The fluorometric method using the aqueous sodium hydroxide solution as the trapping solution possesses a sufficient sensitivity to determine hydrogen sulfide of background levels in the atmosphere. But the sulfide ion collected in the trapping solution is unstable, so the samples must be analyzed soon after sampling. The authors modified the trapping solution by adding a small amount of EDTA for better stability of the sulfide ion in the solution. The EDTA added into the solution masks the metallic ions such as ferrous, ferric and manganse ions, and it lowers the rate of oxidation of sulfide ion as a result of decrease of these ions in the solution which are considered to work as catalysts of oxidation. In case of 1.0×10-5 M of sodium sulfide in 0.1 N NaOH, the concentration decreased at a rate of about 20% in a day at room temperature, but it became more stable by the presence of 0.01 M EDTA and the sulfide solution was stable at least for one week if the solution was kept in a dark room. The sulfide solution containing EDTA was also quite stable to aeration; 5×10-6 M of sodium sulfide in 0.1 N NaOH in the presence of 0.01 M EDTA was stable even after passing air through the solution at a flow rate of 0.5 1/min for 2 hours. The modified trapping solution containing EDTA enables the fluorometric method to be more practical in the determination of hydrogen sulfide of background levels in the atmosphere.
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