Paper

Study of nitrite electro-oxidation mediated (electro-catalyzed??) by Co(II)-[tetra(4-aminophenyl)porphyrin] modified electrodes. Behavior as amperometric sensor.

Abstract
The electro-oxidation of nitrite mediated (catalized?) by Co(II)-[tetra(4-aminophenyl)porphyrin] modified electrodes (4-CoTAPP) is presented in this paper. Catalytic?? Activity toward of electrodes modified with porphyrinpolymeric films (poly-4-CoTAPP) and with multilayer of monomers (m-4-CoTAPP) for the oxidation reaction of nitrite was compared. It was found that the number of cycles for the electrosynthesis of the polymer poly-4-CoTAPP determines the activity of the electrode toward nitrite oxidation since the system loses its catalytic activity as the number of polymerization cycles increases above fifteen. Theeffect of pH on the activity of this reaction is also reported. . It was found that this reaction is pH dependant. Besides, the response for nitrite oxidation of electrodes modified with 4-CoTAPP and with metal-free ligand, i.e., tetra(4-aminophenyl)porphyrin, either as multilayer of monomers or as polymer (m4-H2TAPP and poly-4-H2TAPP respectively), was compared. An increase of activity in thepresence of the metal was found. Furthermore, when the device was tested as measuring the amperometric sensor response as compared to for different nitrite concentrations under the optimum working conditions, only the poly-4-CoTAPP exhibited a linear response throughout the concentration range studied, while m-4-CoTAPP and poly-H2TAPP systems have no linear response throughout the entire range ofconcentration. Finally the number of electrons involved and the Tafel slope for the reaction were estimated. 123mV/dec, indicating that the slow step of the reaction is the first potential-dependent electron transfer.
1. Introduction
Nitrite ion is important for both the environment and biological processes. Nitrite is one of the active intermediate species in the nitrogen cycle. These ions areoften used as fertilizer, which has an environmental impact through pollution of water sources. Nitrites are also added to food as preservative. Concern about the role of nitrite ion as precursor in the formation of N-nitrosamines, many of them carcinogenic compounds found in nature, [1-3] has increased the interest in its research. A variety of techniques have been reported for the determinationof nitrite ion, e.g., chromatography [4,5], spectrophotometry [6-7], and electrochemistry [8-10]. Electrochemical detection techniques offer advantages over other analytical approaches, e.g., faster, cheaper, and safer. The electrochemical determination of nitrite can be performed either by reduction or oxidation. Nitrite oxidation has the advantage of being interference-free from nitrate andmolecular oxygen, which are usually the main constraints in the cathodic determination of nitrite [11,12]. The determination of this ion is difficult at conventional electrodes, first, because nitrite oxidation requires high potentials [13] and secondly, due to poisoning of the electrode by the species produced during the electrochemical process [14 ]. In addition, electrochemical determination ofnitrite is prone to interference from other easily oxidizable substances. Therefore, the modification of electrode surfaces with suitable catalysts has arisen much interest owing to the possibility of obtaining nitrite oxidation at low potentials. Besides, sometimes it is possible to enhance sensitivity and selectivity of the electrode by immobilizing materials which form three-dimensional networkscontaining suitable electro catalytically active sites. In this way, improvement of the electrode activity is possible [15]. Nitrite oxidation at modified electrodes has been extensively described. These methods include carbon paste electrodes chemically modified with N, N'-bis (salicylaldehyde)-4-methyl-1, 2 phenylenediimino oxovanadium (IV) [16], glassy carbon electrodes modified with...