Linear polyamines as carriers in thiocyanate-selective membrane electrodes

The polyamines, octyl-[2-(2-octylamino-ethylamino)-ethyl]-amine (L1) and octyl-{2-[2-(2-octylamino-ethylamino)-ethylamino]-ethyl}-amine (L2), have been used as anion ionophores in PVC-based membrane ion-selective electrodes. Different electrodes were prepared containing L1, or L2, and o-nitrophenyl octyl ether (NPOE) or bis(2-ethylhexyl)sebacate (DOS) as plasticizers. The response of the electrodes was tested in two different buffers, HEPES-KOH (pH 7) and MES-KOH (pH 5.6). Electrodes containing L1 and L2 with NPOE (E1 and E2, respectively) showed a Nernstian response for thiocyanate with a good response time. The detection limit, linear range and slope for electrode E1 were 3.8 × 10−6 mol dm−3, 1 × 10−5 to 1 × 10−1 mol dm−3 and −57.2 mV decade−1 at pH 5.6 and 4.47 × 10−6 mol dm−3, 1.95 × 10−5 to 1 × 10−1 mol dm−3 and −58.1 mV decade−1 at pH 7.0. For electrode E2 the detection limit, linear range and slope found were 2.63 × 10−6 mol dm−3, 7.94 × 10−6 to 1 × 10−1 mol dm−3 and −58.5 mV decade−1 at pH 5.6 and 1.23 × 10−5 mol dm−3, 7.95 × 10−5 to 1 × 10−1 mol dm−3 and −46.0 mV decade−1 at pH 7. In contrast, electrodes containing DOS as plasticizers gave only response at pH 5.6 (detection limit, linear range and slope at pH 5.6 were 3.16 × 10−5 mol dm−3, 1 × 10−4 to 1 × 10−1 mol dm−3 and −52.6 mV decade−1). Selectivity coefficients for different anions with respect to thiocyanate were calculated. The electrode E2 at pH 5.6 was also used for the determination of SCN− by potentiometric titrations with Ag+ ions with good results. The electrode E2 was also used to determine concentrations of thiocyanate in biological samples.