On line dissolution of ZnS for sulfide determination in stabilized water samples with zinc acetate, using spectrophotometry by methylene blue method

Stabilization of sulfide as zinc sulfide is a strategy widely used in batch analysis of this ion in water. A FIA/spectrophotometry system to determine sulfide in stabilized water samples is proposed in this paper. The methodology is based on the formation of methylene blue by the reaction of H2S with a mixture of Fe3+ solution and N,N-dimethyl-p-phenylenediamine (DMPD) in acidic medium. In the system, the sample (480 CLL) containing stabilized sulfide (as ZnS slurry) is injected in a carrier stream formed by mixing acidic Fe3+ and DMPD. In the reactor coil of 1280 mu L, the sulfide is drifted by acid used in the preparation of solutions, releasing H2S that reacts with Fe3+ and DMPD forming methylene blue. The absorbance is measured at 662 nm. The performance of the FIA system was optimized in relation to chemical and flow parameters. The stability of the solutions were studied throughout the period of the experiment. The results did not show any loss of sulfide over a 7 day period. However, in complex matrices an analysis by using the standard addition method is recommended. The analysis of effluent sample through the time showed good agreement between the results obtained each day. The developed system presented the following figures of merit: detection limit of 50 mu g L-1; RSD of 6% at 0.5 mu g mL(-1) (using ultrasonic homogenization) and analytical throughput of 60 samples per hour.