New Ag +- and Pb 2+-selective electrodes with lariat crown ethers as ionophores

Abstract

Two sym-dibenzo-16-crown-5 ethers with sidearms were synthesized and fabricated as ion selective (PVC) membrane electrodes. Among these two ion selective electrodes (ISEs), 3-butenyl sym-dibenzo-16-crown-5 ether 1 gives the best Ag + selectivity and 2′-methoxyethoxyl sym-dibenzo-16-crown-5 ether 2 the best selectivity for Pb 2+. These electrodes are composed of the respective lariat crown ether (LCE) as ionophore, sodium tetraphenylborate (NaTPB) as anion excluder, and 2-nitrophenyl-octyl ether (NPOE) as plasticizer. Electrodes composed of other plasticizer including tris(2-ethylhexyl) phosphate (TOP), bis(2-ethylhexyl) sebacate (DOS) and dibutyl phthalate (DBP) were also investigated. The best performance observed include the two membrane electrodes with the following composition: (1) 3-butenyl sym-dibenzo-16-crown-5 ether–PVC-NaTPB-NPOE, and (2) 2′-methoxyethoxyl sym-dibenzo-16-crown-5 ether–PVC-NaTPB-NPOE, at a ratio of 5:50:1:100. These two membrane electrodes show Nernstain response toward silver and lead (55.4 and 28.9 mV per decade, respectively) over a wide concentration range from 5×10 −5 to 5×10 −1 M. The detection limit for silver(I) and lead(II) ions were estimated as 1×10 −6 M. The response time was less than 30 s and a good reproducibility over a period of 3 months was observed. The newly designed Ag +- 1 and Pb 2+- 2 ISEs appear more reliable than the corresponding commercial Ag +- and Pb 2+-ISEs due to a greater resistance to the interference of Hg 2+ or to Cd 2+. Stability constants (log K s) of 3-butenyl sym-dibenzo-16-crown-5 ether (for Ag +) and 2′-methoxyethoxyl sym-dibenzo-16-crown-5 ether (for Pb 2+) were determined as 3.11 and 3.23, respectively, by potentiometric titration in methanol.