New Ag+- and Pb2+-selective electrodes with lariat crown ethers as ionophores

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), 3butenyl 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 Pb2+. 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 28.9mV per decade, respectively) over a wide concentration range from 5×10-5 to 5×10-1M. The detection limit for silver(I) and lead(II) ions were estimated as 1×10-6M. The response time was less than 30s and a good reproducibility over a period of 3 months was observed. The newly designed Ag+-1 and Pb2+-2 ISEs appear more reliable than the corresponding commercial Ag+- and Pb2+-ISEs due to a greater resistance to the interference of Hg2+ or to Cd2+. Stability constants (logKs) of 3-butenyl sym-dibenzo-16-crown-5 ether (for Ag+) and 2´-methoxyethoxyl sym-dibenzo-16-crown-5 ether (for Pb2+) were determined as 3.11 and 3.23, respectively, by potentiometric titration in methanol.