chemistry journal of moldova

M. Revenco et al./Chem.J. Mold. 2008, 3 (1), 44-47The 4,4’-bipiridil was suitable for this purpose. The interaction of pivalate I with 4,4’-bipyridil in stoechiometric ration2:3, leads to formation of a product III, which one is poorly soluble both in water and in the used for preparation themembrane solvents. Use of a saturate in nitrobenzene solution and incorporated in membrane, does not give rise to anitrates sensitive electrode. This behavior demonstrates the formation of a polymer with very low solubility, whichcannot assure a sufficient concentration to prepare a PVC membrane.To achieve the objective we have tried the synthesis of the active material in situ [8], directly in membraneduring its formation. The complex I, 4,4’-bipiridil and the other components used for preparation of the membrane ofISEI, all together, were mixed stirred and left for evaporation. During this time part of water molecules was substitutedby 4,4’- bipyridil, and a new polymeric structure has appeared and plays the role of ionophore. We suppose, that thepresence of the PVC does not allow the formation of the crystalline polymer, but it ensures the lacing of a number oftrinuclear unities encapsulated by the solvent containing nitrate as counter ion.Washing-out of this compound from membrane is surely hindered. The potentiometric response of ISE3,containing the ionophore synthesized in situ, was linear with a Nernstian slope of 56 mV/decade within the NO 3-concentration range 2 . 10 -5 -1 . 10 -1 mol/l. Detection limit is 3,2 . 10 -6 mol/l (Fig. 1).The reponse time of the electrode was 15 s over the entire linear concentration range of the calibration plot.In order to study the effect of pH on the performance of ISE3, the responses of the sensor to NO 3-ion at differentconcentrations of H + ion were measured. It was observed that the sensor exhibite a stable behavior with a constantpotential in the pH range 2 –11. The Fig. 2 shows the variation of the potential of the sensors immersed in a solutionwith the concentration 0,001 M. To achieve stable parameters the preconditioning time must be close to 12 h in a 0,1mol/l solution of NaNO 3. The drift of the potential for a constant concentration is negligible. The electrodes preparedby this method show stable parameters during 4 months. During this period, the electrode was kept in a 0,1 mol/l NaNO 3solution and calibrated periodically. No significant change in the performance of the electrode was observed.320E, mV3002802600 1 2 3 4 5 6 7 8 9 10 11 12 13 14Fig. 2. Influience of pH on the response of the ISE3 immersed in a solution of NaNO 3(0,001 M)The performances of a selection of different electrodes published in literature are shown in the Table 1. As it canbe observed the parameters of the proposed electrode are comparable with of available nitrate-selective electrodes.Table 1Parameters of a selectoion of nitrate-selective electrodesConcentration range, mol/l Slope, mV/decade Detection limit, mol/l pH range Lifetime Reference1 . 10 -1 – 1 . 10 -6 3,4 . 10 -7 2 - 8 [1]1 . 10 -1 – 2 . 10 -5 58 5 - 11 [2]60 4,2 . 10 -6 1 year [3]54-55 5,0 . 10 -5 [4]1 . 10 -1 – 1 . 10 -5 54,7 2,3 . 10 -6 2 - 12 [5]1 . 10 -1 – 2 . 10 -5 56 3,2 . 10 -6 2 -11 4 months ISE3Selectivity of the electrode ISE3The potentiometric selectivity coefficients were determined by fixed interference method [10]. The potentiometricselectivity coefficients of the proposed electrode are summarized in table 2.Table 2Selectivity coefficients (-lgKNO 3-/X - ) of ISE3 determined using fixed interference methodX ClO 4-I - SCN - Br - BF 4-Cl - CH 3COO - SO 42-F --lgK NO3,X0,83 1,05 1,14 1,38 1,83 3,71 3,83 4,17 4,65pH46