**Hydrogen Sulfide - Methylene Blue Spectrophotometry Method**
**Keywords**: Methylene blue spectrophotometry; hydrogen sulfide; mass spectrometer; UV-1100; UV-1200
This method is widely used for the determination of hydrogen sulfide in water samples. It involves a colorimetric reaction between hydrogen sulfide and methylene blue, which forms a stable blue complex that can be measured at a specific wavelength using a spectrophotometer.
**Procedure:**
1. Pipette 5 mL of an acidic water sample into a 1000 mL volumetric flask, dilute to the mark with distilled water, and mix thoroughly. If using purified water, no dilution is required.
2. Transfer 2 mL of the diluted sample into a centrifuge tube containing 10 mL of borax buffer solution. Then add 20 mL of zinc acetate-acetic acid sodium solution. Mix well and place the tube in a centrifuge. Keep track of the number of tubes.
3. Centrifuge the mixture for 5 minutes at a suitable speed.
4. After centrifugation, discard the supernatant and retain the pellet. Transfer all the pellet to a 100 mL colorimetric tube and add distilled water up to 80 mL.
5. Carefully add 10 mL of a 2 g/L N-N-dimethyl-p-phenylenediamine solution along the wall of the tube. Immediately shake the tube to prevent the loss of hydrogen sulfide and ensure complete dissolution of the precipitate.
6. Add 1 mL of ammonium ferric sulfate solution along the wall of the tube, then dilute the solution to 100 mL with distilled water. Shake the tube immediately and let it stand for 10 minutes.
7. Measure the absorbance at 665 nm using a 1 cm cuvette on a spectrophotometer (such as UV-1100 or UV-1200).
8. Perform a blank test by replacing the sample with distilled water under the same conditions.
**Calculation:**
The concentration of hydrogen sulfide in the acidic water sample (in mg/L) can be calculated based on the direct reading from the spectrophotometer. For purified water, the same procedure applies, but the original sample volume is used instead of the diluted one.
This method is highly sensitive and suitable for routine analysis in environmental monitoring and laboratory settings. It ensures accurate results when performed carefully and under controlled conditions.
For more information, visit [Macylab.com](http://www.macylab.com) or consult your local analytical instrument supplier.
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