Flow-through ion-selective potentiometric sensors

Several classical ion-selective electrodes (some of which are commercially available) have been incorporated into continuous systems via suitable flow-cells. In fact, Lima et al. [112] used a tubular homogeneous crystal-membrane (AgjS or AgCl) sensor for the determination of sulphide and chloride in natural and waste waters. However, the search for new active materials providing higher selectivity and/or lower detection limits continues. Thus, Smyth et al [113] tested the suitability of a potentiometric sensor based on calix[4]arene compounds for use in flow injection systems. They found two neutral carriers, viz. methyl-j3-rerr-butylcalix[4]aryl acetate and 

Ishibashi et al. devised a potentiometric sensor for the determination of non-ionic surfactants which they improved in several steps. Initially, the authors used a sensor based on a PVC membrane plasticized with 2-nitro-phenyl octyl ether that was responsive to cationic complexes formed between a dissolved metal ion and non-ionic surfactants in the sample [116]. At a later stage, they studied the effect of foreign species and elucidated the perturbation from ionic surfactants [117], which they eventually overcame by inserting an ion-exchange column into the base system [118]. 

Some ISEs containing no inner reference solution, as well as tubular potentiometric sensors, has been used in conjunction with FI systems for the determination of vitamins B, and Bg in pharmaceutical preparations. The membranes used for this purpose were prepared from the vitamin tetra(2-chlorophenyl)borate dissolved in o-nitrophenyloctyl ether and immobilized in PVC. The intrinsic behaviour of the tubular electrodes was assessed by using a low-dispersion single-channel FI manifold and compared with those of conventionally-shaped electrodes using the same membrane the results provided by both were very similar [119]. 

The use of selective electrodes for indirect determinations of species to which the sensor responds by a reaction involving the analyte as the primary 

Multideterminations with ISEs entail using as many sensors as species are to be determined. Their placement in the hydrodynamic system used is dictated by both their selectivity and the way in which acquired data are to be subsequently processed. 

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