Method 6

Determination of Sulfuric Acid and Sulfur Dioxide Emissions from Stationary Sources

This method is used for the determination of gaseous sulfur dioxide emissions from stationary sources. A gas sample is extracted from the stack and collected using a midget impinger sampling train. Sulfur dioxide (SO2) and sulfur trioxide (SO3), including these fractions in any sulfuric acid mist, are separated in the train.  Sulfur dioxide is then measured by the barium-thorin titration method. 

A modified Method 5 train consisting of a heated probe, midget bubbler, series of midget impingers, and a silica gel drying tube is used for collection.  The method notes that it is also permissible to substitute the regular sized impingers from the Method 8 sampling train.  The midget bubbler is filled with 15mL of 80% isopropanol.  The first two midget impingers are filled with 15mL of 3% hydrogen peroxide (H2O2), and the third impinger is left empty.

Once the sample has been collected, the contents of midget bubbler can be discarded.  The isopropanol solution is used to trap water-soluble cations and fluorides that may be present in the train and are known interferences to sulfur dioxide.  The contents of the three midget impingers are combined in a polyethylene bottle.  The impingers and connecting glassware are rinsed with deionzied water and the rinse is combined with the impinger contents.

An aliquot is taken from the sample, a thorin indicator added and the sample is titrated with a barium standard.  A separate aliquot is taken and the titration repeated.  The average value is reported and the replicates must agree within 1% or 0.2mL whichever is greater. 

If the data are to be used for compliance with source emission regulations then the method requires the analysis of an audit sample along with the collected field samples.  In order for the audit to pass, the method cites an agreement within 5% from the value determined in the analysis to the accepted true concentration of the audit sample.


(40 CFR Part 60, Appendix A)


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Method Data

Hold Times, Preservatives, Preps, Collection, Analytical & Documentation
Holding Time:   None specified in method.
Preservatives:   None specified in method.
Required Preps:   None specified in method.
Collection Method:   Modified Method 5 or 8 train (midget impingers) following Method 6 procedures.
Analytical Methodology:   Barium-thorium Titration
Documentation:   6

Analyte List*

Analyte Formula CAS Number Detection Limit
Sulfur dioxide

* The analytes and detection limits listed for each method represent the typical detection limits and analytes reported for that particular method. Keep in mind that analyte lists may vary from laboratory to laboratory. Detection limits may also vary from lab to lab and are dependent upon the sample size, matrix, and any interferences that may be present in the sample.