An official website of the United States government.

This is not the current EPA website. To navigate to the current EPA website, please go to This website is historical material reflecting the EPA website as it existed on January 19, 2021. This website is no longer updated and links to external websites and some internal pages may not work. More information »

Can the RQ assigned to hydrochloric acid be used for hydrogen chloride gas?

Hydrogen chloride gas is introduced into water to form hydrochloric acid.  Saturation for this reaction occurs at 38 percent (%).  Therefore, any hydrogen chloride present after the saturation point is reached, does not go into solution and will remain in the gaseous state.  Can the reportable quantity (RQ) assigned to hydrochloric acid, a listed hazardous substance, be used for hydrogen chloride gas?

The RQ for anhydrous hydrogen chloride is 5000 pounds and applies to all forms of hydrogen chloride.  When determining a RQ for a form of hydrogen chloride that occurs in a solution, the Clean Water Act (CWA) "mixture rule" will be used if the percentage of hydrogen chloride in solution is known.  For example: To determine the RQ of a product solution of water and 35 percent (%) hydrochloric acid, the CWA "mixture rule" is applied as follows: divide the RQ of hydrochloric acid, 5000 pounds, by the percentage (expressed as a decimal) of the hydrochloric acid in solution, 0.35.  The product of this equation is the RQ, in pounds, for the previously described solution (5000/0.35 = 14,286 pounds).  If the hazardous substance occurs as a constituent of a hazardous waste, the same rule can be applied when determining the RQ for the hazardous waste.  If the percentage of the hazardous substance in the waste is not known, however, the RQ for the listed or unlisted hazardous substance constituent is to be used.  When more than one hazardous substance is in product or waste solution, always use the lowest applicable RQ for the solution.