UV-VIS Hydrogen Sulfide Content Measurement (Methylene Blue Colorimetric)

Materials Needed

• Sample Solution containing H₂S
• N,N-Dimethyl-p-phenylenediamine sulfate reagent (0.02 M solution)
• Ferric Chloride (FeCl₃) Solution (0.01 M)
• Hydrochloric Acid (HCl) (6 M solution)
• Distilled Water
• Spectrophotometer capable of measuring absorbance at 670 nm
• Cuvettes compatible with the spectrophotometer
• Standard H₂S Solution (for calibration)
• Pipettes for accurate reagent addition

Reagent Preparation

1. N,N-Dimethyl-p-phenylenediamine Sulfate Solution (0.02 M):

   • Dissolve 0.397 g of N,N-dimethyl-p-phenylenediamine sulfate in 100 mL of distilled water.

2. Ferric Chloride Solution (0.01 M):

   • Dissolve 0.162 g of FeCl₃ in 100 mL of distilled water.

Calibration Curve (Optional, but recommended)

1. Prepare Standard Solutions:
   • Prepare a series of standard H₂S solutions with known concentrations (e.g., 0.1, 0.5, 1.0, 2.0, and 5.0 mg/L). These can be made by diluting a stock H₂S solution with distilled water.
2. Prepare Calibration Samples:
   • Add 1 mL of the N,N-dimethyl-p-phenylenediamine sulfate solution, 1 mL of ferric chloride solution, and 2 mL of HCl to each standard solution.
3. Develop Methylene Blue Color:
   • Allow the samples to react for at least 20 minutes to ensure complete color development.
4. Measure Absorbance:
   • Measure the absorbance of each standard solution at 670 nm using the spectrophotometer.
5. Plot Calibration Curve:
   • Plot the absorbance values against the known concentrations to create a standard curve, which will be used to determine the concentration of H₂S in your sample.

Sample Analysis Procedure

1. Sample Preparation:

   • If the sample concentration is unknown and expected to be high, dilute the sample accordingly.

2. Add Reagents:

   • In a clean container, add 1 mL of the N,N-dimethyl-p-phenylenediamine sulfate solution to 100 mL of your sample.
   • Add 1 mL of the ferric chloride solution.
   • Add 2 mL of hydrochloric acid (6 M).

3. Reaction Time:

   • Allow the mixture to sit for at least 20 minutes at room temperature to develop the blue color. This allows the H₂S to react fully with the reagents and form methylene blue.

4. Measure Absorbance:

   • Transfer a portion of the reacted sample to a cuvette.
   • Measure the absorbance at 670 nm using the spectrophotometer.

5. Determine H₂S Concentration:

   • Compare the sample’s absorbance to the calibration curve to determine the H₂S concentration.