The mechanism of the cobalt(II)-catalyzed electro-generated chemiluminescence of luminol in aqueous alkaline solution

Analytica

Chimica

Elsevier Scientific

Acta, 141 (1982) 263-268 Publishing Company, Amsterdam

THE MECHANISM

Printed

in The Netherlands

OF THE COBALT(II)-CATALYZED

GENERATED CHEMILUMINESCENCE ALKALINE SOLUTION

KEIJO

-

ELECTRO-

OF LUMINOL

IN AQUEOUS

E. HAAPAKKA

Department

(Received

of Chemishy,

23rd December

University

of Turfxc, SF-20500

Turku

50 (Finland)

1981)

SUMiMARY A rotating

ring-disc

electrode

system

is used where

the disc electrode

(carbon)

is

maintained at a negative potential to reduce oxygen to hydrogen peroxide, and a symmetric double-step potential is applied to the ring electrode (platinum). Cobalt(LI) catalyzes the electrogenerated chemiluminescence of luminol at the ring electrode during the negative pulse of the double-step potential. A possible reaction scheme for this

cobaIt(II)catalyzed

emission

process

is outlined.

Metal ions generally catalyze luminol chemihuninescence in aqueous alkaline solutions when hydrogen peroxide is used as an oxidant. Much work has been done to elucidate the role of the metal ion [l-5]. It has been proposed that the metal ion-luminol complex is a necessary intermediate for light emission, or that the metal ion acts as a catalyst for hydrogen peroxide decomposition, generating a metal ion-hydrogen peroxide complex which reacts with luminol. According to Gillard and Spencer [6] superoxodicobalt(II1) complexes react with luminol in aqueous alkaline solutions to generate light emission. The mechanism of the electrogenerated chemiluminescence of luminol in aqueous alkaline solutions has been studied [ 7-91, and its feasibility for trace determinations of copper(I1) has been indicated [lo] _ An apparatus which utilizes a rotating ring-disc electrode system has been constructed for mechanistic and analytical studies of luminol electrogenerated chemiluminescence [ll]. A symmetric double-step potential is applied to the ring and the disc is maintained at a negative potential to reduce oxygen to hydrogen peroxide, which is transported to the ring electrode by the electrode rotation. Under these conditions, the luminol luminescence is generated at the ring electrode during the positive pulse of the double-step potential. This has been described in detail [X2]. Cobalt(I1) is also capable of catalyzing the luminol luminescence at the ring electrode during the negative pulse of the double-step potential; this has been utilized for the trace determination of cobalt(I1) [13]. In the present paper, a possible reaction scheme for the cobalt(II)-catalyzed electrogenerated chemiluminescence of luminol in 0003-2670/82/0000-0000/$02.75

0 1982

Elsevier Scientific

Publishing

Company

261

aqueous alkaline solution is proposed. The apparatus, reagents, procedures and method of measurement have already been described [ll-131. RESULTS

AND

DISCUSSION

The cobaIt(II)-catalyzed luminescence, i.e., the cathodic chemiluminescence of luminol, has heen illustrated (see fig. 5 [13])_ An inhibiting effect of cobalt(I1) on the luminescence was found during the positive pulse of the double-step potential, i.e., on the anodic luminescence. Also, in the presence of cobalt(H), the baseline (the intensity detected at zero potential) rose continuously during the luminescence measurement. The cathodic intensity was greatest at the platinum-glassy carbon (R-C) ring-disc electrode [ 131. The mechanism of the cathodic luminescence at this electrode was therefore examined further. A pulsed potential with alternate zero and negative pulses was applied to the ring electrode instead of the symmetric double-step potential, but the cathodic luminescence was not observed. This indicates that some oxidation process at the ring electrode during the preceding p ositive pulse is necessary for the generation of the cathodic chemiluminescence. The effect of disc potential on the cathodic intensity has been discussed [ 13]_ Voltammetric measurements at the glassy carbon disc electrode indicated that oxygen reduction to hydrogen peroxide with a peak potential -0.70 V [12] was the only reduction process occurring under the conditions of the chemiluminescence measurements, and that luminol and cobalt(I1) had no effect on this reduction. Consequently, the enhancing effect of disc electrode potential on the cathodic luminescence must be caused by the generation of hydrogen peroxide by oxygen reduction. Role of cobalt(U) Voltammetric measurements on 1.00 X lo-” M cobalt(I1) under the electrogenerated chemihuninescence conditions revealed no oxidation or reduction process of cobalt(H) at the platinum electrode over the range +l.O to -0-S V. Furthermore, cobalt(I1) had no effect on the luminol electrooxidation [ 121. On this basis, the cathodic luminescence of luminol does not originate from any electrochemical process of cobalt(I1). In the presence of ligands having nitrogen or nitrogen and oxygen donor atoms, cobaIt(I1) is capable of reacting with molecular oxygen to yield peroxo-dicobalt(IIi) complexes [14] which are oxidized by hydrogen peroxide to superoxo-dicobalt(III) complexes [ 151. Superoxo-dicobalt(II1) complexes react with luminol in aqueous alkaline solutions, emitting light [S] . As luminol has one amino and two carbonyl groups, it may induce the formation of a peroxo-dicobalt(III) complex which is oxidized by hydrogen peroxide in the solution flowing between the disc and ring electrodes to give a superoxo-dicobalt(III) complex. The other possible active form of cobalt is a cob&(II)-hydrogen peroxide complex [ 51 also generated in “thesolution flowing between the two electrodes_

In order to shed light on these possible active forms of cobalt, the effects of different ligands on the intensity of the cathodic luminescence were determined. The results are presented in Figs. 1 and 2 (cf. fig. 11 [13] )_ Ammonia, tetraethylenepentamine and triethylenetetramine induce the formation of superoxo-dicobalt(II1) complexes [14, 151 but, as shown in Figs. 1 and 2, these ligands strongly inhibit the cathodic luminescence of luminol. Hence the superoxo-dicobaIt(III) complex is not the species of cobalt(I1) capable of catalyzing the cathodic luminescence. It is suggested, therefore, that the cobalt(II)-hydrogen peroxide complex is the active form of cobalt. As shown earlier [ 131, EDTA strongly inhibits the electrogenerated luminescence. EDTA forms a 1:l cobalt(I1) chelate where all the coordination sites of cobalt(I1) are filled; this prevents the formation of the cobalt(II)-hydrogen peroxide complex, thus inhibiting the effect of cobalt on the luminescence. Because iminodiacetic acid and aminoacetic acid form 1:2 and 1:3 chelates to fill all the coordination sites of cobalt(II), the inhibiting effects of these ligands [ 131 can be explained similarly. Effect

of pulse amplitude

Figure 3 shows the effect of pulse amplitude on the intensity of the cathodic and anodic electrogenerated chemiluminescence of luminol. The cathodic and anodic light pulses at pulse amplitudes of 0.85 V and 1.40 V are shown in Fig.4.

log ‘Ammonia

‘@g =Ligand

Fig. 1. Effect of ammonia on the cathodic luminescence of luminol at the Pt-C ringConditions: 0.100 Iv1 NaCl, disc disc electrode, intensity without ammonia = 100. potential-l.00 V, pulse amplitude 0.625 V, pH lO.O,H,BO,-NaOH buffer, 1.50 X lO_ M solutions luminol, rotation rate 350 rpm, pulse length 35.0 ms, 5.0 x 10d iM cobalt(II), saturated with oxygen. Fig. 2. Effects of (0) tetraethylenepentamine luminescence; (0) and (e) indicate points ligand are equal. Conditions as in Fig. 1.

and (0) triethylenetetramine on the cathodic where the concentrations of cobalt(H) and

266

I

I

0.40

0.90 Pulse

I

1.40

Amplitude(V)

Fig. 3. Effect of pulse amplitude on (a) cathodic and (b) anodic luminescence (arbitrary units) of luminol at the Pt-C ring-disc electrode: (0) without cobalt(I1); (I) with 5.0 x lo* M cobalt(I1). Conditions: 1.00 x 10q M luminol, otherwise as for Fig. 1. Fig_ 4. The variation of luminoi luminescence in the presence of cobalt(I1) ring-disc electrode, with different pulse amplitudes: (a) the symmetric potential; (b) 0.85 V, (c) 1.40 V. Conditions as in Fig. 3.

at the Pt-C double-step

The necessary electro-oxidation process for the generation of the cathodic luminescence can be either the formation of an oxide layer on the surface of the platinum ring electrode or luminol oxidation, with peak potentials of O-50 V and 0.67 V, respectively [12]. Luminol and its oxidation product are not reduced at the platinum electrode. As concluded from Figs. 3 and 4, the anodic luminescence intensity is much suppressed by large pulse amplitudes, but this suppression has practically no effect on the cathodic intensity. This indicates that luminol oxidation is not needed for generation of the cathodic luminescence but the emission is generated only at the oxidecovered surface of the platinum ring electrode. Support for this view is obtained from the fact that the cathodic emission is not generated at a gold ring electrode [133, the surface of which is mainly free from an oxide layer under the electrogenerated chemiluminescence conditions [12]_ The role of the oxide layer in the generation of the cathodic luminescence of luminol can be expkined in the following way. During the positive pulse, . the ring electrode surface is covered by the oxide layer. This lowers the Iurninol oxidation rate so that the luminol adsorbed onto the electrode is

267

not completely oxidized before the end of the pulse (see fig. 5 [ll]

). During the negative pulse, the oxide layer is reduced with the peak potential of

-0.12 V il2]. This releases the “unoxidized luminol” and increases momentarily the luminol concentration in the vicinity of the ring electrode. The

cobalt(H)-hydrogen peroxide complex reacts with this “unoxidized to generate the cathodic luminescence. The role of the oxide layer is strongly supported by the fact that the end of the negative pulse has no effect on the cathodic emission pulse [ 131, which that the lightremitting reaction does not take place at the ring surface but in its vicinity. Possible

mechanism

of the cobalt(II)-catalyzed

cathodic

luminol” proposed potential indicates electrode

luminescence

A mechanism for the uncatalyzed electrogenerated chemihuninescence of luminol has already been proposed [ 121. The emitter is considered to be 3-aminophthalate [8, 161. Under the present conditions, the prevailing species of luminol is its monoanion [l’i, 181 with which cobalt(I1) probably forms a mixed complex that also involves hydroxide ions. The electrode

rotation gives a laminar flow of the solution from the disc to the ring electrade. Some of the oxygen thus transported to the disc electrode is reduced to hydrogen peroxide (pK, = 11.65 [19]), which reacts with the mixed cobalt(I1) complex to yield a cobalt(II)-hydrogen peroxide complex. During the positive pulse, the ring is covered by an oxide layer, which causes the production of “unoxidized luminol” at the beginning of the negative pulse, as described above; this reacts with the cobalt(II)-hydrogen peroxide complex and light is emitted. The luminol anion reacts with the cobalt(II)-hydrogen peroxide complex yielding cobalt(II1) ion and a luminol radical [5]. The luminol radical does not react with hydrogen peroxide or its anion to generate light [20], hence it is proposed that the luminol radical anion is oxidized by cobalt(II1) ion to yield 3aminoazaquinone, which is capable of reacting with the hydrogen peroxide anion with emission of light [21-231. According to Eriksen et al. [22], this reaction directly generates a peroxide intermediate of luminol. On decomposition, the basic form of this intermediate generates excited The acidic peroxide intermediate decomposes via 3aminophthalate. a non-luminescent reaction [12] _ The reaction of the luminol radical with oxygen is the other possible lightemitting pathway, but this involves an additional intermediate, a luminol peroxide radical which is reduced to the peroxide intermediate of luminol.

268

pK,=9.3

ll
Unfortunately, the results of this study do not allow a definitive decision about the light-emitting pathways to be made. However, it is tempting to propose that the pathway based on the reaction of 3-aminozzaquinone with the hydrogen peroxide anion is mainly responsible for the cathodic luminescence of luminol because of the low intensity of the oxygen pathway [12] _ Financial support of this study from gratefully acknowledged.

the Finnish

Cultural

Foundation

is

REFERENCES 1 2 3 4 5 6 7 S 9 10 11 12 13 14 15 16 17 18 19 20 21 32 23

L. Erdey, W. F. Pickering and C. L. Wilson, Talanta, 9 (1962) 653. H. Ojima, Nippon Kagaku Zasshi, 84 (1963) 909; see Chem. Abstr., 61 (1964) 6439f. -4. K. Babko and L. I. Lukovskaya, Russ. J. Phys. Chem., 12 (1967) 87. Y. N. Kozlov, Y. V. Koltypin, B. A. Rusin and Y. I. Skurlatov, Russ. J. Phys. Chem., 49 (1975) 1182. T_ G. Burdo and W. R. Seitz, Anal. Chem., 47 (1975) 1639. R. D- Gillard and A_ Spencer, J. Chem. Sot. A, (1970) 1761. N. Harvey, J. Phys. Chem., 33 (1929) 1456. B. Epstein and T_ Kuwana, Photochem. Photobiol., 4 (1965) 1157. B. Epstein and T. Kuwana, Photochem. Photobiol., 6 (1967) 605. K. E. Haapakka and J. J. Kankare, Anal. Chim. Acta, 118 (1980) 333. K. E. Haapakka and J. J. Kankare, Anal. Chim. Acta, 138 (1982) 253. K. E. Haapakka and J. J. Kankare, Anal. Chim. Acta, 138 (1982) 263. K. E. Haapakka, Anal. Chhn. Acta, 139 (1982) 229. R. D. Jones, D. A. Summerville and F. Basolo, Chem. Rev., 79 (19?9) 139. F. Basolo and R. G. Pearson, Mechanisms of Inorganic Reactions, Wiley, New York, 1967, p. 641. D_ F. Roswell and E. H. White, Methods Enzymol., 57 (1978) 409. L. Erdey. I. Buzas znd K. V&h, Talantz, 13 (1566) 463. K. E. Haapakka, J. J. Kankare and J. A Linke, Anal. Chim. Acta, in press. D. D. Perrin, Dissociation Constants of Inorganic Acids and Bases in Aqueous Solutions, Butterworths, London, 1969, p_ 170. G Merenyi and J. S. Lind, J. Am. Chem. Sot., 102 (1980) 5830. E. H. White, E. G. Nash, D. R. Roberts and 0. C. Zafiiiou, J. Am. Chem. Sot., 90 (1968) 5932. II. Isacsson, J. Kowalewska and G. Wettermark, J. Inorg. Nucl. Chem., 40 (1978) 1653. T. E. Eriksen, J. S. Lind and G. Merenyi, J. Chem. Sot. Faraday Trans. 1, 77 (1981) 3127