Direct Introduction of Water Sample in Multisegmented Flow-Injection Analysis for Sulfide Determination

The present paper describes an inline flow-injection analysis system for the determination of sulfide in water samples, exploiting the Fischer reaction. Water samples were collected and introduced into a reactor of the FIA system. The sulfide released, after sample acidification, was carried out with a nitrogen gas flow and mixed with N,N diethyl-p-phenylenediamine (DEPD) solution in the presence of Fe(III). The blue dye formed was measured in the wavelength range between 672 – 679 nm. An evaluation of the effects of chemical and flow factors was performed using the factorial design of two levels, while optimization was accomplished by a Doehlert matrix. The system presented two linear-response ranges: the first of 0.433 to 400 µg L−1 and the second of 400 to 3500 µg L−1. The detection and quantification limit were found to be 0.130 and 0.433 µg L−1, respectively, while the sample throughput was 12 h−1. The precision was evaluated as the relative standard deviation (n = 10); for 50 and 100 µg L−1 sulfide it was found to be 1.9 and 2.3%, respectively. The method showed satisfactory selectivity regarding the main interference present in environmental samples. The accuracy of the method was successfully evaluated in environmental water samples after a comparison with a literature reference method.