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 salicylate-selective electrodes based on Sn(IV) and OMo(V) (oxomolybdenum) tetraphenylporphyrins (TPP) in o-nitrophenyloctylether plasticized poly(vinyl chloride) membranes are compared. Super-Nernstian salicylate responses (slopes>−100 mV decade−1) are observed for membranes doped with Sn(IV)[TPP]Cl2 and 10–30 mol% of a lipophilic tetraphenylborate derivative (sodium bis[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 ionophores. 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 converted to monomeric porphyrin upon increasing the concentration of salicylate in the sample solution. In contrast, the salicylate response of the membranes 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 selectivity. UV–vis spectroscopic studies of the membranes doped with the molybdenum species confirm the absence of dimer formation, which correlates well with the conventional Nernstian response behavior of such membranes. Electrodes formulated with the optimized membranes containing OMo(V)[TPP]OEt and the borate derivative (10–35 mol%) are shown to exhibit high selectivity for salicylate over many anions, even lipophilic anions such as perchlorate (log Kpot≤−2.5).