Reverse Scan as a Source of Information in Square Wave Voltammetry*

Transcription

1 CROATICA CHEMICA ACTA CCACAA 79 (1) (6) ISSN CCA-364 Original Scientific Paper Reverse Scan as a Surce f Infrmatin in Square Wave Vltammetry* Marina Zeli} Divisin fr Marine and Envirnmental Research, Ru er B{kvi} Institute, P.O. Bx 18, 1 Zagreb, Cratia ( zelic@rudjer.irb.hr) RECEIVED JULY 5, 5; REVISED AUGUST 1, 5; ACCEPTED AUGUST 9, 5 Keywrds square wave vltammetry reverse scan electrde kinetics eurpium electrlyte Under the influence f recently published articles, reverse scans in square wave vltammetry were applied in a study f Eu 3+ /Eu + redx system at increasing cncentratins f several electrlytes, and gradually changing frequency r step ptential. The electrde respnse is highly affected by all these factrs and tw peaks can appear thugh nly ne electractive species is present in the slutin and ne electrn included in the prcess, whereas adsrptin f the reactant r prduct is nt prnunced. Scanning in bth directins appears as a ptential tl fr kinetic studies f the electrde prcesses althugh in the studied system its practical applicatin meets varius prblems. INTRODUCTION When nly the xidized frm f a redx pair is present in the electrlyte slutin at a relatively high cncentratin, vltammetric study is generally perfrmed by scanning in the negative directin. 1 In sme techniques, such as square wave vltammetry (SWV), successful applicatin f the psitive scan is als pssible 3 7 under the same cnditins, thugh (much) mre negative initial ptential shuld be applied.»cmbined«measurements, i.e., scanning in ne and then in the ther directin, give results that cannt be directly cmpared with cyclic vltammetry, 8 in which tw ppsite scans are the parts f the same cycle whereas in the discussed case they are cmpletely independent. In their recently published papers, Faturs 3 5 and cwrkers tested this apprach n reactins f the type: A+e B assuming that, in the experiments perfrmed with a static mercury drp electrde, prduct B remains disslved in the electrlyte slutin. They cncluded that recrding f individual vltamgrams by using bth scan directins can be very useful in studies f electrde kinetics. The pint is that the electrde respnse, even the number f peaks, highly depends n the scan directin if the prcess f interest is nt reversible. Obviusly, the systems in which an amalgam is frmed as the final reductin prduct are nt included in the whle stry. Irrespective f the prblems that appear during theretical treatment f such examples, sme earlier experiments 6,7 pint t the pssible applicatin f reverse scans even when this type f electrde reactin is studied. Suppressin f the effects prduced by adsrptin f the negatively charged, surface active ligand (in Cd + I system) 6 and imprvement f the peak shape when the electrde prcess is cntrlled by the kinetics * Dedicated t the memry f the late Prfessr Mark Branica.

2 5 M. ZELI] f the preceding reactin (as in perchlrate slutins f In 3+ ) 7 can be achieved in the mentined way. On the ther side, the number f systems with sluble prducts, which were studied by means f reverse scans in square wave vltammetry, is small. In fact, the UO + /UO + redx pair in carbnate medium 3 and titanic salts in acidic slutins 4,5 are the nly examples. Therefre it seems imprtant t apply such an apprach t sme ther redx pairs in rder t find its»pluses«and»minuses«and/r t extract sme new infrmatin. In this article, an attempt t apply scanning in bth directins is described taking the reactin Eu 3+ +e Eu + as a mdel. Its prduct is water sluble whereas the electrde reactin rate is gverned by the type and cncentratin f the basic electrlyte. 9 The system is nt f electrchemical interest nly, because eurpium atypical behavir in different electrlyte slutins 1 1 (smetimes described as»anmalus hydratin«) 11 was recrded by cmpletely different methds, such as flurescent r Raman spectrscpy. E 1 /mv t 1 slubility f the reductin prduct in the electrlyte slutin. In Figure 1, the excitatin signals are given fr bth cases. The resulting vltamgram is generally b- t Figure 1. Excitatin signals in square wave vltammetry fr negative (1) and psitive () scan directins. (Arrws indicate that the left y-axis is valid fr curve 1 and the right ne fr curve.) Sampling pints are schematically indicated fr frward (red) and backward currents (blue). t 1 = delay time E /mv EXPERIMENTAL -3 Salt slutins were prepared frm reagent grade chemicals, prduced by Kemika (Zagreb), Ventrn (Karlsruhe) r Fluka, and water purified in a Millipre Milli-Q system. Vltamgrams were recrded using a static mercury drp electrde PAR 33 (Princetn Applied Research) attached t a Autlab System (Ec Chemie, Utrecht). Platinum wire served as a cunter electrde whereas all ptentials were given with respect t the saturated Ag/AgCl (NaCl) reference electrde. All measurements were perfrmed with SW amplitude E sw =4mV(i.e., 8 mv peak t peak) and delay time (defined in Figure 1) t 1 = 3 s while ther ptential and timing parameters were varied. Befre recrding each set f vltamgrams, the slutin in the plargraphic cell was deaerated with high purity nitrgen fr 15 minutes. The rm temperature was kept at 5 C. The influence f electrlyte cncentratin n the electrde respnse (btained by frward and/r reverse scan) was fllwed at a cnstant level f disslved eurpium (.5 mml dm 3 ) and cnstant acidity (.1 ml dm 3 HClO 4 ) RESULTS AND DISCUSSION Electrde Respnse In the intrductry part, the pssibility f recrding SW vltamgrams by starting at psitive r negative ptentials was mentined, assuming the presence f nly the xidized frm f a redx pair in the aqueus phase and Figure. Square wave vltamgram f Eu 3+ (.5 mml dm 3 )in.1 ml dm 3 HCLO 4 and its frward (red) and backward currents (blue). Frequency: s 1 ; amplitude: 4 mv; step ptential: mv; delay time: 3 s; scan directin: negative. Net currents recrded in the same slutin using bth scan directins indicated by arrws. Crat. Chem. Acta 79 (1) (6)

3 REVERSE SCAN IN SQUARE WAVE VOLTAMMETRY 51 tained by sampling the currents under cnditins schematically presented by red and blue circles and recrding their difference as a functin f ptential. In such a way, the peak presented in Figure a was btained applying a frward (i.e., negative) scan t the slutin f Eu 3+ in.1 ml dm 3 HClO 4. The signal crrespnds t a (nearly) reversible reductin prcess as fllws frm the height and shape f its cmpnents, i.e., frward and backward currents btained by sampling at red and blue circles, respectively (Figure 1, curve 1). Reverse (i.e., psitive) scan frm the same slutin (Figure b) als gives ne (but ppsitely riented) signal with the same value f the peak ptential (E p ) as befre but smewhat lwer peak current (i p ). The latter culd reflect the difference in the diffusin cefficients f Eu 3+ and Eu + thugh similar results smetimes appear fr pure kinetic reasns. 3 Strictly speaking, in such a dilute electrlyte slutin (.1 ml dm 3 HClO 4 ) the signal shuld be additinally influenced by IR drp 13 but under applied cnditins, i.e., relatively lw frequency (f =s 1 ) and nt t high metal cncentratin, it is nt prnunced. If eurpium slutin, in additin t.1 ml dm 3 HClO 4, als cntains.1 ml dm 3 NaClO 4 and frequency is increased t 15 s 1, the SW vltamgram btained in a frward (i.e. negative) scan crrespnds t an irreversible reductin prcess (Figure 3a). (Such an effect cannt be achieved frm the dilute perchlric acid by the crrespnding frequency increase withut additin f the disslved salt.) Reverse (i.e., psitive) scan, hwever, gives tw peaks instead, as shwn in Figure 3b, frm which their rigin als becmes bvius. Namely, frward and backward (»red«and»blue«) signals are separated by abut 3 mv and therefre the resulting (i.e., net) peaks are relatively well defined, thugh in the previusly mentined examples 3 5 the separatin was even better and cnsequently verlapping was cmpletely absent. Bth peaks are riented in the same directin because subtractin f negative currents frm the psitive nes (fr the mre negative signal) gives psitive values whereas a mre psitive peak is btained by subtractin f lwer psitive currents frm the higher nes. By inspectin f Figures 3a and 3b it can be cncluded that in the first f them n rexidatin signal appears because at the crrespnding ptentials (with the SW amplitude applied) the reduced frm virtually des nt exist. If the signals btained by reverse scans are recrded in a frequency range 1 3 s 1, the results presented in Figure 4a arise. Fr f =1s 1 nly ne peak is btained at.6 V but fr higher values an additinal peak, lcated nearly 3 mv mre psitively, gradually develps and finally becmes higher than the initially present signal whse height is virtually independent f frequency. If the measured currents are divided by the crrespnding f ½ values, Figure 4a is transfrmed t Figure i= i- i E /mv Figure 3. Square wave vltamgram recrded in acidified.1 ml dm 3 NaClO 4 and its frward (red) and backward (blue) currents. Frequency: 15 s 1 ; scan directin: negative, psitive. Other cnditins as in Figure. 4b. Such a presentatin is analgus t the way in which theretical results are usually given,3 (i.e., in the frm f dimensinless currents prprtinal t i/f ½ ). It indicates that the nrmalized peak current f the mre negative (i.e., cathdic) signal decreases with increasing frequency whereas under the same cnditins the height f the ther (i.e., andic) signal becmes cnstant. At first glance, such a result seems t be ppsite t the theretical predictin (Figure in Ref. 3) but it is nt. The pint is that mathematical mdeling was perfrmed assuming a cnstant rati between delay time (t 1 ) and pulse duratin ( t) whereas the real experiments were perfrmed at a cnstant value f t 1 and decreasing t. Under such cnditins, the results presented in Figure 4b seem acceptable r, in ther wrds, the change f the mentined rati is mre imprtant than the decrease f pulse duratin at increasing square wave frequency, which is analgus t the decrease f the standard rate cnstant (k ). With the negative scans, nly ne peak is btained, which increases and shifts negatively by 4 mv in the frequency range 1 3 s 1. Here, the results are nt influenced by the rati t 1 / t. Crat. Chem. Acta 79 (1) (6)

4 5 M. ZELI] 1/ 1/ / A s if E /mv Anther parameter that mdifies the electrde respnse is step ptential (E s ) als knwn as scan increment. If under therwise identical cnditins its value is -1. Figure 4. Real square wave vltamgrams f Eu 3+ in acidified.1 ml dm 3 NaClO 4. Frequency range: 1 3 s 1. The same curves btained after dividing all the currents by the crrespnding f 1/ values. Other cnditins as befre. Scan directin is indicated by a hrizntal arrw and frequency increase by a vertical ne Figure 5. Influence f step ptential n the electrde respnse in acidified.1 ml dm 3 NaClO 4. Applied value in mv indicated near each vltamgram; frequency: s 1 ; scan directin: indicated by an arrw. Other cnditins as in Figure. 1 gradually increased, the results presented in Figure 5 arise in.1 ml dm 3 NaClO 4. With the frward scan at the frequency f s 1, the peak becmes higher and shifted negatively by 16 mv when step ptential increases frm.5 t 1 mv. In the reverse scans, the secnd, mre psitive signal develps instead. In ther wrds, similar effects appear in measurements perfrmed at a cnstant frequency and increasing scan increment as in the ppsite case, i.e., increasing frequency and a cnstant scan increment.»frmal«scan rate (v) is equal t their prduct (i.e., v = f E s ) but such a value des nt have the same rle here as, fr example, in linear scan vltammetry. 1 In ther wrds, the resulting signals, btained at a cnstant scan rate with different pairs f f and E s values, are similar but nt identical. They were nt btained at a cnstant t 1 / t rati and their similarity culd perhaps be imprved by taking int accunt this factr as well. As already mentined, the electrde respnse is highly affected by the cncentratin f sdium perchlrate in the aqueus phase. 9 The crrespnding vltamgrams, given in Figure 6, clearly indicate that under the influence f the increasing salt cncentratin in the range.1 1. ml dm 3, suppressin f the cathdic and enhancement f the andic signal is btained by the reverse scan. Under the same cnditins, reductin peaks btained by frward scans becme lwer and shifted negatively, 9 indicating that the electrde prcess becmes slwer. (In highly cncentrated slutins such as 8 ml dm 3 NaClO 4, nly ne peak is btained in a reverse scan.) Similar results, i.e., the appearance f tw, thugh prly reslved, peaks, were als btained by reverse scans in slutins f ther salts such as LiNO 3 and CsCl, which cmpared t perchlrate media have smewhat different prperties. Chlride and cesium ins exhibit (relatively weak) surface activity 14 whereas nitrate is a ligand 15 that gives the inner sphere cmplex EuNO + 3. Electrde Kinetics Frm Figure 6 it fllws that the tw peaks btained in a reverse scan are best defined when.1 ml dm 3 sdium perchlrate is used as a basic electrlyte whereas in mre cncentrated salt slutins the height rati changes significantly and verlapping becmes mre prnunced. Accrding t the literature data, 13 with respect t the pssible effects f IR drp (at relatively high frequencies, which shuld be applied), this electrlyte cncentratin is just n the brder. Anyway, an attempt was made t calculate the crrespnding kinetic parameters using the methd f Faturs and Krulic, 3 i.e., prperties f SW signals, recrded by using bth scan directins. In such a prcedure it shuld be pssible t calculate all the kinetic parameters (k, E and bth transfer cefficients) frm the ptentials and widths f the signals (btained under the same well defined cnditins) and the value f the diffusin cefficient. Crat. Chem. Acta 79 (1) (6)

5 REVERSE SCAN IN SQUARE WAVE VOLTAMMETRY Figure 6. Square wave vltamgrams f Eu 3+ btained by reverse scans. Cncentratin f NaClO 4 (in ml dm 3 ) indicated near each curve. Frequency: 1 s 1. Other cnditins as in Figure Accrding t the equatin:. = 1.7RT / FW ½.5 cathdic transfer cefficient ( =.76) was calculated frm the width f the frward signal. In fact, W ½ dentes the distance between the first half peak ptential and the peak ptential but nt the full width f the signal at half f its height, because such a respnse is generally nt symmetric. In the same way, the andic transfer cefficient ( =.34) was calculated frm the reverse andic signal. Literature data fr this inic strength are scarce but in the article by Elzanwska et. al 16 =.8 was given, which is in reasnable agreement with the abve result. Hwever, it indicates that the reactin f interest is asymmetric and, strictly speaking, cannt be studied in mre depth by the equatins given 3 because ne f them, which takes int accunt the measuring cnditins, is valid nly in the range =.3.7. If all the prblems are ignred, kinetic parameters calculated by the straightfrward applicatin f the calculatin prcedure 3 still give reasnable results (lg k /cm s 1 = 3., E = 59 mv vs. SCE). In mre cncentrated perchlrate slutins, the cathdic transfer cefficient becmes lwer as fllws frm the dependence f the peak ptential n SW frequency and the slpe f dc plargrams, but the prblem f verlapping signals increases. (The peaks are nt fully reslved even in.1 ml dm 3 NaClO 4.) Taking int accunt that transfer cefficients are calculated frm the W ½, it is extremely imprtant t have well defined signals. Therefre, reslutin f clse peaks by decnvlutin, 17 ften applied in ur labratry, is nt applicable here because the methd»prduces«better reslved signals by making them narrwer. In sme ther prcedures, based n nnlinear curve fitting, independence f peaks is assumed. 18 Hence their applicability in the present case seems questinable Figure 7. Square wave vltamgrams f Eu 3+ recrded in acidified slutin f.5 ml dm 3 NaClO ml dm 3 NaBr by negative and psitive scans tgether with the crrespnding frward (red) and backward (blue) currents. Frequency: s 1. Other cnditins as in Figure. It fllws that appearance f tw peaks n the square wave vltamgram recrded in a reverse scan des nt guarantee that kinetic parameters f the studied reactin can be calculated withut difficulties. Additinal requirements bviusly include as high as pssible peak separatin and nt t different values f the tw transfer cefficients. In the present case, the prblem f peak separatin in nitrate and chlride slutins is even mre prnunced than in perchlrate media. In additin t the salts that make the electrde prcess less reversible as their cncentratin increases, there are sme anins that can change the trend, i.e., increase the rate 19 at the same inic strength. Brmide and idide belng t this grup and their rle is explained in terms f the bridge effect, i.e., exchange f electrns between the electrde and the reducible species in the slutin via adsrbed anin. If adsrptin f brmide is really crucial fr the whle prcess, it shuld be pssible t find the cnditins under which the apparent reversibility depends n the scan directin. The pint is that the surface cncentratin f adsrbed brmide is much Crat. Chem. Acta 79 (1) (6)

6 54 M. ZELI] lwer at (highly) negative ptentials cmpared t the situatin near. V. Such result was really btained when a mixture f.5 ml dm 3 NaBr and.5 ml dm 3 NaClO 4 was used as the electrlyte. Althugh the effect is nt highly prnunced, it can be clearly seen that backward current btained by the negative scan (Figure 7a) des nt reflect a ttally irreversible prcess whereas with the changed scan directin it des (Figure 7b). in which bth frms f the redx pair are sluble in electrlyte slutins. The impressin is that additinal wrk n the thery and practice f the mentined apprach is needed in rder t clarify the bserved effects and make the whle stry useful and applicable in practice. Acknwledgement. The financial supprt f the Cratian Ministry f Science, Educatin and Sprts is gratefully acknwledged. CONCLUDING REMARKS Recrding f a SW vltamgram frm a reducible analyte slutin by scanning in the psitive directin is a gd way t check the reversibility f the electrde prcess. Such an pprtunity is nt s imprtant in measurements with mdern instruments because frward and backward (i.e.,»red«and»blue«) currents can be used fr the same purpse. With lder plargraphic analyzers, such as PAR 384B fr example, nly the net current can be btained and reverse scan is definitely a tl fr extractin f mre infrmatin. In additin, experimental results shuld depend n the scan directin if the electrde prcess is affected by the presence f a surface active anin. In the Eu 3+ Br system, the frward scan starts at a ptential characterized by a relatively high surface cncentratin f the adsrbed brmide and the electrde prcess is nt ttally irreversible, as fllws frm the shape f the backward current. In a reverse scan that starts at a highly negative ptential and a very lw surface cncentratin f Br, the electrde prcess is bviusly slwer. The mentined apprach culd be als useful in studies f ther ptential dependent adsrptins. Frm the practical pint f view, especially because the whle effect des nt appear in SWV nly, it seems imprtant t accept generally that frmatin f tw vltammetric signals is pssible even in extremely simple cases, i.e., with nly ne electractive species present, which exchanges nly ne electrn in the electrde prcess withut prnunced adsrptin f the reactant r prduct. Accrding t the present state f the art, scanning in bth directins cannt be applied when an amalgam frming metal appears as a reductin prduct (because an adequate thery is lacking) r in measurements at trace cncentratins and remains limited t the systems REFERENCES 1. A. J. Bard and L. R. Faulkner, Electrchemical Methds, Wiley, New Yrk, 1.. M. Lvri}, in: F. Schlz (Ed.), Electranalytical Methds, Springer, Berlin,, pp N. Faturs and D. Krulic, J. Electranal. Chem. 5 () N. Faturs, D. Krulic, and N. Larabi, J. Electranal. Chem. 549 (3) N. Faturs, D. Krulic, and N. Larabi, J. Electranal. Chem. 568 (4) M. Zeli}, I. Pi`eta, and M. Branica, Anal. Chim. Acta 81 (1993) M. Zeli}, M. Mlakar, and M. Branica, Anal. Chim. Acta 89 (1993) F. Marken, A. Neudeck, and A. M. Bnd in: F. Schlz (Ed.), Electranalytical Methds, Springer, Berlin,, pp M. Zeli}, Crat. Chem. Acta 76 (3) and refs. cited therein. 1. S. Lis and G. R. Chppin, Mater. Chem. Phys. 31 (199) H. Kann and H. Ykyama, Plyhedrn 15 (1996) A. Nehlig, M. Elhabiri, I. Billard, A.-M. Albrecht-Gary, and K. Lützenkirchen, Radichim. Acta 91 (3) V. Mir~eski and M. Lvri}, J. Electranal. Chem. 497 (1) F. C. Ansn, Acc. Chem. Res. 8 (1975) NIST Critically Selected Stability Cnstants f Metal Cmplexes. Versin 5., NIST Standard Reference Data, Gaithersburg, H. Elzanwska, Z. Galus, and Z. Brkwska, J. Electranal. Chem. 157 (1983) I. Pi`eta, Anal. Chim. Acta 85 (1994) J. D. Dimitrv, Anal. Bianal. Chem. 379 (4) K. Niki and H. Mizta, J. Electranal. Chem. 7 (1976) Crat. Chem. Acta 79 (1) (6)

7 REVERSE SCAN IN SQUARE WAVE VOLTAMMETRY 55 prmjene ptencijala ka izvr infrmacija u pravkutnvalnj vltametriji Marina Zeli} Pd utjecajem nedavn bjavljenih radva,»brnut«skeniranje u pravkutnvalnj vltametriji primijenjen je u izu~avanju redks sustava Eu 3+ /Eu + pri rastu}j kncentraciji neklik elektrlita i pstepenj prmjeni frekvencije ili kraka ptencijala. Svi vi faktri sna`n utje~u na elektrdni dziv pa se i dva vrha mgu pjaviti iak u tpini pstji sam jedna elektraktivna tvar, u prces je uklju~en sam jedan elektrn, a adsrpcija reaktanta ili prdukta nije izra`ena. Pkazuje se da je skeniranje u ba smjera ptencijaln»ru e«za izu~avanje kinetike elektrdnih prcesa iak je njegva prakti~na primjena u ispitivanme sustavu pvezana s razli~itim prblemima. Crat. Chem. Acta 79 (1) (6)