Quantitative Analysis of a Mixture With Reversible Electrode Processes by Cyclic Voltammetry and...

8/13/2019 Quantitative Analysis of a Mixture With Reversible Electrode Processes by Cyclic Voltammetry and Linear Sweep V

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E L S E V I E R Journal of Electroanalytical Chemistry 402 1996) 1 -17

j O U R N A L O F

uant i ta t ive analys i s o f a m ixture w i th revers ib le e l ec trode processesby c y c l i c v o l ta m m e t r y and l ine a r sw e e p v o l ta m m e t r y

Pingsan Zhao 1 Shen Yun Zhongmin Hu Deyao Q iDepartment o f Chem istry and Chemical Engineering, Shanghai U niversity of Technology, Shanghai 200072, People s Republic of China

Received 5 October 1994; in revised form 23 June 1995

A b s t r a c t

The theories o f the l inea r sweep vo l tam metry (LSV) and cyc l ic vo l tamm etry (CV), including b ranch cyc l ic vo l tammetry (BACV), o f amixture c ontaining tw o comp onents wh ose electrode processes are reversible and independen t are described. The digital s imulationmeth od is used to s imulate the theoretical linear sw eep and cyc lic voltam mo gram s at spherical e lectrodes. A m ixture o f Pb 2+ and C d 2+at hanging m ercury drop electrode s is used to test the theories. Both theor y and experiment sh ow that, in the presence of a species wh ichis reduced at m ore positive potentials , i t is difficult to determine the concentration of another species; considerable er ror could be causedin LSV, bu t CV based on the reverse b ranch of a cyc l ic vo l tamm ogram and BA CV show less e rro r. T he a l lowed concen tra t ion o f themo re positively reduced species with a 10 relative error in CV and BA CV is about four to twenty times mo re than that in LSV .However, in the presence of a species that is reduced at more negative potentials , LSV shows far less error than BACV and CV. Thetheoretical a nd experimental results are consistent.Keywords: Digital simulation; Quantitative analysis; Reversible electrode processes; Cyclic voltammetry; Linear sweep voltammetry

1 I n t r o d u c t i o n

C y c l i c v o l t a m m e t r y ( C V ) i s u s u a l l y u s e d t o d i a g n o s ev a r i o u s e l e c t r o d e r e a c t i o n m e c h a n i s m s a n d t o m o n i t o ri n t e r m e d i a t e p r o d u c t s [ 1 , 2 ] . I n 1 9 8 4 , O l d h a m a n d Z o s k i [ 3 ]e s t a b l i s h e d a n a n a l y t i c a l m e t h o d , b r a n c h a d d i t i o n c y c l i cv o l t a m m e t r y ( B A C V ) , b a s e d o n C V . T h e y a d d e d t h e f o r -w a r d a n d r e v e r s e b r a n c h e s o f t he c y c l i c v o l t a m m o g r a m t oo b t a i n a n e w v o l t a m m o g r a m w i t h a t r o u g h a n d a c r e s t . T h ed i f f e re n c e in c u r re n t s a t t h e t ro u g h a n d c re s t i s d i r e c t lyp r o p o r t i o n a l t o c o n c e n t r a t i o n a n d t o t he s q u a r e r o o t o fs w e e p r a t e . I n B A C V , t h e c h a r g i n g c u r r e n t i s v i r t u a l l ye l i m i n a t e d , t h u s a l l o w i n g a l o w d e t e c t i o n l i m i t a n d i m p r o v -in g th e a n a ly t i c a l a c c u ra c y [3 , 4 ] .

C o m p a r e d w i t h d c p o l a r o g r a p h y , l i n e a r s w e e p v o l t a m -m e t r y ( L S V ) h a s m a n y a d v a n t a g e s [ 5 ] i n c l u d in g i m p r o v e ds e n s i t i v i t y a n d a m u c h s h o r t e r m e a s u r e m e n t t i m e . H o w -e v e r , w h e n s e v e r a l c o m p o n e n t s c o e x i s t i n s o l u t i o n , t h ep r e c e d i n g e l e c t r o d e p r o c e s s e s o f t e n i n t e r f e r e w i t h t h e m e a -s u r e m e n t o f a n a n a l y t e [5 , 6] . S e v e r a l m e t h o d s h a v e b e e np r o p o s e d t o m i n i m i z e a n d / o r e l i m i n a t e t h is in t e r f e r e n c e

Present address: Department of Marine Science, University of Geor-gia, Athens, GA30602, USA.Elsevier Science S ASSDI 0022-0728 95)04214-8

[ 6 ,7 ] , b u t m o s t r e q u i r e r a t h e r a r b i t r a r y e x t r a p o l a t i o n p r o c e -d u r e s t o e s ta b l i s h a b a s e l i n e f o r t h e d e t e r m i n a t i o n o f p e a kh e ig h t [5 ] .

B o t h s e n s i t i v i t y a n d r e s o l u t i o n a r e v e r y i m p o r t a n t f o rt h e q u a n t it a t i ve a n a l y s i s o f a m i x t u r e . T h e i n t r o d u c t i o n o fp u l s e a n d s q u a r e - w a v e t e c h n i q u e s i n c r e a s e s t h e s e n s i t i v i t ys u b s t a n t i a l l y b y e l i m i n a t i n g t h e c h a r g i n g c u r r e n t a n d a l s oi m p r o v e s t h e s y m m e t r y o f t h e v o l t a m m o g r a m [ 8, 9] . H o w -e v e r , it d o e s n o t p r o d u c e m u c h i m p r o v e m e n t i n re s o l u t io na n d , m o r e o v e r , p u l s e p o l a r o g r a p h y i s r e l a t i v e l y s l o w .

M a n y a u t h o r s h a v e c o n c e n t r a t e d o n e n h a n c i n g r e s o l u -t i on b y m e a n s o f c o m p l e x m a t h e m a t i c m e t h o d s w h i c ht r a n s f o r m t h e o r i g i n a l l i n e a r s w e e p , p u l s e a n d s q u a r e - w a v ev o l t a m m o g r a m s , f o r e x a m p l e t h e s e p a r a ti o n o f o v e r la p p i n gp e a k s b y s e m i d i f f e r e n t i a l t r a n s f o r m a t i o n [ 1 0 ] a n d r e s o l u -t i o n e n h a n c e m e n t b y F o u r i e r t r a n s f o r m a t i o n [ 1 1 , 1 2 ] . A l lt h e s e m e t h o d s r e q u i r e n o t o n l y c o m p l e x m a t h e m a t i c t r e a t -m e n t , b u t a l s o k n o w l e d g e o f o r a s s u m p t i o n s a b o u t p e a ks h a p e p a r a m e t e r s , e l e c t r o d e r e a c t i o n m e c h a n i s m e t c . b e -fo re th e t r e a tme n t c a n s t a r t .

S i m p l e , f a s t a n d a c c u r a t e m e t h o d s a r e n e e d e d f o r th ea n a l y s i s o f a m i x t u r e . H e r e w e p r e s e n t a t h e o r e t i c a l a n de x p e r i m e n t a l s t u d y o f C V a n d L S V o f a m i x t u r e c o n t a i n -i n g t w o c o m p o n e n t s w h o s e e l e c t r o d e p r o c e s s e s t a k e p l a c er e v e rs i b ly a n d i n d e p e n d en t l y . T w o n e w C V - b a s e d m e t h o d s


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12 P. Z h a o e t a l. / J o u r n a l o f E l e c t r o a n a l y t i c a l C h e m i s t r y 4 0 2 (1 9 9 6 ) 1 1 1 7fo r min im iz ing the i n f lue nc e o f t he p re c e d ing e l e c t rodep roc e ss a re p re se n te d : t he f i r s t i s B A C V a nd the se c ond i sC V b a s e d o n t h e r e v e r s e b r a n c h o f a c y c l i c v o l t a m m o -g r a m . W e a l s o p r e s e n t a c o m p a r i s o n o f t h e e f f e c t o f afo l low ing e l e c t rode p roc e ss on the c u r re n t pe a k he igh tm e a s u r e m e n t o f t h e d e t e rm i n a n t i n L S V a n d C V .

2 . T h e o r y2 . 1 . T h e c o n c e n t r a t i o n i t e r a t i o n a n d c u r r e n t f u n c t i o n e q u a -t i ons

The the o ry fo r LSV a nd C V i s c ompl i c a t e d , a nd i t i sd i f f i c u l t t o ob ta in a n e xa c t a na ly t i c a l e xp re ss ion e xc e p t fo ra r e ve r s ib l e p roc e ss a t a p l a na r e l e c t rode [13,14 ]. H ow e ve r ,t he d ig i t a l s imu la t ion me thod p ropose d by Fe ldbe rg [15 ]c a n e a s i ly be u se d to s imu la t e l i ne a r sw e e p a nd c yc l i cv o l t a m m o g r a m s u n d e r a l m o s t a l l c o n di t io n s .

A t a sphe r i c a l e l e c t rode , t he fo l low ing c onc e n t ra t i oni te ra t ion equat ions can be deduced: for j > 1CI+ IO.J

( N + j ) 2 ( c ~ . j + , - c ~ . j ) - ( N + j - l ) 2 ( c o . ' _ c O , j _ , )c t , j + D O

e l+ 1R.)= c lR. j + D R

[ ( N + j ) 3 - ( N + j - 1 )3 ]/ 3( l a )

( N - j ) 2 ( c ~ . j + , - C tR .j ) - - ( N - j + l ) 2 ( c lR . j - - (. '~.) , )

and for j = 1e l + l = C I .~_O.1 O,I

[ ( N - j + l ) 3 - ( N - j ) 3 ]/ 3( l b )

N 2 1 o + ( N + 1)2 Do(C~).2 c O , I )[ ( N + 1 ) 3 - N 3 ] / 3 ( 2a )

N 2 I R + ( N + I ) 2 D R ( C ~ , : 'C R , I )1 1 = C l q . _CR. R . l [ ( N + l ) 3 _ N 3 1 / 3 ( 2 b )A l l va r i a b l e s a nd pa ra me te r s i n t he p re c e d ing e qua t ions a red i m e n s i o n l e s s ( e a c h p a r a m e t e r c a n b e m a d e d i m e n s i o n l e s sby the me thod de sc r ibe d in R e f . [6 ] , p . 675 ) , w he re cre p re se n t s c onc e n t ra t i on , 1 a nd j r e p re se n t t ime a nd spa c e ,O a nd R re p re se n t ox id i z e d a nd re duc e d spe c i e s , D i s t hed i f fu s ion c oe f f i c i e n t a nd N i s t he r a d iu s o f t he sphe r i c ale l e c t rode ( in spa c e s t e ps ) . Thus c ~ . j i s t he d ime ns ion le ssc onc e n t ra t i on o f t he ox id i z e d spe c i e s a t t he j t h spa c e a ndl th t ime s t e p , a nd so on .

A t a p l a na r e l e c t rode (N ~ oc ), t he p re c e d ing e qua t ionsc a n be s imp l i f i e d a s fo l low s [2 ,9 ] : Fo r j > 1

l C Ic +l=,s c .,.,+ D i ( c , . j + l - 2 c ~ + i . j - , ) ( 3 )and for j = 1e l + l = i . l C I i ., J f -D i ( c [ . 2 - - C [ . l ) - I - I f ( 4 )w he re i r e p re se n t s O o r R .

The d ime ns ion le ss c u r re n t i n Eqs . (2 ) a nd (4 ) i s g ive nby the e xp re ss ion [2 ]

2 D o ( C o , l - g c l l )I R = 5 )1 + g D o / D R + 2 g " D o / k sw h e r eg = e xp ( E - E ) (6 )He re k s i s a s tandard ra te coeff ic ien t , a i s the t ransfercoeff ic ient , E is the e lec t ro de p otent ia l and E is thes t a nda rd e l e c t rode po te n t i a l ; a l l t he se pa ra me te r s a re d i -me ns ion le ss . Fo r a r e v e r s ib l e p roc e s s (k s ~ z c ) , Eq . (5 )b e c o m e s1 R = 2 0 0 ( C 'O ,I - - g c l ,I ) (1 + g D o / D R ) ( 7 )The c u r re n t g ive n by Eqs . (5 ) o r (7 ) i s de pe nde n t on thesw e e p ra t e V . The re fo re w e se t

w he re v is d ime ns ion le ss .T h e v o l t a m m o g r a m f o r B A C V i s o b t a i n ed b y a d d in g

t h e f o r w a r d a n d r e v e r s e b r a n c h e s o f t h e c y c li c v o l t a m m o -g r a m .

C o n s i d e r a s y s t e m c o n t ai n i n g t w o c o m p o n e n t s O l a n d0 2. Ea c h e l e c t rode r e a c t ion O 1 + n l e -~ R ~ o r 0 2 + n 2e -~R 2 wh ere n~ and n 2 a re the e lec t ron num bers, i sr e v e r s i b le a n d i n d e p e n d e n t. T h e v o l t a m m o g r a m s o f t h em i x t u r e f o r L S V , C V a n d B A C V c a n b e o b t a i n e d b ya dd ing the vo l t a m mo gra m s o f O ~ a nd 0 2 . F ig . 1 show s ther e v e r s i b le c y c l i c v o l t a m m o g r a m s o f O l a n d 0 2 a n d t h e irmix tu re a t sphe r i c a l e l e c t rode s w he re ( a ) r e p re se n t s t hefo rw a rd b ra nc he s a nd (b ) t he r e ve r se b ra nc he s . In f a c t ac y c l i c v o l t a m m o g r a m c o n s i s t s o f t w o l i n ea r s w e e p v o l t a m -m o g r a m s w i t h t h e f o r w a r d b r a n c h c o r r e s p o n d i n g t o L S V .F i g . 2 s h o w s t he r e v e r s ib l e v o l t a m m o g r a m s o f B A C V o fthe mix tu re a t sphe r i c a l e l e c t rode s .2 . 2 . I n f lu e n c e o f t h e p r e c e d i n g e l e c tr o d e p r o c e s s

Thre e me thods c a n be u se d to de t e rmine spe c i e s 0 2 i nthe p re se nc e o f t he more pos i t i ve ly r e duc e d spe c i e s O ~.The f i r s t i s LSV , w h ic h i s ba se d on the fo rw a rd b ra nc h o fthe c yc l i c vo l t a m mo gra m in F ig . l ( a ) . T h i s f i gu re il l us -t r a t e s t he d i f f i c u l ty i n de t e rmin ing the r e a l pe a k he igh t( I C ) r o f s p e c i e s 0 2 in t h e p r e s e n c e o f t h e m o r e p o s i t iv e l yre duc e d spe c i e s O 1 be c a us e the r e fe re nc e c u r re n t (do t t e dc u rve in F ig . l ( a ) ) m us t be e x t ra po la t e d . In suc h a c a se , thef l a t pa r t be tw e e n the pe a ks o f O l a nd 0 2 a nd the i n it i alp a r t o f t h e v o l t a m m o g r a m c a n u s u a ll y b e t a k e n a s t h eba se l ine fo r m e a su r ing the pe a k he igh t o f 0 2 in LSV i f noc o r re c t ion p roc e du re s a re a pp l i e d . The c o r re spond ing tw om e a s u r e m e n t s o f p e a k h e i g h t t o b a s e l i n e a r e r e p r e s e n t e db y 1 ~ a n d I ~ i n F i g . l ( a ). T h e s e c o n d m e t h o d i s t h e C Vb a s e d o n t h e r e v e r s e b r a n c h o f t h e c y c li c v o l t a m m o g r a m i nF ig . l (b ) w he re t he spe c i e s 0 2 c a n be de t e rmine d f rom thep e a k h e i g h t I ~ o f th e s e c o n d p e ak . T h i s m e t h o d i s


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P . Z h a o e t a l . / J o u r n a I o f E l e c t r o a n a l y t i c a l C h e m i s t r y 4 0 2 ( 1 9 9 6 ) l 1 - 1 7 1 3

T a b l e 1C o n c e n t r a t i o n e f f e c t o f a m o r e p o s i t i v e l y r e d u c e d s p e c i e s O ~ o n t h e p e a k m e a s u r e m e n t o f t h e a n a l y t e 0 2 u n d e r c o n d i t i o n s o f c o n s t a n t p e a k p o te n t ia ld i f f e r e n c e a n d e le c t r o n n u m b e rc I B A C V C V L S V

A I 2 ~ / 1 ~2 ~ / 1 ~2 e l I ~' e l0 0 . 6 6 6 4 - - 0 . 4 6 2 7 - - 0 . 4 5 0 8 - - 0 . 4 5 0 8 - -0 . 01 0 . 6 6 6 2 - 0 . 0 3 0 . 4 6 2 7 0 . 0 0 0 . 4 4 9 7 - 0 . 2 4 0 . 4 5 2 2 0 . 310 . 1 0 . 6 6 4 3 - 0 . 3 2 0 . 4 6 2 5 - 0 . 0 4 0 . 4 4 4 1 - 1 . 4 9 0 . 4 6 5 3 3 . 2 20 . 5 0 . 6 5 6 1 - 1 . 5 4 0 . 4 6 1 5 - 0 . 2 6 0 . 4 2 6 0 - 5 . 5 0 0 . 5 2 3 6 1 6 . 1 51 0 . 6 4 6 1 - 3 . 0 4 0 . 4 6 0 4 - 0 . 5 0 0 . 4 0 7 4 - 9 . 6 3 0 . 5 9 6 4 3 2 . 3 0

1 0 0 . 4 8 7 3 - 2 6 . 8 8 0 . 4 3 9 8 - - 4 . 9 5 0 . 2 0 0 5 - 5 5 . 5 2 1 . 9 1 5 5 3 2 4 . 9 12 0 0 . 3 4 4 8 - 4 8 . 2 6 0 . 4 1 7 7 - 9 . 7 2 0 . 0 6 5 7 - 8 5 . 4 2 3 . 3 9 8 3 6 5 3 . 8 4D O = D R = 0 . 4 5 , n t = n 2 = 2 , N = 5 0 0 , t h e p e a k p o t e n t i a l d i f f e r e n c e A E p b e t w e e ns e t a t 1 , c t i s d i e c o n c e n t r a t i o n o f t h e m o r e p o s i t i v e l y r e d u c e d s p e c i e s O ~ a n d e i sr e m a i n i n g s y m b o l s a r e i ll u s t r a t e d i n F i g s . 1 a n d 2 .

s p e c i e s O ~ a n d 0 2 i s 2 0 0 m V , t h e c o n c e n t r a t i o n o f t he a n a l y t e O 2 i st h e r e l a t iv e e r r o r i n t h e p e a k h e i g h t o r c u r r e n t d i f f e r e n c e o f 0 2 . T h e

expected to be more accurate than LSV because the seg-ment CD due to species O~ is fairly flat and species O~ haslittle effect on the peak shape and peak height of species0 2. The third method is based on the vol tammogram ofBACV in Fig. 2 where A I 2 is used as an analyticalparameter. It can be seen from Fig. 2 that when theconcentration e 1 of species O~ increases from zero to fivetimes of that of species O 2, AI2 changes insignificantly.Therefore BACV will also be more accurate than LSV.The three methods are compared using the four parametersA I 2 , I ~ 2 , I ~2 and I~2 in Tables 1 and 2.

Table 1 shows the concentration effect of a more posi-tively reduced species O~ on the peak measurement of theanalyte 02 under conditions of constant peak potentialdifference (AEp = 200 mV) and electron number (n 1 = n 2= 2) where e is the relative error. The value of e for thecurrent difference A I 2 of 2 can be calculated using thefollowing equation:

A I 2 - - ( A l 2 ) te / X 1 0 0( a 6 ) ,where (AI2) represents the true current difference due tospecies O 2 in the absence of the interferent O~ (c~ = 0),i.e. the true value of A I 2 . Similar error equations can be

applied to the remaining analytical parameters I~, I~2 andI~. In Table 1, the concentration of the analyte 02 is fixedas 1 and the concent ration c~ of the interferent O l ischanged from zero to 20. As ct increases, the error in thepeak height or current difference of the determinant 02increases. For example, in BACV e is -3 .0 1 and-4 8. 26 respectively when ct is equal to and 20 timeslarger than the concentration of the analyte 02 (see Table1, third column). Comparing the relative error of thedifferent measuring methods at the same concentration ofthe interferent, Table 1 shows that the error caused by thepresence of the more positively reduced species decreasesin the following sequence:CV < BACV


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14 P . Z hao e t a l. / J o urna l o f E l e c t roana l y t i c a l Che m i s t r y 402 ( 1996) 11 - 1 7

. 5

04 0 0

~)

I

3 0 0 2 0 0 1 0 0 0 -100 -200E-E/mV

192- - - . -. - . T - V . . . . . . . . . . t 1c o 0 , t

H

. 5 I I I I4 0 0 3 0 0 2 0 0 t 0 0 0 - 1 0 0 - 2 0 0E-E/mVFig. 1. The theoretical reversible cyc lic voltammograms of a morepositively reduced species O l ( . . . . . . ), a more n egatively reducedspecies 02 ( . . . . . ) and their mixture ( ) at a spherical electrode:(a) forward branch; (b) reverse branch. The concentration o f O~ and 02 is1; D o = D ~ = 0 . 4 5 ; n l = n z = 2 ; N = 5 0 0 .

t r o d e p r o c e s s b e c o m e s s t r o n ge r . F o r e x a m p l e , i n C V b a s e do n t h e re v e r s e b r an c h o f a c y c l i c v o l t a m m o g r a m , t h ea l l o w e d c o n c e n t r a t i o n o f th e m o r e p o s i t i v e l y r e d u c e d

Table 3Effect of the concentration of a more negatively reduced species 02 onthe peak m easurement o f species O t under cond itions o f constant peakpotential difference and electron number.c a BACV LSV

A11 e / 1~,L e /0 0 .6693 - - 0 . 4508 - -0.01 0.6691 - 0.03 0.4508 0.000.1 0.6671 - 0.33 0.4508 0.001 0.6471 - 3.31 0.4508 0.00

10 0.4757 - 28.92 0.4508 0.001000 No peak 0.4513 0.11The concentration o f species O~ is set at l and c 2 represents theconcentration of the more negatively reduced species O~. See Table 1 forconditions and Figs. 1 and 2 for symbols.

s p e c i e s O t d e c r e a s e s f r o m 2 0 t o 1 2 ti m e s g r e a t e r t h a n t h ec o n c e n t r a t i o n o f t h e a n a l y t e 0 2 w h e n , :I E e d e c r e a s e s f r o m2 0 0 t o 1 0 0 m V a n d n ~ = n 2 = 2 ( s e e T a b l e 2 , f i f t h c o l -u m n ) , a n d i t d e c r e a s e s f r o m 2 0 t o 9 t i m e s g r e a t e r w h e nb o t h n I an d n 2 d e c r e a s e f r o m 2 t o 1 a n d A E r , = 2 0 0 m V( s e e T a b l e 2 , f if t h c o l u m n ) . I t c a n b e s e e n f r o m T a b l e 2t h a t , u n d e r v a r i o u s c o n d i t i o n s , t h e a l l o w e d c o n c e n t r a t i o no f th e m o r e p o s i t i v e l y re d u c e d s p e c i e s O 1 i n B A C V o r C Vi s l a r g e r t h a n t h a t i n L S V , a n d t h e r e f o r e o n e c a n c o n c l u d et h at C V a n d B A C V a r e m o r e a c c u r a t e t h a n L S V . M o r e -o v e r , t w o m o r e c o n c l u s i o n s c a n b e o b t a i n e d f r o m T a b l e 2 :( 1 ) C V i s f a r b e t te r t ha n B A C V ( c o m p a r e t h i rd a n d f o u r t hc o l u m n s i n T a b l e 2 ) ; ( 2) C V i s p r e f e r a b l e t o s m a l l e r p e a ks e p a r a t i o n s ( s e e T a b l e 2 , e i g h t h c o l u m n ) . F o r e x a m p l e , t h ea l l o w e d c o n c e n t r a t i o n o f t h e m o r e p o s i t i v e l y r e d u c e ds p e c i e s i s 2 0 t i m e s l ar g e r in C V t h a n i n L S V w h e nA E p = 2 0 0 m V a nd n j = n 2 = 2 , b u t i s 5 2 t i m e s l a r g e rw h e n A E p = 1 0 0 m V a n d n l = n 2 = 2 ( s e e T a b l e 2 , l as tc o l u m n ) .

t.5 ~ @ ~ 5r ~ ____2

,g .5 ~ 1

. 5

- 1- 1 . 5 i i ~ i i4 0 0 3 0 0 2 0 0 1 o o o - t o o - 2 0 0

E-E /mVFig. 2. The theoretical reversible voltammograms for BACV of a mixturecontaining tw o species O~ and 0 2 at a spherical electrode. The concentra-tion C 2 of 02 is set at 1 and the concentration C l o fO l is set at 0, 1, 2,3, 4 and 5 as shown in the figure. Other conditions are as in Fig. 1.

2 . 3 . In f l u e n c e o f th e f o l l o w i n g e l e c t r o d e p r o c e s s

F i g . 1 s h o w s t h a t t h e p r e s e n c e o f 0 2 h a s l i t t l e e f f e c t o nt h e b a s e l i n e f o r m e a s u r i n g O I a n d t h e p e a k h e i g h t I ~ ,] o f0 1 , i .e . th e s e g m e n t A B i n th e v o l t a m m o g r a m o f 0 2 ( F i g .l ( a ) ) i s a z e r o b a s e l i n e . T h u s t h e f o l l o w i n g e l e c t r o d ep r o c e s s h a s l i tt l e e f f e c t o n t h e p r e c e d i n g p r o c e s s i n L S V .H o w e v e r , i t i s d i f f ic u l t to d e t e r m i n e O ~ b y t h e C V b a s e do n t h e re v e r s e b r a n ch o f t he c y c l i c v o l t a m m o g r a m i n F ig .l ( b ). T h e r e f o r e o n l y a c o m p a r i s o n o f L S V a n d B A C V i sp r e s e n t ed . T a b l e 3 s h o w s t h e c o n c e n t r a t i o n e f f e c t a n d t h en i n t h a n d t e n th c o l u m n s i n T a b l e 2 s h o w t h e e ff e c t o f p e a kp o t e n t i a l d i f f e r e n c e a n d e l e c t r o n n u m b e r . I t c a n b e s e e nf r o m T a b l e s 2 a n d 3 th a t L S V i s f a r m o r e a c c u r a t e t h a nB A C V . F o r ex a m p l e , w h e n t h e c o n c e n t ra t i o n C z o f t h em o r e n e g a t i v e l y r e d u c e d s p e c i e s i s 1 0 0 0 t im e s m o r e t h a nt h a t o f t h e a n a l y t e O ~, t h e r e l a t i v e e r ro r is 0 . 1 1 i n L S Vb u t p e a k 0 2 d i s a p p e a rs i n B A C V u n d e r t h e sa m e c o n d i -t i o n s ( s e e l a s t l i n e o f T a b l e 3 ) .


5/7

P . Z h a o e t a l . / Jo u r n a l o f E l ec t r o a n a l y t i ca l C h em i s t r y 4 0 2 1 9 9 6 ) 1 1 -1 7 1 5

3. Experiments3 1 Reagents

0.1 M KC1 was used as the supporting electrolyte.Solutions 0.01 M) of Pb 2+ and Cd 2+ were prepared fromnitrate. The reagents used were analytical grade and allwater was treated with an ion exchanger and redistilled.3 2 Instrumentation

All experiments were carried out with a PARC model273A potentiostat controlled by a model 50 z IBM PS/2computer with model 270 electrochemical analysis soft-ware PARC). The working electrode was a PARC model303 SMDE and the reference electrode was AgCI lag 10.1M KC1.3 3 Experimental proced ures

One drop of 0.5 M HC1 and 5 ml of 0.1 M KC1 wereadded to the electrolyte cell and the solution was deaeratedfor 15 min. Solutions of reduced species Pb 2 a nd /o rCd 2 ) were injected into the bulk solution by a microinjec-tor and the cyclic voltammograms were recorded. Twoseries of measurements were performed: 1) the concentra-tion of Cd 2+ w as fixed and the concentration of Pb 2+ w aschanged to test the influence of the preceding electrodeprocess Fig. 3); 2) the concentration of Pb 2 was fixedand the concentration of Cd 2+ was change d to test theinfluence of the following electrode process Fig. 4).

It - ~ . . . . . . . . .

V aI~)/ ~ ,, ,~ SV ~-e)

< i ~ cvL ( I~~ . . . . . . ~ k _ __ _ j t. ~ . ~ . _ . = . : . . . . .

i I

1 I ~ ,

i i: :0 I I I I ~ I 13 3 . 5 4 4 . 5 5

- l o g ( c / t o o l 1 - 1 )Fig. 3. The effect of the mor e positivel y reduced ion Pb 2+ on thedetermination of Cd 2+ in LSV, CV and BACV ([Cd 2+ ]= 5X 10 5 M;sweep rate, 100 mVs -l ) O AI2 in BACV; , I~2 in CV; B, 1~,2 inLSV; E3, I~ in LSV. The broken lines and the numbers on these linesindicate the allowed concentrati ons of the more positivel y reduced speciesPb 2+ (logarithmic value) in different me thods for a 10% relative error.The solid curves join the points.

1000See Figs. 1, 2 and 5 for symbols.a At this concentration or ratio, the relative error is less than 10 .


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P. Zhao et al. / Journal of Electroanalytical Chemistry 402 1996) 11-17 17

t h e c y c l i c v o l t a m m o g r a m i s f a r m o r e f a v o r a b le t h a n B A C Va n d t h e e x p e r i m e n t a l s y s t e m s c o r r e s p o n d i n g t o t h i s c a s ea r e u n d e r i n v e s t i g a t i o n .

A c k n o w l e d g m e n t

W e a r e m o s t g r a t e f u l to D r . C . G . Z o s k i f o r h e r c o m -m e n t s a n d a d v i c e o n t h e manuscr ipt

e f e r e n c e s

[1] G.A. Mabbot t , J . Chem. Educ. , 60 (1983) 697 .[2] P.T. Kissinger and W.R. Heinem an, J . Chem. Educ. , 60 (1983) 702 .[3] K.B. Oldham and C.G. Zoski , Anal . Chem., 55 (1983) 1992 .[4] P. Zhao and D. Qi , Acta Chim. Sin . , 45 (1987) 1163 .[5] A.M. Bond (Ed.) , Modem Polarographic Methods in Analy t ica l

Che mis try, Dekker, N ew York, 1980, p. 207.

[6] A.J . B ard and L.R. Faulkn er (Eds.), E lec t rochemical Methods: Fun-dam enta ls and Appl ica tions, Wi ley , Londo n, 1980 , p. 232 .

[7] P.S. Polcyn and I . Shain , Anal Chem., 38 (1966) 370 .[8] J. Osteryo ung an d J.J. O De a in A .J. Bard (ed.), Electroanalytical

Chem ist ry , Vol . 14 , Dekker, New York , 1986 , p . 209 .[9] J. Osteryo ung and R. Osteryoung, Ana l. Chem . Symp. Ser., 25

(1986) 3 .[10] M. Palys , T. Korba , M. Bos and W.E. Van Der Linden , Talan ta , 38

(1991) 723 .[1 l] S.O. En gblom , J. Electroanal. C hem., 2 96 (1990) 371.[12] I. Pizeta, M. Lovric and M. Branica, J. Electroanal. Chem., 296

(1990) 395 .[13] R.S. Nicholson and R.I. Shain, Anal. Chem., 36 (1964) 706.[14] J.C. M yland and K.B. Oldharn, J. Electroanal. Chem ., 153 (198 3)

43.[15] S.W . Feldbe rg in A.J. Bard (Ed.), Electroanalytical Che mistr y, Vol.

3, Dekker, New York, 1969, p. 199.[16l K . Tokuda, N . Eno mo to and H. Matsuda, J. Electroanal. Chem ., 159

(1983) 23 .[17] J.C. M yland, K.B. Oldham an d C.G. Zoski, J. Electroanal. Che m.,

206 (1986) 1 .[18] R.C. Rooney, J. Polarogr. Soc., 9 (1963) 45.

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