Lead-selective poly(vinyl choride) membrane electrodes based on heterocyclic thiocarboxylic acid

Heterocyclic thiocarboxylic acids have been designed to prepare polymeric membrane ion-selective electrode (ISE) for Pb²⁺. Construction, response characteristic and application of the lead ISEs are investigated. Better results have been obtained with membranes containing ligands L1∼L3 with bis(2-ethylhexyl) sebacate (DOS) as a plasticizer. Ionophores L1∼L3 are [(4,6-dimethyl-2-pyrimidinyl) thio] acetic acid (L1), (1,3,4-thiadiazole-2,5-diyldithio) diacetic acid (L2) and (1,3,4-thiadiazole-2,5-diyldithio) dipropionic acid (L3). The optimum electrodes have the composition of L1 (1.6): PVC (32.7): DOS (65.3): KT _p ClPB (0.4) (w/w), L2 (1.0): PVC (32.8): DOS (66.0): KT _p ClPB (0.2) (w/w), and L3 (1.0): PVC (32.7): DOS (65.4): KT _p ClPB (0.9) (w/w). The optimized membrane electrodes work well over a wide range of concentrations (1.0 × 10⁻⁵ ∼1.0 × 10⁻² M, 1.0 × 10⁻⁶ ∼1.0 × 10⁻² M, and 1.0 × 10⁻⁶ ∼1.0 × 10⁻² M) with the response slope of 27.4, 30.1 and 29.2 mV/decade, respectively. Potentiometric selectivities of the ISEs based on L1 ∼ L3 for Pb²⁺ over other interfering ions are determined with the fix interference method. The electrodes display good selectivity over a number of alkali, alkaline earth, transition and heavy metal ions. The lifetime of the electrodes is about 2 months and their response time is 20 s. Applications of these electrodes for the determination of lead in real samples and as indicator electrodes for potentiometric titration of Na₂SO₄ using Pb²⁺ solution are reported.