Flow injection potentiometry by poly(vinyl chloride)-membrane electrodes with substituted azacrown ionophores for the determination of lead(II) and mercury(II) ions

Abstract

A flow cell with a poly(vinyl chloride) (PVC) neutral-ionophore liquid-membrane ion-selective electrode has been developed for flow injection potentiometry (FIP). The flow system was optimised and five substituted azacrown ethers: 7,16-dithenoyl-1,4,10,13-tetroxa-7,16-diazacyclooctadecane (DTODC), 7,16-di-(2-thiopheneacetyl)-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane (DTAODC), 7,16-dithenyl-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane (DTDC), 1,10-dioxa-4,7,13,16-tetraazacyclooctadecane (TC) and 4,7,13,16-tetrathenoyl-1,10-dioxa-4,7,13,16-tetraazacyclooctadecane (TTOTC) synthesised and investigated as the ionophores in Pb 2+ (DTAODC, TTOTC) and Hg 2+ (DTDC, DTODC, TC, TTOTC) selective electrodes. The short contact times between analyte and ionophore in FIP allow the measurement of a strongly complexing ion such as Hg 2+ that saturates the electrodes in batch analysis, or in continuous flow analysis that comes to a steady state. For the mercury-selective electrodes with ionophores with amide functional groups (TTOTC and DTODC) a carrier of 10 mM potassium nitrate was found to increase the speed of response and recovery to baseline. The linear calibration range for a DTAODC lead-selective electrode was pPb=2.0 to 5.0 with a slope of 32.5 mV decade −1 and for a TTOTC mercury-selective electrode, pHg=3.0 to 5.5 with a slope of 28.4 mV decade −1. Highly reproducible measurements were obtained (RSD <2%) at a flow rate of 3.0 ml min −1 giving a typical throughput of 40 samples h −1 for Pb 2+ and 30 samples h −1 for Hg 2+.