Chen1. Anal. (Warsaw), 44, 623 (1999)

Membrane-BasedEleetrodes forFlow-In~ectionDetermination of Noble Metal Ions

by Yuri A. Zolotov, Lilia K. Shpigun,Natalia E. Kopytova and Patimat M. Kamilova

Kurnakov Institute o/General & Inorganic Chemistry, Russian Academy o/Sciences,Leninsk.yProspect31, iV!oscow 117907, Russia

Key lvords:flow-injection analysis, noble metals, membrane electrodes

T"vo types of polyvinyl chloride (PVC) membrane-basedelectrodes\,yereprepared andstudied under the tlo"v-injection (FI)conditions in order to develop high-performanceelectrochemical sensors for noble metalions. Thefirst type of electrodes was similar to aclassical PVC-model, "vhereas in the second, the PVC-matrix "vas modified by carbonpo"vder and sensorhad no internal filling solution. As ion-sensing components{recogni-tion elements), a series of 0, }i, S containing mono- and dibenzo-cro"vn ethers "veretested. The strong dependence ofthe electrochemical signal to silver(I) ions as "veIl as toacido-complexes ofAu(III), Pd(II), Pt(IV) and Ir(IV) onthe chemical structure ofamem-brane-immobilized macrocyclic ligand "vas found, for both groups ofthe electrodes. It"vas clearly shown thatthe response characteristics ofcomposite membrane-based elec-trodes (CMEs), having carbon po"vder "vithin the PVC-matrix, exhibit higher selectivityto noblem.etal ions than the electrodes ofa conventional type. It "vas also recognized thatsuch electrodes might operate as bifunctional sensorsinFI potentiometric/amperometricdetection ofAu(In) or Ir(IV). The response mechanism ofCMEs "vas discussed and therole of both chemically active agents and carbon species in the forming ofresulting sig-nals "vas clarified.

Wykonano i przebadano, "V warunkach przeplywo"vej analizy "vstrzyko"vej, d"varodzajeelektrod membrano"vych z polichlorku "vinylu (PVC), przeznaczonych do konstrukcjiwysoko"vydajnycll elektrochemicznych czujnik6"v do oznaczania jono"v metali, szla-chetnych. Jedna z elektrod jest podobna doklasycznej elektrody PVC, •• natomiast "Vdrugiej matrycaPVC zostala zmodyfiko"vana dodatkiem pylu wyglo"vego i czujnik nierna "vewnytrznego rozt"voru wypelniaj~cego. Przebadano szereg etero"v mono- i di-benzo-koron· zawierajtl.cych tlen, azot i siarky pod "vzglydem czulosci na jony (rozpo-znawanie pierwiastko"v). W przypadku obu elektrod zaobser"vo"vano siln~ zaleznosc

t Dedicated to Professor Adam Hulanicki, on the occasion of his 70th birthday.

624 J.A. Zolotov et al.

elektrochemicznego sugnalujon6w srebra (1), a tak:Ze kwasowych kompleks6w Au(III),Pd(II), Pt(IV) i Ir(IV) od chemicznej budowy makrocyklicznego ligandu immobilizowa-nego w melnbranie. Wykazano, ze charakterystyki odpowiedzi zlozonych elektrodmembrano,-,vych (CMEs), zawieraj~cych pyl ,-,v~glowy w matrycy PVC, wykazuj~ sto-sunko,-,vo wi~ksz~ selektywnosc w stosunku do jon6w metali szlachetnych niz elektrodykonwencjonalne. Stwierdzono, ze takie elektrody mog9: dzialac jako czujniki dwu-funkcyjne w metodzie przeplywowo-wstrzykowej z potencjometrycznq/amperometry-czn9: detekcj9:Au(III) lub Ir(IV). Przedyskutowano i wyjasniono mechanizm odpo,-,viedziCMEs oraz roly chemicznie akty,-,vnych czynnik6,-,v i wygla w tworzeniu si~ optymalnychsygna16w.

Electrochemical sensors like membrane-based ion-selective electrodes (ISEs)1101d avery promising position among flow-injection (F I) detectors due to their sensi-tivity, wide measuring range, simplicity, low cost and automation[I-3]. For the firsttime, the significant analytical possibilities ofthis technique have been demonstratedby Pungor et ala [4]. Since then, FI electrochemical detection hold great promise frommany points of view [5-7].

As Hulanicki mentioned [8], there is a practical possibility to construct ISEs fornearly all ionic species, but realistic determination is not possible in most complexmatrix. One of the rather difficult problem could be related to the design of sensorable for individual detennination ofnoble metal ions o"ving to speed of analysis andreliable analytical characteristics. Until now, you can find only several examples inthe literature concerning FI electrochemical techniques for the detennination of thisgroup oftransitioll metals [9-11]. Therefore, during the last years our investigationswere focllsed on the development ofdifferent approaches to FI detection ofdefferentchemical forms of110ble metals which would be proved to reach superior selectivityand sensitivity of electrochemical sensors [12-14].

In general, the key to successful FI electrochemical detection of an analyte, is theproper choice of chemically active components "vhich can provide rapid, strong andreversible chemical interactions between the analyte and the electrode material. Be-sides, there are otller requirements for selectivity, potential stability, reprodllcibilityand lifetime of sensors that depends on the lipophility ofplastisizer and other mem-brane-fonning components that play an important role in tIle overall response pro-cess. So, Inain questioll that should be taken into account when llsing ISEs for FIdetermination of noble metals can be attributed to the limited selectivity of mem-brane-fonning components and insufficient membrane conductivity. It should bepointed out that the choise of appriopriate ion-sensing agents and the optimization ofmembrane composition playa critical role for solving this problem.

In earlier contributions [15-1 7] it was shown that the potelltiometric selectivityofmembrane-based electrodes for d-transition metal cations could be sometimes im-proved by incorporating electrically neutral carries like synthetic macrocyclic conl-pounds into the solvent in the polymeric membranes.The role of neutral carriers inanion permeation "vas also discussed [18]. On the other hand, there was no infoffila-tion about application ofthese compounds in membrane-based electrodes for detect-

Deter111ination ofnoble metal ions 625

ing anionic forms ofnoble metals. However it was claimedthat macrocyclic ligands Llike crown ethers may behave as liquid extractants for acido-complexes ofgold (III, I)and platinum metals when forming ion-pairs with the cationic macrocyclic com-plexes as (H+, Kt+) MXriL [19]. Futhermore, the selectivity of liquid-liquid extrac-tion of Au(III, I) was .found to ·be increased when using aza- and thiacrowncompounds [20]. According to data reported [21];. it could be explained by formingmore stable macrocyclic complexes LAu3+.

Inthepresentpaper, recent results ofour research in the field ofFI electroanalysiswith polymeric membrane-based ISEs are reported. Comparative evaluation of thetwo types ofelectrochemical· sensors for detection'of noble metal ions was carriedout:platisizedpolyviny1chloride (PVC) :membrane-based electrodes (PVC-MES) 0 fthe classical IIwdel and the new •. PVC-caroon composite me1lloraneelectrodes(PVC-CMEs). The compositions ofmentioned electrodes are given in the Table 1. Asion-sensing agents, a variety of the following synthetic compounds were immobi-lizedwithin the membrane phase of electrodes:

a:) ('o~()(~. x X.,.'.',.•X)~.,.,~ I .•. /A 0 0 ~~{ R

I. II.

III.

IV.

V.

VI

VII.

VIII.

IX.

n=O X=O.

n=OX=S

n=1X=O

n= 1 X=NH

n=1X=S

n=2 X=O

n=2 X=NH

n=2X=S

X.

XI.

XII.

XIII.

X=O, R=H

X=NH, R=H

X=S,R=H

X = S, R = C(CH3)3

Table 1. The prepared ,electrode composition

Electrode Electrode substrate,m % Ion-sensing Resistancetype Conductor Insulator Solvent mediator modifier kn

I. non Poivinyl Didecyl Electrically 1000PVC-E chlori e ~owder phtalate neutral

(30- 3) (60-65) compound

n. Carbon Polyvinyl Didecyl (2-10) 8PV-CME towder chloride ~owder phtalate

30-35) (20 (40)

626 JA. Zolotov et al.

EXPERIMENTAL

Instrumentation

Pump 1

All experiments were perfonned by using FIAStar-5020 equipment (Tecator). The Chemifold I wasused in order to provide the necessary FImanifold configuration (Fig. 1). To avoid the effectofthe stream-ing potential, a carrier solution was mixed with a baseline supporting electrolyte (BLSE) stream. All tub-ing was PTFE (0.5 mm ID.).

The flow cell (FC) and detecting membrane-based electrodes used in this study were of laboratoryfabrication. The preparation ofthe PVC-ME is described elsewhere [22]. For preparing the other type ofindicator electrodes, PVC-CME, 20 mg of PVC and 40 mg ofdidecylphtalate (DDPh) weredissolved inabout 0.5-1.0 ml oftetrahydrofuran (THF). This solution was mixed with 10 mg ofthe macrocyclic com-pound and then with 30 mg ofthe spectroscopic carbon powder. A polished spectroscopic carbon rod (ca

15 mm2 x 40 mm) was connected to a copperwire at one end and from other end was inserted into a plastictube. The tube was inverted, and its upper cavity was filled with the above mentioned sensor cocktail,which was allowed to solidify at room temperature.

After conditioning each prepared electrode for 24 h in the appriopriate solution, it was mounted inthe flow cell constructed according to [23].

The heightHof the analytical signal caused by the electrode response such as potential till or current

M changes was measured at 25°C by using electrochemical elements. of the two following types:

Hg; Hg2Clz, KC.II KN03 I Sample I PVC-membrane IKCI, AgC.1, Ag(satd.) I (1 mol r 1) I (0.01 mol r 1)II ~

PVC-ME

Hg; Hg2C12, KCl I KN03 I Sample I PVC-carbon I Electronic (Cu)(satd.)I (1 mol r 1) I composite membrane conductor

I I. ~

PVC-CME

~A (mt/min).~

Carrier \

BLSE

Figure 1. Schematic diagram of the FI manifold used for electrochemical measurements

Detennination ofnoble metal ions 627

The reference electrode was saturated calomel electrode, Radelkis type, with a double salt-bridgefilled with 1 mol 1-1 potassium nitrate.

A digital millivoltmeter, type OP-206 (Radelkis, Hungary) was used for EMF (mV) observations re-corded with an analogue pen recorder (Linear 1200, USA). Amperometric and cyclic voltammogrammeasurements were performed using a polarograph, model PA-3 (Russia), with·a two or three-electrodecell using a platinum wire auxiliary electrode.

The dynamic selectivity characteristics of the electrodes were determined with help of the relativeselectivity factors F, the values of which were calculated by using the following equation:

(1)

whereHx and Hy-thepeakheight obtained from the same concentration ofpure primary X and pure for-eign Y ions, respectively, in injected solutions under the fixed FI conditions.

Chemicals and solutions

All chemicals used were of analytical-reagent grade and solutions were prepared with double dis-tilled .deionized water. All reagents and standard solutions were stored in clean polyethylene bottles.

The PVC, THF and DDPh (Fluka, Switzerland) were used to prepare polymeric membranes. Allelectrically neutral compounds I-XIII were synthesized and supplied by the organic chemistry depart-ment ofMoscow State University (Russia).

RESULTS AND DISCUSSIONS

T.he response behaviorofPVC-MEs and. 'PVC-CMEs was evaluated inapotentiometric. FI system (Fig. 1). The exp.erimentalresu1ts indicated that theelec-trode response characteristics were affected significantly by changes inthe ch.emicalstructure ofalnembrane..immobilized compound and in other components ofan elec-trode composition as well as baseline supporting electrolyteu.sed..TheFigure2dem-onstrated the EMF. response peaks of the electrode based ona plasticized membranecOlltaining dibenzo-18-dithiacrown ether XII (DB[18]S2C6), which .were obtainedby injection ofthe solutions ofnoble and some other metal ions. As can.be seen, thiselectrode .responds 'not .only to· Ag+-ions .(positive response) but also. to .• chloro-complexes of Au(III), Pd(II), Pt(IV) and Ir(IV) (negative response).In the case ofother d-transitionmetal ions, the values ofpeak height Hwere found tobe negligible.

It Inight be supposed that the potentiometric FI signal is caused by the extractionexchange reactions. at the membrane (m)/aqueous solution (aq) interface (Kt+A- -BLSE, Mn+:L == 1:1) as follows:

M ll+' +KtLn+A-MTu+ +nA- +Kt+(aq) • n(m) .L(m) (m) (aq)

or

(2a)

(2b)

628

(a)

J.A. Zolotov et al.

(b)Hg

Ag

12smv

Pt

4 min

Figure 2. Thetypieal EMF signalsreeordedforFI systemusing DB[18]S2C6-eontaining PVC-ME by in-j eetion of0.01 mol r l metal ion solutions. Vo = 200 J.LI, L = 60 em, VI =1.2 ml min-1, V2 =0.8 mlmin-I, BLSE (0.4 moll-I): NaCI (a), NaCI04 (b)

The later experiments showed that the introduction of carbon powder in the bulkofthe PVC-membrane composition (typeII in the Tab. 1) provides not only decreas-ing electrical resistance ofthe membrane but gives significant changes in selectivityand other dynamic response characteristics ofelectrode. So far, the remarkable differ-ence·between potentiometric response characteristics of the ·PVC-MEs and thePVC-CMEs was observed. For example, the response characteristics ofthe electrodebased onthePVC-matrixmembrane with dibenzo-18-dithiacrown-6 were cOlnparedwitll those obtained for a PVC-CME containing a similar crown ether. The selectedexperimental data are given in the Table 2. It includes some values of a peak height Has well as a peak width tb measured by injecting 200 III of solutiol1S ofAg(I), Au(III)or Ir(IV) ions into theFI system (Fig. 1). As it can be seen, the new composite meln-brane-based electrode has a much stronger dynamic response to the above mentionedions than the conventional electrode. It was also observed that su.ch electrode exhibitsslower recovery to baseline than the analogous PVC-ME.

FiguTe 3 graphically shows the values ofthe relative selectivity factors Fk~~ cal-culated for the DB[18]S2C6-containing PVC-CME compared with those obtail1edfor the two conventional PVC-matrix membrane electrodes modified with a typicalion-exchal1ger (Aliquat 336) and DB[18]S2C6, respectively. There are marked differ-ences for these electrodes. A typical selectivity pattern conforming to the usualHoflneister series e.g. lipophilic inorganic anions being preferred to hydrophobicions, was observed for the electrode based on the ion-exchanger. In the case ofcon-

Deterlnination ofnoble lnetal ions 629

ventional PVC-MEs modified with DB[18]S2C6, the higher relative selectivity to sil-ver ions and tochloroaurate(III) - ions was observed, but large excesses of.hydrophobic inorganic anions like ClO~ ions depressed the. signals of these metalions. In contrast, hydroph0 bic anions have v.erylittle effect on the electrode potentialof the PVC-CME. This new electrode with the membrane containing carbon addi-tives shows auniqu.e response to iridium-ions (fig. 4). Furthemore, the peak heightrecorded with PVC-CMEs foracido-c01nplexes.ofAu(III) anqplatinuillmetals.in-creased in the following sequence of BLSE used: HCl, KCl > NaCl < NaN03 <NaCl04•

Table 2. Comparative characteristics of the peakprofiles corresponding to the potentiometric detection ofsome metal ions by using FIsystem with DB[18]S2C6-containing PVC-ME andPVC-CME.Metal ion concentration -10 mmol r l . BLSE: 0.4 mol r l NaNOj (n = 4, p;;; 0.95)

Parameter of PVC-CME PVC-ME

PI signal Ag(I) Au(IH) Ir(IV) Ag(I) Au(III) Ir(IV)

H,MV 400±5 469 ±5 330±4 70±2 187 ±3 41 ±1

tb, c 368 ± 8 932 ± 18 576 ± 12 162±4 288 ±6 90±3

pot Au.X potAu.X

o ~ AuC; ....------ ------....------------- AUO; 0

PVC-ME PVC-MEs PVC-CMEmodified with modified with DB[18]S2C6Aliquat 336

Figure 3. The diagram of the relative selectivity factors F obtained for the PVC-ME and PVC-CME,both modified with DB[18]S2C6, compared to that obtained for PVC-ME modified with typi-cal ion-exchanger (Aliquat 336) under the fixed PI conditions: Vo ;;; 200 ~d, L ;;; 60 cm, VI == 1.5ml min-I, V2 == 0.8 m} min-I

630 J.A. Zolotov et al.

~.... ~ ~

0.4 •..... -:::•..;.••••:.:- -

I .. ---." , .

o Pd.__;..,:: ...~"

.. -- · Pd

(b)

.... .....·····_Ir.

.........::': ~

I

:: -........

.........~

(a)

.......~ -..~.. ,

1.2

0.8

rPotr- Pd,X

1.6

.0.4 Pt _ _ _..~Fe ...

...........~ _ Pt• Fe

·0.8 QA_ ~ ... ...········..·· .. - ......-.Cu

o1S 11= 2\.""-- ~---_.-J ,, -....,-_--.-Jnon X= a("o~

~X XY')V-o o;V

~O~

Figure 4. Influence ifring size (b) and nature ofheteroatoms (a) ofcrown ethers on the potentiometricre-sponse selectivity of PVC-MCE. The conditions as in Figure 2a

Our special attention was paid to the description ofPVC-CME response mecha-nismunder the FI conditions. In this connection, the effects ofmacrocyclic ligand andother membrane-fonning compounds on the dynamic electrode response and its se-lectivitywereexamined. In accordance with·experiments, it was found that the incor-poration of mono-benzo-crown ethers II-IX and dibenzo-18-crown-6 derivativesX-XIII in the PVC-carbon composite membranes improved remarkably the electroderesponse towards some noble metal ions. The influence of the ligand structure onpotentiometric selectivity factors detennined with PVC-CMEs is shown in the Fig-ure 5. It was found that in all cases the largest selectivity was recognized for Ag(l),AU.(III) and. Ir(IV) ions. The data also indicate that the relative selectivity of aPVC-CME response towards Au(III)· and Ag(l) ions increases with increasing thesize of a macrocycle and was found to be higher when using mono- ordibenzo-dithia-crown ether derivatives. In case ofAg+ ions, the effect ofvariation ofthe ring size as well as introduction of an alkyl substituent into the benzyl fragmentwas more pronounced. Obviously, these effects are compJetely dependent on thecomplexing affinity of a macrocyclic ligand that was immobilized within the mem-brane.

Detennination ofnoble metal ions

H,lnV500 __......-----r----"""""":""---~-~---.,.......__r-_r_-~__r-_r____,

:::--I----t------·--r-i-·-~- .~--350-

300-

250

200-

150

100-

50-

0--

IX VIlli INu,uber f

o cOluPOUlJd

631

Figure 5. Effect of introduction of the electrically neutral compounds into PVC-CME on thepotentiometric peak height recorded for some platinum metal ions. The conditions as in Figure2a

Because the selectivitypattem for electrodes based on PVC membranes with andwithout carbon additives showed a deep contrast,. we. carried out experilnents usingcyclic voltammetry.• The Figure 6a shows a cyclic voltammogram of silver on theDB[18]C6 (X) - containing PVC-CME obtained by recording potential with a scanrate of 50 mV S-l. As it can be seen frolntheFigure 6b, it gave a pair ofredox-peaks.The magnitudes and potentials ofthese peaks were dependent on the chemical struc-ture ofthe membrane-immobilized molecules ofa membrane-immobilized ligand. Inshort, the experimentaldataindicate that the mechanism ofvoltammetric behavior ofsilver at this type of ISEs inclu.des the complex formation of silver ions with themacrocyclic compound. Similar results were obtained for Au(III), Pd(II) and Ir(IV)ions.

The established experimental regularities allow to suggest that t~e response ofPVC-CMEsappears to bea result of combining two processes - the ion transferthrough the aqueous solution/membrane interface and the redox-reactions at carbonspecies within the surface layer ofthe PVC membrane phase. In general, themagni-tude of the potentiometric·· signal depends· strongly on the· extractability. andelectroactivity ofthe analyte.

An interesting phenomena were observed whenPVC:.-.-CMEs were operated asbifunctional sensors for Au(III) and Ir(IV) ions. In another words, such electrodes canbe utilized simu.1taneously for both potentiometric and amperornetric measu.rementsof the same ionic species without applying any outer voltage between the indicatorand. reference electrode potential. Figure 7 shows the calibration graphs. for irid-iurn(IV) ion concentration with a PVC-CME containing dithiacrown compou.nd VI

Ip'JJ.lA1.5 2 2.6 3 3.6 4 4.5 S

632

(a)

(b)

I.A. Zolotov et al.

i000oi

=a. I,..=oC.J III~~~~~~~

c:..-o~ VI ~!!!!!!!_~~~::!!!!!!!!!!!!!I!!!!!!!!!!!!!!!!!:::IlI:I:I==Z:::rzD

..Q

§IX)!!!!!!!!!!!!!!!~~~~~!!!!!!~!!!!!!!!!!l!!!!!!!:l:--=-:&:a*D~Z -0.4 -0.3 -0.2 -0.1 () 0.1 0.2 o. 0.5

Ep, V- -Ipc~ -Ipa ""- -Epc-o.. -Epa

Figure 6. The cyclic voltammogram ofPVC-CME obtained inthe solution containing 1xl0-4 mol 1-1 sil-ver ions in 0.5 mol r 1 NaN03 (a) and its dependence on the structure of membrane-formingcompound (b)

(B[15]S2C5). The regular peaks and good linearity ofthe calibration graphs were ob-tainedoverthe wide concentration range for both electrode potential M(t) and cur-rent AI(t) changes as output signal in the FI measurements. The explanation of thepotentiometric response slope increasing up to the super-Nerstian values can be obvi-ously based 011 the differences of the rates of electron-transfer reactions with thecllange ofprilnary ion concentration. The data obtained indicate the excellent poten-tial and responses to IrCI~- ions over the group I, II and some d-transition metal ions aswell as. over all hydrophobic inorganic anions (Fig. 7b).

Themaill analytical characteristics ofthe proposedpotentiometric/amperometricISEs are listed in the Table 3. They were proven to be suitable for routine FI detectionofAu(III) and Ir(IV) concentration in the aqueous solutions. Thus, it seelns that bothextractable electroinactive species and non-extractable electroactive species do notinterfere in the detecting ofAu(III), Ag(I) and Ir(IV) ions. Evidently, the choice oftheFI operation conditions is also very important.

Detennination ofnoble lnetal ions 633

-0.2

.. -0.4

~Pcl .• -0 .. 6

.. ~~ .. -0.8---c"

pot lr,.X

-----/ Ir 0.0

(b)

0.0 1,.----

1.2 ~

0.8

0.4

prop Ir,X1.6 Au_

80

60

40

100

(a)

20 .0.4 N----....o Pt

o -0.8 'Ru~ •L---&..i--4-~2...--;~,3_+t--=-S""2:'C c g~ ~~. ... •.•...._---_...----_....

400

100

300

200

6E,mV

Figure 7. Calibration graphs(a) and relative selectivity factors (b) for Ir(IV). FI system with B[15]S2C5-containing PVC-CME

Table 3. Performance characteristics for the FI systems with PVC-CMEs

Metal ionIon-sensing Detection Linear measuring range LOD Sampling Vocomponent mode molr1 J.Lmolr1 rate, h-

1J.Ll

Ir(IV) B[15]S2C5 Pot. 1 x 10-5-0.01 5.0 6-60 500Amp. 1 x 10-5-5 X 10-3 20.0 20-70 200

Au(III) DB[18]S2C6 Pot. 5 x 10-7-1 x 10-4 0.1 12-75 400Amp. 1 x 10-6-1 x '1 0-3 5.0 30-120 400

CONCLUSION

The solid-contacted PVC-CMEs of the type described in this work offer manypotential advantages and promise to have a high analytical applicability in. FIelectroanalysis. In comparison to conventional-size PVC-ME, the proposed units aresmaller and much more convenient for incorporation into the FI Inanifold, exhibitgood stability of baseline and reproducibility of signals in the flowing stream. Themost remarkable future of the new sensors is that they can be used in the differentmodes ofelectrochemical measurements. by choosing a highly selective ion-sensingcomponents it is possible to receive improved potentiometric/amperometric signalsto some noble metal ions in the presence of other non-complexing redox-ions.

Aclmo,vledgment

This research was supported by the Russian National Science Foundation (Grant No. 97-03-33216a).

634 lA. Zolotovet al.

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Received October 1998AcceptedApril 1999