All-solid-state calcium-selective electrode prepared of soluble electrically conducting polyaniline and di(2-ethylhexyl)phosphate with ETH1001 as neutral carrier Original Research Article

A new type of an all-solid-state PVC-free Ca2+-selective electrode is described. The electrode membrane is prepared of soluble electrically conducting polyaniline (PANI), di(2-ethylhexyl)phosphate (H+DEHP-) and Ca2+-ionophore ETH1001. The influence of an incorporated lipophilic salt, potassium tetrakis(4-chlorophenyl)borate (KB(ClPh)4), in the electrode membrane was also studied. The non-conducting form of PANI is made electrically conductive with H+DEHP-, which is also a complexing agent for Ca2+. PANI is then dissolved in tetrahydrofuran (THF) together with the other membrane components and electrode membranes are easily prepared in a single step on a glassy carbon (GC) substrate by solution-casting. An important role of PANI is also to facilitate the charge transfer at the substrate | membrane interface. It is shown that incorporation of ETH1001 and KB(ClPh)4 improves the Ca2+-sensitivity of the electrodes studied. The best Ca2+-sensitivity, 28.6 ± 1.1 mV/log aCa (10−1–10−3 M CaCl2) in 0.1 M NaCl, was obtained for an electrode membrane containing 40% (m/m) PANI, 30% (m/m) ETH1001 and 30 % (m/m) KB(ClPh)4. No redox sensitivity was observed for a PANI electrode membrane consisting of 30% ETH1001 and 5% KB(ClPh)4 in a solution of 10 mM Fe(CN)63−/4− with 10−1–10−3 M CaCl2 as the ionic background. The selectivity coefficient (logKCa,jpot) of this electrode is −1.8 ± 0.1, −1.7 ± 0.1, −1.7 ± 0.1 and approximately −2 towards j = Na+, K+, Li+ and Mg2+, respectively. A model describing the working mechanism of the different PANI-based ion-selective electrodes is also presented.