Method for sampling and analysis of hydrogen sulfide

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.