Rules Of Thumb Ammonia/Ammonium Ion

1. The percentage of dissolved total ammonia (NH3 + NH4+) present in the unionized form (NH3) increases as either pH or temperature increases.

2. Ammonia toxicity to aquatic life is mainly due to unionized  NH3 form.

3. An increase in either pH or temperature shifts the equilibrium of equation NH3 + H2O <-> NH4+ + OH , to the left, favoring NH3, the toxic form.

4. At 20 oC and pH>9.3, the equilibrium of equation NH3 + H2O <-> NH4+ + OH is mainly to the left, with NH3 (the toxic form) > 50%

5. At 20 oC and pH<9.3, the equilibrium of equation NH3 + H2O <-> NH4+ + OH is mainly to the right, with NH4+ (the non toxic form) > 50%

6. NH3, concentrations >0.5 mg NH3-N/L cause significant toxicity to fish

7. The unionized form is volatile (air-strippable), and the ionized form is nonvolatile

Reference

Eugene R. Weiner. Application of Environmental Aquatic Chemistry. A Practical Guide. Third edition. CRC Press