All-solid-state calcium-selective electrode prepared of soluble electrically conducting polyaniline and di(2-ethylhexyl)phosphate with tetraoctylammonium chloride as cationic additive

A novel all-solid-state Ca2+-selective electrode was prepared of soluble el ectrically conducting polyaniline (PANI), di(2-ethylhexyl)phosphate (H+DEHP -) and tetraoctyl ammonium chloride (TOA(+)Cl(-)). PANI is made soluble and electrically conducting in tetrahydrofuran (THF) with H+DEHP-. The DEHP- a nion is a complexing agent of the charged carrier type for Ca2+. TOA(+)Cl(- ) is added to this solution and electrode membranes are then prepared by dr op casting on a GC substrate. PANI membranes containing 0-40% (m/m) TOA(+)C l(-) has been studied in this work. The Ca2+-sensitivity was significantly improved by incorporation of 20-30% (m/m) TOA(+)Cl(-) in the PANI electrode membrane. The best Ca2+-sensitivity , 27.0 +/- 0.4 mV/log a(Ca) (10(-1)-10(-3) M CaCl2, n = 3, LOD = 10(-4) M) in 0.1 M NaCl, was obtained with an electrode membrane containing 25% TOA()Cl(-) (PANI25). The reproducibility of the standard potential of three ide ntical PANI25 electrodes was also very good. The selectivity coefficient (l ogK(Ca,j)(pot)) of this electrode towards j = Na+, K+ and Li+ is -1.6. Howe ver, Mg2+ shows severe interference in determination of Ca2+. No redox sensitivity was observed for the PANI25 electrode in a 10 mM Fe(CN )(6)(3-/4-) solution with 0.1 M CaCl2 as the ionic background and only a we ak redox response, 5 mV/decade, could be detected with 10(-3) M CaCl2 as th e ionic background. The pH sensitivity of the PANI25 electrodes studied was found to be approximately 5 mV/pH within the pH range of 4.5-9.7. Furthermore, the impedance spectrum and the cyclic voltammogram of the PANI 25 electrode reveal that TOA(+)Cl(-) improves the ionic mobility within the PANI membrane. Finally, it is shown that the working mechanism of the PANI electrode membrane can be explained with the charge carrier model, which i s usually applied to PVC-based ion-selective electrodes. (C)2000 Elsevier S cience B.V. All rights reserved.