Proposed flow system for spectrophotometric determination of fluoride in natural waters

An environmentally-friendly flow system was developed for the determination of fluoride in natural waters, based on its reaction with zirconium ions and 2-(parasulfophenylazo)-1,8-dihydroxy-3,6-naphthalene-disulfonate (SPADNS). The flow system was designed with solenoid valves in order to increase the versatility and improve the analytical performance, since each device handles the solution independently leading to a more efficient use of reagents. The minimization of reagent consumption and waste generation favors the development of a noteworthy system. Under optimized conditions, a linear response was observed within the range of 0.1-2.2 mg L(-1), with the detection limit, coefficient of variation and sampling rate estimated as 0.02 mg L(-1), 4.1% and 60 determinations per hour, respectively. In order to analyze samples containing high fluoride content a wider linear range (0.3-6.6 mg L(-1)) can be obtained by using a low sample flow rate and low sample volume. In comparison with other methods, the proposed system offers advantages such as wider analytical range (1.7-2.2 times), higher sample throughput (1.5-4.0 times) and lower quantification limit (by a factor of 5-68). Along with the consumption of reagents being 20-55 times lower, the generation of wastes is 1.2-3.0 times lower, which contributes to the development of a 'greener' system. This method is fast, amenable to automation, environmentally-friendly and of low-cost. In addition, it could be successfully applied to the determination of fluoride in water samples, the results obtained being in agreement with those of the ISE method.