SECTION 10 

 SOME FACTORS AFFECTING THE TOXICITY OF AMMONIA TO FISHES 

 Robert V. Thurston^ 



INTRODUCTION 



Ammonia can be a serious toxicant to fishes and other aquatic life. It 

 can enter natural water systems from several sources, including industrial 

 wastes, sewage effluents, coal gasification and liquefaction conversion pro- 

 cess plants, and agricultural discharges including feedlot runoff. It is 

 also a metabolic waste product of fishes, and as such presents a major pro- 

 blem in fish culture. 



In aqueous solutions, ammonia assumes two chemical species, illustrated 

 by the following equation. 



NH 



3(g) ' ^^2^{l) ^ N"3-"^20(aq) ^ ''^" ' 0^" ' ^'-'^""z^U) 



These species are the gaseous or un-ionized form (NH3), bound to at least 

 three water molecules, and the ionized form (NH4"'"). In this presentation, 

 the term NH3 will refer to un-ionized ammonia, NH4"'" will refer to ionized 

 ammonia, and total ammonia will refer to the sum of these. The aqueous am- 

 monia equilibrium is strongly dependent upon the pH of the solution, and to 

 a lesser extent upon temperature and ionic strength. As the pH increases, 

 increasing the hydroxide ion concentration, the equilibrium shift of ammonia 

 is toward the un-ionized (NH3) species. Within the pH range acceptable to 

 most freshwater fishes, an increase of one pH unit will increase the NH3 

 concentration approximately tenfold (Thurston et al_. 1974). Temperature in- 

 crease also favors the NH3 species, but to a lesser extent; ionic strength 

 increase, at low concentrations, favors the NH4"'" species (ibid). 



Early reported research on the toxic effect of ammonia (Chipman 1934; 

 Wuhrmann et a^. 1947; Wuhrmann and Woker 1948) implicated NH3 as being the 

 toxic form of ammonia, and NH4+ was considered non-toxic or appreciably less 

 toxic. Because of the recognized toxicity of NH3, and the belief that NH4"'" 

 is not significantly toxic, most toxicity values reported in the literature 

 are as NH3. Sometimes total ammonia values have also been reported, but too 



'Fisheries Bioassay Laboratory, Montana State University, Bozeman, Montana 

 59717. 



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