Nitrate and pH in a Fishlessly-cycled Aquarium

[Frank Pike]

Homework Statement

Prior to introducing livestock, a fishkeeper may establish colonies of nitrosifying and nitrifying bacteria by repeatedly inoculating an aquarium with a solution of ammonia (to simulate the presence of fish). At the end of the process there is a high concentration of nitrate in the water - typically of the order of 80mg/l.

May such a concentration of nitrate lower pH?

Homework Equations

[none understood]

The Attempt at a Solution

And anyway, what kind(s) of nitrate is it? Cations can come only from the virgin water (usually tapwater) in which there is typically a preponderance of calcium and magnesium. Is calcium nitrate more likely than magnesium nitrate? I have read that the former's pH is around 4 or 5 ; while the latter's is around neutral. So if calcium nitrate forms there'll be a heap of extra H+ ions around ...

... which is fine so long as there's sufficient buffer (in the form of bicarbonate) to soak them up. If not, then presumably high nitrate in low-buffered water would indeed tend to acidify? Would nitric acid form?

Or am I on the wrong track altogether?

 

[Borek]

The only thing I can think of is that oxidation of ammonia can acidify the solution:

NH₃ + 2O₂ -> HNO₃ + H₂O

You don't produce calcium or magnesium nitrate, these cations were in water and they are still there, nothing have changed.