Solid-phase extraction and spectrophotometric determination of hydrogen sulfide in air and water utilizing ethylene blue formation

Abstract

Hydrogen sulfide in water stabilized with or absorbed from air in an alkaline mixture of zinc acetate and EDTA was treated with 4-N,N-diethylaminoaniline and iron(III) in sulfuric acid medium and the absorbance of ethylene blue was measured at 670 nm after 5 min. The molar absorptivity was found to be 9.42 × 105 l mol−1 cm−1. The ethylene blue dye could be preconcentrated by solid-phase extraction on a CN sorbent and eluted by potassium bromide solution in water–methanol leading to an enrichment factor of at least 50. Coloured samples were subjected to clean-up by using Lichrolut EN and the colourless eluate was treated for colour development. Rectilinear calibration graphs were obtained over the range 10–800 μg l−1 sulfide (r = 0.9997) without solid-phase extraction, and 1–100 μg l−1 sulfide (r = 0.9989) after solid-phase extraction of the dye. The limits of detection were 5 μg l−1 and 0.2 μg l−1 sulfide without and after solid-phase extraction, respectively. The interference of up to 100 mg l−1 thiols, 50 mg l−1 sulfite and 10 mg l−1 thiosulfate could be avoided by their masking with N-ethylmaleimide. Up to 10 mg l−1 fluoride and 100 mg l−1 thiocyanate did not cause any interference. Interference of up to 100 mg l−1 fluoride was avoided by masking with zirconyl nitrate. The method has been validated by analysing spiked river water samples, when the average recovery was 99% (range 96–104%) with an RSD of 3% (range 2–4%). Sulfide has been detected in many river waters at low μg l−1 levels.