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Polarography of eosin and erythrosin in solutions of varying pH at the DME
Electrochhica
Acta,
1973. Vol.
18, pp. 265-270.
Pergamon
Press. Printed in Great Britain.
POLAROGRAPHY OF EOSIN AND ERYTHROSIN IN SOLUTIONS OF VARYING pH AT THE DME I. M. ISSA, R. M. ISSA, M. M. GHONEIMand Y. M. TEMERK Chemistry Department, Faculty of Science, Assiut University, Assiut, A. R. Egypt (Received 8 March 1972)
Abstract-The polarographic behaviour of eosin and erythrosin in buffer solutions of pH 4-11 is discussed. The observed waves are assigned to the reduction of furan and pyrone rings of the molecule of erythrosin at pH 4-11 and for eosin at pH <7. The nature of the waves is also investigated. In alkaline media the electrode reaction is essentially diffusion controlled of the reduction products contributes in the electrode process.
INTRODUCTION Polarographic studies on fluorescein and its derivatives have been mainly confined to the parent compound[l-71, and a few workers examined the behaviour of eosin[8, 91 and erythrosin[9, lo]. Delahay[l] proposed a two-electron reduction mechanism for ffuorescein in solutions of pH 2-l 1. Below pH 7 the polarograms show a single wave but within the pH range 7-105 two waves are observed; the total current is independent of the pH. Bannerjee and Vig[4, 5] also obtained a two-electron reduction wave in different media of pH 34-5.4. At pH 11.2 the dye gave rise to two reduction waves; the first wave was reversible, while the second was irreversible. The behaviour of eosin[8, 91, erythrosin[9, lo] and some other halogenated fluoresceins[9] were also conducted in alkaline media, pH 2 11-O. All substances were reduced in two one-electron steps to give radical anions as intermediates. The present work is devoted to an investigation of the polarographic behaviour of eosin and erythrosin in the universal buffer series[ll].
whereas at pH -C 8 adsorption
The polarograms were recorded by a Radiometer P04F type polarograph using a dropping electrode with m = 2.016 mg/s and t = 4.24 s at a height it of 49 cm Hg. The pH of the solutions were checked by a Radiometer pH-meter model 28. The working procedure was the same as that given before[l2]. RESULTS AND DISCUSSION The polarograms of eosin and erythrosin (Figs. 1 and 2) show clearly that the compounds exhibit different behaviour due to changes in the nature of the substituent’; also both compounds differ markedly from fluorescein in so much as fluorescein itself gives simpler polarograms in acid media[l-51. (a) The electroreduction
of eosin
The polarograms representing the electroreduction of 4.4 x 10m4 M eosin in universal buffer solutions
EXPERIMENTAL TECHNIQUE Solution andprocedure
10m2 M eosin or erythrosin solutions were prepared by dissoIving the accurately weighed quantity of the purified solid (BDH grade) in the appropriate volume of @02 M sodium hydroxide. The solids were purified by recrystallization from ethanol and their purity and individuality were checked by mp determination, element analysis and thin-layer chromatography. As supporting electrolytes the universal buffer series of Britton and Robinson[l l] were used.
45E.
VISCOI
of 4.4 x 10T4 M eosin in universal butler solutions of different pH. 1,4-O; 2, 5.15; 3, 6.0; 4, 7.02; 5, 8.08; 6, 9.0; 7, 10-O; S,ll-01.
Fig. 1. Polarograms
26.5
266
I. M. ISSA,R. M.
ISSA,
M. M. GHONEIMAND Y. M.
TEMERK
so-
w-
4
-
a
‘i
2.0 -
I
‘00 -0.4
-24
Fig. 2. Polarograms of 5.8 x lo-4 M erythrosin universal buffer solutions of different pH. 1,4.0; 2, 5.15; 3,6.0; 4,9-O; 59-S; 6, 10.0; 7,ll.O.
in
are given in Fig. 1. The simplest polarogram is that obtained in a solution of pH cu 8, which consists of a single reduction wave. From the height of the reduction wave and the value of the diffusion coefficient[2], the number of electrons involved in the electrode reaction is two. As the pH of the medium increases from 9. to 11 a second more negative wave starts to develop at the expense of the first one. This behaviour recalls that of fluorescein[l-51. In weakly acid solutions (pH G 7) eosin yields a polarogram which consists of two waves at concentrations as low as lo-” M of the dye. A third wave develops as the concentration is increased above 1.92 x lo-* M. The height of the first wave does not increase indefinitely with increasing eosin concentration, but tends to a constant value at a concentration depending upon the pH of the solution. This concentration is 4 x lo-+ M at pH 5.15, and 1-8 x 10T4 M at pH cu 7.0. The tirst wave, as gathered from the effect of Hg-pressure and concentration of the depolarizer on the limiting current, proved to be controlled by adsorption of the reduced form at the electrode surface. The adsorption is visualized from the electrocapillary curves, showing a remarkable lowering in the surface tension of eosin solution compared to the supporting electrolyte (cf Fig. 5, later). This is not the case with fluorescein. The height of the second wave increases on increasing the pH of solution from ca 5.0 to ca 7.0, whereas those of the first and third waves decrease with the same pH change. With increase. of pH, the first and second waves shift to more negative potentials while the E1,2 value of the third becomes more or less constant at pH 5 and 6, then shifts to less negative potentials at pH 7-O (cf Fig. 3). Also, the heights of the first wave (adsorp-
4-o
0.0
12.0
PH
Fig. 3. E&pH curves --sin: a, 1st wave (adsorption); b, 2nd wave at pH < 7.0 = 1st wave at pH > 8.0; c, 3rd wave at pH Q 7.0; c’, 2nd wave at pH > 9.0. - - -erythrosin: d, 3rd wave at pH < 9.0 = 1st wave at pH > 9.0; e, 4th wave; f, 5th wave at pH < 9.0 = 2nd wave at pH > 9.0.
tion wave) and third wave decrease till they vanish more or less completely at pH ca 8.0. The plot of i, for the second wave US pH gives a Z-shaped curve, Fig. 4A, indicating an acid-base equilibrium. The diffusion current starts to decrease at pH ca 8.0, until it becomes constant at about pH 11.0. At the same time the current for the new wave, which develops again at pH > 9.0, tends to increase on increasing the pH, attaining a constant value at pi-I cc 11.0. The reduction of eosin at the dme can be explained as follows: (i) The reduction at pH cu 8-O involves two electo fluorecorresponding in analogy trons, scein[l-51, to the reduction of the furan ring. The excessive current observed at lower pH, ie the third wave, represents the reduction of another active centre apart from the furan ring. This may be either the pyrone ring[l] or the C-Br bond,[lO] and its appearance seems to be related to the adsorption of the reduction product. The heat of adsorption would, at least, be partly consumed in activating the reduction of this centre. (ii) The second wave at lower pH (~7) represents the reduction of the furan ring. As the pH increases above pH 60 adsorption decreases and the corresponding waves (1st and 3rd) decrease in consequence.
Polarography of eosin and erythrosin in solutions of varying pH at the dme
DH
Fig. 4. i,/pH curves A, eosin: a, 1st wave (adsorption); b, c, 2nd wave at pH Q 7.0~ 1st wave at DH z 8.0: b. total heiaht: c. net height. d, 3rd wave; e, 2nd wave at pH 5 9:O B, erythrosin: a, 1st wave (adsorption); b, 2nd wave; c, 4th wave; d, e, 3rd wave at pH < 9.0- 1st wave at pH a9.0; d, net height; e, total height; f, 5th wave at pH < 9-O = 2nd wave at pH > 9.0.
The splitting of the reduction wave at pH > 8 is due to the formation of a stable intermediate free radical. the stabilitv of which increases with increase of pH[i, 3, 61. (b) The electroreducfion of erythrosin The polarograms representing the electro reduction of 5.8 x lo-4 M erythrosin in buffer solutions of different pH are given in Fig. 2. These poiarograms were recorded in presence of cu 6 x 10e4% Triton X- 100 to eliminate the maxima that appear in the region of the third wave at pH < 8 and the first at pH > 9.0. The simplest polarograms of erythrosin are obtained in solutions of pH 9-9-5. These consist of two waves, the first of which is more or less twice as high as the second. Under such conditions, the
267
first wave corresponds mainly to the reduction of the furan ring as in the case of fluoresce.in[l, 31 and its dihalogen derivatives[l3], and the second is due to the reduction at the pyrone ring or the carbon -haIogen bond[lOJ. With increasing pH (>9*5) the second wave increases in height at the expense of the first wave until its height becomes more or less double that of the first at pH 2 10. At pH G 8-O the polarograms comprise three to five waves. Thus at pH.ca 5, the polarogram, contrary to previous statements[9] consists of five waves of which the last is vitiated by the hydrogen wave. The first wave is controlled by adsorption of the reduced form as in the case of eosin. The second and third correspond to the second wave in the case of eosin at pH < 7 and represent the reductiQn of the furan ring. Separation of the furan ring reduction into two steps is due to the reduction of the oxidized form on the mercury surface already partly covered with the reduction product. Thus the first daughter wave (2nd wave) corresponds to reduction occuring on the uncovered mercury surface, whereas the other (3rd wave) corresponds to a retarded reduction on the covered part of the mercury drop. With rise of pH, the adsorption wave (the first wave) and the wave due to the unretarded reduction (the second) shift to more negative potentiaIs and decrease in height till they disappear more or less completely, in addition to the fourth wave, at pH 2 9-O. Reduction along the third wave is retarded because of the adsorbed film. The destruction of the adsorbed film on increasing pH lowers the surface resistance, hence the observed shift of the second daughter wave (the third) and the fourth wave to less negative potentials up to pH cu 8. When the adsorption disappears at pH ca 9-0, the direction of the shift is reversed as a result of the effect of pH on the reduction process (Fig. 3). At the same time the second and third waves are replaced by a single wave, the behaviour of which simulates the first wave of eosin under these conditions. Since the fourth wave is largely connected with the first and second waves, as gathered from the i,/pH curves shown in Fig. 4B, one may conclude that this represents the reduction of another active centre for the unretarded molecules. The fifth wave, which increases in height on increasing the pH and which remains after the disappearance of the fourth wave, is undoubtedly due to the reduction of the same active centre for the retarded erythrosin species as in the case of diiodofluorescein[13]. Furthermore, the first wave in alkaline solufions (pH > 9.5) should represent the first step of reduction of the furan ring, while the second cumulative wave corresponds to the second reduction step of the furan ring and the other active centre, which takes place at the same potentials. These deductions are based on the variation of i, and EllZ with pH as shown in Figs. 3 and 4B.
I. M. ISSA, R. M. WA, M. M. GHONELMAND
268
The active centre mentioned above may be either the halogen atom or the pyrone ring. The reduction at the carbon-halogen bond leading to liberation of the halide ion takes place according to I -C-X+2e+H+
I
-C-H+X-. I
-
I
The reduction at the four C-X bonds, as stated by Broad[lO], necessitates the uptake of 8 electrons. Accordingly the total reduction current should correspond to 10 electrons, which is not the case. Since, however, the total reduction current corresponds to 4 electrons, two of which represent the reduction of the furan ring, then only one C-X bond should be reduced, which seems improbable: since at least two C-X bonds should be reduced, the total current should corresponds to 6 electrons, which is not in agreement with experimental results. Controlled-potential electrolysis of erythrosin solution did not reveal the formation of iodide ions in solution and thus the second reduction centre should be the pyrone ring rather than the C-I bond[lO]. The same should hold also for eosin. The behaviour of eosin and erythrosin in solutions of varying pH denotes that both compounds exhibit more or less the same reduction mechanism at pH < 8 whereas at higher pH they differ. Thus for the lower pHs reduction occurs at both furan and pyrone rings for the two compounds whereas above pH 8 the reduction of eosin does not involve the pyrone ring. This behaviour shows the easier reduction of the pyrone ring in case of the iododerivative, which is supported by the higher current of the waves due to the reduction of this centre. Also it seems that the reduction at the pyrone ring in eosin is enhanced by adsorption, since the corresponding
(A) In solutions
Y.
M. TEMERK
wave is observed only in media favouring adsorption of the reduced form. Both compounds differ from fluoresceinrl-51, in that the reduction at the pyrone ring is not achieved with fluorescein. It was stated by Zuman and Wawzonek[l4] that the C-O linkage is not reducible at the electrode surface except when a neighbouring substituent causes a decrease in the electronegativity of the carbon atom. It seems thus that Br- or I-substituents lower the interaction of the carbon atom with the lone pairs of electrons on the oxygen atom, leading to a lower bond order and hence to a lower cleavage energy. Also the increased charge density on the oxygen atom facilitates its protonation during the reduction process. Analysis of the ir spectra of the three compounds[l5] reveals that the band due to the C-O-C linkage lies at 1000, 990 and 970 cm-’ for fluoresceine, eosin and erythrosin respectively. Also the band assigned by Davis and Jones[ld] for the C-O-C at 1209 cm-’ in fluorescein was found to shift to 1205 and 1200 cm-’ for eosin and erythrosin. Both cases denote a lowering of the C-O bond order and hence a lowering of the cleavage energy in the direction fluorescein > eosin > erythrosin. The lower value of bond order in case of erythrosin can be attributed to the electronic character of the iodine atom, which lowers the electronegativity of the carbon atoms of the phenyl rings and hence the mesomeric interaction with the pyrone oxygen. This is in accordance with the fact that the C-O bridge in the iodo-compound (erythrosin) is readily reduced, whereas it is hardly reduced in the case of fluoresceinrl-71. However, since the change in bond order is but small, the reduction of the C-O bond in the halogenated derivatives is favoured through adsorption, as in the case of eosin. The electrode reaction can be represented as follows :
of pH 4 7. X
X
x
(1st
step)
(2nd
step)
Polarography of eosin and erythrosin in solutions of varying pH at the dme
(B) In solutions
of pH > 9, the depolarizer
exists mainly as triphenylcarbonium
The three steps corresponding to the high pH values occur in the case of erythrosin, but only steps I + II for eosin. (c) The nature
of the waves
The effect of Hg-pressure on the limiting current of both eosin and’erythrosin revealed that the first wave observed in both cases at low pH (C7) is controlled by adsorption, while the other waves at all pHs are essentially diffusion-controlled. The value of x in the relation i = Kh” is 0.9-1.0 for the adsorption-controlled wave (the first wave) and O-49-0.55 for the other waves. The current/concentration curves exhibit the character of an adsorption isotherm and the plot of log i us log c is a linear relation, in accordance with
269
ion,[3]
the Freundlich isotherm. Adsorption is also suggested from the electrocapillary curves of Fig. 5. Analysis of the waves of eosin and erythrosin at different pH using the fundamental equation for polarographic waves indicated that all electrode processes proceed more or less irreversibly, and hence the number of electrons involved in the electrode process cannot be evaluated by this method. Heat of udrorption of the reducedproduct
Brdicka[l7] derived a simple relation by which the energy of adsorption of the reduced form can be calculated under conditions when the oxidized form is surface-active. Although the oxidized forms of eosin and erythrosin are surface-active, their effects can be neglected, especially under conditions when the surface becomes saturated with the reduced form, ie when the limiting current of the first (adsorption) wave becomes constant. The constancy of the limiting current is observed in presence of a certain concentration of these two dyes, which depends on the pH of the solution. The relation used by Brdicka is #=2ArlF+RTlng, B
Fig. 5. Electrocapillary curves at pH 5.15 a, in absence of the depolarizer b, in presence of 9 x 10eQ M eosin c, in presence of 9 x lo-“ M erythrosin.
where 4 is the energy of adsorption, AII the difference of the half wave potential of waves I and II, F the faraday, V the dilution of the depolarizer in the bulk of the solution and V. the volume of one gram-molecule in the adsorbed state. The area of the eosin molecule as given by Reilly[ll] is 114 AZ, compared to 106 A2 given by Kalousek[lS]. Taking this value for the sake of simplicity as 100 AZ, then the volume of one molecule is 1000 A3 and hence the volume of one gram-
J. M. ISSA, R. M. WA, M. M. GHONEIM AND Y. M. TEMER~C
270
Table 1. r) values of eosin and erythrosin Concn. c M
PH (A) Eosin 4.00 5.15 6.00 7.08 (B) Erythrosin 5.15 6.00
2 4 2 1.8
x x x x
as determined v= l/c l/m01
from polarographic
AI-I
results
4
V
cal.
1o-4 10-d 10-4 1O-4
4000 2500 5000 5556
0.18 0.22 0.26 0.21
13529 15512 17769 15474
3 x 1o-4 4 x 10-d
3333 2500
0.25 0.15
17075 12280
in the adsorbed state is 0.6 1. The values of 4 at the different pHs have been calculated and are collected in Table 1.
molecule
These values indicate that the adsorption of eosin or erythrosin is not caused by Van der Waals forces but through chemisorption. This is in con-
firmity with the findings of Kalousek and Blahnik[l9] and of Lssa[8]. The heat of adsorption in solutions permitting adsorption may be partly consumed in activating the reduction of the pyrone ring. REFERENCES
1. P. Delahay, BUM.Sot. chim. Fr. 15, 348 (1948). 2. P. A. Gollmick and H. Berg, Bet-. Bunsenges Phys. Chem. 69, 196 (1965). 3. R. M. Issa, F. M. Abd-El-Halim and A. R. Hassanein, Electrochim. Acta 14, 561 (1969) 4. N. R. Bannerjee and S. K. Vig, Proc. Symp. Elecfrode Processes, 113 (1966). 5. N. R. Bannerjee and S. K. Vig, J. them. SM. (B) 484 (1967). 6. S. K. Vig, Indian J. Chem. 7, 501 (1969).
7. S. K. Vig and N. K. Bannerjee, Electrochim. Act? 16, 157 (1971). 8. I. M. Jssa, Extr. of the Assiut Sci. & Technol. Bull. 11 (1959). 9. M. Cardinalli, L. Rampazza and A. Trazza (Unni Reme) Ric. Sci., Rend., Ser. AB (6), 1361 (1965). 10. P. W. Board, D. Britz and R. V. Hotland, Electrochim. Acta 13, 1575 (1968). 11. H. T. S. Britton, Hydrogen Zons, 4th edn, p. 313. Chapman and Hall, London (1952). 12. R. M. Issa, B. A. Ahd-El-Nahey and H. Sadek, Electrochim. Acta 13, 1827 (1968). 13. I. M. Issa, A. El-Samahy, R. M. Issa and M. M. Ghoneim, Electrochim. Acta 17, 1195 (1972). 14. P. Zuman and I. M. Kolthoff in Progress in Pofarography, ed. P. Zumman and S. Wawzonek, Vol. 1, ch. XIII, p, 303. Wiley, New York, London (1962) 15. I. M. Issa, R. M. lssa and M. M. Ghoneim,J. Chem.. Egypt, in press. 16. M. Davies and R. L. Jones, J. them. Sot. 120 (1954). 17. R. Brdicka, Elecktrochem., 48, 278 (1941); CoiZtt. Czech. them. iTommun. 12, 270 (1947). 18. R. W. Schmidt and C. N. Reilly, J. Am. them. Sot. 80. 2087 (1958). 19. M: Kalousek -and R. Blahnik, Colln. Czech. them. Commun. 6, 782 (1955).
Acta,
1973. Vol.
18, pp. 265-270.
Pergamon
Press. Printed in Great Britain.
POLAROGRAPHY OF EOSIN AND ERYTHROSIN IN SOLUTIONS OF VARYING pH AT THE DME I. M. ISSA, R. M. ISSA, M. M. GHONEIMand Y. M. TEMERK Chemistry Department, Faculty of Science, Assiut University, Assiut, A. R. Egypt (Received 8 March 1972)
Abstract-The polarographic behaviour of eosin and erythrosin in buffer solutions of pH 4-11 is discussed. The observed waves are assigned to the reduction of furan and pyrone rings of the molecule of erythrosin at pH 4-11 and for eosin at pH <7. The nature of the waves is also investigated. In alkaline media the electrode reaction is essentially diffusion controlled of the reduction products contributes in the electrode process.
INTRODUCTION Polarographic studies on fluorescein and its derivatives have been mainly confined to the parent compound[l-71, and a few workers examined the behaviour of eosin[8, 91 and erythrosin[9, lo]. Delahay[l] proposed a two-electron reduction mechanism for ffuorescein in solutions of pH 2-l 1. Below pH 7 the polarograms show a single wave but within the pH range 7-105 two waves are observed; the total current is independent of the pH. Bannerjee and Vig[4, 5] also obtained a two-electron reduction wave in different media of pH 34-5.4. At pH 11.2 the dye gave rise to two reduction waves; the first wave was reversible, while the second was irreversible. The behaviour of eosin[8, 91, erythrosin[9, lo] and some other halogenated fluoresceins[9] were also conducted in alkaline media, pH 2 11-O. All substances were reduced in two one-electron steps to give radical anions as intermediates. The present work is devoted to an investigation of the polarographic behaviour of eosin and erythrosin in the universal buffer series[ll].
whereas at pH -C 8 adsorption
The polarograms were recorded by a Radiometer P04F type polarograph using a dropping electrode with m = 2.016 mg/s and t = 4.24 s at a height it of 49 cm Hg. The pH of the solutions were checked by a Radiometer pH-meter model 28. The working procedure was the same as that given before[l2]. RESULTS AND DISCUSSION The polarograms of eosin and erythrosin (Figs. 1 and 2) show clearly that the compounds exhibit different behaviour due to changes in the nature of the substituent’; also both compounds differ markedly from fluorescein in so much as fluorescein itself gives simpler polarograms in acid media[l-51. (a) The electroreduction
of eosin
The polarograms representing the electroreduction of 4.4 x 10m4 M eosin in universal buffer solutions
EXPERIMENTAL TECHNIQUE Solution andprocedure
10m2 M eosin or erythrosin solutions were prepared by dissoIving the accurately weighed quantity of the purified solid (BDH grade) in the appropriate volume of @02 M sodium hydroxide. The solids were purified by recrystallization from ethanol and their purity and individuality were checked by mp determination, element analysis and thin-layer chromatography. As supporting electrolytes the universal buffer series of Britton and Robinson[l l] were used.
45E.
VISCOI
of 4.4 x 10T4 M eosin in universal butler solutions of different pH. 1,4-O; 2, 5.15; 3, 6.0; 4, 7.02; 5, 8.08; 6, 9.0; 7, 10-O; S,ll-01.
Fig. 1. Polarograms
26.5
266
I. M. ISSA,R. M.
ISSA,
M. M. GHONEIMAND Y. M.
TEMERK
so-
w-
4
-
a
‘i
2.0 -
I
‘00 -0.4
-24
Fig. 2. Polarograms of 5.8 x lo-4 M erythrosin universal buffer solutions of different pH. 1,4.0; 2, 5.15; 3,6.0; 4,9-O; 59-S; 6, 10.0; 7,ll.O.
in
are given in Fig. 1. The simplest polarogram is that obtained in a solution of pH cu 8, which consists of a single reduction wave. From the height of the reduction wave and the value of the diffusion coefficient[2], the number of electrons involved in the electrode reaction is two. As the pH of the medium increases from 9. to 11 a second more negative wave starts to develop at the expense of the first one. This behaviour recalls that of fluorescein[l-51. In weakly acid solutions (pH G 7) eosin yields a polarogram which consists of two waves at concentrations as low as lo-” M of the dye. A third wave develops as the concentration is increased above 1.92 x lo-* M. The height of the first wave does not increase indefinitely with increasing eosin concentration, but tends to a constant value at a concentration depending upon the pH of the solution. This concentration is 4 x lo-+ M at pH 5.15, and 1-8 x 10T4 M at pH cu 7.0. The tirst wave, as gathered from the effect of Hg-pressure and concentration of the depolarizer on the limiting current, proved to be controlled by adsorption of the reduced form at the electrode surface. The adsorption is visualized from the electrocapillary curves, showing a remarkable lowering in the surface tension of eosin solution compared to the supporting electrolyte (cf Fig. 5, later). This is not the case with fluorescein. The height of the second wave increases on increasing the pH of solution from ca 5.0 to ca 7.0, whereas those of the first and third waves decrease with the same pH change. With increase. of pH, the first and second waves shift to more negative potentials while the E1,2 value of the third becomes more or less constant at pH 5 and 6, then shifts to less negative potentials at pH 7-O (cf Fig. 3). Also, the heights of the first wave (adsorp-
4-o
0.0
12.0
PH
Fig. 3. E&pH curves --sin: a, 1st wave (adsorption); b, 2nd wave at pH < 7.0 = 1st wave at pH > 8.0; c, 3rd wave at pH Q 7.0; c’, 2nd wave at pH > 9.0. - - -erythrosin: d, 3rd wave at pH < 9.0 = 1st wave at pH > 9.0; e, 4th wave; f, 5th wave at pH < 9.0 = 2nd wave at pH > 9.0.
tion wave) and third wave decrease till they vanish more or less completely at pH ca 8.0. The plot of i, for the second wave US pH gives a Z-shaped curve, Fig. 4A, indicating an acid-base equilibrium. The diffusion current starts to decrease at pH ca 8.0, until it becomes constant at about pH 11.0. At the same time the current for the new wave, which develops again at pH > 9.0, tends to increase on increasing the pH, attaining a constant value at pi-I cc 11.0. The reduction of eosin at the dme can be explained as follows: (i) The reduction at pH cu 8-O involves two electo fluorecorresponding in analogy trons, scein[l-51, to the reduction of the furan ring. The excessive current observed at lower pH, ie the third wave, represents the reduction of another active centre apart from the furan ring. This may be either the pyrone ring[l] or the C-Br bond,[lO] and its appearance seems to be related to the adsorption of the reduction product. The heat of adsorption would, at least, be partly consumed in activating the reduction of this centre. (ii) The second wave at lower pH (~7) represents the reduction of the furan ring. As the pH increases above pH 60 adsorption decreases and the corresponding waves (1st and 3rd) decrease in consequence.
Polarography of eosin and erythrosin in solutions of varying pH at the dme
DH
Fig. 4. i,/pH curves A, eosin: a, 1st wave (adsorption); b, c, 2nd wave at pH Q 7.0~ 1st wave at DH z 8.0: b. total heiaht: c. net height. d, 3rd wave; e, 2nd wave at pH 5 9:O B, erythrosin: a, 1st wave (adsorption); b, 2nd wave; c, 4th wave; d, e, 3rd wave at pH < 9.0- 1st wave at pH a9.0; d, net height; e, total height; f, 5th wave at pH < 9-O = 2nd wave at pH > 9.0.
The splitting of the reduction wave at pH > 8 is due to the formation of a stable intermediate free radical. the stabilitv of which increases with increase of pH[i, 3, 61. (b) The electroreducfion of erythrosin The polarograms representing the electro reduction of 5.8 x lo-4 M erythrosin in buffer solutions of different pH are given in Fig. 2. These poiarograms were recorded in presence of cu 6 x 10e4% Triton X- 100 to eliminate the maxima that appear in the region of the third wave at pH < 8 and the first at pH > 9.0. The simplest polarograms of erythrosin are obtained in solutions of pH 9-9-5. These consist of two waves, the first of which is more or less twice as high as the second. Under such conditions, the
267
first wave corresponds mainly to the reduction of the furan ring as in the case of fluoresce.in[l, 31 and its dihalogen derivatives[l3], and the second is due to the reduction at the pyrone ring or the carbon -haIogen bond[lOJ. With increasing pH (>9*5) the second wave increases in height at the expense of the first wave until its height becomes more or less double that of the first at pH 2 10. At pH G 8-O the polarograms comprise three to five waves. Thus at pH.ca 5, the polarogram, contrary to previous statements[9] consists of five waves of which the last is vitiated by the hydrogen wave. The first wave is controlled by adsorption of the reduced form as in the case of eosin. The second and third correspond to the second wave in the case of eosin at pH < 7 and represent the reductiQn of the furan ring. Separation of the furan ring reduction into two steps is due to the reduction of the oxidized form on the mercury surface already partly covered with the reduction product. Thus the first daughter wave (2nd wave) corresponds to reduction occuring on the uncovered mercury surface, whereas the other (3rd wave) corresponds to a retarded reduction on the covered part of the mercury drop. With rise of pH, the adsorption wave (the first wave) and the wave due to the unretarded reduction (the second) shift to more negative potentiaIs and decrease in height till they disappear more or less completely, in addition to the fourth wave, at pH 2 9-O. Reduction along the third wave is retarded because of the adsorbed film. The destruction of the adsorbed film on increasing pH lowers the surface resistance, hence the observed shift of the second daughter wave (the third) and the fourth wave to less negative potentials up to pH cu 8. When the adsorption disappears at pH ca 9-0, the direction of the shift is reversed as a result of the effect of pH on the reduction process (Fig. 3). At the same time the second and third waves are replaced by a single wave, the behaviour of which simulates the first wave of eosin under these conditions. Since the fourth wave is largely connected with the first and second waves, as gathered from the i,/pH curves shown in Fig. 4B, one may conclude that this represents the reduction of another active centre for the unretarded molecules. The fifth wave, which increases in height on increasing the pH and which remains after the disappearance of the fourth wave, is undoubtedly due to the reduction of the same active centre for the retarded erythrosin species as in the case of diiodofluorescein[13]. Furthermore, the first wave in alkaline solufions (pH > 9.5) should represent the first step of reduction of the furan ring, while the second cumulative wave corresponds to the second reduction step of the furan ring and the other active centre, which takes place at the same potentials. These deductions are based on the variation of i, and EllZ with pH as shown in Figs. 3 and 4B.
I. M. ISSA, R. M. WA, M. M. GHONELMAND
268
The active centre mentioned above may be either the halogen atom or the pyrone ring. The reduction at the carbon-halogen bond leading to liberation of the halide ion takes place according to I -C-X+2e+H+
I
-C-H+X-. I
-
I
The reduction at the four C-X bonds, as stated by Broad[lO], necessitates the uptake of 8 electrons. Accordingly the total reduction current should correspond to 10 electrons, which is not the case. Since, however, the total reduction current corresponds to 4 electrons, two of which represent the reduction of the furan ring, then only one C-X bond should be reduced, which seems improbable: since at least two C-X bonds should be reduced, the total current should corresponds to 6 electrons, which is not in agreement with experimental results. Controlled-potential electrolysis of erythrosin solution did not reveal the formation of iodide ions in solution and thus the second reduction centre should be the pyrone ring rather than the C-I bond[lO]. The same should hold also for eosin. The behaviour of eosin and erythrosin in solutions of varying pH denotes that both compounds exhibit more or less the same reduction mechanism at pH < 8 whereas at higher pH they differ. Thus for the lower pHs reduction occurs at both furan and pyrone rings for the two compounds whereas above pH 8 the reduction of eosin does not involve the pyrone ring. This behaviour shows the easier reduction of the pyrone ring in case of the iododerivative, which is supported by the higher current of the waves due to the reduction of this centre. Also it seems that the reduction at the pyrone ring in eosin is enhanced by adsorption, since the corresponding
(A) In solutions
Y.
M. TEMERK
wave is observed only in media favouring adsorption of the reduced form. Both compounds differ from fluoresceinrl-51, in that the reduction at the pyrone ring is not achieved with fluorescein. It was stated by Zuman and Wawzonek[l4] that the C-O linkage is not reducible at the electrode surface except when a neighbouring substituent causes a decrease in the electronegativity of the carbon atom. It seems thus that Br- or I-substituents lower the interaction of the carbon atom with the lone pairs of electrons on the oxygen atom, leading to a lower bond order and hence to a lower cleavage energy. Also the increased charge density on the oxygen atom facilitates its protonation during the reduction process. Analysis of the ir spectra of the three compounds[l5] reveals that the band due to the C-O-C linkage lies at 1000, 990 and 970 cm-’ for fluoresceine, eosin and erythrosin respectively. Also the band assigned by Davis and Jones[ld] for the C-O-C at 1209 cm-’ in fluorescein was found to shift to 1205 and 1200 cm-’ for eosin and erythrosin. Both cases denote a lowering of the C-O bond order and hence a lowering of the cleavage energy in the direction fluorescein > eosin > erythrosin. The lower value of bond order in case of erythrosin can be attributed to the electronic character of the iodine atom, which lowers the electronegativity of the carbon atoms of the phenyl rings and hence the mesomeric interaction with the pyrone oxygen. This is in accordance with the fact that the C-O bridge in the iodo-compound (erythrosin) is readily reduced, whereas it is hardly reduced in the case of fluoresceinrl-71. However, since the change in bond order is but small, the reduction of the C-O bond in the halogenated derivatives is favoured through adsorption, as in the case of eosin. The electrode reaction can be represented as follows :
of pH 4 7. X
X
x
(1st
step)
(2nd
step)
Polarography of eosin and erythrosin in solutions of varying pH at the dme
(B) In solutions
of pH > 9, the depolarizer
exists mainly as triphenylcarbonium
The three steps corresponding to the high pH values occur in the case of erythrosin, but only steps I + II for eosin. (c) The nature
of the waves
The effect of Hg-pressure on the limiting current of both eosin and’erythrosin revealed that the first wave observed in both cases at low pH (C7) is controlled by adsorption, while the other waves at all pHs are essentially diffusion-controlled. The value of x in the relation i = Kh” is 0.9-1.0 for the adsorption-controlled wave (the first wave) and O-49-0.55 for the other waves. The current/concentration curves exhibit the character of an adsorption isotherm and the plot of log i us log c is a linear relation, in accordance with
269
ion,[3]
the Freundlich isotherm. Adsorption is also suggested from the electrocapillary curves of Fig. 5. Analysis of the waves of eosin and erythrosin at different pH using the fundamental equation for polarographic waves indicated that all electrode processes proceed more or less irreversibly, and hence the number of electrons involved in the electrode process cannot be evaluated by this method. Heat of udrorption of the reducedproduct
Brdicka[l7] derived a simple relation by which the energy of adsorption of the reduced form can be calculated under conditions when the oxidized form is surface-active. Although the oxidized forms of eosin and erythrosin are surface-active, their effects can be neglected, especially under conditions when the surface becomes saturated with the reduced form, ie when the limiting current of the first (adsorption) wave becomes constant. The constancy of the limiting current is observed in presence of a certain concentration of these two dyes, which depends on the pH of the solution. The relation used by Brdicka is #=2ArlF+RTlng, B
Fig. 5. Electrocapillary curves at pH 5.15 a, in absence of the depolarizer b, in presence of 9 x 10eQ M eosin c, in presence of 9 x lo-“ M erythrosin.
where 4 is the energy of adsorption, AII the difference of the half wave potential of waves I and II, F the faraday, V the dilution of the depolarizer in the bulk of the solution and V. the volume of one gram-molecule in the adsorbed state. The area of the eosin molecule as given by Reilly[ll] is 114 AZ, compared to 106 A2 given by Kalousek[lS]. Taking this value for the sake of simplicity as 100 AZ, then the volume of one molecule is 1000 A3 and hence the volume of one gram-
J. M. ISSA, R. M. WA, M. M. GHONEIM AND Y. M. TEMER~C
270
Table 1. r) values of eosin and erythrosin Concn. c M
PH (A) Eosin 4.00 5.15 6.00 7.08 (B) Erythrosin 5.15 6.00
2 4 2 1.8
x x x x
as determined v= l/c l/m01
from polarographic
AI-I
results
4
V
cal.
1o-4 10-d 10-4 1O-4
4000 2500 5000 5556
0.18 0.22 0.26 0.21
13529 15512 17769 15474
3 x 1o-4 4 x 10-d
3333 2500
0.25 0.15
17075 12280
in the adsorbed state is 0.6 1. The values of 4 at the different pHs have been calculated and are collected in Table 1.
molecule
These values indicate that the adsorption of eosin or erythrosin is not caused by Van der Waals forces but through chemisorption. This is in con-
firmity with the findings of Kalousek and Blahnik[l9] and of Lssa[8]. The heat of adsorption in solutions permitting adsorption may be partly consumed in activating the reduction of the pyrone ring. REFERENCES
1. P. Delahay, BUM.Sot. chim. Fr. 15, 348 (1948). 2. P. A. Gollmick and H. Berg, Bet-. Bunsenges Phys. Chem. 69, 196 (1965). 3. R. M. Issa, F. M. Abd-El-Halim and A. R. Hassanein, Electrochim. Acta 14, 561 (1969) 4. N. R. Bannerjee and S. K. Vig, Proc. Symp. Elecfrode Processes, 113 (1966). 5. N. R. Bannerjee and S. K. Vig, J. them. SM. (B) 484 (1967). 6. S. K. Vig, Indian J. Chem. 7, 501 (1969).
7. S. K. Vig and N. K. Bannerjee, Electrochim. Act? 16, 157 (1971). 8. I. M. Jssa, Extr. of the Assiut Sci. & Technol. Bull. 11 (1959). 9. M. Cardinalli, L. Rampazza and A. Trazza (Unni Reme) Ric. Sci., Rend., Ser. AB (6), 1361 (1965). 10. P. W. Board, D. Britz and R. V. Hotland, Electrochim. Acta 13, 1575 (1968). 11. H. T. S. Britton, Hydrogen Zons, 4th edn, p. 313. Chapman and Hall, London (1952). 12. R. M. Issa, B. A. Ahd-El-Nahey and H. Sadek, Electrochim. Acta 13, 1827 (1968). 13. I. M. Issa, A. El-Samahy, R. M. Issa and M. M. Ghoneim, Electrochim. Acta 17, 1195 (1972). 14. P. Zuman and I. M. Kolthoff in Progress in Pofarography, ed. P. Zumman and S. Wawzonek, Vol. 1, ch. XIII, p, 303. Wiley, New York, London (1962) 15. I. M. Issa, R. M. lssa and M. M. Ghoneim,J. Chem.. Egypt, in press. 16. M. Davies and R. L. Jones, J. them. Sot. 120 (1954). 17. R. Brdicka, Elecktrochem., 48, 278 (1941); CoiZtt. Czech. them. iTommun. 12, 270 (1947). 18. R. W. Schmidt and C. N. Reilly, J. Am. them. Sot. 80. 2087 (1958). 19. M: Kalousek -and R. Blahnik, Colln. Czech. them. Commun. 6, 782 (1955).