Spectrophotometric Determination of Sulfide in Water

The hydrosulfide ion HS^- has high absorbance in the ultra violet. It predominates in solution at pH above 8, but is negligible below pH 5. Sulfide up to 5 mg S^2-/liter in water can be determined by the difference between absorbance measurements at 228 nm, first on the sample made alkaline with sodium hydroxide, and then after acidification. Samples can be stabilized for some hours if made alkaline when collected, and then kept in the dark. Interference may be caused if calcium and magnesium are precipitated by alkali, and difficulty may be found with colored waters with high ultraviolet absorbance. Standard deviation was ±0.1 at the 4 mg S^2-/liter level.