Membrane-dialzer injection loop for enhancing the selectivity of anion-responsive liquid-membrane electrodes in flow systems : Part 1. A sensing system for NOx and nitrite

A general approach for enhancing the selectivity of anion-responsive liquid-membrane electrodes is described. The method is based on use of the receptor channel of a flow-through dialyzer unit as the injection loop in a flow-injection system. An appropriate membrane in the dialyzer prevents ionic interferents from reaching an inselective electrode in the final flow stream. The concept is demonstrated by developing a highly selective semi-automated method for the determination of dissolved NO_x or nitrite at levels greater than 5 μM. Nitrogen dioxide is trapped across a teflon membrane in the dialyzer and converted to nitrate by a buffered peroxide recipient solution. This solution is injected and carried to a tubular nitrate electrode. The final selectivity and detection capabilities of this gas-sensing arrangement are examined and compared to those of the nitrate electrode alone and more conventional sensing systems based on pH electrodes. A dialyzer unit with wider channels improves the efficiency of gas transfer and thus the detection capabilities of the system.