Feature Article Highly Selective PVC-Membrane Electrodes Based on Three Derivatives of (Tetraphenylporphyrinato)Cobalt(III) Acetate for Determination of Trace Amounts of Nitrite Ion Mojtaba Shamsipur,* a Mehran Javanbakht, b Ali Reza Hassaninejad, c Hashem Sharghi, c Mohammah Reza Ganjali, d Mir Fazlollah Mousavi b a Department of Chemistry, Razi University, Kermanshah, Iran * e-mail: mshamsipur@yahoo.com b Department of Chemistry, Tarbiat Modarres University, Tehran, Iran c Department of Chemistry, Shiraz University, Shiraz, Iran d Department of Chemistry, Tehran University, Tehran, Iran Received: October 28, 2002 Final version: January 29, 2003 Abstract Three different para-substituted derivatives of (tetraphenylporphyrinato) cobalt(III) acetate were examined as suitable ionophores for the preparation of polymeric membrane (PME) and coated graphite (CGE) nitrite-selective electrodes. The electrodes revealed a Nernstian behavior over very wide NO 2  ion concentration ranges (1.0  10 6  1.0  10 1 M for the PME and 5.0  10 8  5.0  10 2 M for the CGE based on the p-isopropyl derivative) and very low detection limits (8.0  10 7 M for the PME and 2.0  10 8 M for the CGE). The potentiometric response is independent of the pH of solution in the pH range 4.5 ± 7.0. The electrodes manifest advantages of relative long lifetime, fast response and, most important, good selectivities relative to a wide variety of inorganic and organic anions. In fact the selectivity behavior and detection limit of the proposed NO 2  ion-selective electrodes show great improvements compared to the previously reported electrodes for nitrite ion. The proposed PMEs and CGEs can be used for at least 2 and 4 months, respectively, without any divergence in potentials. The electrodes were successfully applied to the determination of nitrite ion in different real samples such as sausage, milk, flour and cheese. Keywords: Nitrite ion-selective electrodes, PVC membrane, Coated graphite, Co(III)-porphyrin, Potentiometry 1. Introduction There is a current urgent need for the quick determination of trace quantities of anionic species by simple methods in a variety of chemical, clinical and environmental samples. Due to their ease of preparation, simple operation, fast response, reasonable selectivity and wide linear dynamic range, the solvent polymeric membrane ion-selective elec- trodes incorporating suitable ionophores have shown to be very useful tools for these purposes [1 ± 6]. However, anion complexation and the design of suitable sensing elements for anionic species are far less developed than the field of cation receptors [7, 8]. Because of the important role of nitrite ion in producing toxic, carcinogenic and/or mutagenic N-nitrosamines in human body, in one hand [9], and its increasing use as a preservative in the food industry, in the other hand [10, 11], the determination of this anion in trace levels is of special interest. Besides a large number of analytical methodologies applicable to nitrite quantification based upon electro- chemical [12 ± 17], spectrophotometric [18 ± 22], chromato- graphic [23 ± 27] and electrophoretic techniques [28 ± 30], several nitrite ion-selective electrodes based on Co(III)- aquocyanocobyrinate [31], Co(III)-phthalocyanine [32], Co(III)-tetraphenylporphyrin chloride and nitrite deriva- tives [33,34], In(III)-tetraphenylporphyrin chloride deriva- tives [35], benzylbis(triphenylphosphine) palladium(II) [36] and UO 2 -salophen [37] have also been reported in the literature. However, these nitrite ion-selective electrodes suffer from narrow linear ranges, high limits of detection and, especially, strong interference effects, from such anionic species as perchlorate, thiocyanate and iodide. We have recently shown that by the application of coated graphite technology to the PVC-membrane potentiomen- tric sensors via replacing an internal solution of the target ion, in the case polymeric membrane electrodes, by a copper wire, in the case of coated graphite electrodes, the selectivity and the sensitivity of the potentiometric sensors will enhance significantly. Our recently reported coated graph- ite ion-selective electrodes for such ionic species as Ce 3 [38], Cu 2 [39], Pb 2 [40] Ag  [41], SCN  [42] and SO 4 2 [43] clearly revealed a great enhancement in the selectivity behavior as well as a highly improved limit of detection down to some 10 8 M. In this article, we report on the use of the acetate salts of three Co(III)-tetraphenylporphyrin derivatives (i.e., Co(III)T(p-X)PP acetate with X  H (I1), X  Me (I2) and X  isopropyl (I3)) as excellent ionophores for the preparation of new highly selective and sensitive polymeric membrane (PME) and coated graphite (CGE) electrodes for nitrite ion. 1251 Electroanalysis 2003, 15, No. 15-16 ¹ 2003 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim DOI: 10.1002/elan.200302805