Use of cobalt(II) phthalocyanine to improve the sensitivity and stability of glassy carbon electrodes for the detection of cresols, chlorophenols and phenol
Introduction
Analysis of water for phenolic compounds is of importance since many of these compounds have established a reputation for their toxicity and persistence in the environment. Electrochemical methods have been used successfully in the detection of some phenolic compounds since most phenols are oxidised at readily accessible potentials. There have been several reports on the voltammetric behaviour of phenol 1, 2, 3, 4and cresols 5, 6. Mechanisms for the oxidation of cresols and phenol have been proposed 1, 5. It has been suggested that a free radical is formed during the initial one-electron oxidation of phenol or cresol. The coupling of the radicals followed by intramolecular rearrangements resulted in the formation of electroactive dimeric products and/or long chain polymeric species 1, 5. This coupling was more favoured in acidic media. The voltammetric behaviour of chlorophenols has also received a considerable amount of attention 6, 7, 8.
The main disadvantage of the use of voltammetry for the determination of the phenolic compounds is the fouling of the electrode by the dimeric or polymeric oxidation products 1, 5, 6. Thus the voltammetric response for the detection of the phenolic compounds decreases on the second and subsequent scans, making the use of electrochemical methods for accurate determination of these species problematic. Many investigations have been directed at solving this problem. Ways of reactivating the electrode following voltammetry have been explored [9], but the best results were obtained by modification of the electrode surface with polymeric substances [6]. Metallophthalocyanine (MPc) complexes have been used successfully in modifying electrodes, hence aiding in the detection of many substances including thiols, amino acids, nitrates and nitrites 10, 11, 12, 13, 14, 15. In this work, we report on the use of MPc-modified glassy carbon electrode (MPc-GCE) for the detection of o-, m-and p-cresols, and of 2-chlorophenol, 4-chlorophenol and phenol. MPc complexes are known to improve the sensitivity of glassy carbon electrodes by increasing the current response and/or lowering the oxidation potentials for species such as thiols and amino acids 11, 14.
Cobalt(II) phthalocyanine (CoPc) and its derivatives show remarkable catalytic activity for many reactions 12, 14, 15. The catalytic activity of MPc complexes is a function of the central metal ion, and MPc complexes that contain electroactive central metal, e.g. CoPc and FePc, show higher catalytic activities than those MPc complexes with ring-based redox processes, e.g. NiPc and ZnPc. We compare the catalytic activities of the CoPc species with those of the other first row transition metal phthalocyanine complexes.
Section snippets
Experimental
o-cresol (BDH), m- and p-cresols (Saarchem), 4-chlorophenol, 2-chlorophenol and phenol (BDH) were used without further purification. The phthalocyanine complexes of cobalt (CoPc), iron (FePc), manganese (MnPc), nickel (NiPc), copper (CuPc) and zinc (ZnPc) and metal-free phthalocyanine species were purchased from Kodak or Aldrich and used without further purification. The tetrasulfophthalocyanine complex of cobalt(II), Na4[Co(II)TSPc], was prepared and purified according to the method of Weber
Cyclic voltammetry of the cresols and phenols on MPc-GCE
Fig. 1(a) shows the cyclic voltammogram obtained for a CoPc-GCE immersed in the blank 0.05 mol dm−3 H2SO4 solution. As has been observed before [14]for CoPc carbon paste electrodes, the CoPc-GCE exhibited a single anodic wave when cycled in the blank solution. Three relatively small reductions were observed on the reverse scan. The anodic wave was observed at 0.88 V vs. Ag/AgCl (Table 1) and is associated with the oxidation of Co(II)Pc to Co(III)Pc species [14]. Fig. 1(b) and (c) show the cyclic
Acknowledgements
This work was supported by Rhodes University and by the Foundation for Research Development of South Africa. TM thanks the Lesotho Government for a graduate bursary.
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