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 electrically 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⁻ anion 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 drop casting on a GC substrate. PANI membranes containing 0–40% (m/m) TOA⁺Cl⁻ 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 identical PANI25 electrodes was also very good. The selectivity coefficient (logK_Ca,j^pot) of this electrode towards j = Na⁺, K⁺ and Li⁺ is −1.6. However, 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 weak redox response, 5 mV/decade, could be detected with 10⁻³ M CaCl₂ as the 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 PANI25 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 is usually applied to PVC-based ion-selective electrodes.