Determination of Hydrogen Halide and Halogen Emissions from Stationary Sources (Isokinetic Method) – USEPA Method 26A

USEPA Method 26A is used to determine emissions of hydrogen halides (HX) [HCl, HBr, and HF] and halogens (X2) [Cl₂ and Br₂] from stationary sources when specified by the applicable subpart.  This method collects the emission sample isokinetically and is therefore particularly suited for sampling at sources, such as those controlled by wet scrubbers, emitting acid particulate matter (e.g., hydrogen halides dissolved in water droplets).
 
Gaseous and particulate pollutants are withdrawn isokinetically from the source and collected in an optional cyclone, on a filter, and in absorbing solutions.  The cyclone collects any liquid droplets and is not necessary if the source emissions do not contain them; however, it is preferable to include the cyclone in the sampling train to protect the filter from any liquid present. 
 
The filter collects particulate matter including halide salts but is not routinely recovered or analyzed.  Acidic and alkaline absorbing solutions collect the gaseous hydrogen halides and halogens, respectively.  The halides/halogens are vaporized to gas and collected in the impingers by pulling conditioned ambient air through the sampling train.  They are solubilized in the acidic solution and form chloride (Cl-), bromide (Br-), and fluoride (F-) ions.  The halogens pass through to the alkaline solution and are hydrolyzed to form a proton (H+), the halide ion, and the hypohalous acid (HClO or HBrO). 
 
Sodium thiosulfate is added to the alkaline solution to assure reaction with the hypohalous acid to form a second halide ion such that 2 halide ions are formed for each molecule of halogen gas. The halide ions in the separate solutions are measured by ion chromatography (IC).  If desired, the particulate matter recovered from the filter and the probe is analyzed following the procedures in USEPA Method5.
 
Volatile materials, such as chlorine dioxide (ClO₂) and ammonium chloride (NH₄Cl), which produce halide ions upon dissolution during sampling, are potential interferents.  Interferents for the halide measurements are the halogen gases which disproportionate to a hydrogen halide and a hypohalous acid upon dissolution in water.  The use of acidic rather than neutral or basic solutions for collection of the hydrogen halides greatly reduces the dissolution of any halogens passing through this solution. 
 
Other potential interferents of the method include:

• the simultaneous presence of HBr and Cl₂, which causes a positive bias in the HCl result with a corresponding negative bias in the Cl₂ result, as well as affecting the HBr/Br₂ split;
• high concentrations of nitrogen oxides (NOx) which produce sufficient nitrate (NO₃-) to interfere with measurements of very low Br- levels;
• HF being outgassed from new Teflon components.