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

Abstract

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.