Response function and analytical parameters of nitrite-selective optode membranes in absorbance and fluorescence mode

Four different nitrite-selective optode membranes were studied. Each membrane consisted of a nitrite-selective ligand and four different H+-selective chromoionophores dissolved in a plasticized poly(vinyl chloride) bulk medium. The membranesʹ analytical parameters differ with respect to optical spectra, dynamic range and detection limit. The unique features of each membrane are discussed and the versatility of the optical sensors based on the membranes is evaluated. The response functions of the optical sensors, derived from the changes in the absorbance and/or fluorescence of the membranes with varying nitrite concentration, are described by a theoretical model based on coextraction equilibria. The dynamic range of the sensors extended from 0.5 to 5000 mg l−1 nitrite with a detection limit of 0.24 mg l−1. The response time, τ95, of the membranes placed in a continuous flow measuring cell was 1.2 min when changing from 0.5 mg l−1 nitrite to 0.05 mg l−1, and 3.6 min in the opposite direction. The selectivity coefficients determined by the separate solution method were: 10−3.3 for nitrate, 10−3.7 for chloride, 10−3.0 for bromide, 10−3.8 for sulphate, and 10−5.0 for phosphate. Various applications of these optode membranes are discussed.