Mercury(ii) ion-selective electrode. Study of 1,3-diphenylthiourea as ionophore

A new liquid membrane, mercury(II) ion-selective electrode (ISE) based on 1,3-diphenylthiourea as a neutral carrier was developed. In solubility tests of 1,3-diphenylthiourea in six different plastizicers, tris(2-ethylhexyl) phosphate (TEHP) showed the best performance and was selected for the membrane. Several ISEs were conditioned and tested for the selection of seven metal ions (Hg2+, Cu2+, Zn2+, Pb2+, Ca2+, Mg2+ and Ag+). The best Nernstian response was obtained for Hg2+ with an experimental slope of 58.6 ± 0.8 mV per decade in the concentration range 6.0 × 10−6–5.0 × 10−4 mol l−1; the presence of the complex Hg(OH)+ ion explains the slope of the response curve. The ISE showed stable detection reproducibility during 6 weeks, with a practical detection limit of 10−6 mol dm−3 and a high selectivity for Cd2+, Pb2+, Co2+, Cu2+, Sr2+, Zn2+, Ca2+, Mg2+, Mn2+, K+, Na+ and NH4+ ions. Only one interference for Ag+ was found. A second linear response curve for Hg2+ was obtained at pH 4 with a slope of 30.8 mV per decade in the concentration range 2.0 × 10−6–2.1 × 10−4 mol l−1.