Polarography of polycyclic aromatic hydrocarbons

ELECTROANALYTICAL CHEMISTRY AND INTERFACIAL ELECTROCHEMISTRY Elsevier Sequoia S.A., L a u s a n n e - Printed in The N e t h e r l a n d s

159

P O L A R O G R A P H Y OF POLYCYCLIC AROMATIC HYDROCARBONS III. T H E ENHANCING EFFECT OF VARIOUS HYDROCARBONS ON T H E POLAROGRAPHIC MAXIMUM OF METHYL-p-BENZOQUINONE

KIYOKO TAKAMURA

Tokyo College of Pharmacy, Uenosakuragi, Taito-ku, Tokyo (Japan) TSUTOMU TAKAMURA

Central Research Laboratory, Tokyo Shibaura (TOSHIBA) Electric Co., Ltd., Komukai, Kawasaki (Japan) (Received December 28th, 1967)

INTRODUCTION

I t has previously been reported1 that anthracene enhances the polarographic m a x i m u m of methyl-p-benzoquinone (I) in 2-methoxyethanol solution. This is contrary to the generally accepted view that polycyclic aromatic hydrocarbons behave as m a x i m u m suppressors 2. The enhancing effect of anthracene was explained on the basis of YON STACKELBERG'Stheory 3 by assuming the charge-transfer complex between (I) and anthracene to be adsorbed strongly on the electrode surface. Such an enhancement was also found for other polycyclic aromatic hydrocarbons and was particularly marked on the addition of compounds of higher ring number. Polycyclic aromatic hydrocarbons, especially those having higher ring number, adsorb strongly on the surface of the mercury electrode in the positive region of the electrocapillary curve 4. Adsorption of such compounds on the electrode surface lowers the surface tension of the mercury-solution interface. In the present paper, the problem of whether or not the adsorption of hydrocarbons on the mercury surface is an important step in the maximum-enhancing phenomenon, is discussed polarographically in an a t t e m p t to support the theories put forward in the previous paper. EXPERIMENTAL

The polarographic measurements were carried out at 25 ° with the apparatus previously described 1. (I) and 2-methoxyethanol were purified as described earlier 1. o.25 M lithium chloride was usually used as a supporting electrolyte. Aromatic hydrocarbons of guaranteed reagent-grade were further purified according to the following procedures. Benzene: distilled at atmospheric pressure, and the distillate (8o.o,-8o.2 °) collected. Naphthalene: purified by repeated sublimation. Phenanthrene and pyrene: recrystallized repeatedly from ethanol. Perylene and 1,2-benzanthracene were used without further purification and anthracene was the same reagent as in the previous experiment 1. j . Electroanal. Chem., 18 (1968) 159-164

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RESULTS AND DISCUSSION

The polarogram obtained with a freshly-prepared solution of (I) showed a single reduction wave having a typical m a x i m u m 1. The half-wave potential was about - o . 1 7 V v s . the mercury pool electrode. Most aromatic hydrocarbons used in the present experiment did not give the reduction waves in the potential range investigated because of their highly negative reduction potentials. The enhancing effect on the m a x i m u m height of (I) was examined for benzene, naphthalene, anthracene, phenanthrene, pyrene, perylene and 1,2-benzanthracene and all showed an enhancing effect. The effect was more marked for the compounds of higher ring number. Gradual increase of hydrocarbon concentration caused the m a x i m u m to become higher and wider up to a limiting value. The addition of hydrocarbon scarcely affected the diffusion current of (I). These results were similar to those obtained in the (I)-anthracene system 1. In order to compare the enhancement produced b y the various hydrocarbons, the heights of maxima at a fixed concentration of (I) were measured for solutions containing the same amounts of hydrocarbons. The results are shown in Fig. I. The addition of 6.8 m M benzene to the solution barely enhanced the m a x i m u m of (I), but in the presence of the same concentration of naphthalene, pyrene etc., a distinct enhancement was observed as seen in Fig. I. In the case of perylene, although the concentration was lower than that of other additives because of its low solubility in the solvent, the enhancing effect was clearly apparent. 24

I

I

I

I

I

o 0

20

d

16

o

E

12

O

CD 8

4

i

0

•

o 1

T

1

I 2 3 Number of ring

I

I

4

5

Fig. I. Comparison of t h e e n h a n c e m e n t of the m a x i m u m of (I) reduction w a v e b y v a r i o u s h y d r o c a r b o n s at 25 °. Max. heights (/max) were obtained in t h e solns, containing i.z- lO -3 M (I) a n d 6.8. lO -3 M h y d r o c a r b o n (perylene, 3.4" io-3 M, nearly satd. soln). (O), in t h e absence of h y d r o c a r b o n .

As described in the previous report 1, the enhancing effect of hydrocarbons can also be explained on the basis of YON STACKELBERG'Stheory. The m a x i m u m of the first kind in the positive branch of the electrocapillary curve is caused b y the increased rate of supply of electro-reducible substances to the electrode, as a result of the streaming brought about b y the difference in the interfacial tension between the neck and the base of the mercury drop. The streaming of the solution around the mercury drop could j . Electroanal. Chem., 18 (1968) 159-164

EFFECT OF HYDROCARBONS ON POLAROGRAPHY OF METHYL@-BENZOQUINONE

161

be detected by observing the flow of talc powder suspended in the solution. The addition of hydrocarbons to the solution caused the positive branch of the eleetrocapillary curve to become steeper than that of base electrolyte solution. The steeper the eleetroeapillary curve, the larger the streaming induced and, consequently, the higher the maximum obtained. The relation between the maximum height and the amount of adsorption will be treated in some detail. At present, for simplicity, it is assumed that only the hydrocarbon adsorbs strongly on the mercury surface. In the presence of specific adsorption of anion, the hydrocarbon adsorbs through a competitive process, but the following conclusion is not altered seriously if the adsorptive force of the anion is weaker than that of the hydrocarbon and if we consider only the effective maximum height brought about solely by the hydrocarbon. If the rate of charge transfer is very fast, the maximum current,/max, is proportional to the rate of supply of electro-reducible substance to the electrode. The rate of supply is proportional to the streaming velocity 3, v, which is brought about by the difference in the interracial tension, A ~, between the neck and the base of the mercury drop. At constant quinone concentration, the maximum height can then be ex. pressed by the equation, /max =ktv,

(I)

where kl is a constant defined by the concentration, diffusion coefficient, etc. The streaming velocity is given by: v =k2Ay,

(2)

where k2 is a constant which includes hydrodynamic properties. At a constant applied potential E, owing to the potential difference, AE, between the neck and the base of the mercury drop, AV is given by eqn. (3), since AE is a very small change in E,

A7 = (@/~E)uAE.

(3)

It can readily be shown by eqn. (3) that A7 is proportional to the slope of the electrocapillary curve at fixed AE. Since it can be assumed that AE is constant for any given capillary and electrolytic conditions, eqn. (3) can be rewritten

A~ =k~F

(4)

where ka is a constant a n d / ~ is the amount of adsorption of hydrocarbon. In the derivation of eqn. (4) from eqn. (3), we assume t h a t / " is proportional to the effective charge of the dipole which is caused by the aromatic ~-electron system through adsorption. This assumption is verified qualitatively by the increasing slope of the electrocapillary curve with increase of concentration of aromatic hydrocarbon; this is seen clearly in Fig. 2. The maximum height at constant potential will then be given by the following expression :

imax = K F

(5)

where K is a constant. The deduction holds for the effective height of the maximum brought about by the addition of aromatic hydrocarbon. I f / ' o b e y s the Langmuir isotherm, we can rewrite eqn. (5) as a function of concentration (activity a) by eqn. (6), /max =Kfla/'s/(I +fla)

(6) j. Electroanal. Chem., 18 (1968) 159-I64

162

K. TAKAMURA, T. T2~KAMURA.

where fl represents the c o n s t a n t p a r t of the free energy a n d Fs, the s a t u r a t e d a m o u n t of adsorption. F r o m eqn. (6) we have

1/imax ~---(I/KJXsfl) ( I / a ) Jr I / K F s The plot of 1/im~x a t c o n s t a n t p o t e n t i a l a g a i n s t t h e reciprocal of the h y d r o c a r b o n conc e n t r a t i o n was a l m o s t a s t r a i g h t line. Two e x a m p l e s of this plot, o b t a i n e d for the (I) - n a p h t h a l e n e a n d ( I ) - p y r e n e systems, are given in Fig. 3. S i m i l a r results were also obt a i n e d in other cases. A l t h o u g h the value of fl for h y d r o c a r b o n s could be e x p e c t e d to be o b t a i n e d from the plot using t h e relative concentration, C/C~,(C~: s a t u r a t e d concentration) I

r

.I

I

I

-0.4

-0.6

-0.8

-I.0

4.1 u

4.0

b

.E_ 3.9 to Q.

5.7 0

-0.2

V vs.

Hg pool

Fig. 2. Drop time curves of base electrolyte solns, containing: (a), o; (b), o.94. Io-2; (c) 4.13. io-~; (d), iI.O. io-~ M pyrene, at 25 °. 0.20

//

0.15

E e--

0.10 0

I

I

I

I

2

4

6

8

1

c

I0

M -1 x 10-2

Fig. 3. Plots of l/imax against the reciprocal concn, of added hydrocarbon./max were observed at --0.2 V vs. mercury pool electrode in i . i . l O -3 M (I) soln. containing various concns, of (a), naphthalene; (b), pyrene.

j. Electroanal. Chem., 18 (1968) 159-164

EFFECT OF HYDROCARBONS ON POLAROGRAPHY OF METHYL@-BENZOQUINONE

163

instead of concentration C, the lack of solubility data (or solvation energy data, crystal energy data, etc.) of hydrocarbons in 2-methoxyethanol solution precludes any discussion of the adsorption energy of hydrocarbons on the mercury electrode. The results obtained for all the hydrocarbons investigated can be explained from the same standpoint as above. The experimental results show that the addition to the solution of aromatic compounds of higher ring number causes greater streaming around the mercury drop. Accordin~ to FRUMI~IN4, the adsorptive poweI of aromatic hydrocarbons on a mercury electrode increases with increase of ring number. These compounds adsorb strongly in the positive region of the electrocapillary curve because the adsorption is due to the mutual attraction between the ~-electron system of the hydrocarbons and the electrode surface. This fact was confirmed under the present experimental condition as follows: The drop-time, i.e., the interfacial tension of the meTcury-solution interface, obtained in a solution containing hydrocarbons but no (I) decreased in the positive region with increasing concentration of added hydrocarbons. The results are shown in Fig. 4, in which the drop-time at o V (vs. Hg pool) are plotted against the hydrocarbon concentration. In Fig. 4, the lowering of the drop-time b y the addition of hydrocarbons is intensified in the order: naphthalene, phenanthrene, pyrene. 4.1--

s

T

f

--

"5 0

4.0 G

0

.E 3.9 b

0

L.

0

I

k

I

5

I0

15

j 20

Concentration, M × 10 2

Fig. 4- The relation b e t w e e n d r o p - t i m e (measured at o V vs. m e r c u r y pool electrode) a n d h y d r o carbon concn, in t h e s u p p o r t i n g electrolyte soln. in t h e presence of : (a), n a p h t h a l e n e ; (b), p h e n a n t h r e n e ; (c), pyrene.

Although there is a rational explanation for the effect of the aromatic hydrocarbon on the m a x i m u m of the quinone wave, the hydrocarbon did not increase the m a x i m u m of the reduction wave of Cu 2÷ in the same solution. In fact, the original m a x i m u m of the Cu 2+ reduction wave was rather suppressed b y the addition of the aromatic hydrocarbon and this effect cannot be explained on the basis of the above considerations alone. In the above explanation we assumed that the rate of electron transfer is much faster than the rate of supply of electro-reducible substances. This would be true for the quinone-hydrocarbon system because of the considerable affinity between them. We cannot expect such an affinity between the hydrocarbon and Cu 2+ because of the very low solubility of the copper salt in the hydrocarbon. When the adsorbed species repelthe electro-reducible substances and the rate of electron transfer is hind ered j . Electroanal. Chem., 18 (1968) 159-164

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K. TAKKMURA, T. TAKAMURA

by the adsorbed film, insufficient current can flow although the amount of electroreducible substance at the electrode surface is adequate. This causes a decrease in the rate of streaming, and, consequently, the maximum height will be decreased. The depression of the maximum of the Cu 2+ reduction wave by the hydrocarbon is considered to correspond to such a case. On the contrary, molecular interaction is expected between quinones and hydrocarbons, for example, the formation of charge-transfer complexesS-L Such complexes have a high adsorptive power because of their large dipole moment. In fact, the solution containing both (I) and hydrocarbon gave a lower and steeper interracial tension curve than that obtained in the solution containing either (I) or hydrocarbon, as already shown in the previous paper. SUMMARY

The effect of polycyclic aromatic hydrocarbons on the polarographic maximum of methyl-p-benzoquinone (I) has been investigated in 2-methoxyethanol solution containing 0.25 M lithium chloride. (I) gave a single reduction wave having a typical maximum which was enhanced by the addition of hydrocarbons. Increasing the concentration of hydrocarbons caused the maximum to become higher and wider. The enhancing effect was more marked for compounds of higher ring number. This phenomenon was explained on the basis of YON STACKELBERG'Stheory b y assuming the Langmuir adsorption of hydrocarbons on the electrode surface. REFERENCES

i I-{.TAKAMURAAND T. TAKAMURA,Trans. Faraday Soc., 61 (1965) 127o. 2 C. E. SEARLE,Nature, 184 (1959) 1716. 3 M. vo~ STACKELBERGA~D R. I)OPPELFELD,Advances in Polarography, Vol. I, Pergamon Press, Oxford, 196o, pp. 68-1o4 . 4 A. N. FRUMKII'¢,Proc Intern. Congr. Surface Activity, 2nd London, I957, Vol. III, Butterworths, London, 1957, pp. 58-66. 5 M. CHOWDHURY,Trans. Faraday Soc., 57 (196I) 1482. 6 S. K. CHAKRABARTIAND S. BASU, Trans. Faraday Soc., 60 (1964) 16. 7 M. E. P~OV~R, Trans. Faraday Soc., 60 (1964) 417. J. Electroanal. Chem., 18 (1968) 159-164