Determination of sulfide in waters by flow-injection solid phase spectrophotometry

A highly sensitive flow injection solid-phase spectrophotometric method was developed for the determination of sulfide in waters. The method is based on the formation of Methylene Blue (MB) by reaction between sulfide and N,N-dimethyl-p-phenylenediamine chloride in presence of Fe(iii) in acidic medium. The MB formed was adsorbed on C-18 bonded silica, located inside a laboratory-made flow cell, which was placed in the optical path of the spectrophotometer. Analyte retention and detection at 666 nm were performed simultaneously, followed by elution with a mixture of methanol and hydrochloric acid. Several variables of the system, such as amine concentration, acidity of the reaction medium, reactor coil volume, sample volume and flow rate of the cleaning solution, were studied in order to achieve the best performance in the analysis of natural samples. This procedure provides an alternative for the determination of sulfide in the range 5-50 g L-1 with a relative standard deviation of 4.0% for 10 independent determinations at a concentration level of 50 g L-1. A detection limit of 1.7 g L-1, for a confidence level of 99.7%, was found. The system presented an analytical throughput of 12 h(-1).