Method for sampling and analysis of hydrogen sulfide

Abstract

A sensitive spectrophotometric method for the determination of trace amounts of hydrogen sulfide after fixing it in a zinc acetate–citrate–sodium hydroxide trapping solution is described. The method is based on the reaction of bromate with hydrogen sulfide in acid medium to liberate bromine, which brominates 2′,7′-dichlorofluorescein to form a dibromo-derivative. The brominated product is extracted into a solvent mixture of 15% isoamyl acetate in isoamyl alcohol and the organic layer is re-equilibrated with acetate buffer (pH 6.0) in the presence of a cationic surfactant (cetrimide) before its absorbance is measured at 535 nm. The colour system obeys Beer's law over the range 0–2 µg of H₂S. The relative standard deviation is 3.8% for 10 determinations at 1 µg of H₂S. The effect of interfering gases on the determination is discussed. The developed trapping solution has been evaluated with the use of an H₂S permeation device. This new trapping solution has an absorption efficiency of >98% at a flow rate of 1 l min^–1 for a sampling period of 150 min. The collected samples are stable for 15 d. The method has been applied to the determination of residual H₂S in a laboratory fume hood. The results obtained are comparable to those obtained using the standard Methylene Blue method. The method can be used to determine concentrations of H₂S of as low as 0.07 µg.