Zn(II) complex-based potentiometric sensors for selective determination of nitrate anion

Polymeric membranes containing new Zn(II) complexes as anion carriers were prepared for determination of nitrate anion present in water samples. Two Zn(II) complexes coordinated by neutral tetradentate ligands, N,N′-ethylene-bis(N-methyl-(S)-alanine methylamide) and N,N′-ethylene-bis(N-methyl-(S)-alanine dimethylamide), worked well as anion-selective carriers, while common phthalocyanine Zn(II) complex rarely responded to any anions. The combination of these new Zn(II) complexes with dioctylsebacate as a plasticizer particularly offered high sensing selectivity for nitrate anion. They exhibited near-Nernstian slopes in the wide linear concentration range of 5.0 × 10−5 to 1.0 × 10−1 M, and operated well in the wide pH range from 4 to 11 with the response time of less than 25 s. The potentiometric selectivity coefficients were evaluated using the fixed interference method, indicating that the two Zn(II) complexes exhibited better selectivity for nitrate anion with respect to a wide variety of inorganic anions. Although chloride anion worked as an interfering species at a concentration higher than 1.0 × 10−3 M, the new Zn(II) complex-based sensors were applicable in determination of the nitrate anion after adding silver sulfate to remove the chloride anion.