Electroanalytical Chemistry and Interfacial Electrochemistry, 41 (1973) 367-372 © Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
ADSORPTION OF METHYLENE BLUE ON A MERCURY ELECTRODE
S. ROFFIA and G. FEROCI
Centro di Studio di Elettrochimica Teorica e Preparativa lstituto Chimico "G. Ciamician", Universitd di Bologna, Bologna (Italy) (Received 1st May 1972; in revised form 1st September 1972)
In the last few years the study of adsorption phenomena at the electrode/ solution interphase has been of increasing interest because of the influence of adsorption on double layer structure and the mechanism of the electrode processes. One of the substances which has often been considered from the point of view of adsorption is methylene blue. After the pioneering work of Brdicka 1 on the polarographic behaviour of methylene blue, this substance has been examined by several workers. Lorenz and Schmalz 2 have determined the surface excess of both the oxidized and reduced forms as a function of applied potential by constant current potentiometry. Methylene blue has also been studied by Gupta 3 and Sancho and Hurtado 4 by a.c. current techniques. Mirri and Favero have investigated its behavior 5 with potential-sweep voltammetry, succeeding in giving a first quantitative account 6 of the shape and properties of the adsorption peaks. Using the same technique methylene blue has been investigated by Kemula et al. 7 A more rigorous treatment of the properties of the.potential-sweep voltammetric curves in the case of redox processes with adsorption has been carried out by Wopschall and Shain 8 and by Laviron9, who considered the reduction of methylene blue as a test for their theories. Direct measurement of the surface excess of methylene blue on the mercury electrode has been made by Los and Tompkins I 0 using a spectrophotometric method. A comparison of quantities describing adsorption on mercury of methylene blue and its leucoform as derived from polarographic and chronopotentiometric methods has been made by Tedoradze et al. ~ . Results on the determination of the surface excess of methylene blue by integration of potential-sweep voltammetric curves have recently been reported by the present authors ~2. Since these results appear to be in some respects in disagreement with those reported in the literature for similar experimental conditions we have thought it appropriate to reconsider the problem. Voltammetric results have now been compared with those obtained by interfacial tension measurements, which have been treated taking into account recent results on self-association of methylene blue. EXPERIMENTAL
All substances used were reagent grade chemicals. The methylene blue was used as 3,9-bis(dimethylamino)phenazothionium chloride (MBC1). Solutions were prepared as previous reported 13. All measurements were made at 25_0.1 °, in 0.1 M KC1 as supporting electrolyte. A saturated calomel electrode (SCE) was
S. ROFFIA, G. FEROCI
used as a reference electrode and all potential are referred to it. Potential sweep voltammetric curves were recorded by means of a model 448 instrument manufactured by Amel-Milano using a hanging drop mercury electrode (HMDE) of a type recently described 14. Values of charges reported throughout were derived by graphic integration of voltammetric curves suitably enlarged. The integration interval was 0 to - 1.2 V. The determination of the mercury/solution interfacial tension under conditions of adsorption equilibrium has been carried out with the HMDE, according to a method previously described ~5. THERMODYNAMIC ANALYSIS The thermodynamic analysis of the system studied in the preceding paper lz (solution of MBC1 in Britton-Robinson buffer pH 7.9) shows that it is impossible to determine the surface excess of the cation of MBC1, MB +, from the dependence of interfacial tension on MBC1 concentration. In fact from the basic electrocapillary equation 16, one can see that it i~s impossible to vary #MBCl while keeping all other chemical potentials and the applied voltage constant. However approximately constant values of chemical potentials other than PMacl can be obtained when the concentration of the counterion of MB ÷ is much higher than the MB ÷ concentration. This is why the adsorption of MB ÷ has been studied, in this work, i n the presence of excess counterion, i.e. CI-, supplied by 0.1 M KC1, instead of using Britton-Robinson buffer*. In this milieu (0.1 M KCI), assuming the constancy of the activity coefficients with changing MBC1 concentration, which is justifiable owing to the excess of KC1 present, the adsorption of MB ÷ can be obtained by the following approximate equation: / \ O7 - ,[ R T O In c,.,.+//T.,,.E...~, = nr°""÷ +"" + 2r, M,r + V,~B+ (1) where, apart from T, p, R that have the usual significance, ~ is the interfacial tension, E the voltage applied to the cell,/] the surface excess relative to water of species i and cj the concentration of species j. In deriving eqn. (1) from the basic electrocapillary equation16~ the electrode has been considered ideally polarized since the concentration of the reduced form is negligible at the potential at which the surface excess of MB ÷ has been determined by interfacial tension measurements (a potential which is the same as the initial potential of the voltammetric experiments). Furthermore, the assumption has been made that self-associationofMB + to formdimers(MB)22+ , trimers(MB)33+ , and higher multimers (MB),~ + occur. Account has also been taken of the fact that chemical potentials of various polymers of MB + are not independent, owing to the following relations: 3/.tMB+ = / / ( M B ) 3 3 + n~MB+ = ~ B ~
* The use of a non-bufferedsystem does not seem to introduce any appreciable error in comparison with a bufferedsystem(see later).
A D S O R P T I O N O F METHYLENE BLUE O N Hg
which stem from the following equilibria: 2 MB + = (MB) 2 + 3 MB + = (MB) 3÷ n MB + = (MB)."÷ E X P E R I M E N T A L RESULTS AND DISCUSSION
Owing to the fact that a different supporting electrolyte has been used in this study with respect to the previous one 12, we had to redetermine the charge Qads due to the reduction of MB ÷ adsorbed at the potential E = 0. This was done by means of linear potential-sweep voltammetry, using the same procedure cited in ref. 12. In particular, the voltammetric curves were recorded after a prepolarisation time long enough to make the total charge independent of it. For the lowest concentrations, for example, the constant maximum value of the total charge was reached after about half-an-hour with stirring. In Fig. 1 values of the total charge Qtot a r e plotted against the reciprocal of the square root of the potential sweep rate v and Qtot is shown to depend linearly on v -½ for all concentrations examined. The slopes of these lines are proportional to the bulk concentration of the depolarizer c. A c o m m o n intercept Qtot,~ equal to 1.65/~C, is found. On the basis 17"t8 of the following relation: ~2tot =
kc/v~ -J Qads "Jr-Qd .1.
where the constant k depends only upon the integration interval for a given tem10
Fig. 1. Charge/sweep rate plots. Methylene blue concns.: (i), 1 x 10 -4, (2), 2 x 10 -4, (3), 3× 10 -4, (4), 5 x 10 -4 M. Integration interval, 0 to - 1.2 V v s . SCE. Supporting electrolyte: 0.1 M KC1. Electrode surface: 2.3 x 10 -2 cm z.
S. ROFFIA, G. FEROCI
perature and surface area, a knowledge of the double layer contribution Qo.I. is necessary in order to evaluate from Qtot,o~the term Q,ds due to adsorption phenomena. The necessary datum has been obtained with a procedure already described 13. Briefly, the double layer charging current is considered constant over all the integration interval and in particular over the range where the faradaic process dominates. Its value is taken as the one, practically constant, obtained under adsorption equilibrium conditions for the oxidized form in the region where the faradaic process is negligible, i.e. at t h e beginning of the voltammetric curve. Since Qd.~. is found to be constant for the concentrations examined, its value being 0.14/~C, it follows that Qads is also concentration independent and equal to 1.51 #C. Taking into account the value of 1.04 for the ratio between the two surface areas utilized in this work and the previous one 12, the same charge per cm 2 obtains, i.e. 65 #C cm-2. This value evidently corresponds to conditions of surface saturation as is shown by its concentration independence. On the assumption that the number of electrons involved in the reduction process of dimer, trimer and n-polymer is four, six and 2n respectively, in accordance with the number of pairs of electrons found per molecule of monomer, one can write the following relation: nF(MB),"+ + . . . + 2F(MB)I+ + FM,+ = Q,as/2FA
where A is the surface area. For the left-hand side of (2), 3.4(1 +0.1)x 10-lo mol cm-2 is obtained. Its reciprocal, divided by the Avogadro constant, gives also the average surface available for a monomer molecule. This area was found to be 50 ~z. We have compared this experimental value with that calculated on the basis of a model where the MB + aromatic rings lie parallel to the electrode surface, using bond distances and van der Waals radii reported in the literature 19. This orientation seems the most likely because of the favourable interaction of the aromatic rc electrons with the positively charged mercury surface. This model gives 98 /~2. The disagreement between the two values can be rationalized if it is assumed that, under equilibrium conditions, the MB + is adsorbed, on average, as a bimolecular layer. The fact that other authors 1'2'8 have concluded in favour of a monomolecular layer is probably due to the lack of equilibrium conditions in their experiments. The prepolarization times used by us were much longer than those of the authors cited above. In order to check the surface excess value obtained by potential-sweep voltammetry and to test the hypotheses, we have also proceeded to determine this quantity independently by means of equilibrium interfacial tension measurements with varying MBCI concentrations. Comparison of(l) with (2) shows in fact that the left-hand side of (2) can be obtained experimentally from: - (OT/RT 6 In CMB*)T,P,E,cKc,
The equilibrium interfacial tension has been determined at 0 V in solutions of 10-4-10 -3 M MBC1 in 0.1 M KC1. Owing to the excess KCI pre~ent, the MBC1 concentration dependence of the liquid junction potential, associated with the use of a fritted disk, was within the error limits and consequently ignored. The evaluation of the differential coefficient mentioned above requires a knowledge of the monomeric MB + concentration, CMB+. According to the literature data derived from vapor
A D S O R P T I O N O F M E T H Y L E N E B L U E O N Hg
pressure osmometry, absorption spectra 2° and polarographic 21 measurements, dimers and trimers are the only multimers present in relevant concentration and these should be taken into account. The concentration of MB ÷ has been calculated using for the equilibrium constants K 2 = C(MB)~+/C2B+ and K 3 = C(MB)] +/K2C3MB* , respectively, the values 2 × 103 and 3 × 103 1 mol- 1. Values of CMB+ differing by only a few percent are obtained using the equilibrium constants K 2 and K 3 reported by Ghosh and Mukerjee 22.
log (CMB÷/rnol 1-1) Fig. 2. Dependence of mercury/solution interracial tension, ?, upon concn, of methylene blue cation in monomeric form, CMB+. Polarisation potential: 0 V v s . SCE.
Figure 2 shows the dependence of 7 on log CMB+. This plot is linear, in agreement with the results of voltammetric experiments under surface saturation conditions. The value of the differential coefficient ( O 7 / R T ~ In CMB+)r,P,e,~:c, obtained from the slope of this straight line is 3.4( 1 + 0.05) x 10-10 mol cm- 2, which is in very good agreement with that obtained from potential-sweep voltammetry. This result lends support to the correctness of the assumptions made in deriving the surface excess from voltammetric experiments. On the other hand, neglecting association phenomena altogether in the evaluation of CMB*,a value of surface excess lower by about 40~o from that derived from voltammetric experiments is obtained. A negative difference of 17~ is observed taking into account only the dimer. Evidently, consideration of association phenomena in terms of the presence of both dimers and trimers in solution leads to a better consistency as between thermodynamic and voltammetric results. ACKNOWLEDGEMENT
The authors are indebted to Professor E. Vianello for valuable suggestions and stimulating discussion. They also thank Dr. R. Parsons for very useful comment and advice. This work has been supported by CNR under contract No. 70.00102/03 115.2113. SUMMARY
The adsorption of methylene blue cation has been studied on the mercury electrode by linear potential-sweep voltammetry and by interfacial tension measure-
s. ROFFIA, G. FEROCI
ment. T h e a m o u n t of m e t h y l e n e blue c a t i o n a d s o r b e d at different c o n c e n t r a t i o n s has been d e t e r m i n e d b y e x t r a p o l a t i o n f r o m p l o t s o f the integrals o f the c u r r e n t / p o t e n t i a l curves v e r s u s the r e c i p r o c a l o f the s q u a r e r o o t o f the p o t e n t i a l - s w e e p rate. T h e surface c o v e r a g e thus d e r i v e d is in very g o o d a g r e e m e n t with the surface excess o b t a i n e d f r o m e q u i l i b r i u m m e r c u r y / s o l u t i o n interfacial t e n s i o n m e a s u r e m e n t s , which have been t r e a t e d t a k i n g into a c c o u n t the self-association o f m e t h y l e n e blue cation. T h e results o b t a i n e d a r e c o n s i s t e n t with the h y p o t h e s i s of the f o r m a t i o n , o n average, o f a b i m o l e c u l a r layer of m e t h y l e n e blue c a t i o n with fiat o r i e n t a t i o n of the molecule with respect to the e l e c t r o d e surface.
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