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solution interface
359
Bioelectrochemistry and Bioenergetics, 26 (1991) 359-366 A section of J. Electroanai. Chem.. and constituting Vol. 321 (1991)
Elsevier Sequoia S.A., Lausanne J E C BB 01394 Short communication
V o l t a m m e t r i c studies o n the surface activity and o r i e n t a t i o n of 2-thiobarbituric acid at the mercury~solution interface M o s t a f a M. K a m a l Chemistry Department, Faculty of Science, Assiut University, Assiut (Egypt)
(Received 12 March 1991; in revised form 20 April 1991) INTRODUCTION T h e first h y p n o t i c b a r b i t u r a t e , 5 , 5 - d i e t h y l b a r b i t u r i c acid (barbital), w a s i n t r o d u c e d in m e d i c i n e in 1905; the s e c o n d was 5 - e t h y l - 5 - p h e n y l - b a r b i t u r i c acid ( p h e n o barbitol), used as a l o n g a c t i n g C N S d e p r e s s a n t . Similarly, t h i o b a r b i t u r i c acid derivatives were used as i n t e r m e d i a t e s for t h e s y n t h e s i s of d i a l k y l barbituric, acid until the a n a e s t h e t i c p r o p e r t i e s of s o m e t h i o b a r b i t u r i e acids w e r e d i s c o v e r e d [1]. T h i o b a r b i t u r a t e s , like o t h e r s u l p h u r - c o n t a i n i n g c o m p o u n d s , e x h i b i t a n a n o d i c o x i d a t i o n w a v e at the m e r c u r y electrode surface [ 2 - 4 ] c o r r e s p o n d i n g to the f o r m a t i o n o f slightly soluble m e r c u r y c o m p o u n d s . I n d e e d , a n o d i c waves of t h i o b a r b i t u r a t e s in 0.1 M s o d i u m h y d r o x i d e h a v e b e e n u s e d for t h e a n a l y s i s o f p h a r m a c e u t i c a l p r e p a r a t i o n s [4-7]. T h e p o l a r o g r a p h i c a n d v o l t a m m e t r i c b e h a v i o u r of t h i o b a r b i t u r i c a c i d d e r i v a t i v e s h a s b e e n i n v e s t i g a t e d extensively [2-7], f r o m b o t h t h e a n a l y t i c a l a n d t h e m e c h a n i s tic view. In the p r e s e n t p a p e r the a d s o r p t i o n p r o p e r t i e s o f T B A are s t u d i e d b y o u t - o f - p h a s e sensitive ac v o l t a m m e t r y . A f u r t h e r a i m of this s t u d y w a s to c o m p a r e the surface activities o f T B A ( d i h y d r o x y m e r c a p t o p y r i m i d i n e ) a n d t h e n u c l e i c acid base uracil ( d i h y d r o x y p y r i r a i d i n e ) . D u e to t h e b i o l o g i c a l s i g n i f i c a n c e o f b o t h c o m p o u n d s it is o f great interest to investigate h o w t h e i n t r o d u c t i o n of a n - S H g r o u p in the h y d r o x y p y r i m i d i n e s y s t e m will i n f l u e n c e its a d s o r p t i o n effects a t t h e m e r c u r y / s o l u t i o n interface. EXPERIMENTAL 2 - T h i o b a r b i t u r i c acid ( T B A , 4 , 6 - d i h y d r o x y - 2 - m e r c e . p t o p y r i m i d i n e ) w a s p u r c h a s e d f r o m Sigma a n d was u s e d w i t h o u t f u r t h e r p u r i f i c a t i o n . S o l u t i o n s c o n t a i n i n g d i f f e r ent c o n c e n t r a t i o n s of T B A were p r e p a : e d b y d i s s o l v i n g a k n o w n a m o u n t o f the c h e m i c a l l y p u r e p r o d u c t i n t o a d e f i n i t e v o l u m e o f B r i t t o n - R o b i n s o n buffer. B u f f e r s 0302-4598/9~/$03.50 © 1991 Elsevier Sequoia S.A. All rights reserved
360
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--I.2
E/V Fig. 1. A c capacitive curves of T B A at p H 3.2, 0.5 IV[ B r i t t o n - R o b i n s o n buffer w i t h N a C l at the HMDF_.. E|ectrode area 1.75 x 10 - 2 cm z, scan rate 2 m V s - l , frequency of ac potential 330 Hz, amplRude 10
mVpp,phase angle 90 0. waiting time
t s of 180 s (adsorption time) and 22°C. (1) pure buffer, (2) 0.99, (3) 1.49, (4) 2.49, (5) 3.48, (6) 4.48, (7) 5.47, (8) 6.45, (9) 11.7, (10) 15.7×10 - s M TBA.
w i t h a c o n s t a n t i o n i c s t r e n g t h of 0.5 M w i t h N a C I w e r e u s e d as s u p p o r t i n g electrolytes. All c h e m i c a l s were r e a g e n t grade. A P r i n c e t o n A p p l i e d R e s e a r c h ( P A R ) M o d e l 174 p o l a r o g r a p h i e a n a l y s e r c o u p l e d w i t h P A R M o d e l 1 7 4 / 5 0 ac p o l a r o g r a p h i c an&l.yser i n t e r f a c e a n d , a P A R M o d e l 5101 l o c k i n g a m p l i f i e r / p h a s e d e t e c t o r were e m p l o y e d for ac v o l t a m m e t r i c m e a s u r e m e n t s . T h e cell u s e d w a s a t h e r m o s t a t e d M e t r o h m cell w h i c h w a s e q u i p p e d w i t h a t h r e e - e l e c t r o d e system. A M e t r o h m H M D E , T y p e E-140, s e r v e d as w o r k i n g ' elect r o d e w h i l e a s a t u r a t e d c a l o m e l e l e c t r o d e w a s used as r e f e r e n c e e l e c t r o d e a n d a coiled p l a t i n u m w i r e as a u x i l i a r y electrode. All m e a s u r e m e n t s w e r e carri_'ecl o u t at 22°C. RESULTS AND
DISCUSSION
Phase-sensitive ac voltammograms of T B A in solutions of varying p H at the HMDE are shown in Figs. •1-3. At relatively low bulk concentrations of T B A the ac voltammograms indicate a progressive decrease of the capacitive ac signal around
361
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E/V Fig. 2. Ac capacitive curves of T B A at p H 7.3. (1) Pure buffer, (2) 0.99, (3) 2.49, (4) 3.98, (5..%5.47, (6) 6.95, (7) 8.43, (8) 9.41, (9) 9.9, (10) 13.8, (11) 18.6X10 - s M TBA° Other conditions as in Fig. 1.
the potential of m a x i m u m adsorption. This depression of the ac signal c o r r e s p o n d s to the progressive coverage of the m e r c u r y surface b y a dilute a d s o r p t i o n layer (stage I of adsorption). A t m o r e elevated bulk concentrations of T B A a clear capacitance pit is observed. T h e conclusions of Vetterl [8] a n d N f i r n b e r g a n d ~ co-workers [9] suggested that the capacitance pit reflects the association of the oriented molecules on the electrode surface b y inte, molecular a t t r a c t i o n forces a n d the formation of a c o m p a c t a d s o r p t i o n film (stage I I of adsorption). A t m o r e negative potentials, a r o u n d --0.8 V, the ac v o l t a m m o g r a m s of T B A exhibit a desorption p e a k corresponding to the desorption of less strongly b o u n d species existing in the adsorption film a t the negatively charged electrode surface. It a p p e a r s that the negatively charged electrode surface destabilizes the a d s o r p t i o n f'flm of neutral molecules ( p H < 7.3) due to repulsion between the negatively c h a r g e d surface a n d the ~r-electron system of the a d s o r b e d molecules. H o w e v e r the destabilization of the a d s o r p t i o n film in a l k a l i n e solutions is m o r e r e m a r k a b l e at less negative potentials b e c a u s e n o w the repulsion of the anionic fo~,~ o f the a d s o r b e d molecules b y the negatively charged electrode surface is m o r e p r o n o u n c e d .
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E/V Fig. 3. Ac c a p a c i t i v e c u r v e s o f T B A at p H 9.35. (1) P u r e b u f f e r , (2) 0.99, (3) 1.99; (4) 3.48, (5) 4.97, (6) 6.95, (7) 7.93, (8) 8.92, (9) 12.3, (10) 1 2 . 7 x 10 - s M T B A . O t h e r c o n d i t i o n s as in Fig. 1.
At p H >t 7.3 (TBA is fully anionic, pK~ = 7.3 [7]), the voltammograms s h o w sharp ac peaks at the positively charged electrode surface (Figs 2 and 3). These peaks are seen clearly when stage II of adsorption is reached. The frequency and phase angle dependence of the sharp peaks at ca. --0.1 V confirm the faradaic nature of these ac peaks. The appearance of such faradaic peaks is probably due to the electroreduction of the Hg(II) ion of the [ H g ( R S - ) 2 ] in the adsorption film according to the well k n o w n charge transfer mechanism [10] of the interaction between adsorbed R S - anions and the mercury electrode surface. The charge transfer reaction is represented by the following equations: at p o l e n t i a l s
H g ~lose to 0.0 v
Hg2++ 2 e-
2 R S H ~- 2 R S - + 2 H ÷
363
The overall reaction is Hg+ 2 RSH ~ Hg(RS)2 + 2 e-+ 2 H + The p H dependence of this p e a k confirms the latter overall reaction, whereas the peak is shifted, on a average by a b o u t 50 m V / p H , to m o r e negative potentials in the p H range 7.2-10.2. It is worth mentioning that TBA, like o t h e r s u l p h u r containing compounds, exhibits anodic polarographic waves at the m e r c u r y electrode surface corresponding to the formation o f slighdy soluble m e r c u r y c o m p o u n d s [2,6]; the H g ( I I ) ion of this c o m p o u n d undergoes cathodic reduction showing a well developed reduction p e a k [2,11]. A t p H >t 9.35, the faradaic p e a k at m o r e positive potentials splits into two consecutive peaks. The a p p e a r a n c e of these two v o l t a m m e t r i c p e a k s is n o t a n unusual observation. In his study [12], Florence a s s u m e d that the a p p e a r a n c e of the second peak was d u e to the change in the m o r p h o l o g y of the H g ( I I ) c o m p o u n d deposited on the electrode surface. Miller a n d T e v a [13] attributed the double p e a k
2.0
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.105c/M Fig. 4, Dependence of the capacitive current decrease ~ia= on the bulk concentration of T B A at the maximum adsorption potentials, and various p H values. (1) p H 3.2, (2) pH 5.2, (3) pH 7.3 and (4) p H 9.35.
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to the presence of b o t h Hg(I) a n d Hg(II) c o m p o u n d s in the a d s o r p t i o n film species Hg2(RS)2 a n d Hg(RS)2. C o m p a r i s o n of the Aiac (the decrease of the capacitive ac current with respect to the ia¢ of the supporting electrolyte for a given b u l k c o n c e n t r a t i o n ) versus c ( c o n c e n t r a t i o n of TBA) plot at different p H values a n d at the m a x i m u m a d s o r p t i o n potential, indicates a two-step a d s o r p t i o n isotherm (Fig. 4). The first stage reflects the dilute adsorption layer for re!ativeiy low bulk c o n c e n t r a t i o n s o f TBA. A t higher concentrations, o~e oblains the second stage, which is due to strong lateral interactions and high interaction coefficients between the a d s o r b e d molecules, giving rise to a c o m p a c t layer. It is concluded that at the m a x i m u m a d s o r p t i o n p o t e n t i a l the T B A molecules are oriented parallel to the electrode surface in a dilute layer w h e n the interaction of the or-electron system with the interface favours adsorption. W i t h an increase of the bulk c o n c e n t r a t i o n the stacking interactions between vertically oriented molecules lead to association a n d f o r m a t i o n of the c o m p a c t layer in the pit potential region. As shown in Fig. 4, the c o n c e n t r a t i o n o f T B A required for pit f o r m a t i o n in alkaline solution (pH >/7.3) is lower t h a n the c o r r e s p o n d i n g value in acidic solutions, showing that the surface activity of the anionic form ( R S - ) of T B A at the H g / s o l u t i o n interface is stronger t h a n that of the neutral form (RSH). It is possible that the electrostatic interaction of the a n i o n i c fotn~ of T B A ( R S - ) with the positively charged electrode surface e n h a n c e s the o r i e n t a t i o n process from the parallel to the vertical position and hence the f o r m a t i o n of a c o m p a c t a d s o r p t i o n film. The electrostatic interaction of the negatively charged sulphur a t o m with the positively charged electrode surface is s u p p o r t e d by the a n o d i c shift of the pit potential ragion as well as the m a x i m u m a d s o r p t i o n p o t e n t i a l u p o n increasing the p H of the solution from 3.2 to 10.2. In order to calculate the various p a r a m e t e r s of a d s o r p t i o n of T B A at different p H values, the equilibrium values o f the ac capacitive current at a given bulk c o n c e n t r a t i o n and at the m a x i m u m a d s o r p t i o n p o t e n t i a l were measured a n d the degree of coverage 0 was calculated using the relation
Co-C o= Co-Cm = (ai.o)m
(z) where the C ' s are the different capacitances in the s u p p o r t i n g electrolyte (Co), at a given bulk c o n c e n t r a t i o n of TBA ( C ) a n d at a b u l k c o n c e n t r a t i o n c o r r e s p o n d i n g to full coverage (Cm); Aiac is the decrease of the capacitive ac current for a given bulk c o n c e n t r a t i o n and (Ai.~c)m is the m a x i m a l decrease c o r r e s p o n d i n g to full coverage. T h e double step adsorption isothe~iii (Fig .4) observed at the m a x i m u m a d s o r p t i o n potential can be treated as c o m p o s e d of two isotherms [14]. The (Ai~=)~ values for stages I a n d i i of adsorption were t a k e n at the first a n d second p l a t e a u of the d o u b l e step isotherm, respectively. T h e results were fitted ~o several a d s o r p t i o n isotherms. F r o m a comparison of the experimental results with the theory it seems that the isotherm is of F r u m k i n type, given b y the equation: O(1
0)-' exp(--2a0)
=
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10.5c/M Fig. 5. D e p e n d e n c e o f the surface coverage o n the bulk concentration o f T B A for the dilute adsorption layer (stage I) at the H D M E and at various p H values. (1) p H 3.2, (2) pH 5.2, (3) p H 7.3 and (4) p H 9.35.
w h e r e 0 is the degree of coverage, a the interaction coefficient, fl the a d s o r p t i o n coefficient and c the bulk c o n c e n t r a t i o n o f T B A . T o c o n f i r m that the e x p e r i m e n t a l results fit the F r u m k i n isothe,,r, well (Fig. 5), the logarithmic plot o f the linear eqn. (2) w a s subjected to a least-squares refinement; the values o f the regression coefficient o f the fit were found to lie in the range 0.992 to 0.996 at the various p H values. T h e interaction coefficient a w a s d e t e r m i n e d from the slope o f the l o g a r i t h m i c plot o f eqn. (2), and the a d s o r p t i o n coefficient fl from the values at half c o v e r a g e (_0----0.5). T h e free enerb~J o f a d s o r p t i o n ( - - A G ° ) v, as then calculated from the adsorption coefficient fl a c c o r d i n g to the e q u a t i o n
1
(
°)
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366
C o m p a r i s o n of the a d s o r p t i o n p a r a m e t e r s of T B A at different p H values shows a significant dependence on the p H for b o t h stages (I a n d II) of adsorption. T h e surface activity and the degree of association expressed b y the value of the adsorption coefficient 13 increases with increasing p H (fl = 2.9 -i- 0.1 × 104 I / m o l at p H < 7.3 a n d 3.4 4- 0.1 x 104 1 / m o l at p H >i 7.3 for stage 1 of a d s o r p t i o n , fl = 3.3 a n d 3.9 x 1 0 3 i / m o l for stage II at p H < 7.3 a n d p H >~ 7.3, respectively). T h e lower interaction coefficient a at higher p H s ( a = 1.1 a n d 0.9 for stage I at p H s 3.2 a n d 9.3, respectively) indicates a m o d e r a t e lateral attractive i n t e r a c t i o n of the a n i o n i c form of the adsorbed molecules. In general the results of the a d s o r p t i o n of T B A ( d i h y d r o x y m e r c a p t o p y r i m i d i n e ) are in agreement with our previous results o b t a i n e d with uracil base ( d i h y d r o x y pyrimidine) [15] a n d support the theory that most p y r i m i d i n e derivatives exhibit two consecutive adsorption stages at the mercury surface. Moreover, the surface activity of T B A is significantly greater t h a n that of uracil, as reflected b y the h i g h e r values o f / ~ (by a factor of ~ 200) a n d - A G ~ (by a factor 1.5) c o m p a r e d with uracil at various p H values. This indicates a higher h y d r o p h o b i c i t y of T B A c o m p a r e d with uracil. It can be concluded that i n t r o d u c t i o n of an - S H group i n t o the p y r i m i d i n e system facilitates its electrostatic interaction with the mercury electrode surface and c o n s e q u e n t l y enhances the surface activity a n d the a d s o r p t i o n properties of the p y r i m i d i n e contair.ing thiol system at the charged mercury interface. REFERENCES 1 E. Miller, J.C. Munch, F.S. Crossely a n d W . H . H a r t u n g , J. A m . C h e m . Soc., 58 (1936) 1090. 2 C.A. Malresse-Ducarmois, G.J. Patriarche a n d J.L. V a n d e n b a l c k , Anal. Claim. Acta, 79 (1975) 69. 3 J. Rodriguez, L. C a i r o , C. M a t i n a n d A. Sanchez, J. Electroanal. C h e m . , 237 (1987) 105. 4 0 . M a n o u s e k a n d P. Z u m a n , Pharmazie, 11 (1956) 530. 5 W.F. Smyth, G. Svehla a n d P. Z u m a n , Anal. Chim, Acta, 51 (1970) 463. 6 W.F. Smyth, (3. SvehIa a n d P. Z u m a n , Anal. Chim. Acta, 52 (1970) 129. 7 W.F. Smyth, P. Z u m a n a n d G . Svehla, J. Electroanal. Chem., 30 (1971) 101. 8 V. Vetterl, Bioeleetrochem. Bioenerg., 3 (1976) 338. 9 D. Krznaric, P. Valenta a n d H . W . Nfirnberg, J. Electroanal. C h e m . , 65 (1975) 863. 10 M.T. Stankovich a n d A.J. Bard, J. Electroanai Chem., 75 (1977) 487. 11 C.A. Mairesse-Ducarmois, G.J. Patriarche a n d J.L. V a n d e n b a l e k , Anal. Chim. Acta, 71 (1974) 1 6 5 : 7 6 (I 975) 299. 12 T.M. Florence, J. Electroanal. Chem., 99 (1979) 219. 13 I.R. Miller a n d J. T e v a , J. Electroanal. Chem., 36 (1972) 157. 14 A.N. F r u m k i n a n d B.B. D a m a s k i n in J . O ' M . Bockris a n d B.E. C o n w a y (Eds.), M o d e r n Aspects o f Electrochemistry, Vol. 3, Butterworths, L o n d o n , 1964, p. I49. 15 Y.M. T e m e r k , M . M . K a m a l , Z.A. A h m e d a n d M.S. I b r a l ~ m , J. Electroanal. C h e m . , 260 (1989) 201.
Bioelectrochemistry and Bioenergetics, 26 (1991) 359-366 A section of J. Electroanai. Chem.. and constituting Vol. 321 (1991)
Elsevier Sequoia S.A., Lausanne J E C BB 01394 Short communication
V o l t a m m e t r i c studies o n the surface activity and o r i e n t a t i o n of 2-thiobarbituric acid at the mercury~solution interface M o s t a f a M. K a m a l Chemistry Department, Faculty of Science, Assiut University, Assiut (Egypt)
(Received 12 March 1991; in revised form 20 April 1991) INTRODUCTION T h e first h y p n o t i c b a r b i t u r a t e , 5 , 5 - d i e t h y l b a r b i t u r i c acid (barbital), w a s i n t r o d u c e d in m e d i c i n e in 1905; the s e c o n d was 5 - e t h y l - 5 - p h e n y l - b a r b i t u r i c acid ( p h e n o barbitol), used as a l o n g a c t i n g C N S d e p r e s s a n t . Similarly, t h i o b a r b i t u r i c acid derivatives were used as i n t e r m e d i a t e s for t h e s y n t h e s i s of d i a l k y l barbituric, acid until the a n a e s t h e t i c p r o p e r t i e s of s o m e t h i o b a r b i t u r i e acids w e r e d i s c o v e r e d [1]. T h i o b a r b i t u r a t e s , like o t h e r s u l p h u r - c o n t a i n i n g c o m p o u n d s , e x h i b i t a n a n o d i c o x i d a t i o n w a v e at the m e r c u r y electrode surface [ 2 - 4 ] c o r r e s p o n d i n g to the f o r m a t i o n o f slightly soluble m e r c u r y c o m p o u n d s . I n d e e d , a n o d i c waves of t h i o b a r b i t u r a t e s in 0.1 M s o d i u m h y d r o x i d e h a v e b e e n u s e d for t h e a n a l y s i s o f p h a r m a c e u t i c a l p r e p a r a t i o n s [4-7]. T h e p o l a r o g r a p h i c a n d v o l t a m m e t r i c b e h a v i o u r of t h i o b a r b i t u r i c a c i d d e r i v a t i v e s h a s b e e n i n v e s t i g a t e d extensively [2-7], f r o m b o t h t h e a n a l y t i c a l a n d t h e m e c h a n i s tic view. In the p r e s e n t p a p e r the a d s o r p t i o n p r o p e r t i e s o f T B A are s t u d i e d b y o u t - o f - p h a s e sensitive ac v o l t a m m e t r y . A f u r t h e r a i m of this s t u d y w a s to c o m p a r e the surface activities o f T B A ( d i h y d r o x y m e r c a p t o p y r i m i d i n e ) a n d t h e n u c l e i c acid base uracil ( d i h y d r o x y p y r i r a i d i n e ) . D u e to t h e b i o l o g i c a l s i g n i f i c a n c e o f b o t h c o m p o u n d s it is o f great interest to investigate h o w t h e i n t r o d u c t i o n of a n - S H g r o u p in the h y d r o x y p y r i m i d i n e s y s t e m will i n f l u e n c e its a d s o r p t i o n effects a t t h e m e r c u r y / s o l u t i o n interface. EXPERIMENTAL 2 - T h i o b a r b i t u r i c acid ( T B A , 4 , 6 - d i h y d r o x y - 2 - m e r c e . p t o p y r i m i d i n e ) w a s p u r c h a s e d f r o m Sigma a n d was u s e d w i t h o u t f u r t h e r p u r i f i c a t i o n . S o l u t i o n s c o n t a i n i n g d i f f e r ent c o n c e n t r a t i o n s of T B A were p r e p a : e d b y d i s s o l v i n g a k n o w n a m o u n t o f the c h e m i c a l l y p u r e p r o d u c t i n t o a d e f i n i t e v o l u m e o f B r i t t o n - R o b i n s o n buffer. B u f f e r s 0302-4598/9~/$03.50 © 1991 Elsevier Sequoia S.A. All rights reserved
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--I.2
E/V Fig. 1. A c capacitive curves of T B A at p H 3.2, 0.5 IV[ B r i t t o n - R o b i n s o n buffer w i t h N a C l at the HMDF_.. E|ectrode area 1.75 x 10 - 2 cm z, scan rate 2 m V s - l , frequency of ac potential 330 Hz, amplRude 10
mVpp,phase angle 90 0. waiting time
t s of 180 s (adsorption time) and 22°C. (1) pure buffer, (2) 0.99, (3) 1.49, (4) 2.49, (5) 3.48, (6) 4.48, (7) 5.47, (8) 6.45, (9) 11.7, (10) 15.7×10 - s M TBA.
w i t h a c o n s t a n t i o n i c s t r e n g t h of 0.5 M w i t h N a C I w e r e u s e d as s u p p o r t i n g electrolytes. All c h e m i c a l s were r e a g e n t grade. A P r i n c e t o n A p p l i e d R e s e a r c h ( P A R ) M o d e l 174 p o l a r o g r a p h i e a n a l y s e r c o u p l e d w i t h P A R M o d e l 1 7 4 / 5 0 ac p o l a r o g r a p h i c an&l.yser i n t e r f a c e a n d , a P A R M o d e l 5101 l o c k i n g a m p l i f i e r / p h a s e d e t e c t o r were e m p l o y e d for ac v o l t a m m e t r i c m e a s u r e m e n t s . T h e cell u s e d w a s a t h e r m o s t a t e d M e t r o h m cell w h i c h w a s e q u i p p e d w i t h a t h r e e - e l e c t r o d e system. A M e t r o h m H M D E , T y p e E-140, s e r v e d as w o r k i n g ' elect r o d e w h i l e a s a t u r a t e d c a l o m e l e l e c t r o d e w a s used as r e f e r e n c e e l e c t r o d e a n d a coiled p l a t i n u m w i r e as a u x i l i a r y electrode. All m e a s u r e m e n t s w e r e carri_'ecl o u t at 22°C. RESULTS AND
DISCUSSION
Phase-sensitive ac voltammograms of T B A in solutions of varying p H at the HMDE are shown in Figs. •1-3. At relatively low bulk concentrations of T B A the ac voltammograms indicate a progressive decrease of the capacitive ac signal around
361
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E/V Fig. 2. Ac capacitive curves of T B A at p H 7.3. (1) Pure buffer, (2) 0.99, (3) 2.49, (4) 3.98, (5..%5.47, (6) 6.95, (7) 8.43, (8) 9.41, (9) 9.9, (10) 13.8, (11) 18.6X10 - s M TBA° Other conditions as in Fig. 1.
the potential of m a x i m u m adsorption. This depression of the ac signal c o r r e s p o n d s to the progressive coverage of the m e r c u r y surface b y a dilute a d s o r p t i o n layer (stage I of adsorption). A t m o r e elevated bulk concentrations of T B A a clear capacitance pit is observed. T h e conclusions of Vetterl [8] a n d N f i r n b e r g a n d ~ co-workers [9] suggested that the capacitance pit reflects the association of the oriented molecules on the electrode surface b y inte, molecular a t t r a c t i o n forces a n d the formation of a c o m p a c t a d s o r p t i o n film (stage I I of adsorption). A t m o r e negative potentials, a r o u n d --0.8 V, the ac v o l t a m m o g r a m s of T B A exhibit a desorption p e a k corresponding to the desorption of less strongly b o u n d species existing in the adsorption film a t the negatively charged electrode surface. It a p p e a r s that the negatively charged electrode surface destabilizes the a d s o r p t i o n f'flm of neutral molecules ( p H < 7.3) due to repulsion between the negatively c h a r g e d surface a n d the ~r-electron system of the a d s o r b e d molecules. H o w e v e r the destabilization of the a d s o r p t i o n film in a l k a l i n e solutions is m o r e r e m a r k a b l e at less negative potentials b e c a u s e n o w the repulsion of the anionic fo~,~ o f the a d s o r b e d molecules b y the negatively charged electrode surface is m o r e p r o n o u n c e d .
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-0.8
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E/V Fig. 3. Ac c a p a c i t i v e c u r v e s o f T B A at p H 9.35. (1) P u r e b u f f e r , (2) 0.99, (3) 1.99; (4) 3.48, (5) 4.97, (6) 6.95, (7) 7.93, (8) 8.92, (9) 12.3, (10) 1 2 . 7 x 10 - s M T B A . O t h e r c o n d i t i o n s as in Fig. 1.
At p H >t 7.3 (TBA is fully anionic, pK~ = 7.3 [7]), the voltammograms s h o w sharp ac peaks at the positively charged electrode surface (Figs 2 and 3). These peaks are seen clearly when stage II of adsorption is reached. The frequency and phase angle dependence of the sharp peaks at ca. --0.1 V confirm the faradaic nature of these ac peaks. The appearance of such faradaic peaks is probably due to the electroreduction of the Hg(II) ion of the [ H g ( R S - ) 2 ] in the adsorption film according to the well k n o w n charge transfer mechanism [10] of the interaction between adsorbed R S - anions and the mercury electrode surface. The charge transfer reaction is represented by the following equations: at p o l e n t i a l s
H g ~lose to 0.0 v
Hg2++ 2 e-
2 R S H ~- 2 R S - + 2 H ÷
363
The overall reaction is Hg+ 2 RSH ~ Hg(RS)2 + 2 e-+ 2 H + The p H dependence of this p e a k confirms the latter overall reaction, whereas the peak is shifted, on a average by a b o u t 50 m V / p H , to m o r e negative potentials in the p H range 7.2-10.2. It is worth mentioning that TBA, like o t h e r s u l p h u r containing compounds, exhibits anodic polarographic waves at the m e r c u r y electrode surface corresponding to the formation o f slighdy soluble m e r c u r y c o m p o u n d s [2,6]; the H g ( I I ) ion of this c o m p o u n d undergoes cathodic reduction showing a well developed reduction p e a k [2,11]. A t p H >t 9.35, the faradaic p e a k at m o r e positive potentials splits into two consecutive peaks. The a p p e a r a n c e of these two v o l t a m m e t r i c p e a k s is n o t a n unusual observation. In his study [12], Florence a s s u m e d that the a p p e a r a n c e of the second peak was d u e to the change in the m o r p h o l o g y of the H g ( I I ) c o m p o u n d deposited on the electrode surface. Miller a n d T e v a [13] attributed the double p e a k
2.0
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.105c/M Fig. 4, Dependence of the capacitive current decrease ~ia= on the bulk concentration of T B A at the maximum adsorption potentials, and various p H values. (1) p H 3.2, (2) pH 5.2, (3) pH 7.3 and (4) p H 9.35.
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to the presence of b o t h Hg(I) a n d Hg(II) c o m p o u n d s in the a d s o r p t i o n film species Hg2(RS)2 a n d Hg(RS)2. C o m p a r i s o n of the Aiac (the decrease of the capacitive ac current with respect to the ia¢ of the supporting electrolyte for a given b u l k c o n c e n t r a t i o n ) versus c ( c o n c e n t r a t i o n of TBA) plot at different p H values a n d at the m a x i m u m a d s o r p t i o n potential, indicates a two-step a d s o r p t i o n isotherm (Fig. 4). The first stage reflects the dilute adsorption layer for re!ativeiy low bulk c o n c e n t r a t i o n s o f TBA. A t higher concentrations, o~e oblains the second stage, which is due to strong lateral interactions and high interaction coefficients between the a d s o r b e d molecules, giving rise to a c o m p a c t layer. It is concluded that at the m a x i m u m a d s o r p t i o n p o t e n t i a l the T B A molecules are oriented parallel to the electrode surface in a dilute layer w h e n the interaction of the or-electron system with the interface favours adsorption. W i t h an increase of the bulk c o n c e n t r a t i o n the stacking interactions between vertically oriented molecules lead to association a n d f o r m a t i o n of the c o m p a c t layer in the pit potential region. As shown in Fig. 4, the c o n c e n t r a t i o n o f T B A required for pit f o r m a t i o n in alkaline solution (pH >/7.3) is lower t h a n the c o r r e s p o n d i n g value in acidic solutions, showing that the surface activity of the anionic form ( R S - ) of T B A at the H g / s o l u t i o n interface is stronger t h a n that of the neutral form (RSH). It is possible that the electrostatic interaction of the a n i o n i c fotn~ of T B A ( R S - ) with the positively charged electrode surface e n h a n c e s the o r i e n t a t i o n process from the parallel to the vertical position and hence the f o r m a t i o n of a c o m p a c t a d s o r p t i o n film. The electrostatic interaction of the negatively charged sulphur a t o m with the positively charged electrode surface is s u p p o r t e d by the a n o d i c shift of the pit potential ragion as well as the m a x i m u m a d s o r p t i o n p o t e n t i a l u p o n increasing the p H of the solution from 3.2 to 10.2. In order to calculate the various p a r a m e t e r s of a d s o r p t i o n of T B A at different p H values, the equilibrium values o f the ac capacitive current at a given bulk c o n c e n t r a t i o n and at the m a x i m u m a d s o r p t i o n p o t e n t i a l were measured a n d the degree of coverage 0 was calculated using the relation
Co-C o= Co-Cm = (ai.o)m
(z) where the C ' s are the different capacitances in the s u p p o r t i n g electrolyte (Co), at a given bulk c o n c e n t r a t i o n of TBA ( C ) a n d at a b u l k c o n c e n t r a t i o n c o r r e s p o n d i n g to full coverage (Cm); Aiac is the decrease of the capacitive ac current for a given bulk c o n c e n t r a t i o n and (Ai.~c)m is the m a x i m a l decrease c o r r e s p o n d i n g to full coverage. T h e double step adsorption isothe~iii (Fig .4) observed at the m a x i m u m a d s o r p t i o n potential can be treated as c o m p o s e d of two isotherms [14]. The (Ai~=)~ values for stages I a n d i i of adsorption were t a k e n at the first a n d second p l a t e a u of the d o u b l e step isotherm, respectively. T h e results were fitted ~o several a d s o r p t i o n isotherms. F r o m a comparison of the experimental results with the theory it seems that the isotherm is of F r u m k i n type, given b y the equation: O(1
0)-' exp(--2a0)
=
tic
(2)
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10.5c/M Fig. 5. D e p e n d e n c e o f the surface coverage o n the bulk concentration o f T B A for the dilute adsorption layer (stage I) at the H D M E and at various p H values. (1) p H 3.2, (2) pH 5.2, (3) p H 7.3 and (4) p H 9.35.
w h e r e 0 is the degree of coverage, a the interaction coefficient, fl the a d s o r p t i o n coefficient and c the bulk c o n c e n t r a t i o n o f T B A . T o c o n f i r m that the e x p e r i m e n t a l results fit the F r u m k i n isothe,,r, well (Fig. 5), the logarithmic plot o f the linear eqn. (2) w a s subjected to a least-squares refinement; the values o f the regression coefficient o f the fit were found to lie in the range 0.992 to 0.996 at the various p H values. T h e interaction coefficient a w a s d e t e r m i n e d from the slope o f the l o g a r i t h m i c plot o f eqn. (2), and the a d s o r p t i o n coefficient fl from the values at half c o v e r a g e (_0----0.5). T h e free enerb~J o f a d s o r p t i o n ( - - A G ° ) v, as then calculated from the adsorption coefficient fl a c c o r d i n g to the e q u a t i o n
1
(
°)
(3~
366
C o m p a r i s o n of the a d s o r p t i o n p a r a m e t e r s of T B A at different p H values shows a significant dependence on the p H for b o t h stages (I a n d II) of adsorption. T h e surface activity and the degree of association expressed b y the value of the adsorption coefficient 13 increases with increasing p H (fl = 2.9 -i- 0.1 × 104 I / m o l at p H < 7.3 a n d 3.4 4- 0.1 x 104 1 / m o l at p H >i 7.3 for stage 1 of a d s o r p t i o n , fl = 3.3 a n d 3.9 x 1 0 3 i / m o l for stage II at p H < 7.3 a n d p H >~ 7.3, respectively). T h e lower interaction coefficient a at higher p H s ( a = 1.1 a n d 0.9 for stage I at p H s 3.2 a n d 9.3, respectively) indicates a m o d e r a t e lateral attractive i n t e r a c t i o n of the a n i o n i c form of the adsorbed molecules. In general the results of the a d s o r p t i o n of T B A ( d i h y d r o x y m e r c a p t o p y r i m i d i n e ) are in agreement with our previous results o b t a i n e d with uracil base ( d i h y d r o x y pyrimidine) [15] a n d support the theory that most p y r i m i d i n e derivatives exhibit two consecutive adsorption stages at the mercury surface. Moreover, the surface activity of T B A is significantly greater t h a n that of uracil, as reflected b y the h i g h e r values o f / ~ (by a factor of ~ 200) a n d - A G ~ (by a factor 1.5) c o m p a r e d with uracil at various p H values. This indicates a higher h y d r o p h o b i c i t y of T B A c o m p a r e d with uracil. It can be concluded that i n t r o d u c t i o n of an - S H group i n t o the p y r i m i d i n e system facilitates its electrostatic interaction with the mercury electrode surface and c o n s e q u e n t l y enhances the surface activity a n d the a d s o r p t i o n properties of the p y r i m i d i n e contair.ing thiol system at the charged mercury interface. REFERENCES 1 E. Miller, J.C. Munch, F.S. Crossely a n d W . H . H a r t u n g , J. A m . C h e m . Soc., 58 (1936) 1090. 2 C.A. Malresse-Ducarmois, G.J. Patriarche a n d J.L. V a n d e n b a l c k , Anal. Claim. Acta, 79 (1975) 69. 3 J. Rodriguez, L. C a i r o , C. M a t i n a n d A. Sanchez, J. Electroanal. C h e m . , 237 (1987) 105. 4 0 . M a n o u s e k a n d P. Z u m a n , Pharmazie, 11 (1956) 530. 5 W.F. Smyth, G. Svehla a n d P. Z u m a n , Anal. Chim, Acta, 51 (1970) 463. 6 W.F. Smyth, (3. SvehIa a n d P. Z u m a n , Anal. Chim. Acta, 52 (1970) 129. 7 W.F. Smyth, P. Z u m a n a n d G . Svehla, J. Electroanal. Chem., 30 (1971) 101. 8 V. Vetterl, Bioeleetrochem. Bioenerg., 3 (1976) 338. 9 D. Krznaric, P. Valenta a n d H . W . Nfirnberg, J. Electroanal. C h e m . , 65 (1975) 863. 10 M.T. Stankovich a n d A.J. Bard, J. Electroanai Chem., 75 (1977) 487. 11 C.A. Mairesse-Ducarmois, G.J. Patriarche a n d J.L. V a n d e n b a l e k , Anal. Chim. Acta, 71 (1974) 1 6 5 : 7 6 (I 975) 299. 12 T.M. Florence, J. Electroanal. Chem., 99 (1979) 219. 13 I.R. Miller a n d J. T e v a , J. Electroanal. Chem., 36 (1972) 157. 14 A.N. F r u m k i n a n d B.B. D a m a s k i n in J . O ' M . Bockris a n d B.E. C o n w a y (Eds.), M o d e r n Aspects o f Electrochemistry, Vol. 3, Butterworths, L o n d o n , 1964, p. I49. 15 Y.M. T e m e r k , M . M . K a m a l , Z.A. A h m e d a n d M.S. I b r a l ~ m , J. Electroanal. C h e m . , 260 (1989) 201.