Toxicity of Ammonia

I have written a short explanation on how the toxicity of ammonia is affected by pH and temperature. This has been added to the ‘Nitrogen Cycle’ page!

The toxicity of ammonia is affected by both temperature and pH. This can be explained by examining the equilibrium of the protonation of ammonia. When ammonia is protonated it forms relatively harmless ammonium ions (NH4+).

According to Le Châtelier’s principle, if you impose a change on an equilibrium the equilibrium will shift to oppose the change.

Temperature

NH3 + H3O+ ↔ NH4+ + H2O

Firstly, this reaction is exothermic (it gives out heat). It follows that:

  • An increase in temperature will cause the equilibrium to shift to the left-hand side, taking in the heat energy. This increases the amount of ammonia.
  • A decrease in temperature will cause the equilbrium to shift to the right-hand side, giving out heat energy. This decreases the amount of ammonia.

Lowering the temperature therefore lowers the toxicity of ammonia, simply by converting a higher amount to ammonium.

pH

NH3 + H3O+ ↔ NH4+ + H2O

pH is inversely proportional to the amount of H+ in the water, which are present in the form H3O+. We can therefore rewrite this equilibrium as follows:

NH3 + H+ ↔ NH4+

It follows that:

  • An increase in pH (decrease in H+) will cause the equilibrium to shift to the left-hand side, producing H+ ions. This increases the amount of ammonia.
  • A decrease in pH (increase in H+) will cause the equilibrium to shift to the right-hand side, taking in H+ ions. This decreases the amount of ammonia.

Lowering the pH therefore lowers the toxicity of ammonia, simply by converting a higher amount to ammonium.

Hope it’s useful!

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