Salicylate-selective membrane electrodes based on Sn(IV)- and O=Mo(V)-porphyrins: differences in response mechanism and analytical performance

The potentiometric response characteristics (slope and selectivity) of sali cylate-selective electrodes based on Sn(IV) and O=Mo(V) (oxomolybdenum) tet raphenylporphyrins (TPP) in o-nitrophenyloctylether plasticized poly(vinyl chloride) membranes are compared. Super-Nernstian salicylate responses (slo pes > -100 mV decade(-1)) are observed for membranes doped with Sn(IV)[TPP] Cl-2 and 10-30 mol% of a lipophilic tetraphenylborate derivative (sodium bi s[trifluoromethyl]phenylborate, NaTFPB). The borate derivative is required to optimize potentiometric selectivity for salicylate over other anions in accordance with existing theory for charged carrier type anion selective io nophores. The super-Nernstian response of Sn(IV) porphyrin doped membranes is shown, via UV-vis spectroscopy, to result from spontaneous formation of hydroxide ion bridged porphyrin dimers in the membrane phase., which are co nverted to monomeric porphyrin upon increasing the concentration of salicyl ate in the sample solution. In contrast. the salicylate response of the mem branes doped with O = Mo(V)[TPP]OEt are Nernstian (-56 to 60 mV decade(-1)) , even in the presence of added borate derivative to optimize salicylate se lectivity, UV-vis spectroscopic studies of the membranes doped with the mol ybdenum species confirm the absence of dimer formation, which correlates we ll with the conventional Nernstian response behavior of such membranes. Ele ctrodes formulated with the optimized membranes containing O=Mo(V)[TPP]OEt and the borate derivative (10-35 mol%) are shown to exhibit high selectivit y for salicylate over many anions, even lipophilic anions such as perchlora te (log K-pot less than or equal to -2.5). (C) 2001 Elsevier Science B.V. A ll rights reserved.