur

rent

/ µA

a

b

Fig. 5. (a) SWV forward current segments analysis after background subtrac1 mM as aqueous phase electrolyte). Concentrations of Cr (VI): 43.2 lM (1)current (in absolute value) vs. concentration. Inset: peak current vs concentratsweeps from 0.84 to 0.1 V.

defined peak on this blank voltammogram. Furthermoreafter the additions of ammonium dichromate to theaqueous phase, difference (Fig. 4, voltammogram 4),forward and reverse (not shown) currents have the sameresponse behaviour. The only difference recorded is thatin this cases a new signal, in the high potential region, isrecorded. This new peak increases with the increase ofCr(VI) concentration. In order to improve the qualityof the data analysis the background subtraction ap-proach [18] was adopted. To achieve this, initial SWVsof blank (no Cr(VI) added) were subtracted from theSWVs after additions of Cr(VI). It was also found thatuse of the forward current was better than using eitherthe difference or the reverse current with backgroundsubtraction.

Fig. 5(a) shows examples of SWV peak after back-ground subtraction, based on the forward current sam-

ples. The peak around 0.8 V is associated with theCr(VI) transfer process, as indicated in Fig. 4, voltam-

y = 8.2576Ln(x) + 47.57R2 = 0.9505

2 2.5 3 3.5

ration / mM

y = 0.348x + 4.5747R2 = 0.9789

0 10 20 30 40 50 60

Concentration / µM

0.6 0.8 1

V vs Ag|AgCl

1 23

4 5

tion and using cell 5 (DCH organic solvent, and LiCl 10 mM and HCl, 52.7 lM (2), 148.9 lM (3), 242.1 lM (4) and 335.4 lM (5). (b) Peakion in the linear range. The concentration of NeoTT was 0.3 mM. SWV


mogram 4. The large positive peak around 0.6 V is be-lieved to be an artefact associated with the Cl� transferprocess which is not negated by the background sub-traction procedure employed. Fig. 5(b) shows the back-ground-subtracted forward current sample responseversus concentration of Cr(VI). The current responseis linear with Cr(VI) concentration in the lower range,and reaching a plateau at higher concentrations. Thelinear response range, as shown by the inset inFig. 5(b) was in the range 4.8–148 lM (or 0.25–7.7 partsper million, ppm, mg kg�1). The lowest detectable con-centration was 4.8 lM of Cr(VI), or 0.25 ppm. In pHcontrol experiments, it was found that there was no dif-ference in signal for aqueous phases containing LiCl10 mM with pH adjusted to the range pH 2–3 or inHCl 0.1 M.

4. Conclusion

Preliminary results presented show that hexavalentchromium, in the form of HCrO�

4 , can be transferredacross the ITIES when a suitable ionophore, in this caseNeoTT, is present in the organic phase. The ionophorewas the penta protonated form of polyamine macrocy-cle 2,5,8,11,14-pentaaza[15]-16,29-phenanthrolinophane(NeoTT). The ability to transfer Cr(VI) in this waycan be useful as the basis of an analytical methodologyfor the determination of this toxic species. The lowestconcentration detectable was 0.25 ppm Cr(VI). Furtherinvestigations are required to elucidate the transfermechanism undergone by the ionophore–chromatecomplex.

Acknowledgements

The authors acknowledge Science Foundation Ire-land (02/IN.1/B84) for support.

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Documents

Voltammetry of chromium(VI) at the liquid|liquid interface