Use of a zeolite-modified electrode for the study of the methylviologen–sodium ion-exchange in zeolite Y

Journal of Electroanalytical Chemistry 463 (1999) 100 – 108

Use of a zeolite-modified electrode for the study of the methylviologen – sodium ion-exchange in zeolite Y A. Walcarius a,*, P. Mariaulle b, L. Lamberts b a

Laboratoire de Chimie Physique pour l’En6ironnement, Unite´ Mixte de Recherche UMR 7564, CNRS-Uni6ersite´ H. Poincare´ Nancy I, 405 rue de Vandoeu6re, F-54600 Villers-les-Nancy, France b Faculte´s Uni6ersitaires Notre-Dame de la Paix, Laboratoire d’Electrochimie et de Chimie Analytique, 61 rue de Bruxelles, B-5000 Namur, Belgium Received 8 June 1998; received in revised form 6 November 1998

Abstract The electrochemical response of zeolite-modified carbon paste electrodes (ZMCPEs) has been exploited to plot the normalised ion-exchange isotherm of methylviologen and sodium in zeolite Y. Results observed from the proposed methodology agree well with those obtained using the conventional procedure. Selectivity was very high for the large organic divalent cation over sodium species, but the degree of exchange of methylviologen did not exceed 64%. Electrochemistry of ZMCPEs allows the in situ quantitative determination of methylviologen in the zeolite phase, without significant modification in the solution-phase concentrations, so that the ion-exchange isotherm can be plotted very rapidly and without any other chemical analysis. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Zeolites; Carbon paste electrode; Ion-exchange; Isotherm; Methylviologen; Voltammetry; Preconcentration

1. Introduction An important intrinsic property of most zeolites is their ability to undergo ion-exchange. Although fundamental ion-exchange equilibria involving alkali metal, alkaline earth, rare earth and ammonium cations were fully characterised a long time ago [1 – 6], the applications and uses of natural and synthetic zeolites have attracted much attention and considerable progress in recent years because of their utilisation as ion-exchangers for water softening [7,8], for the removal and storage of radionuclides [9,10] as well as the uptake of heavy metals from polluted effluents [11 – 15] and the removal of ammonium ions from waste waters [16–18]. The exchange of ions between zeolite and solution phases can be analysed by calculating distribution coefficients from chemical analyses performed, most of* Corresponding author. Tel.: +33-383-91-6343; fax: + 33-383-275444; e-mail: [email protected].

ten, only on the solution phase (see [19], for example). However, the full characterisation of the ion-exchange equilibrium is best achieved by plotting ion-exchange isotherms [1,3,20,21]. This requires the quantitative analysis of the exchanging cations in both solution and zeolite phases after equilibration [20,21]. These steps, and especially the analysis of the zeolite phase, are often tedious and time consuming. They are however absolutely necessary, in particular when the zeolite displays a strong affinity for one of the two exchanging cations, because of the small variation (or the low values) of the solution phase concentrations. Some complications with zeolites may arise due to the existence of several different ion exchanging sites and the rigid tridimensional lattice made of pores and channels of molecular dimensions, conferring to zeolites a unique size (and shape) selectivity with volume exclusion effects which could result in incomplete exchange [1– 6,20,21].

0022-0728/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 2 - 0 7 2 8 ( 9 8 ) 0 0 4 5 2 - 5

A. Walcarius et al. / Journal of Electroanalytical Chemistry 463 (1999) 100–108

Recently, zeolite-modified electrodes (ZMEs) have attracted considerable attention from electrochemists [22 – 27]. The combination of electrochemical methods with the unusual properties of zeolites (size, shape and size selectivity, ion-exchange capacity, chemical and thermal stability) was exploited in many fields. For example, ZMEs were successfully applied as new selective amperometric sensors [23,27 – 34], by exploiting the ion-exchange between electroactive and non-electroactive cations in zeolite particles located at the electrode surface. The amperometric response of the sensor was directly related to the ion-exchange properties of zeolites [34]. Basically, the electrochemical response of a ZME exchanged with an electroactive cation, Em + , immersed in an electrolyte solution containing a cation C + (chosen as monovalent for writing convenience), can be explained by the following electron transfer mechanism [35]: + + m+ + Em (Z) +m C(S) X E(S) +m C(Z)

(1)

+ − (m − n) + X E(S) Em (S) +ne

(2)

where subscripts S and Z refer to solution and zeolite, respectively. The electroactive species is first exchanged for the electrolyte cation at the electrode solution interface (Eq. (1)) and then undergoes the charge transfer in the solution phase (Eq. (2)). The voltammetric response of the electrode is therefore strongly affected by the ion-exchange reaction (Eq. (1)). Accordingly, it was recently reported that electrochemistry at ZMEs could be used for the qualitative characterisation of ion-exchange reactions in Ref. [34]. Among other electroactive species, methylviologen (a well-known herbicide whose the electrochemical activity was previously reviewed [35]) was found to give an amperometric response when incorporated in zeolite Y [31,34,35,37–39]. The electron transfer mechanism responsible for its amperometric response at a zeolitemodified carbon paste electrode (ZMCPE) immersed in a sodium chloride electrolyte solution was demonstrated to proceed according to Eqs. (1) and (2) [39]. On the other hand, methylviologen is known to be incorporated into the zeolite Y lattice [37 – 41]. Due to its large size, methylviologen is excluded from the sodalite cages, being therefore located only in the supercages or in the channels linking the supercages to each other. As a consequence, the kinetics associated with the methylviologen–sodium exchange in zeolite Y is thought to be controlled by the diffusion of the species through the 12-ring apertures of 0.74 nm, and not to any intracrystalline dynamics in interconnected cages of different size (as was the case for the exchange of cations able to reach sites located in pores or channels of various sizes [3]). The aim of this paper is to provide a new and original way to produce the ion-exchange isotherm of

101

the methylviologen –sodium system under in situ conditions. After having plotted the isotherm following the conventional method [20], we will point out the usefulness of electrochemistry at a zeolite Y modified carbon paste electrode for measuring, in a non destructive way, the equivalent fraction of methylviologen in zeolite Y. Zeolite particles will contact the liquid phase by immersing the ZME into a solution containing the exchanging cations at a constant total normality of 1.0× 10 − 2 N (i.e. concentrations of uniequivalent anions of 10 − 2 M). A major advantage of this approach is that the solution phase concentrations will be affected neither by the ion-exchange reaction itself nor by the electrochemical determination of the equivalent fraction of methylviologen in zeolite, because of the very high liquid-to-solid ratio, so that the equivalent fractions in solution are not to be measured because they do not change.

2. Experimental

2.1. Apparatus Electrochemical experiments were performed at 25°C in an undivided 50 ml three electrode cell. Solutions were purged by bubbling pure nitrogen during 15 min before measurements. Working electrodes were homemade carbon paste electrodes (unmodified or zeolitemodified, see section 2.3.). The counter-electrode was made of a platinum wire, and the saturated calomel (model TR 100, Radiometer) and Ag AgCl (Metrohm) reference electrodes were used, respectively for cyclic voltammetry and square wave voltammetry experiments. Cyclic voltammetric measurements were made by using the EG&G Princeton Applied Research model 362 potentiostat/galvanostat, and voltammograms were recorded on a Philips model PM8271 x–y–t recorder. Square wave voltammetry experiments were performed with the EG&G Princeton Applied Research model 303A static mercury drop electrode associated to the EG&G Princeton Applied Research model 273 potentiostat/galvanostat. Atomic absorption experiments were carried out with a Philips PU 9200X atomic absorption spectrometer.

2.2. Chemicals and reagents All solutions were prepared with high purity water (18 MV cm − 1) from a Millipore Milli-Q water purification system. Methylviologen (N,N%-dimethyl-4,4%-bipyridinium dichloride, MVCl2) was purchased from Acros. Sodium chloride, sodium dodecylsulfate (SDS) and citric acid were all analytical grade (Aldrich). Stock solutions of 0.50× 10 − 2 M MVCl2 and 1.00× 10 − 2 M NaCl were used for the ion-exchange experiments. Zeo-

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lite Y was Linde Molecular Sieve Cat. Base L-Y54 powder (UOP, Molecular Sieve Division) and used in its sodium form (average particle size: 0.5 – 2 mm).

2.3. Electrode preparation Zeolite-modified carbon paste electrodes were prepared as previously described [31,32,34,38,39]. Briefly, 280 mg of an organic binder (Nujol, Aldrich) was added to a homogeneous mixture of 100 mg zeolite particles and 620 mg carbon graphite (B 325 mesh, Johnson Matthey), and mixed together with the aid of an automated mortar (Pulverisette II, Fritsch) until a uniformly wetted paste was obtained. The paste was then packed into the end of a home-made PTFE cylindrical tube (ed 8 mm, id 6 mm) equipped with a screw-in stainless steel piston. When necessary, a new surface was obtained by pushing an excess of paste out of the tube (typically 200 to 500 mm thickness) and polishing it on a weighing paper. Unmodified carbon paste was prepared in the same way without adding zeolite to the mixture.

2.4. Procedures 2.4.1. Cyclic 6oltammetry Cyclic voltammograms were recorded immediately after immersion of the electrode into the electrolyte solution (typically 0.1 M NaCl). Solutions were purged with nitrogen for 15 min before starting measurements. Four different working electrodes were used, containing various amounts of methylviologen (MV2 + ) within the zeolite framework (1.6, 2.6, 3.8 and 5.3 MV2 + per supercage). The surface of the electrolyte solution was protected by pure nitrogen during measurements, in order to avoid interference by oxygen. Experiments performed with zeolite-modified electrodes were repeated three times in order to check their reproducibility. 2.4.2. Ion-exchange The MV2 + /Na + ion-exchange isotherm was produced according to the recommendations of Dyer et al. [20]. The chemical composition of the zeolite Y used was: Na56Al56Si136O384 ·  250 H2O. It was pre-exchanged with Na + to remove ionic impurities and the sodium content was checked by atomic absorption spectrometry. The maximum ion-exchange capacity of zeolite Y for MV2 + species was determined by successive equilibration of 200 mg zeolite (hydrated beforehand) in fresh 10 − 3 M MVCl2 solutions (50 ml) until obtaining a constant maximum value for the methylviologen content in the zeolite phase. Kinetic tests following procedures previously described [39] showed that equilibrium was rapidly obtained (in less than 1 h), so that equilibration of ion-exchange experiments was

only over 1 day. Isotherm solutions were prepared from the stock solutions of MVCl2 and NaCl. They contained various known proportions of the methylviologen and sodium chloride, at a total constant normality of 10 − 2 N (i.e. constant concentration of 10 − 2 M Cl − 1). 150 mg of either the homoionic sodium form of the zeolite (NaY) or of the maximally exchanged methylviologen zeolite (MVNaY) was added to 100 ml solution. The use of these two compounds checked the reversibility of the exchange process. Suspensions were allowed to react under constant stirring. After equilibration, zeolite particles were filtered, thoroughly washed with pure water and allowed to dry in air. The extent of exchange was determined by the analysis of both solid and liquid phases. Zeolites were analysed after decomposition of the framework in 0.5 M aqueous citric acid, as previously described [37,39,41]. This treatment is known to hydrolyse all the aluminium centres of the zeolite (not the silicon ones), resulting in the complete leaching of all the exchanged cations in a less aggressive medium than that obtained with HF treatment. The efficiency was ascertained with control experiments. The energy dispersive X-rays analysis of the resulting solids revealed only silica oligomers: no more aluminium species were detected, demonstrating their complete solubilisation. Sodium ions were determined by atomic absorption spectrometry while methylviologen was analysed by square wave voltammetry at the static mercury drop electrode, after appropriate calibrations in the corresponding medium. The voltammetric analysis of methylviologen was made according to a previously optimised procedure [42]. Typically, experiments were carried out using a square wave frequency of 100 Hz, a potential step height of 2 mV and scanning potentials from − 0.85 V to − 0.25 V in the presence of 0.003% gelatin.

2.4.3. Ion-exchange isotherm by electrochemistry This new methodology will be fully described in the ‘results and discussion’ section. Few experimental details will be given hereafter. The zeolite-modified electrode was immersed in an accumulation cell filled with 50 ml of either a mixture of methylviologen and sodium ions in various proportions ([Cl − ] constant at 10 − 2 M). After a given preconcentration time in stirred solutions and at open circuit, the electrode was taken out, rinsed with water and transferred into the voltammetric cell containing 50 ml 0.1 M NaCl as the supporting electrolyte. Then, a cyclic voltammogram was recorded immediately after immersion of the electrode into the solution, at a scan rate of 50 mV s − 1. The electrode surface was regenerated by 5 min immersion into a cleaning cell containing 50 ml of stirred 0.1 M SDS solution and rinsed with water before any new measurement. SDS was used as an ion-pairing agent to

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ensure a fast and complete leaching of MV2 + species from zeolite particles. When necessary, new surfaces of ZMCPE were obtained by mechanical polishing.

Table 1 Results of the ion-exchange experiments: corrected selectivity coeffi2+ cients (K MV ) and activity coefficients of the two cations in zeolite, Na+ as a function of the normalised equivalent fraction of methylviologen in zeolite (MVZ)

3. Results and discussion

MVZ

2+

K MV Na+

3.1. The MV 2 + /Na + ion-exchange equilibrium The MV2 + /Na + ion-exchange of zeolite Y can be represented by the following equilibrium (Eq. (3)): + + + 2 Na(Z) + MV2(S)+ v 2 Na(S) +MV2(Z)

(3)

where subscripts Z and S refer to zeolite and solution phases, respectively. The isotherm corresponding to the exchange is expressed as MVZ =f(MVS) at a constant total ion concentration where MVZ and MVS are the equivalent fractions of methylviologen in the zeolite and in solution, respectively. The exchange isotherm is shown in Fig. 1. The main features of Fig. 1 are (1) a strong preference of the zeolite for the divalent organic cation and (2) a maximum methylviologen exchange level less than 1. Moreover, the exchange reaction within the zeolite was found to be fully reversible over the entire range of methylviologen content (from 0.05 to 1.08 mmol MV2 + (g of zeolite) − 1. The process selectivity is best characterised by calculating the corrected selectivity coefficients which are defined as the rational selectivity coefficients corrected for the activity coefficients of the solution-phase species (Eq. (4)) [1]. 2+

K MV Na + =

MV2Z+ × (NaS+ )2 g 2Na × (NaZ+ )2 × MVS gMV

(4)

Fig. 1. Isotherm for the binary MV2 + and Na + ion-exchange of zeolite Y at 298 K and total normality of 0.01 eq l − 1, obtained from the conventional method [20].

0.048 0.12 0.22 0.47 0.59 0.64 0.70 0.72 0.75 0.80 0.84 0.86 0.91 0.92 0.95 0.97 0.98 0.99 1.00 1.00

205 118 90 86 75 50 48 42 45 22 19 16 14 5 3.5 1.6 0.65 0.6 0.15 0.05

Activity coefficients in zeolite (ln g) Na+

MV2+

−0.02 −0.09 −0.19 −0.32 −0.42 −0.55 −0.60 −0.65 −0.64 −0.93 −1.0 −1.1 −1.2 −1.7 −1.9 −2.3 −2.7 −2.8 −3.4 −3.9

−2.6 −2.1 −1.8 −1.6 −1.4 −1.1 −1.1 −1.0 −1.0 −0.60 −0.49 −0.40 −0.32 0.10 0.25 0.55 0.92 0.93 1.5 1.9

where MVZ, MVS, NaZ and Na are the equivalent fractions of methylviologen or sodium in the zeolite and in solution, respectively, and g represents the activity coefficients in the liquid phase. The corrected selectivity coefficients were determined by calculating the g 2Na/gMV ratio from values of the activity coefficients for NaCl in the presence of MVCl2 and inversely [43]. These values have been evaluated using the general equation of Glueckauf [44] for a binary mixture with a common (chloride) anion. The corrected selectivity coefficients related to the isotherm of Fig. 1 are compiled in Table 1. The process selectivity has been found to vary, which increased the methylviologen exchange. The entering divalent ion was initially largely preferred up to MVZ = 0.5, then decreased at intermediate loadings, and a reversal of selectivity was even found to begin at the highest MV values (0.63–0.64). The great affinity of the negatively charged zeolite lattice for the large organic cation can be related to the highly diffuse character of the positive charges which can be delocalised around the aromatic cycles of methylviologen. The MV2 + /Na + ion-exchange displays a higher selectivity for the divalent ion than most of the M2 + /Na + exchanges reported in the literature, M2 + being alkaline earth or metal cations [2,3,45,46]. If methylviologen is known to be accommodated in the supercages of zeolite Y [37,35,38–41], no detailed studies on the ion-exchange equilibrium involving this cation are however available. Depending on the experimental conditions, various MV2 + loadings were found,

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but never higher than 1 mmol g − 1. As shown in Fig. 1, and in agreement with these previous investigations [37 – 41], an incomplete removal of the indigenous sodium ions by the in-going methylviologen species was observed. The maximum exchange capacity of zeolite Y for MV2 + was accurately determined by successive equilibrations in fresh MV2 + solutions. A value of 1.08 90.02 mmol g − 1 was observed, corresponding to 64% of the total ion-exchange capacity (5.8 MV2 + per supercage). The structure of zeolite Y is well-known [3], and helps us to understand that methylviologen can enter the supercages via the channels through 12-ring apertures but cannot fit inside the sodalite cages connected to the supercages by rings of six tetrahedra with free diameters of about 0.25 nm, because of its rather large critical dimensions (0.63 nm× 1.34 nm in its planar configuration). On this basis, 71% of the exchanging sites could theoretically be reached by MV2 + species. The 64% observed maximum level of exchange was less than that one would expect from the molecular sieving properties of zeolite Y. This makes it necessary to take into account volume exclusion effects. The volume of MV2 + is such that there is not enough space within the supercages to confine a number of MV2 + cations corresponding to the complete charge balance of the negatively charged sites of the zeolite framework. This effect is common with zeolites [6]. It explains, for example, why the maximum capacity of zeolite Y for Ru(NH3)26 + and Ru(NH3)36 + species is 1.5-fold higher for the trivalent cation. With approximately the same size, one Ru(NH3)36 + species neutralise three negative charges of the framework, while only two charges are counter-balanced by the corresponding divalent species [47]. The thermodynamic equilibrium constant, Ka, can be estimated from the normalised Kielland plot (ln ‘corrected selectivity coefficients’ versus normalised MVZ), according to the following equation (Eq. (5)) [1,3]. ln Ka = − 1+

&

1

2+

ln K MV Na + dMVZ

3.2. Voltammetric response of the methyl6iologenexchanged zeolite-modified carbon paste electrode The electrochemical behaviour of methylviologen-exchanged zeolites has been previously investigated in both aqueous and non aqueous media [37–39]. To understand the principle of the MV2 + /Na + ion-exchange characterisation by electrochemistry, we need however to know some basic aspects of the voltammetric response of the methylviologen-exchanged zeolitemodified carbon paste electrode. It should be also ensured that voltammetric currents can be related directly to the amount of methylviologen exchanged within the zeolite.

3.2.1. General considerations The voltammetric behaviour of the methylviologenexchanged zeolite-modified carbon paste electrode is shown in Fig. 2. As is well-known [36], the divalent cation MV2 + is first reduced (peak C1) into the corresponding cation-radical MV’ + which then undergoes a further mono-electron transfer at peak C2, giving the neutral species MV0. Their anodic peaks, A2 and A1 appear on scan reversal. The reaction process, MV2 + / MV’ + , is fast and so-called ‘electrochemically reversible’ with a difference between the C1 and A1 peak potentials close to 60 mV, together with peak currents of approximately the same magnitude. The second redox process, MV’ + /MV0 is less reversible due to the existence of the conproportionation reaction occurring between MV2 + and MV0 [36]. This is shown in Fig. 2, where signal A2 appears as a shoulder rather than a peak when large amounts of MV0 are produced. One of the requirements for this work is the quantitative analysis of the methyl–viologen content into the zeolite particles, so that the voltammetric response must

(5)

0

A value of 18 was obtained, corresponding to a standard Gibbs energy of exchange per equivalent equal to − 3.6 kJ mol − 1. The activity coefficients of both MV2 + and Na + in the zeolite phase have been estimated for each point of the isotherm, from plots of either MVZ or NaZ versus ln ‘corrected selectivity coefficients’, using equations described elsewhere. Integrations were made from the best polynomial fitting of the Kielland plots. These values are reported in Table 1. As expected, activities of both methylviologen and sodium ions inside the zeolite structure can vary to a large extent, because of the strong interactions that could exist in this confined medium. The effect is more marked when approaching the homoionic forms of the zeolite.

Fig. 2. Cyclic voltammetry of methylviologen supported on zeolite Y (1.08 mmol g − 1) modified carbon paste electrode. Potential sweeps were reversed respectively at −1.5, −1.2, − 1.05 and −0.85 V. Supporting electrolyte: 0.1 M NaCl. Scan rate: 1 V s − 1.

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Fig. 3. Dependence of the peak currents sampled at zeolite-modified carbon paste electrodes on the methylviologen content in the zeolite, as a function of the supporting electrolyte (NaCl) concentration: (a) 0.005 M; (b) 0.01 M; (c) 0.02 M; (d) 0.03 M; (e) 0.05 M; (f) 0.1 M; (g) 0.2 M and (h) 0.3 M. Currents are those corresponding to the monoelectronic reduction of methylviologen, obtained at a scan rate of 50 mV s − 1.

be directly proportional to the MV2 + concentration. Peak C1 was therefore chosen as the indicative signal because it was not perturbed by any side chemical reaction. Currents for this peak are directly related to the amount of MV2 + species reduced at the electrode surface, while those involving the subsequent transformations could be affected by the conproportionation reaction and are thus not appropriate for quantitative purposes.

3.2.2. Influence of supporting electrolyte Cyclic voltammetric peaks of Fig. 2 are due to free MV2 + species, which were leached from the zeolite into the solution after being exchanged for the supporting electrolyte cation, Na + . From the quantitative point of view, it must be ensured that all the zeolite-phase MV2 + species are expelled from the substrate to be analysed in solution or, as an alternative, that the amount of leached species (MV2(S)+ ) is directly proportional to that initially located + within the zeolite (MV2(Z) ). This was ascertained by using zeolites variously exchanged with methylviologen and recording voltammograms in solutions containing various concentrations of supporting electrolyte (i.e. NaCl). Results are presented in Fig. 3. They demonstrate the need for a supporting electrolyte concentration higher than 0.1 M in order to achieve a linear evolution of the voltammetric peak currents with respect to the methylviologen content in zeolites. As stated earlier, the non linear dependence at lower concentrations is explained by both the kinetic control of the electrochemical measurement itself, and the variation of the exchange selectivity as a function of the solution and zeolite composition [39].

3.2.3. Ion-exchange 6oltammetry The above results were obtained from zeolites exchanged with methylviologen prior to their incorporation into the carbon paste. The accumulation can also be performed after the dispersion of zeolite particles (in their NaY form) into the electrode. Therefore, the exchange of Na + species for MV2 + cations occurs directly with the zeolite particles located at the electrode surface in contact with the methylviologen, according to Eq. (3). After a selected preconcentration time (typically 1–6 min), the electrode is transferred to the analysis cell and the voltammogram is recorded. This sequence ‘preconcentration at open circuit-voltammetric measurement’, when applied to electrodes modified with ion-exchangers, is often called ion-exchange voltammetry [48]. The electrode surface is then renewed before the next analysis. This analytical procedure will be used in our proposed methodology for plotting the ion-exchange isotherm by means of voltammetry at zeolite-modified electrodes. 3.2.4. Regeneration and reproducibility One major advantage of carbon paste and modified carbon paste electrodes is their ability to allow an easy surface renewal, by simple mechanical smoothing [49,50]. With zeolite-modified carbon paste electrodes, this cleaning step is rather complicated by the difficulty of obtaining homogeneous dispersion of zeolite particles within the paste [27]. Typically, ten measurements are required to get a standard deviation less than 10% of the average value. This result can be improved by replacing the mechanical surface renewing step by a chemical cleaning. The first idea is the back-exchange in the presence of an excess Na + species. This reaction is

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however not favourable because of the strong selectivity of zeolite Y for MV2 + over Na + . This problem can be overcome by associating to Na + an anion able to interact with MV2 + , and acting somehow as a driving force to shift the back-exchange reaction to the right. Dodecyl sulfate was chosen as an ion-pairing agent. A 5 min cleaning step in 0.1 M SDS (sodium dodecyl sulfate) gave satisfactory results: more than 95% renewal. This solution was chosen as the cleaning medium for further experiments. Using these conditions, a series of 25 successive measurements (1 min preconcentration in 10 − 3 M MV2 + ; analysis in 0.1 M NaCl; cleaning in 0.1 M SDS) was performed. The results display a 6% standard deviation without any signal deterioration.

3.3. Ion-exchange isotherm by electrochemistry 3.3.1. Principle Plotting the isotherm for the MV2 + /Na + ion-exchange requires the knowledge of the equilibrium equivalent fractions MVZ and MVS, from MVS =0 to MVS =1. The system under study will be firstly defined. The goal is to characterise the ion-exchange of sodium by methylviologen using a zeolite-modified carbon paste electrode immersed into a solution. Thus, the zeolite particles concerned with this reaction are those located at the electrode surface, i.e. those contacting the solution. Particles situated deeper into the paste cannot be reached by the solution so that they will not participate into the exchange process. For the small times of experiment encountered here, and in agreement with previous works on modified carbon paste electrodes [38,51], one can assume that the paste thickness involved in the reaction is of the same order of magnitude as the size of solid particles (about 50 mm). Therefore, the mass of zeolites which is expected to contact the 50 ml solution can be evaluated at ca. 0.05 mg. The ‘zeolite mass’ to ‘solution volume’ ratio is so low (10 − 6 g ml − 1) that the solution composition will be not affected by the ion-exchange reaction itself. This is the first advantage of the proposed method: the equivalent fractions of the solution-phase cations remain unchanged (macroscopically) during the whole reaction. For a selected initial solution composition, the equivalent fraction MVS is thus known at any moment and has not to be determined by any chemical analysis. On the other hand, results of Fig. 3 demonstrate that voltammetric response of the zeolite-modified electrode is directly proportional to the methylviologen content of the zeolite, for a selected composition of the detection medium. Under such conditions, peak currents are directly proportional to the equivalent fraction of methylviologen in zeolites. In conclusion, MVS is known and MVZ can be measured via the peak currents sampled at the zeolite-modified carbon paste electrode.

The general procedure for the proposed methodology may be described as follow. Two kinds of solutions were prepared: Solutions 1: containing MV2 + and Na + species in varying ratio, from MVS = 0 to MVS = 1, at a total normality of 10 − 2 N. Solutions 2: the same as solutions 1 but without Na + species, that is solutions containing only MV2 + ions at concentrations corresponding to those of the first series. Each solution was placed into the preconcentration cell and ion-exchange voltammetry was applied after selected accumulation times. Peak currents observed with solutions 1, Ip(sol 1), will be proportional to MVZ (Eq. (6)), each MVZ being related to one well-defined MVS. Peak currents observed with the corresponding solutions 2, Ip(sol 2), will be proportional to the maximum equivalent fraction of methylviologen in zeolites, MVZmax (Eq. (7)), because no competition will occur with Na + . For the same accumulation time, and after normalisation of MVZmax to unity, the currents ratio between solutions 1 and 2 is a direct measurement of the normalised MVZ (Eq. (8)). Ip(sol

1)

= k×MVZ

(6)

Ip(sol

2)

= k×MVZmax

(7)

MVZ normalisation Ip(sol 1) = [ MVZ Ip(sol 2) MVZmax

(8)

When using solutions 1, the accumulation of MV2 + ions within the zeolite is limited by the presence of Na + ions in the medium, and the resulting voltammetric currents are related to the amount of methylviologen able to reach an ion-exchange site in the presence of the sodium interferent species. When using solutions 2, the accumulation of MV2 + ions within the zeolite is not limited by any other cation in the medium, so that the electrochemical response is now directly related to the maximum ion-exchange capacity of the zeolite for MV2 + species. After normalisation of these last currents to unity, the currents recorded in the presence of sodium are then related directly to the equivalent fractions of MV2 + within the zeolite. The establishment of the ion-exchange isotherm requires the use of MV2 + / Na + mixtures in a wide range of MV2 + /Na + concentration ratios at a selected constant total normality [20]. Therefore, the direct monitoring of the preconcentration process by recording repeated potential scans during the preconcentration step, largely applied to clay modified electrodes [52–55], was prevented because of the lack of supporting electrolyte in the medium. The addition of any supporting electrolyte would result in significant side-effects on the MV2 + /Na + ion-exchange equilibrium under study. Finally, it is noteworthy that the two kinds of solutions are necessary because the accumulation process is diffusion-controlled. Concentration gradients at the electrode solution interface are

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then very different depending on the methylviologen concentrations in solution (MVS). Currents obtained for MV2 + + Na + mixtures must be rationalised to those observed without the competing Na + species. Our method results in plotting the normalised isotherm, the real MVZmax value having to be determined from batch experiments.

3.3.2. Application The above methodology was applied to the MV2 + / Na + exchange in zeolite Y. Results are presented in Fig. 4. The solid line was obtained by fitting the isotherm of Fig. 1 after normalisation to MVZmax = 1. Our method as shown gave excellent results in agreement with those of the conventional method, but more rapidly because of the skipping of solution-phase analysis and the fast electroanalytical measurement of zeolite-phase methylviologen. Data of Fig. 4 were obtained from ion-exchange voltammetry carried out for a 3 min accumulation period in the preconcentration cell. Of course, in the same series of experiments, the accumulation period was the same in both solutions 1 and 2 (MV2 + alone and the mixture MV2 + /Na + ) in order to be able to apply Eq. (8) quantitatively. On the other hand, several series of experiments were performed by varying the preconcentration time in the range 1 – 6 min (keeping it constant in the same series). They led to similar isotherms to that of Fig. 4, indicating a fast completion of the exchange reaction. It should be emphasized, however, that this analytical methodology for determining the MV2 + /Na + exchange isotherm is valid because the equilibrium is reached rapidly, so that voltammetry can reflect a thermodynamic function as it can be used to determine a formal potential for a fast heterogeneous electron transfer species.

Fig. 4. Normalised isotherm for the binary MV2 + and Na + ion-exchange of zeolite Y at 298 K and total normality of 0.01 eq l − 1, obtained from the conventional method (line) and by electrochemistry at zeolite-modified electrodes (points).

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4. Conclusions By using a zeolite-modified carbon paste electrode, it has been shown that cyclic voltammetry is a good tool for the quantitative characterisation of the binary MV2 + and Na + ion-exchange in zeolite Y. Good correlation was found between the normalised isotherms plotted either following the conventional method or using the proposed methodology. The resort to electrochemistry at zeolite-modified electrodes is attractive because it provides a rapid quantification of the normalised equivalent fraction of methylviologen in zeolite and, furthermore, no analysis of the solution-phase concentrations is required.

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