Optical chloride sensor based on [9]mercuracarborand-3 with massively expanded measuring range

Bulk optodes for monovalent ions typically exhibit a relatively narrow measuring range of about two-orders of magnitude. Here, the measuring range was expanded to about six-orders of magnitude concentration change by optimizing a fluorescent chloride optical sensing film based on a plasticized poly(vinyl chloride) film incorporating the halide-selective ionophore 9]mercuracarborand-3 and the H⁺-chromoionophore 9-dimethylamino-5-4-(15-butyl-1,13-dioxo-2,14-dioxanonadecyl) phenyl-imino]benzoa]phenox-azine (ETH 5418). This was achieved with the recently established two-step ionophore binding mechanism: the expanded sensing range sequentially makes use of the 1:2 and 1:1 binding stoichiometries between ionophore to chloride. The relevant complex formation constants were found here as log K₁=9.1 and log K₂=2.7, which are consistent with previously reported values. The selectivity of this optode is very good, with an excellent discrimination of thiocyanate and lipophilic anions such as salicylate, nitrate and perchlorate. As with earlier work, the main interferences are the other halides bromide and iodide.