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 3 10 5 l mol 21 cm 21 . 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 mg l 21 sulfide (r = 0.9997) without solid-phase extraction, and 1-100 mg l 21 sulfide (r = 0.9989) after solid-phase extraction of the dye. The limits of detection were 5 mg l 21 and 0.2 mg l 21 sulfide without and after solid-phase extraction, respectively. The interference of up to 100 mg l 21 thiols, 50 mg l 21 sulfite and 10 mg l 21 thiosulfate could be avoided by their masking with N-ethylmaleimide. Up to 10 mg l 21 fluoride and 100 mg l 21 thiocyanate did not cause any interference. Interference of up to 100 mg l 21 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 mg l 21 levels.
Experimental EquipmentAll spectrophotometric measurements were made with an ATI-Unicam UV2-100 UV/visible spectrophotometer (Cambridge, UK) using 1 cm matched quartz cells.Solid-phase extraction (SPE) cartridges, 2.8 ml, packed with CN sorbent (500 mg) (Alltech, Deerfield, IL, USA) were used.