USEPA Method 26

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

USEPA Method 26 applies to the determination of emissions of hydrogen halides (HX) [HCl, HBr, and HF] and halogens (X2) [Cl₂ and Br₂] from stationary sources. Sources such as those controlled by wet scrubbers that emit acid particulate matter must be sampled using USEPA Method 26A.

An integrated sample is extracted from the source and passed through a prepurged heated probe and filter into dilute sulfuric acid and dilute sodium hydroxide solutions which collect the gaseous hydrogen halides and halogens, respectively. The filter collects particulate matter including halide salts but is not routinely recovered and analyzed. The hydrogen halides are solubilized in the acidic solution and form chloride (Cl-), bromide (Br-), and fluoride (F-) ions. The halogens have a very low solubility in the acidic solution and pass through to the alkaline solution where they are hydrolyzed to form a proton (H+), the halide ion, and the hypohalous acid (HClO or HBrO). Sodium thiosulfate is added in excess to the alkaline solution to assure reaction with the hypohalous acid to form a second halide ion such that two halide ions are formed for each molecule of halogen gas. The halide ions in the separate solutions are measured by ion chromatography (IC).

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 hydrohalous acid upon dissolution in water. However, 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.

The simultaneous presence of HBr and Cl₂ 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) produces sufficient nitrate (NO₃-) to interfere with measurements of very low Br- levels.

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Last Update: October 25, 2006