Measuring Sulfur Dioxide (SO₂) in Ambient Air: A Practical Laboratory Guide.
Monitoring sulfur dioxide (SO₂) in ambient air is a key component of air‑quality assessment and public health protection. SO₂ is a major atmospheric pollutant generated primarily from fossil‑fuel combustion, power plants, refineries, and other industrial activities. Prolonged exposure can harm human health, damage vegetation, and contribute to acid rain formation.
This blog presents a practical, laboratory‑based guide to measuring ambient SO₂ using the widely accepted para‑rosaniline colorimetric method, explaining the principle, reagents, procedures, and calculations in a clear and user‑friendly manner.
Why Measure Ambient SO₂?
Accurate measurement of sulfur dioxide is essential because:
- SO₂ irritates the respirat hiory system and aggravates asthma
- It damages crops, forests, and building materials
- It contributes to acid rain and secondary particulate formation
- Regulatory agencies require routine monitoring for compliance
Reliable laboratory analysis supports environmental decision‑making and pollution‑control strategies
Principle of the Method
Ambient air is drawn through an absorbing solution of potassium tetrachloromercurate (TCM). Sulfur dioxide reacts with TCM to form a stable dichlorosulphitomercurate complex, which is resistant to oxidation by oxygen, ozone, and nitrogen oxides. This stability allows samples to be stored prior to analysis without significant SO₂ loss.
For analysis, the complex reacts with para rosaniline in the presence of formaldehyde, forming a colored compound. The color intensity is directly proportional to the amount of SO₂ present and is measured spectrophotometrically at 560 nm.
Roles and Responsibilities
- Laboratory Chemist: Conducts sampling, analysis, and calculations
- Technical Manager: Reviews analytical procedures and results
- Quality Manager: Ensures SOP implementation and quality control
Key Reagents Used
The following reagents are critical for accurate SO₂ analysis:
- Distilled water (free from oxidizing agents)
- Potassium tetrachloromercurate (0.04 M) – absorbing solution
- Sulphamic acid (0.6%) – removes nitrogen oxide interference
- Formaldehyde (0.2%) – supports color development
- Para rosaniline dye – produces measurable color
- Iodine and sodium thiosulphate solutions – for standardization
- Standard sulphite solution – used to prepare calibration standards
All reagents must be freshly prepared or stored under specified conditions to maintain analytical accuracy.
Preparation of Standards and Calibration
Standard Sulphite Solution
A sulphite solution is prepared using sodium sulphite or sodium metabisulphite and standardized by iodine–thiosulphate titration. This step determines the exact SO₂ concentration in the standard solution.
Working Sulphite–TCM Solution
A measured volume of the standardized sulphite solution is diluted and mixed with TCM. This working solution is stable for up to 30 days when stored under refrigeration and is used for preparing calibration standards.
Calibration Curve
Different volumes of the working sulphite–TCM solution are added to volumetric flasks to prepare standards containing known amounts of SO₂. After reagent addition and color development, absorbance is measured at 560 nm.
A straight‑line plot of absorbance versus SO₂ mass (µg) confirms proper calibration. The slope of this line is used to calculate the calibration factor (B).
Sample Analysis Procedure
- Prepare a reagent blank, control, and sample solutions
- Add sulphamic acid to remove nitrite interference
- Add formaldehyde followed by para rosaniline
- Allow color to develop for 30 minutes
- Measure absorbance between 30–60 minutes at 560 nm using a 1 cm cuvette
- Use distilled water, not the reagent blank, as the spectrophotometer reference
Strict temperature control is essential, as color intensity is temperature‑dependent.
Handling High Absorbance Samples
- If absorbance lies between 1.0 and 2.0, dilute the sample 1:1 with reagent blank
- Highly concentrated samples may require dilution up to six times
- Always apply the correct dilution factor (D) during calculations
Calculations
SO₂ Concentration in Air
SO₂ concentration is calculated using:
SO₂ (µg/m³) = (SA × B × D) ÷ V₁
Where:
- SA = Sample absorbance
- B = Calibration factor
- D = Dilution factor
- V₁ = Volume of air sampled at STP (m³)
Conversion to ppm
The calculated mass concentration can be converted to ppm using standard gas‑law relationships.
Quality Control and Good Laboratory Practice
- Analyze control samples with known SO₂ concentrations
- Recalibrate if reagent blank absorbance deviates significantly
- Clean cuvettes immediately after use
- Maintain consistent temperature during calibration and analysis
Final Thoughts
The para rosaniline method remains a dependable and sensitive technique for measuring sulfur dioxide in ambient air. When performed with careful reagent preparation, calibration, and quality control, it provides accurate and reproducible results essential for air‑quality monitoring and regulatory compliance.
Consistent application of this method helps laboratories contribute reliable data toward protecting public health and the environment.
