The list of regulated toxic substances at 40 CFR Section 68.130 includes both "ammonia (anhydrous)" and "ammonia (conc 20% or greater)," but does not include a specific listing for "ammonium hydroxide." The Chemical Abstract Registry Service (CAS) number for ammonium hydroxide is 1336-21-6, and the CAS number for ammonia is 7664-41-7. Ammonium hydroxide is, however, simply a mixture of ammonia and water. Must a stationary source owner or operator consider the amount of ammonia present in ammonium hydroxide that is contained in a process when determining whether the threshold for ammonia is exceeded?

Yes. For the purposes of the risk management program regulations at 40 CFR Part 68, ammonium hydroxide must be treated as a solution of ammonia and water, regardless of the fact that ammonium hydroxide may be identified by a unique CAS number. The Agency has made it clear that the listing for "ammonia (conc 20% or greater)" applies to aqueous solutions of ammonia (List Rule Response to Comments document, page 50). If the concentration of ammonia in the ammonium hydroxide is 20% or greater, then the mixture is subject to threshold determination for "ammonia (conc 20% or greater)" under 40 CFR Section 68.115.