The methylene blue method for the colorimetric determination of aqueous sulfide was modified and evaluated for field use in the concentration range of 0.3 to 1500 µM sulfide. Aqueous sulfide (H₂S, HS⁻, and S⁼) reacts with N,N-dimethyl-ρ-phenylenediamine sulfate and ferric chloride in an acidic solution, forming a methylene blue complex which is measured spectrophotometrically at 670 nm. Modifications to the method permit rapid and repetitive sampling, produce reproducible standard curves, and define the limitations on stability of reagents and of the color-complex. Field measurements are made easily after the reagents have been standardized in the laboratory. The method has a relative standard deviation of ±3.8% at 80 µM S⁼ and ±0.84% at 420 µM S⁼.

The method was laboratory tested for possible interferences from phenols, polynuclear aromatic hydrocarbons and salts. Sulfide concentrations up to 70 µM were measured in ground water contaminated with creosote waste products including phenols and aromatic hydrocarbons. No interferences were found from these constituents.