The role of the depolarizer adsorption on the polarographic behaviour of isothionicotinamide

J. Electroanal. Chem., 73 (1976) 219--233 © Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands

219

THE ROLE OF THE DEPOLARIZER ADSORPTION ON THE POLAROGRAPHIC BEHAVIOUR OF ISOTHIONICOTINAMIDE

S. MUSUMECI, I. FRAGAL.£,, S. SAMMARTANO and R. MAGGIORE Istituto di Chimica Generale, Universit5 di Catania, Viale A. Doria 8, 95125 Catania (Italy) (Received 13th October 1975; in revised form 19th January 1976)

ABSTRACT The polarographic behaviour of isothionicotinamide in aqueous-organic system and at various values of pH has been studied. The reaction pattern has been explained on the basis of the protolytic equilibria and surface activity of the depolarizer.

INTRODUCTION

The polarographic behaviour of the heterocyclic thioamides has been investigated [ 1--5], but the literature data give information on the electroreduction stoichiometry only; no conclusion has been drawn as regards the mechanism of the electrode processes. On the other hand, the heteroatoms present in such molecules can influence the surface activity on the Hg cathode and can affect protolytic equilibria with the proton donors present in the solutions. The aim of this work is to investigate the polarographic behaviour of isothionicotinamide in aqueous-organic systems with different composition. The effect of the solution composition has been shown to be a powerful tool to elucidate the relations between polarographic behaviour and the structure of organic compounds [6]. Isothionicotinamide has a pharmacological interest owing to its antitubercular activity [ 7 ]. EXPERIMENTAL

Isothionicotinamide (TINA), prepared according to Meltzer et al. [8], has been crystallized from ethanol (m.p. 140°C). Reagent grade products (C. Erba, Merck), recrystallized when necessary, have been used. Twice distilled water, dioxane purified according to Vogel [9] and acetonitrile purified by Coetzee's [10] procedure, were used for the solutions. The ionic strength, except when expressly indicated, was kept to 1 mol 1-1 by adding KC1. The solutions, containing 33% of dioxane except when indicated, were buffered by the following systems: pH 2.0--8.0 (McIlvaine) [11]; 8.2--10.3 (Michaelis: NH4C1 0.2 M,

220

NH3 0.2 M); 9.2--11.2 (Michaelis: Na2B407 0.1 M, NaOH 0.2 M). In these systems, the concentration of the undissociated acid or base was more ~han 10 - 2 M. The pH values of the mixed systems were checked by a PW 9408 Philips digital pH-meter. They must be considered as operational values, as proposed by Bates [12]. An AMEL 557/SU potentiostat, equipped with an AMEL 463 programmer unity and a Metrohm 351 Polarecord, was used in d.c. voltammetry. An AMEL 448 multifunction plug-in system was used in linear potential sweep and chronoamperometric measurements. Sargent capillaries (5--12 s), equipped with an electromechanic hammer, were used for drop-times longer than 1 s ( m 213t 116 = 1.26; h = 30 cm; t = 1 s; cap. A). Shorter drop-times were obtained with a Metrohm E 354 S apparatus ( m 2/3_ t 116 = 1.49; h = 30 cm; t = 1 s; cap. B). All the capillary constants were measured in de-oxygenated water at open circuit and at 25 + 0.1°C. Drop-time measurements were performed with an electronic digital timer [13]. The potentials were always referred to an internal saturated calomel electrode. All the solutions, de-oxygenated with 99.99% pure nitrogen, were kept in such an atmosphere during all the measurements, performed in pyrex cells at 25 + 0.1°C. RESULTS

Adsorption tests In 1 M KC1 aqueous solution, TINA adsorbs on the Hg cathode starting from --0.100 V up to --1.200 V (Fig. lc). More positive potentials were n o t

t/s

1

12.5

H ~se~

5

10.5

10.5 i

i

I

0.1

0.5

0.9

0.1

0.5

0.9

0.1 0.5 0.9 -E,/V vs S.C.E.

Fig. 1. D r o p - t i m e curves o f T I N A in various s y s t e m s at 25°C. (a) 1 M HC1, d i o x a n e 33%; (b) McIlvaine's b u f f e r , p H c = 6.8, d i o x a n e 33%; (c) 1 M KC1, d i o x a n e 0%; (d) c h r o n o a m p e r o m e t r i c curve r e c o r d e d at - - 1 . 1 5 V. CTINA : (O) 0.0 M; (+) 2.5 X 10 - 4 M ; (A) 1.0 X 10 - 3 M.

221

investigated. Therefore the dioxane was chosen as organic solvent in the aqueous-organic mixtures owing to its adsorbability in the same potential range [14]. In Fig. 1, drop-time curves for aqueous-organic systems (33% dioxane) are also reported at two different hydrogen ion concentrations. In 1 M HC1 system (Fig. la), TINA adsorbs starting from --0.050 V up to --1.300 V and, taking into account the two reduction processes undergone by the depolarizer, in the same potential range, we can conclude t h a t the reduction products are also adsorbed. At pH 6.6, TINA adsorbs moderately near 0.200-0.300 V and then desorbs up to --1.00 V (Fig. lb). This potential corresponds to the polarographic step and the adsorption detected at more cathodic potentials can be attributed to the reduction of the products. Owing to the different adsorbability of the depolarizer and the reduction product, inhibited polarographic processes can be supposed. In this system, chronoamperometric curves, recorded at this potential, are indicative of an inhibition (Fig. l d ) [15]. In more acidic media, i vs. t curves show a normal behaviour. Further evidence of some inhibition was observed in 1 M HC1 (33% dioxane) at high depolarizer concentration and at low temperature (10°C). Some irregularities on the i vs. t curves were observed in these conditions during the drop life.

Polarographic data In acidic media, TINA undergoes reduction on the DME giving two waves of approximately equal height. Before pH 2, at more cathodic potentials, a third wave appears. The characteristics of this latter wave suggest a catalytic process, as found, on the other hand, in the polarographic behaviour of pyridine derivatives [16]. This wave has n o t been studied in detail in this work. (E1/2) 1 and (E1/2)2, in 1 M HC1 (33% dioxane), depend linearly on the logarithm of the drop time with a slope respectively of 26.2 mV and 32.8 mV. In the pH range 3.4--5, the second wave decreases to zero, while the first one increases to a value which keeps constant up to pH 8.5. The total limiting current (il + i2) equals the m a x i m u m value achieved by the first wave before pH 5 (Fig. 2). The second wave, starting from pH 3.4, was followed by a sudden increase in the limiting current in the form of a m a x i m u m of the second kind. On increase of t h e pH value, this wave merges with the m a x i m u m so that it was impossible to detect the wave. By adding an electroinactive surfactant such as Triton X 100, the second wave can be detected up to pH 6. In tl:ie pH-range where the second wave can be well detected, the added Triton does not affect the parameters of this wave. In the more alkaline region, the height of the unique wave tends to the same value achieved by the first wave in acidic media. The current constants, as calculated for the two waves in 1 M HC1 and for the unique wave at pH 5.4, correspond respectively to a bi-electronic and to a four-electronic process, taking into account the effect of dioxane on the depolarizer diffusion constant [17]. The pH dependence of the half-wave potentials of the first two waves are also shown in Fig. 2. The general pattern closely resembles that described by

222 -E/V

vs S.C.E.

1.2

1,0 ~

0.8

~-~

o

®

a

0.6

1

3

5

7

9

11

pH

13

Fig. 2. Dependence of E l l 2 and i values on the pH for the first and second wave in various systems. CTINA = 5 X 10 _ 4 M; t = 25 C. (o) (E1/2)l, dioxane 33%, cap. B; (o) (E1/2)l, dioxane 33%, cap. A; (A) (E1/2)1, dioxane 5%, cap. A; (t)) (El/2)2, dioxane 33%, cap. B; (D) i l , dioxane 33%, cap. B; (m) i2, dioxane 33%, cap. B.

Lund [4], but it differs by the sigmoidal shape of the first wave E1/2--pH plot. The diffusive control of the polarographic waves on the whole pH range was checked through the limiting current dependences on the square root of the height of the Hg-reservoir and through the slopes of the straight lines obt a i n e d b y p l o t t i n g l o g i vs. l o g t. T h e l o g a r i t h m i c a n a l y s i s i n d i c a t e s n o r e v e r s i b l e p r o c e s s e s in t h e w h o l e p H r a n g e . T y p i c a l d a t a c o n c e r n i n g t h e w a v e s s t u d i e d a t v a r i o u s p H v a l u e s a r e r e p o r t e d i n T a b l e 1.

TABLE 1 b = d Ei/d log//(id - - i ) ; B = d log i/d t; C = 2 . 3 / A T log (i)w2[(i)w 1 ; CTINA

=

5 X 10 - 4 M;

dioxane 33%; t = 25°C; cap. B pH

--(E1/2) 1 /V(SCE)

--bl/mV

B

C /% °C - 1

--(El/2) 2 /V(SCE)

--b 2 /mV'

B

C /% °C - 1

1 M HC1 3.4 5.0 5.7 8.5 10.7 12.3

0.395 0.545 0.670 0.730 0.950 1.070 1.140

38.2 36.3 38.1 33.0 34.8 41.3 43.2

0.22 0.24 0.25 0.26 0.38 0.40 0.38

1.52 2.3 -2.5 . 1.8 .

0.80 0.83 0.97 1.03 . . . .

72.3 68.8 72.5 73.3 . . . .

0.21 0.26 0.23 0.28

1.24 2.0 ---

. .

. .

223

Effect o f the depolarizer concentration, o f the buffer capacity and of the ionic strength In the pH range 0.5--3 (i 1 = i2) , the currents of the t w o waves d e p e n d linearly on the dePolarizer c o n c e n t r a t i o n in a wide range (2.25 X 1 0 - 5 - - 8 . 3 5 X 10 - 3 M). On increase of the depolarizer concent rat i on, (E 1/2)2 values stay constant, while (El/2) 1 values shift to more anodic potentials. T he slopes of t he semilogarithmic plots of b o t h waves increase progressively (Table 2). T he temperature coefficients, as measured in 1 M HC1 systems, were 1.52%/°C for the first wave and 1.24%/°C for the second one. In phthalic acid + potassium phthalate buffer systems (Cac/Csalt ---- 1; pHc = 3.6), the undissociated acid c o n c e n t r a t i o n does n o t influence the E 1/2 or the b values of b o t h waves (Table 6). In less acidic media (i 1 ~ i2) , the limiting current of the unique wave depends linearly on the depolarizer concentration. The effects on the E1/2 and b values can be evaluated only by taking into account t hat these parameters depend properly on the ratio of the depolarizer c o n c e n t r a t i o n to the undissociated acid concentration o f the buf f er system. In acetic buffer (pH c = 5.4; Cac/Csalt = 1; ca¢ = 5 X 10 - 3 M) by increasing the depolarizer concent rat i on, the E 1/2 value shifts cathodically while a progressive drawing out o f the wave appears. T h e polarograms for two concentrations are r e p o r t e d in Fig. 3. At very high depolarizer c o n c e n t r a t i o n ( > 5 X 10 - 4 M), when Cac was less than 5 X 10 - 2 M, t he limiting cu r r en t shows a sudden increase near the wave t op (Fig. 3). This m a x i m u m was observed by operating with drop-times shorter than 1 s. For longer droptimes, the c h r o n o a m p e r o m e t r i c curves suggest the presence of an inhibition. On increasing Cac, Ell 2 shifts positively, while the slope of the wave lowers. This slope does n o t de pe nd on the drop-time (Table 3). The semilogarithmic plot o f the wave, for a given Cac, deviates f r o m linearity by increasing the depolarizer concentration. Higher temperatures give a more p r o n o u n c e d deviation (Fig. 4), while the slope of the initial p o r t i o n o f the semilogarithmic plot becomes larger and the E 1/2 value shifts cathodically (Table 4). The temperature coefficients also depend on the depolarizer concent rat i on. Typical data are r e p o r t e d also in Table 4. By varying KC1 c o n c e n t r a t i o n in the range 0.1--1M, the half-wave potential of the t w o waves in 0.1 M HC1 (33% dioxane) changes,

TABLE 2 1 M HC1; dioxane 33%; t = 25°C; cap. A CTINA/mOl 1-1

--(E1/2)I]V(SCE )

--bl/mV

--(E1/2)2/V(SCE)

--b2]mV

2.5 3.5 5.0 5.0 5.0 8.3

0.405 0.400 0.395 0.390 0.390 0.380

34.3 36.6 38.3 36.8 41.7 43.8

0.840 0.830 0.830 0.835 0.830 0.840

49.8 52.1 60.4 59.1 64.4 72.8

x x x x x x

10- 5 10- 5 10_5 10_4 10- 3 10- 3

224

O f

f

I 0.7

0.8

0.I/uA

o.9 to E/V (S.C.E.)

~.~

Fig. 3. Polarograms concerning acetate buffer. Cac = 5 X 1 0 - 3 M; p H c = 5 . 4 ; d i o x a n e 33%; t = 25°C;cap. A. CTINA: (a) 5 X 10 - 5 M ; ( b ) 1.16 X 10 - 3 M.

w h i l e t h e El~ 2 o f t h e u n i q u e w a v e i n a c e t a t e b u f f e r ( 3 3 % d i o x a n e ) k e e p s r o u g h l y c o n s t a n t (Fig. 6).

Effects o f the composition o f aqueous-organic systems and o f the added inactive surfactant B y i n c r e a s i n g t h e d i o x a n e p e r c e n t a g e ( 5 - - 6 5 % ) in 1 M HC1 s y s t e m s , a c h a n g e

TABLE 3 CTINA

=

5 × 10 - 4 M; dioxane 33%; t = 25°C; cap. A. Drop-times are given in s

Cac/mol 1-1

--El~ 2

i/pA

/V(SCE) 100 75 50 25 10 7.5 5

x x x x x x ×

10 - 3 10 - 3 10 - 3 10 - 3 10 - 3 10 _ 3 10 _ 3

0.746 0.751 0.744 0.756 0.770 0.783 0.782

3.7 3.8 3.8 3.75 3.65 3.8 3.8

--bt=1

--bt=2

--bt=3

--bt=4

--bt=5

/mV

/mV

/mV

/mV

/mV

27.1 30.0 27.2 30.3 35.9 38.7 45.1

. . 29.3 28.7 37.3 35.6 44.3

. . 28.2 31.6 34.4 38.1 43.3

. . 32.5 32.9 37.4 38.4 43.5

34.2 34.3 36.5 37.3 45.3

. .

225 - E / V (S£.E3 900

85(

80(

75(

70C

-0.5

0

0.5

log i/id-i

I

Fig. 4. S e m i l o g a r i t h m i c plots o f the polarographie waves in acetate buffer s y s t e m s , p H c = 5 . 4 ; Cac ~ 5 X 1 0 - 3 M ; c a p . A . (m) CTINA = 5 X 1 0 - 4 M , t = 6 . 5 C; ( o ) CTINA = 5 X 1 0 - 4 M , t = 3 4 . 3 C; (•) CTINA = 1 . 1 6 X 1 0 - - ~ M , t = 6 . 5 ° C ; ( © ) CTINA = 1 . 1 6 X 1 0 . 3 M , t = 3 8 . 8 C; ( X ) Cac = 1 X 1 0 - 2 M, CTINA = 5 X 10 - 4 M, t = 2 5 C.

in the morphology of the first wave was observed (Fig. 5). This tendency parallels a decrease in the current, an increase of the wave slope and a shift of the E1/2 value to more negative potentials (Table 5). In acetate buffer containing a small amount of dioxane (5%, pHc = 4.8), in respect to the system containing 33%, a dramatic positive shift ( 2 200 mV)

i/)JAi 0.04 V

0.25, V

- E / V S.C.E.

Fig. 5. I n f l u e n c e o f the d i o x a n e percentage o n the first wave m o r p h o l o g y . 1 M H C I ; CTINA = 5 X 1 0 - 4 M ; t = 2 5 ° C ; c a p . A . D i o x a n e : (a) 5 % ; ( b ) ( 3 3 % ; (c) 6 6 % .

226 TABLE 4 Cac = 5 X 10 - 4 M; d i o x a n e 33%; t = 25°C; CTINA: ( a ) 1.16 x 10 - 3 M; (b) 5 x 10 - 4 M a

b

t/°C

--E1/2/V(SCE )

--b/mV

t/°C

--E1/2/V(SCE )

--b/mV

6.5 11 15 20 25 31.5 34.8

0.770 0.780 0.790 0.800 0.805 0.815 0.820 --

57.2 63.2 68.8 74.1 79.0 80.3 81.0 --

6.5 10.5 15.5 20 25 27 29.5 34.3

0.750 0.760 0.765 0.775 0.780 0.790 0.790 0.795

38.7 38.4 39.6 44.6 45.3 45.6 46.3 49.9

--

Current temperature coefficient: 2.6%/° C

Current temperature coefficient: 2.8%/° C

~/mv 0.82 O.8O

40

O.78

20

0.76

0

0.74 y

0.46 0.44

J

i

-1

i

-0.5

i

0 tog (p/mot [-~)

Fig. 6. D e p e n d e n c e o f t h e half-wave p o t e n t i a l s a n d o f t h e d o u b l e l a y e r p o t e n t i a l d r o p o n t h e KC1 c o n c e n t r a t i o n . CTINA = 5 X 10 - 4 M, t = 25°C, d i o x a n e 33%, cap. A. (o) A ~ I ; (o) A ( E 1 / 2 ) 2 , 0.1 M HC1; (~) A ( E 1 / 2 ) l , a c e t a t e b u f f e r , p H c = 5.4; (0) A ( E 1 / 2 ) 2 , 0.1 M HC1.

TABLE 5 CTINA -- 5 X 10 - 4 M; t = 2 5 ° C ; cap. A

System

il /PA

1 M H C l / d i o x a n e 5% 1 M HC1/dioxane 33% 1 M HC1/dioxane 66%

2.28 1.8 1.44

i2/PA 2.28 1.72 , ~1.44

--bl/mV

--b2/mV

(--E1/2 )I /V

(--El/2)2/V

24.5 35.4 44.6

67.4 56.1 47.8

0.34 0.39 0.42

0.78 0.83 0.83

;>

~ . o~

I---

I---

I---

ol L~3

..~

0 0 ~ 0 0

IIu

T~

I---

I--. C~ ~J

G

~

"~

~ o

II

'-~

'

0

T

x

Im .<

°

IIIII

228 of the wave was observed, the wave assuming a characteristic peak shape. Typical data concerning the two buffer systems are reported in Table 6. The EI/2--pH plot for the first wave, in the pH range 0.5--7 (5% dioxane), gave a straight line with a slope of 82.5 mV/pH unit. No modification was observed in the pH dependence of the wave currents. In phthalate buffers (5% dioxane, pile = 2.8), while the il/i2 ratio has the same value as in the 33% system, the half-wave potentials shift positively. In 5% dioxane systems, the (E 1/e)l value depends slightly on the undissociated acid concentration. Typical data, including the slope of the waves are reported in Table 6. In linear sweep voltammetry in aprotic medium (CHaCN), two waves are observed, with ( E p ) 1 = --1.43 V and ( E p ) 2 = --2.53 V. The first wave appears to be reversible. The addition of an electroinactive surfactant (Triton X 100), in both 5% and 33% dioxane systems, generally causes the suppression of irregularities on the wave morphology and a negative shift of the E1/2 values, together with an increase of the wave slope. These effects were more marked for the first wave. Moreover, they do n o t depend on the undissociated acid concentration, in 33% acetate systems (Table 7). DISCUSSION

All the experimental data suggest a dependence of the reduction path on the hydrogen ion concentration. Protolytic equilibria involved on the whole pH range can be written as: NCsH4C n

--~ ~3

S)K,(HN0,H,0

(NO,H,<

NH2 H+

NH2

~1

NH 2

-+ ~2

H÷# K2 HNCsH4 C

where pK1 ~- 1.5--2 taking into account the reported value for isomeric thiopicolinamide [18] and pK2 certainly lies on the negative H0 range. The E 1/2 vs. pH curves for the first and the second wave, together with the irreversibility of the electron transfer, suggest a depolarizer preprotonation preceding a slow reduction step. In the more acidic pH range, where i 2 = i l , both oxidized

229

TABLE 7 5 X 10- 4 M; dioxane 33%; t = 25°C; pHc = 5.4 (acetate buffer); Cac: (a) 5 × 10-3 M, (b) 7.5 × 10- 3 M; cap. A

CTINA

=

Triton (2%)/ml

0.4 0.8 1.0 1.6

a

b

(--E1/2)l/V

--bl/mV

(--E1/2)I/V

0.805 0.815 0.825 0.835

58.8 70.3 76.1 79.0

0.795 0.805 0.815 0.825

"_bl/mV 58.5 58.8 67.3 73.2

and reduced forms adsorb on the Hg cathode. The i vs. t curves show a normal shape, as previously described. The (El/2)1 values, close to EM, p r o m o t e high coverage on the electrode surface. The observed shift of the (El/2) 1 toward more positive potentials, by increasing the depolarizer concentration, indicates an adsorption following Frumkin's S-shaped isotherm. The increase of the effective fi0 value, paralleling the depolarizer concentration, shifts the E 1/2 value according to the equation: b' (

El/2 - b b__ b log

0.81 k'el tl]2

D 112

) + log/30 l ~ + 0.43 a~p2/2

[191

where the symbolism is the conventional one. The (El/2)Jlog t value, which is substantially higher than one half the slope of the semilogarithmic plot, agrees with such an isotherm type, as found, on the other hand, in the reduction of fi-iodopropionitrile [20]. The shape and the irregularities observed on the i vs. t curves in 1 M HC1, at high depolarizer concentration and at low temperature, suggest the presence of an inhibiting depolarizer film, due to higher coverage. This film is broken during the drop life. Similarly, the morphology of the wave, in 5% dioxane 1 M HC1 systems, can be explained. The sudden increase of the current to values higher than the diffusional limit can be connected with a removal of the inhibition due to a more favorable orientation of the particles in the adsorbed state. The positive shift of the (El/2)1 values and the negative shift of the E M lower the surface coverage so that a reorientation of the depolarizer becomes possible at the discharge potential [21]. The equilibrium is displaced on the electrode surface toward the protonated species also for pH values far from its pK1. The surface nature o f the preprotonation and the double layer effect on the effective pH0 [22] value near the electrode surface can explain this effect. Quasi-diffusive surface waves have been observed even at pH values higher by 7--8 units than the actual pKa value [23]. The following scheme can de-

230 scribe the reduction path for the first step (all species are assumed in the adsorbed state): ,~/~-~

UN+

.C

/(+) + e ~ H N

HN ~

C

/<')

*

/~S

C']

H+

/ (.)

('r) + H+

( 1)

(2)

(3)

pK=lO

(4)

\NH2.j

i- H ~ F I N ~ ~

CH / \NH,J

(5)

The second step involves protonation of the first step product and then a reductive breaking of the C--S or C--N bond. The data concerning this step agree well in the whole pH range with the pattern for quasi-diffusive waves. This step is also surface in nature and the value of d(El/2)2/dpH higher than the value of semilogarithmic plot of the second wave can be connected with the influence of the organic solvent. The dependence of (E1/2)2 on the ionic strength in 0.1 M HC1 (33% dioxane), when compared to the potential drop in the diffusion part of the double layer (Fig. 6), calculated through the relation [24] : ~1 ~ --0.06 +

(.RT/F) In c -- (2RT/F) In ~a

where c is the concentration of the 1--1 valent electrolyte (mol 1-1) and ~a is the potential referred to the E M value, suggests that the depolarizer carries a positive charge. For the ion discharge, the Ell 2 value can be related to the A@I value by the relation:

AEll2 ~ A~I ((~na--Z)/(0~na) Calculation of the z value run for the aforesaid system gives a charge near +2 for the particle originating the second wave. Similar calculation cannot be done for the first wave because the (El/2)1 lies close to the EM.

231 Moreover, the variation of the (El/2)1 with KC1 concentration (Fig. 6) also suggests a positive charge for the depolarizer. The fact that A(El/2)1 is smaller than A(E1/2)2 could be concerned with an anodic shift of the (E1/2)1 caused by the change of the pH 0 by double layer effect. The sigmoidal shape of the (EI/2)I--PH plot, together with the fact that TINA can accept the first electron even in an aprotic medium, suggests that El/2 of such quasi-diffusive surface waves is n o t determined only by the kinetics of the antecedent protonation, b u t also by the protolytic equilibrium, by which the concentration of the electrochemically active species is decreased owing to the dissociation. This behaviour was observed in the polarography of nitro-compounds [25]. In phthalate buffer (33% dioxane, pHc = 3.7), the (El/2) 1 value remains constant by varying the undissociated acid concentration and does n o t depend on the buffer nature. Therefore, the diprotonated species are reduced at the electrode and the equilibrium constant K1 controls the E1/2 value. When the dioxane content is lowered, the (El/2)1 value shifts positively, while EM shifts toward more negative potentials. These combined effects lower the surface coverage, so that the rate of the surface preprotonation decreases. In phthalate buffer (5% dioxane, pile = 2.8), (El/2)1 depends on the undissociated acid concentration (Table 6) and on the buffer nature. The (E112h-pH plot loses its sigmoidal shape and this fact can probably be explained. The lower dioxane a m o u n t also decreases its competition with TINA in the adsorption. The change in the wave slope with the dioxane addition agrees well with this assumption. In the pH range where il > i2, the dependence of (ElI2)1 on the undissociated acid concentration observed in acetate buffer (33% dioxane, pile = 5.5), together with the shape of the (EI/2)I--pH plot, suggests that both monoprotonated and diprotonated forms are reduced in the first polarographic step. The ratio of both species in the electrode layer affects the half-wave potential. This ratio depends on the protonation kinetic. For the m o n o p r o t o n a t e d particle the main reductive process can be written:

//N.2

//-..2j

(6)

followed b y protonation (reversed reaction (3) which is fast in this pH-range) and reduction of the radical in reaction (4); in this pH range, the (El/2)1 values also depend on the buffer nature (Table 7). The decre&se of the second wave, whereas the first one increases, observed as the hydrogen ion concentration is lowered, can be related to an increase of the m o n o p r o t o n a t e d particles, whose reduction involves a tetra-electronic process, due to the instability o f the intermediate product formed, in accordance with the mechanism proposed for the more acidic systems, after the transfer of the first two electrons. S-shaped

232

Frumkin's isotherms are present also in these systems. The deviation from the linearity, observed in the semilogarithmic plot (Fig. 4), can be explained in terms of smaller coverage due to the increase of the potential. This effect, observed also b y increasing the temperature, can probably be explained similarly. Therefore, the aforesaid protonation is a surface process and the decrease in the wave slope, on lowering the temperature and increasing the depolarizer concentration, depends on the amount of the adsorbed depolarizer. The fact that adsorption of TINA plays a role in its reduction is shown by the unusual change of (El/2)1 with the temperature. The values of the ratio AEx 1 2 / A t ° are generally positive, while, for TINA in acetate buffer (dioxane 33%, pH c = 5.6), A E 1/2/At ° is negative (Table 4). This fact can be explained by TINA desorption at higher temperature and by the decrease of the protonation rate. Similar effects are even observed by adding surface active c o m p o u n d s (Triton X 100) (Table 7). In the more alkaline pH range, when il lowers to the value corresponding to a bi-electronic process, a different reduction pattern becomes operating. This effect may depend on a different orientation of the molecule, in the adsorbed state, as observed for other pyridine derivatives [26], but the deprotonation equilibrium of the thioamide group (pK5 = 11.58) [27] could also play a role. On the other hand, Lund [4] proposed the formation of a dihydro derivative of TINA, in this pH range. In conclusion, this latter process is worth of a more detailed investigation. ACKNOWLEDGEMENT

The National Research Council (C.N.R., Italy) is gratefully acknowledged for a research grant.

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233 14 R.G. Barradas and P.G. Hamilton, Can. J. Chem., 43 (1965) 2468. 15 E. Laviron, Bull. Soc. Chim. Fr., (1962) 418. 16 S.G. Mairanovskii, Catalytic and Kinetic Waves in Polarography, Plenum Press, New York, 1968, p. 241. 17 J.L. Walter and M. Rosalie, Anal. Chem., 37 (1965) 45. 18 J.C. Poriand, J. Dumas and R. Mauger, Bull. Soc. Chim. Fr., (1969) 743. 19 S.G. Mairanovskii, Catalytic and Kinetic Waves in Polarography, Plenum Press, New York, 1968, p. 212. 20 N.G. Feoktistov and S.I. Zhdenov, Izv. Akad. Nauk. SSSR, Otd. Khim. Nauk., 12 (1962) 2127. 21 S.G. Mairanovskii, Catalytic and Kinetic Waves in Polarography, Plenum Press, New York, 1968, p. 107. 22 S.G. Mairanovskii, J. Electroanal. Chem., 4 (1962) 166. 23 S.G. Mairanovskii, N.V. Barashkova and F.D. Aleshev, Zh. Fiz. Khim., 36 (1962) 562. 24 S.G. Mairanovskii, Catalytic and Kinetic Waves in Polarography, Plenum Press, New York, 1968, p. 157. 25 P.J. Elving, Pure Appl. Chem., 7 (1963) 423. 26 J. Volke and A.M. Kardos, Collect. Czech. Chem. Commun., 33 (1968) 2560. 27 C. Tissier and M. Tissier, Bull. Chem. Soc. Ft., (1970) 3752.