Method 6
Determination of Sulfur Dioxide Emissions From Stationary Sources
40 CFR Part 60, Appendix A-4
From the method — regulatory text
Sections 1.0 and 2.0 of Method 6—Determination of Sulfur Dioxide Emissions From Stationary Sources, as published at 40 CFR Part 60, Appendix A-4. Retrieved 10/07/2026. The full method is on the eCFR.
1.0 Scope and Application
1.1 Analytes.
| Analyte | CAS No. | Sensitivity |
|---|---|---|
| SO2 | 7449-09-5 | 3.4 mg SO2/m3 (2.12 × 10)−7 lb/ft3 |
1.2 Applicability. This method applies to the measurement of sulfur dioxide (SO2) emissions from stationary sources.
1.3 Data Quality Objectives. Adherence to the requirements of this method will enhance the quality of the data obtained from air pollutant sampling methods.
2.0 Summary of Method
2.1 A gas sample is extracted from the sampling point in the stack. The SO2 and the sulfur trioxide, including those fractions in any sulfur acid mist, are separated. The SO2 fraction is measured by the barium-thorin titration method.
EssentialRegs notes
Our plain-English reading of Method 6. Not the method text.
What it measures
The concentration of sulfur dioxide (SO2) in stack gas, determined from gas collected in a wet-chemistry sampling train.
How it works
A sample of stack gas is drawn through a probe and a filter, then through a series of impingers. An isopropanol impinger first removes sulfuric acid mist and sulfur trioxide so they are not counted. The sulfur dioxide is then captured in hydrogen peroxide impingers, where it is converted to sulfate, and the sulfate is measured in the laboratory by titration with barium perchlorate. The result is related to the volume of gas sampled.
Equipment
A Method 6 sampling train: a probe with a filter, an isopropanol impinger, two hydrogen peroxide impingers, a pump and a dry gas meter, followed by laboratory titration.
When a rule cites it
Performance tests to show that a sulfur dioxide limit or a sulfur dioxide removal requirement is met, including tests on sulfur recovery units, process heaters and combustion equipment. It is the wet-chemistry method; rules that allow an instrumental analyzer name Method 6C instead.
Notes for compliance staff
Method 6 reports sulfur dioxide only; sulfuric acid mist and sulfur trioxide are removed ahead of the measurement, and Method 8 is the method that determines them. Method 6C is the instrumental-analyzer alternative for sulfur dioxide. A concentration from Method 6 becomes an emission rate when it is combined with a gas flow measurement from Methods 1 through 4, or with an oxygen or carbon dioxide reading and an F-factor under Method 19.
Cited by
Provisions in the corpus whose text cites Method 6. Subscribers land on the provision in the regulation reader; visitors see a preview of it (the GP05 sample opens in full).
Regulation 1
Regulation 6
- II.E.3. — Method 3 for gas analysis to be used when applying Reference Methods 5 and 6;
- II.E.5. — Method 6 for concentration of SO2.
- III.E.3. — Method 3 for gas analysis to be used when applying reference Methods 5 and 6;
- III.E.5. — Method 6 for concentration of SO2.
- IV.D.3. — Method 3 for gas analysis to be used when applying Method 6; and
- IV.D.4. — Method 6 for concentration of SO2.
40 CFR Part 60 Subpart OOOO
40 CFR Part 60 Subpart OOOOa
40 CFR Part 60 Subpart OOOOb
49 CFR Part 192
The sections marked as regulatory text above are reproduced from the CFR. Everything under ‘EssentialRegs notes’ is our reference copy, not the method. The method as published in the CFR controls.