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CO2 and NH3 into water


I use Phreeqc to simulate fish breathing to see what happen when I dissolve CO2 into seawater and freshwater, with and without NH3. (NH3 is a gas from nitrogen break down in the metabolism). If I, in seawater only use CO2 (for instance 0.3 mmol/l) the reaction is normal with expected fall in pH. But if I also adding NH3 (0.03 mmol/l) the pH falls further? If I simulate the same procedure in fresh water the opposite happen, which I think is normal.

For me it looks that NH3 have functions as acids in seawater (it convert HCO3 to H2CO3) but is a base in freshwater (it convert CO2 to HCO3)?

CO2 + H2O « H2CO3 « (HCO3-) + (H+) « (CO3--) + (H+)

NH3 + H2O « (NH4+) + (OH-)

In freshwater (simplified) CO2 + OH- « HCO3 (pH increase)

In seawater (simplified) HCO3 + OH- « H2CO3 (pH decrease)????

I hope you understand my not so god English.


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