RESUMO
The toxicity of un-ionized ammonia, NH3 (aq), in anaerobic digestion of high-nitrogen wastes has been researched extensively. Previous estimates of NH3 (aq) concentration have relied on a simple speciation approach, based only on the acid dissociation constant and the sample pH and total ammonia concentration. The distinction between concentration and chemical activity has generally not been made, despite the potential for resulting errors in the calculation of NH3 (aq) concentration, and the greater applicability of activity to toxicity work. The currently accepted approach for estimating NH3 (aq) concentration is based on assumptions that are not valid in digested animal manure or other concentrated wastes. This work presents an approach for directly measuring NH3 (aq) activity in complex mixtures using gaseous/aqueous equilibrium across microporous tubing. Application of this approach to anaerobic digester samples confirms that the currently accepted approach is not accurate; it overestimated NH3 (aq) activity in unaltered samples by 45-200%. Previous work on the toxicity of ammonia to methanogenesis has probably overestimated the tolerance of consortia to NH3 (aq), due to overestimation of concentrations. The method introduced here is expected to be useful in a range of research on ammonia toxicity and volatilization.