EPA Method 3762

Methylene blue solution I: Dissolve 1.0 g of methylene blue in distilled water in a 1 liter volumetric flask and dilute to the mark. Use U.S.P. grade or one certified by the Biological Stain Commission.

The dye content reported on the label should be 84% or more. Standardize (5.8) against sulfide solutions of known strength and adjust concentration so that 0.05 mL (1 drop) equals 1.0 mg/L sulfide. 5.7 Methylene blue solution II: Dilute 10.00 mL of adjusted methylene blue solution I (5.6) to 100 mL with distilled water in a volumetric flask.

Add somewhat less than enough water to cover the crystals. 5.8.3 Stir occasionally for a few minutes. Pour the solution into another vessel. This reacts slowly with oxygen but the change is insignificant over a few hours.

Make the solution daily. 5.8.4 To 1 liter of distilled water add 1 drop of solution and mix. 5.8.5 Immediately determine the sulfide concentration by the methylene blue procedure (6) and by the titrimetric iodide procedure (Method 376.1, this manual). 5.8.6 Repeat using more than one drop of sulfide solution or less water until at least five tests have been made in the range of 1 to 8 mg/L sulfide. 5.8.7 Calculate the average percent error of the methylene blue procedure (6) as compared to the titrimetric iodide procedure (Method 376.1). 5.8.8 Adjust by dilution or by adding more dye to methylene blue solution I (5.6)