Standard Methods: 4500-S2- D:  Sulfide by Methylene Blue

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Official Method Name
4500-S2- D. Methylene Blue Method
Current Revision
Standard Methods Online
Spectroscopy (Colorimetry; Photometry)
Method Subcategory
Method Source
  Standard Methods
  Standard Methods Online - Standard Methods for the Examination of Water and Wastewater
Brief Method Summary
The methylene blue method is based on the reaction of sulfide, ferric chloride, and dimethyl-p-phenylenediamine to produce methylene blue. Ammonium phosphate is added after color development to remove ferric chloride color.
Scope and Application
The method can be used for a variety of waters and wastewaters.
Applicable Concentration Range
0.1 to 20.0 mg-S2-/L (See 4500-S2- A)
Strong reducing agents interfere in the methylene blue method by preventing formation of the blue color. Thiosulfate at concentrations about 10 mg/L may retard color formation or completely prevent it. Ferrocyanide produces a blue color. Sulfide itself prevents the reaction if its concentration is very high, in the range of several hundred milligrams per liter. Iodide, which is likely to be present in oil-field waste waters, may diminish color formation if its concentration exceeds 2 mg/L. Many metals (e.g., Hg, Cd, Cu) form insoluble sulfides and give low recoveries. Eliminate interferences due to sulfite, thiosulfate, iodide, and many other soluble substances, but not ferrocyanide, by first precipitating ZnS, removing the supernatant, and replacing it with distilled water. Use the same procedure, even when not needed for removal of interferences, to concentrate sulfide.
Quality Control Requirements
See Section 4020 Quality Assurance/Quality Control.
Sample Handling
Collect water samples with minimum aeration. Either analyze samples immediately after collection or preserve with zinc acetate solution for later analysis. To preserve a sample for a total sulfide determination put zinc acetate and sodium hydroxide solutions into sample bottle before filling it with sample. Use 0.2 mL 2M zinc acetate solution per 100 mL sample. Increase volume of zinc acetate solution if the sulfide concentration is expected to be greater than 64 mg/L. The final pH should be at least 9. Add more NaOH if necessary. Fill bottle completely and stopper.
Maximum Holding Time
28 days; 7 days for regulatory use (See Section 1060)
Relative Cost
Less than $50
Sample Preparation Methods
See Section 4500-S2- C