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Chemistry Journal <strong>of</strong> Moldova. M. Revenco General, et al./Chem.J. Industrial Mold. and 2008, Ecological 3 (1), Chemistry. 44-47 2008, 3 (1), 44-47NITRATE-SELECTIVE ELECTRODES BASED ON THE TRINUCLEARCHROMIUM(III) PIVALATESMihail Revenco a *, Mariana Martin a , Waell A.A. Dayyih baState University <strong>of</strong> Moldova, A.Mateevici street, 60 MD 2009, Chisinau, Republic <strong>of</strong> MoldovabUniversity Petra, Amman, Jordan*E-mail: revenco@usm.md; phone 373-22-57-74-94, 373-22-43-22-98, 373-69-19-48-91;fax 373-22-24-42-48Abstract: The paper describes the analytical potentialities <strong>of</strong> the trinuclear chromium(III) complexes as potentiometricionophores for the construction <strong>of</strong> electrodes sensitive to the presence <strong>of</strong> nitrate anion. The electroactive materialcontaining 4,4’-bipyridil was synthesized in situ. The membrane was prepared using dioctylphthalate as a solventmediator and poly (vinyl chloride) as a polymeric matrix. The electrodes presented a slope <strong>of</strong> 56 mV/decade, a lowlimit <strong>of</strong> detection (3,2 . 10 -6 mol/l), an adequate lifetime (4 months), and suitable selectivity characteristics whencompared with other nitrate electrodes. The good parameters <strong>of</strong> this electrode made possible its application to thedetermination <strong>of</strong> nitrate in different types <strong>of</strong> fertilizers.Keywords: Trinuclear chromium(III) pivalate, nitrate-sensors.IntroductionNitrate is an anion <strong>of</strong> major importance in environmental and biological areas. The necessity for nitratemonitoring in drinking water and food products is recognized by most health authorities. This ion, after chemicaltransformation, could have a direct impact on health because <strong>of</strong> its reaction with amines to produce nitrosamines, whichare known to be one <strong>of</strong> the most powerful carcinogens to mammalians. This is why, a lot <strong>of</strong> various methods includingspectrophotometry, electrochemical methods based on modified electrodes and nitrate-selective electrodes have beendeveloped for the determination <strong>of</strong> nitrate in different samples. Among these methods, potentiometric detection usingthe ion-selective electrodes [1-5] provides a rapid, simple, reasonably selective and inexpensive way for determinationthe content <strong>of</strong> this ion.Recently the trinuclear chromium(III) benzoates were investigated as ionophores in the membrane <strong>of</strong> the anionicsensors [7]. The lipophilic complex cation can be combined with some inorganic and organic anions, which are part <strong>of</strong>a number <strong>of</strong> pharmaceutical materials. Variation <strong>of</strong> the carboxylic acid anion and use <strong>of</strong> different amines <strong>of</strong>fer a realpossibility to elaborate new sensors <strong>of</strong> high performance level. Recently we have described an electrode, sensitive tothe presence <strong>of</strong> the perchlorate anion, based on the trinuclear chromium(III) pivalates [8].Therefore, using this type <strong>of</strong> chromium(III) complexes, we were prompted to study the response <strong>of</strong> a membraneto the presence <strong>of</strong> nitrate anions. One <strong>of</strong> these electrodes shows improved selectivity for some <strong>of</strong> the interferences whencompared with those for the commercially available nitrate-selective electrodes.ExperimentalReagents and solutionsDioctylphtalate (DOP) and poly (vinyl chloride) (PVC) <strong>of</strong> relative high molecular weight were used. All thereagents and solvents were <strong>of</strong> analytical reagent grade.A standard nitrate solution (0,1 mol/l) was prepared by carefully weighing solid sodium nitrate. Standardsolutions <strong>of</strong> all other anions were prepared from their respective sodium or potassium salts as 0,1 mol/l stock solutions.The pH adjustments were made with dilute sulphuric acid or sodium hydroxide solutions, as required. All solutionswere prepared with twice distilled water.Synthesis <strong>of</strong> ionophoresThe initial compound - [Cr 3O(C 5H 9O 2) 6(H 2O) 3]NO 3,(I) - was prepared using the described previously procedure[9] by dissolving 2 g <strong>of</strong> [Cr 3O(C 5H 9O 2) 6(H 2O) 3] C 5H 9O 2in 10 ml acetone , containing 0,5 ml <strong>of</strong> HNO 3solution (31%).The precipitated green crystals were filtered and washed with ether.The compounds [Cr 3O(C 5H 9O 2) 6(C 9H 7N) 3]NO 3II and [Cr 3O(C 5H 9O 2) 6(C 10H 8N 2) 3]NO 3(III) were prepared bydissolving I in acetone and addition <strong>of</strong> quinoline or 4,4’-bipyridil in stoechiometric ratio. The precipitated after aslow evaporation complexes were filtered, washed and dried at 105 0 C. The composition <strong>of</strong> the complexes has beenconfirmed by the elemental analysis for C, H, N, and Cr.The complexes I and II are soluble in ethanol, acetone, tetrahydr<strong>of</strong>uran (THF), nitrobenzene (NB), while thecompound III is poorly soluble in this solvents. All pivalates are insoluble in water.44
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