Na+-selective nasicon electrode used as potentiometric and amperometric sensor in a wall-jet FI system

Abstract

A Na+-selective NASICON-membrane electrode (Na+-SNME) was tested in a single-line flow-injection (FI) system, both in potentiometric and amperometric operational modes. The Na+-SNME was homemade, using a NASICON pellet and an Ag/AgCl, 0.02 M NaCl internal reference electrode, and it was inserted into a wall-jet electrochemical cell. For a 28 μL sample loop and an injector-cell connecting tube (i.d. 0.5 mm) as short as possible, the optimum experimental conditions (flow rate, jet-nozzle membrane distance, and applied potential value) were established. The FI results obtained in the steady-state potentiometric operational mode confirmed those previously reported for batch measurements: lower limit of the linear range 4 × 10−4 M; slope 58.6 mV/pNa; detection limit 2 × 10−4 M; selectivity coefficients KNaK = 2 × 10−2, KNaLi = 1.6 × 10−2, KNaH = 3.8. For FI amperometric operational mode, the analytical parameters were better than those corresponding to the potentiometric one: detection limit 10−5 M; selectivity coefficients KNaK = 7.1 × 10−3, KNaLi = 7.6 × 10−3, KNaH = 3.1; a linear dependence between the current intensity and the Na+-concentration in the low concentration domain (20–100 μM). The FI amperometric sensitivity was quite high, leading to a good repeatability: the relative standard deviations for 10 repeated injections were 2.3% and 0.7% for pNa = 4 and pNa = 3, respectively. © 1998 John Wiley & Sons, Inc. Lab Robotics and Automation 10: 339–345, 1998