Polarography of dihalogen fluoresceins in solutions of varying pH at the dme

Ekctrochimica

Acta.

1972, Vol. 17, pp. 1195 to 1uIz.

POLAROGRAPHY SOLUTIONS

Pwgamon

Prcas.

Printed in Northern

Ireland

OF DIHALOGEN FLUORESCEINS OF VARYING pH AT THE DME*

IN

I. M. ISSA, A. A. EL-SAMAHY, R. M. ISSA and M. M. GHONEIM Chemistry Department, Faculty of Science, Assiut University, Assiut, Egypt, U.A.R. Ahstraet-The polarographic behaviour of dichloro-, dibromo- and di-iodofluorescein has been studied in buffer solutions (pH 4-12) at the dme. The reduction of dichloro- and dibromo-derivatives involves the uptake of two electrons whereby the furane ring is reduced. At low pH (67) a small wave appears, which is attributed to the partial reduction of the pyrone ring. In the case of diiodofluorescein, four electrons are consumed, corresponding to the reduction of both furane andpyrone rings at all pHs. The nature of the reduction waves is discussed. R&mm&Etude en solutions tampon&es (pH 4 B 12) et sur &&rode ti mercure gouttante, du comportement polarographique des dichloro- dibromo- et cliiodofluoresc6ines. La rkduction des d&h& dichloro- et dibromo- implique la fixation de deux &ctrons qui reduisent le cycle furane. Aux faibles pH (17) une petite vague apparalf, elle est attribuk & la reduction partielle du cycle pyrone. Dans le cas de la diiodofluoresc&me, quatre 6lectrons sont consomm&, correspondant, quelque soit le pH, 1 la r&luction des deux cycles furane et pyrone. La nature des vagues de rbduction est discut&e. Zusammenfass~Das polarographische Verhalten von Dichlor-, Dibrom- und Diiodfluorescein ist in Puffertosungen (pH 4-12) an der Tropfelektrode untersucht worden. Die Reducktion der Dichlor-, und Dibromderivate vertauff aus Furanring durch die Aufnahme von zwei Elektronen. Bei der Diiod-verbindung werden vier Elektronen aufgenomen, wobei die Reduktion aus Furanund Pyronring statt6ndet. Die Eigenschaften der Reduktionsstufen werden ebenfalls diskutiert. INTRODUCTION THE POLAROGRAPHIC behaviour

of fluorescein has been the subject of many investigations. 1-6 These studies classified the nature of species existing in solution and as well that of the electrode process. It seems, however, that no polarographic studies on the dihalogen ffuorescein derivatives has been carried out. Also, little work has been mentioned on eosin,‘J*’ erythrosin and some other ffuorescein derivatives7 in certain media. The present work is an investigation of the polarographic behaviour of dihalogen fluoresceins in buffer solutions of varying pH in order to throw light on the nature of species present in solution. We also discuss the electrode reaction. EXPERIMENTAL

TECHNIQUE

10m2 M dichloro-, dibromo-, or di-iodofluorescein solutions were prepared by dissolving the accurately weighed solid (BDH grade) in the appropriate volume of O-02 M NaOH solution. As supporting electrolytes the universal buffer series of Britton and Robinson* was used. The polarograms were recorded on a Radiometer P04F type polarograph using a dme having m = 2.061 mg/s and t = 4.24 s at 40 cm Hg. The pH of the solutions were checked by a Radiometer pH meter model 28. The working procedure was the same as given before.6 RESULTS

AND

DISCUSSION

The polarograms of dichloro-, dibromo- and diiodofluorescein (Figs. l-3 below), show clearly that these compounds exhibit different behaviour in accordance with the nature of the substituent. They also differ markedly from the parent compound fluorescein. * Manuscript received I8 June 1971. 1195

I.

1196

M.

ISSA, A.

A. The electroreduction

A.

E-SLAMAHY,

R.

M.

ISSA and

M.

M.

GHONEIM

of dichlorofuorescein

of 4.58 x lOA M dichloroPolarograms representing the electroreduction fluorescein in buffer solutions of different pHs are given in Fig. 1. At pH > 5.0 the compound is completely soluble, and the polarogram consists of two waves, the first of which is larger than the second. In comparison with the behaviour of fluoresceinx*s the first wave would represent the cumulative reduction of neutral molecules (HA)

E, V( see) FIG. 1. 1,pH

4.58 x 1O-4 M dichlorofluorescein.

l-85;2,4-O;3,515; 4,6-O;5,7-08;6,8.08;7,9-O;8,9*95;9,10+98.

and singly charged anions (HA-) together with the first reduction step of the doubly charged anions (A”) if existing. The second wave might represent the second reduction step of a doubly-charged species. However, at this pH, the quantity of this species should not be appreciable. g The second reduction wave should rather correspond to the reduction of another centre in the molecule, which may be the pyrone ring or the C-Cl bond.lO Also, the height of this wave relative to that of the first decreases as the pH of the solution increases, till it almost vanishes at pH w 8.0. Hence the second wave cannot be assigned to the reduction of the doubly-charged anions, the concentration of which increases with rise of pH. At pH > 9-O the polarogram consists of two waves, one lying within the same potential range of the wave at pH M 8.0, while the second is at more negative values. With increasing pH, the height of the second wave increases at the expense of the first until both waves exhibit a more or less constant limiting current (pH s 11-O). Over the whole pH range studied (5-O-ll.O), the total current is but slightly dependent on the pH. At pH 9.0 and 9.5 a small maximum is observed at the first wave which can be suppressed by the addition of 6 x 1O-s% Triton X-100. 3.

The electroreduction of dibromofluorescein

Polarograms of 4.58 x lOa M dibromo-derivative in buffer solutions of different pH are shown in Fig. 2. At pH > 4-O two waves are observed with an apparently higher limiting current for the first wave. At pH 5 and 6 the polarogram consists of two waves, the second characterized by a small maximum, which can be suppressed by 1.6 x lOA % Triton X-100. This wave is higher than that in case of the dichlorocompound. As the pH increases, the El,+ of the two waves shift to more negative potentials, but to a small extent in case of the second. At pH 7 and 8, the second wave

Polarography of dihalogen

0.0

-04

ffu~rescein~

-0-0

in solutions of

-1.2

varying

-1.6

1197

pH

-2.0

E.Vkce)

1, pH2,98;

2, 4.0;

FIG. 2. 4.58 x 1O-4 M dibromofluorescein.

3, 5.15;

4, 6.0;

5, 7.08;

6, 8.08;

7, 9vO; 8, 9.95;

9,10@.

decreases in height and suffers no change in E,,,. In comparison with the behaviour of fluorescein and dichlorofluorescein the reduction of the dibromo-compound at pH < 8-O occurs at the furane ring and another eIectro-active centre. Above pH 8.0 the reduction wave splits into two well-defined waves, at more negative potentials. The height of the second wave increases with pH 9-l 1, while that of the first decreases. This is apparent from i,JpH curves of the two waves (Fig. 4, below). Also, as in the case of dichforo- compounds a small maximum is observed at the rising portion of the first wave in solutions of pH 9 and 9.5, which can be eliminated by the addition of 4 x 1O-s Triton X-100. C. T’e electroreduction

of di-iodofluorescein

The polarograms of 4.58 x lOA di-iodofluorescein in universal buffer solutions of varying pH are recorded in Fig. 3. In a buffer solution of pH 5.15, the polarogram consists of three more or less well-developed waves. The first wave has a sloping nature, which may be due to the presence of an ill-developed adsorption wave at Iess

7.0

5.0 u a_ < 30

IO

-0.4

-0.6

-0.8

-1-O

-1.2

-I

6

-2.0

E*V(sce) 1, pH 4-O; 2, 5.15;

FIG. 3. 4.58 x lo-’ M di-iodofluorescein. 3, 6.00; 4, 7.08; 5.8.08; 6, 9.0; 7, 9.5; 8, 10.0; 9, 11.0;

10. 11.7.

1198

I. M. Is%%,A. A. ELSAMAHY, R. M. 1%~ and M. M. GHONEW

PH Fro. 4. ia/pH curves. a, total id d, total ia e, 1st wave di-iodofluorescein. b, 1st wave dichlorofluorescein; c, 2nd wave f, 2nd wave I

negative potentials. The first two waves may correspond to the reduction of the furane ring for both adsorbed and non-adsorbed molecules. Within the pH range 6-9, the first wave shifts to more negative potentials and the second shifts to less negative, until both waves intermingle forming a single wave. At the same time the third wave splits into two waves. The limiting current of the first wave formed by the fusion of the two small waves remains more or less constant within the pH range 6-9, whereas those of the two other waves decrease. The E,,, of the first and third waves shifts to more negative potentials, while the second remains almost constant on increasing pH. The simplest polarograms of the di-iodo-compound are obtained in solutions of pH > 9.5. These consist of two waves, of which the first is characterized by a well-developed plateau. At pH 10 the two waves are of equal height, whereas at pH M 11-O the height of the second wave becomes double that of the first. Under such conditions, the first wave corresponds to the first reduction step of the furane ring. The second wave is due to the completion of the reduction of the furane ring and the partial reduction of the other electro-active centre. The total current, however, shows an apparent decrease with pH in alkaline solutions. By comparison to the results obtained in the case of dichloro- and dibromoderivatives, the mechanism of reduction of di-iodofluorescein, especially at higher pHs, must be different from those of the other two compounds. D. The electrode

reaction

The electroreduction of fluorescein, eosin and erythrosirP7 proceeds according to two main mechanisms. The first mechanism involves the consumption of two

Polarography of dihalogen fluoresceins in solutions of varying pH

1199

electrons, in which case the furane ring is reduced. The reduction process may proceed either along a single or two waves depending on the pH of the medium and the nature of the reducible species. The second mechanism involves the uptake of four electrons, two of which are consumed in the reduction of the furane ring as in the first case and the other two for the reduction of the other electro-active centre. The dichloro- and dibromo-compounds are reduced via the first mechanism. However for these two compounds a small wave is observed at more negative potentials in buffer solutions of pH Q 7, which disappears at pH w 8. Investigation of that wave revealed that it is a composite irreversible one controlled by diffusion and could not accordingly be ascribed to adsorption of the oxidized form. Since the small wave is not observed in case of fluorescein, it may be due to the reduction of the pyrone ring or C-halogen bond. However, experimental data are not in favour of the reduction of the halogen atoms from the halogenated derivatives. The height of this small wave increases from the dichloro-, to dibromo-, to di-iodocompounds, ie the height decreases as the polarity of C-halogen bond increases_ Trials to test the splitting of halide ion during the electroreduction of eosin or erythrosin, by controlled-potential electrolysis, did not reveal the reduction of carbonhalogen bond. l1 Accordingly it seems that this wave is due to a partial reduction of the pyrone ring. Thus the electrode reaction can be considered to take place in the case of dichloroand dibromo-derivatives in accordance with the following scheme: (a) For solutions of pH < 7.0, the reduction of the furane ring proceeds along a single wave corresponding to two electrons. The second wave represents the partial reduction of the pyrone ring. The reduction of monovalent anions, which predominate at lower pH, can be represented as

1. First wave, with all reducible particles (main current)

II. Second wave. At pH < 7 for dichloroand dibromo-compounds with a certain proportion of the reducible species.

(b) For solution of pH > 10, the wave due to the reduction of the pyrone ring is not observed for dichloro- and dibromo-derivatives; the two waves observed are of almost equal height, each involving one electron. The splitting of the wave has been explained through the formation of an intermediate free radical established at higher PH.~ Since at higher pH, EII2 of the first wave of the two compounds is but slightly influenced by pH, it is assumed that H+ ions are not consumed in the electrode process proper. For the second wave, however, a proton shares in the electrode reaction. The reducible form could be the triphenylcarbonium ion, which exhibits varying

1200

I. M. ISSA, A. A. EL-SAMAHY, R. M. ISSA and M. M. GHONEIM

stability according to the nature of the halogen atom.lr The process of reduction be assumed to be

I. First wave.

II. Second wave.

In the case of di-iodofluorescein, the pre-wave observed at low pH and at the tail of the first reduction wave is an adsorption wave. As a result of the existence of the adsorption phenomenon, the reducible species comprises those leading to adsorption and others that are not adsorbed_ Thus the reduction wave splits into two waves lying less negative or more negative to the actual reduction potential, which may be evaluated from polarograms at higher pH where no adsorption is apparent. Accordingly, our discussion of the behaviour of di-iodofluorescein will be restricted to the behaviour above pH 8.0, where adsorption has little influence on the waves. In general the electroreduction of di-iodofluorescein involves the consumption of four electrons at all pHs, as in the case of erythrosin.ll Within the pH range 8-9 the electrode reaction can be represented as

zc2H+

Polarography of dihalogen fluoresceinsin solutionsof varying pH

1201

At pH > 9.5 the reduction of the furane and pyrone rings takes place along two waves, one due to the partial reduction of the furane ring (I) and the other to completion of the furane ring reduction and with that the pyrone ring (II + III).

The nature of the waves Analysis of the waves using the fundamental wave equation indicates that all the reduction waves at the different pHs are irreversible, and hence the number of electrons involved in the electrode process cannot be evaluated by this method. The plots of E,,,/pH for the dihalogen fluoresceins show a number of segments, Fig. 5. Generally the curve for the first wave consists of three segments covering the

.+6

-

-1.2 -

b.a

0.6

-

-

PH

FIG.5. E1,21pH curves.

c, 1st wave d, 2nd wave di-iodofluorescein. e, 3rd wave I

dichloroffuorescein;

pH ranges 2-7, 7-10 and >10, indicating that the electrode reaction changes within these pH ranges. The slopes of E,,,/pH curves are not the same for all the three compounds. This variation in the slope is due to the change in the value of transfer coefficient, oc, which can be calculated from4 0.059 AE,,,/A pH = zH +* a-h

This equation is valid only if the number of electrons in the rate-determining step is equal to that involved in the over-all electrode process. The calculated values of a vary between O-39-0.86. The values obtained for alkaline solutions are higher than those found at lower pHs, indicating that the electrode reaction tends to be less irreversible at higher pH, The effect of Hg pressure on the limiting current reveals that, at all pHs, the different waves at potentials more negative than the electrocapillary maximum are essentially

1202

I. M. ISSA, A. A. EL-SAMAHY, R. M. ISSA and M. M. GHONEIM

diffusion-controlled, especially in the case of dichloro- and dibromo-compounds; they show a partial kinetic contribution in case of diiodofluorescein. The value of x in the relation i = K/P is O-36-0-5. REFERENCES 1. P.D BWIHAY, Bull. Sot. chim. Fr. 348 (1948). 2. P. A. GOLLMICK and H. BERG, Ber. Bunsenges.phys. Chem. 69, 196 (1965). 3. N. R. BAand S. K. VIQ, J. them. Sot. 484 (1967). 4. H. Bwcs, Chem. Tech., Beri. 6, 585 (1954). 5. R. M. Iss~, F. M. AEDBL-HALIM and A. A. HAS~ANEIN, Electrochim. Actu 14, 561 (1969) 6. I. M. WA, Extr. Assiur Sot. Tech. Bull. 11, 41 (1959). 7. M. CARDMALL~, L. RAUEAZZA and A. TRAZZA (Unni Reme). Ric. sci. Rend. Sez. A 8, 1361

(1959). 8. I-S.T. S. BRITTON, Wjuirogen Ions, 4 ed., p. 313. Chapman & Hall, London (1952). 9. F. M. ABDEL&ALIM, R. M. ISSA, M. S. EL-EZABY and A. A. HASSANEIN,2. Phys., in press. 10. P. W. BOARD, D. BRITZ and R. V. HOLLAND, Electrochim, Actu 13, 1575 (1968). 11. I. M. ISSA, R. M. WA and M. M. GHONEIM, Electrochim. Actu, in press.