Poly(vinyl chloride)-based macrocyclic membrane sensors for magnesium

Poly(vinyl chloride)-based membranes of macrocycles 4,11-dimethyl-2,4,9,11-tetraethyl-1,5,8,12-tetraaza cyclotetradeca-1,8-diene (I) and 4,11-dioxa-2,9-dimethyl-1,5,8,12-tetraaza cyclotetradeca-1,8-diene (II) with sodium tetraphenyl borate (STB) as an anion excluder and dibutyl phthalate (DBP), dioctyl phthalate (DOP), dibutylbutyl phosphonate (DBBP) and 1-chloronaphthalene (CN) as plasticizing solvent mediators were prepared and investigated as magnesium selective electrodes. The best performance was observed having the composition (II)–PVC–STB–DBP in the ratio 2:10:1:7, which works well over a wide concentration range (1.9×10−6 to 1.0×10−1 M) with a Nernstian slope of 29 mV per decade of activity between pH 2.5 and 6.5. These electrodes have been found to be chemically inert showing a fast response time of 15 s and were used over a period of 3 months with good reproducibility (S=±0.2 mV). The selectivity coefficient values for mono-, di- and trivalent cations indicate excellent selectivity for Mg2+ over a large number of cations. The electrodes have also been used successfully in partially non-aqueous medium and as an indicator electrode in the potentiometric titration of Mg2+ with EDTA. Anions such as Cl− and SO42− do not interfere in the working of the electrode. The practical utility of the membrane sensor has also been observed in solutions contaminated with detergents (cetyltrimethyl ammonium bromide and sodium dodecyl sulphate). Above all, the membrane sensor has been very successfully used to analyse some babyfood products and soft drinks for the determination of Mg2+.