Ammonia To Nitrate

[rdd1952]

Here's another one for the scientific types. I have heard it said that 1 ppm of ammonia will be processed into 1 ppm nitrite and then into 1 ppm of nitrate but never found anything to back that up. Today I saw the information below here. So, my question is, how true is this?

1 ppm of ammonia can lead to almost 3 ppm of nitrite because one Nitrogen atom in a molecule of ammonia (molecular weight of 17) forms one Nitrogen atom in a molecule of nitrite (molecular weight of 46), so 17 ppm of ammonia would lead to 46 ppm of nitrite. In other words, the ratio of the molecular weights (46/17) can potentially multiply the ammonia levels by 2.7 times.

1 ppm of nitrite can similarly lead to 1.35 ppm of nitrate (62/46).

1 ppm of ammonia can for the above reasons lead to 3.65 ppm of nitrate (62/17).

 

[Bignose]

You have to remember that ppm isn't really what is being measured. Because it is dilute concentrations ppm and mg/L are pretty close to equivalent, but the real measurement is mg/L.

Even more confusing, the chemical reactions will be balanced on a molar basis. If you recall, one mole of a chemical is equal to 6.022*10^23 molecules of that substance. The molecular weight is used to convert moles to weight, which is what we will need to convert from moles/L to mg/L. The molecular weight of ammonia is 17 g/mole.

On a molecular basis, NH3 --AOB--> NO2 --NOB--> NO3 where the notation --AOB--> means "is converted by ammonia oxidizing bacteria" and the --NOB--> notation means "is converted by nitrite oxidizing bacteria". It is one mole of ammonia becomes one mole of nitrite becomes one mole of nitrate.

So, if we started with one mole of ammonia per liter of water, that would be equal to 17g/L of ammonia in the water. This is converted to one mole of nitrite, which would be 46g/L of nitrite. This is then converted into 62 g/L of nitrate. Yes, the information is correct.

It is 1 to 1 to 1 on a mole basis, but it is 1 to 2.7 to 3.7 on a mass basis. And, ppm is really mg/L at the dilute concentrations, which is a mass per unit volume, so the mass ratios determine the ppms that result.

 

[rdd1952]

No wonder I hated chemistry in high school.

So in essence, if I understand correctly, during a fishless cycle, that 4 ppm of ammonia we start with will actually become over 10 ppm of nitrite. No wonder it takes forever for the nitrite to drop.