Question

The laboratory reports that the concentration of ammonia as nitrogen in a sample is 3.4 mg/l. The pH value of the sample is 5.5 units. In what form is almost all of this ammonia? could this sample be toxic to aquatic organisms?

Answer 1

Ammonia occurs naturally in water bodies arising from the microbiological decomposition of nitrogenous compounds in organic matter. Fish and other aquatic organisms also excrete ammonia. Ammonia may also be discharged directly into water bodies by some industrial processes or as a component of domestic sewage or animal slurry. Ammonia can also arise in waters from the decay of discharged organic waste. Natural (unpolluted) waters contain relatively small amounts of ammonia, usually <0.02 mg/l as N.

Ammonia exists in aqueous solutions in two forms, ionised (NH₄⁺) and un-ionised (NH₃) and the un-ionised fraction is toxic to fresh water fish at very low concentration. The relative proportions of ionised and un-ionised ammonia in water depend on temperature and pH and to a lesser extent on salinity. The concentration of un-ionised ammonia becomes greater with increasing temperatures and pH and with decreasing salinity.

Ammonia is in the unionised form and this sample is toxic to aquatic organisms.