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A Technical Note on Nitrogen Balance


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A Technical Note on Nitrogen Balance

Or how to lose your mind watching nitrogen oxidize

We talk a lot about nitrate accumulation in the aquarium water as an indicator of the need for water changes, or as a macronutrient we may need to add to planted tanks. We also talk about nitrification, the oxidation process certain aerobic bacteria perform on nitrogenous wastes, oxidizing fish waste in the form of ammonia to nitrite by one group of bacteria, then that nitrite on to nitrate by another group of bacteria. We talk a lot about cycling with or without fish and testing for those same nitrogen-containing ions or molecules.

But I have only just realized that many people do not understand the balance of these materials, or why we have two different measuring or reporting systems for nitrogen-containing waste in its various stages of oxidation or reduction. So I want to spend a few minutes on tests and simple nitrogen balances for anyone who might want a little better understanding. I am not going to detail the reactions or the particular conditions they need, just a little on nitrogen cycle tests and balances.

The portion of the total nitrogen cycle of special interest to aquarists is the two-step process oxidizing toxic ammonia to much less toxic nitrate that was mentioned in the first paragraph. We have test kits for ammonia, for nitrite, and for nitrate. Some of those kits report as the concentration of the molecule (ammonia, NH3) or the ion (ammonium, NH4; nitrite, NO2; nitrate, NO3 – note that I am omitting the charges on the ions for simplicity). The numbers may be defined as parts per million (ppm), or as milligrams per liter (mg/l) – which for our purposes are the same. That is, 1ppm = 1mg/l. But the number scales in some kits introduce another term, nitrite-nitrogen or nitrate-nitrogen. The kits using the more familiar names of ammonia, nitrite, or nitrate are reporting the whole molecule or ion. This means it is saying that there is 1ppm ammonia, NH3, in the sample (or nitrate, or nitrite). That means it is reporting on in the case of ammonia one nitrogen atom plus three hydrogen atoms as a unit. The kits reporting nitrite-nitrogen or nitrate-nitrogen are reporting on the nitrogen in those ions only, not on the two or three oxygen atoms which would be part of the ions.

When we work on nitrogen balances, the ammonia-nitrogen, nitrite-nitrogen, and nitrate-nitrogen figures are the figures used. This is done because at each step we are looking at the nitrogen (N) only, not the three hydrogen atoms associated with nitrogen in ammonia, nor the two oxygen atoms associated with the nitrogen in nitrite, or the three oxygen atoms associated with the nitrogen in nitrate. There is only a single unit nitrogen at each step (one N in the chemical symbol for each of the materials): ammonia (NH3) is oxidized to nitrite (NO2), which is in turn oxidized to nitrate (NO3). To make the numbers balance we need to ignore the hydrogens which go elsewhere, or the at first two and later three oxygens which are added. If we were writing it out as an equation (we won’t), we would symbolically say that 1ppm ammonia-nitrogen became 1ppm nitrite-nitrogen, which in turn became 1ppm nitrate-nitrogen. At each step we have one unit of nitrogen (which is likely to be at least many millions to billions of atoms, but it is still a unit throughout) - One to one to one. The balance is obvious (I hope).

If we had used the more familiar form of test reporting – ammonia, nitrite, and nitrate – the situation become much more complex: Ammonia is NH3, one nitrogen and three hydrogens. The test is reporting the whole molecular unit, not just the nitrogen. Similarly, nitrite, NO2, results represent the whole ion, one nitrogen and two oxygens. Nitrate adds a third oxygen to the ionic unit.

To make the equations balance with the more commonly used test reports, we need to covert the readings or figures for ammonia to ammonia-nitrogen, nitrite to nitrite-nitrogen, and nitrate to nitrate-nitrogen. The conversion factors are simple, but I want to show you how they are derived. Deriving these conversions requires the atomic weights of each of the components of the molecule or ion. We only have three different atoms involved here:

Hydrogen: atomic weight = 1

Nitrogen: atomic weight = 14

Oxygen: atomic weight = 16

With those numbers we can derive approximate conversion factors:

Ammonia = NH3 = 14 (for one N) + 3 (for the 3 H) = 17 atomic weight, but only 14 is nitrogen, so 17/14 = ~1.2

Our conversion factor for ammonia-nitrogen to ammonia is 1.2 times, or from ammonia to ammonia-nitrogen is dividing by 1.2

If we have a titer of 1ppm ammonia, then 1ppm/1.2 = ~0.83ppm ammonia-nitrogen.

Nitrite = NO2 = 14 (for one N) + 32 (two times 16 for the two O) = 46 atomic weight, but only 14 is nitrogen, so 46/14 = ~3.3

Our conversion factor for nitrite-nitrogen to nitrite is 3.3 times, or from nitrite to nitrite-nitrogen is dividing by 3.3

In our ammonia example above we had 0.83ppm ammonia-nitrogen, which will oxidize to 0.83ppm nitrite-nitrogen (remember it is one unit on N passing along).

To show that as nitrite, we multiply by 3.3, so:

0.83ppm nitrite-nitrogen x 3.3 = ~2.7ppm nitrite

So our more familiar test gives us what appears to be confusing results- our first example tested at 1ppm ammonia, but the same nitrogen now represents 2.7ppm nitrite. That is because we have in effect removed three hydrogens and added two oxygens, significantly increasing the net weight, even though the same quantity of nitrogen is maintained.

Nitrate magnifies the apparent confusion.

Nitrate = NO3 = 14 (still just one N) + 48 (three times 16 for the three O) = 62 atomic weight, but only 14 is nitrogen, so 62/14 = ~4.4

Our conversion factor for nitrate-nitrogen to nitrate is 4.4 times, or from nitrate to nitrate-nitrogen is dividing by 4.4

In our ammonia and nitrite examples we had 0.83ppm ammonia-nitrogen carried to 0.83ppm nitrite-nitrogen, and the nitrate-nitrogen will still be the same figure, so 0.83pp nitrate-nitrogen. We have oxidized a single unit measure of nitrogen all the way through the tank process.

To show that as nitrate, we multiply by 4.4, so:

0.83ppm nitrate-nitrogen x 4.4 = ~3.6ppm nitrate.

If we trace nitrogen only, we see:

1ppm ammonia-nitrogen oxidized to 1ppm nitrite-nitrogen, which is oxidized in turn to 1ppm nitrate-nitrogen.

If we trace the molecule/ions, we see:

1ppm ammonia oxidized to 2.7ppm nitrite, which is in turn oxidized to 3.6ppm nitrate.

Comparing the two seems irrational, but it is not. The first reporting system is handy for certain things, the second for others.

But the real point of this whole exercise is not to confuse you with the details and the development of the conversions, but instead to help avoid future confusion if you are looking at something like fishless cycling titers and the numbers do not seem to match up. Well, from our standard hobby kits they are not supposed to 'match', that is to equal, each other. The nitrogen-containing material is bigger and heavier at each step – from 17 to 46 to 62.

Another complication enters with the fact that we measure titers –the concentration of ammonia, or nitrite, or nitrate in the water sample right now. Not how much total was added, not the total end product, but a snapshot of this instant only. This is especially confusing with nitrite titers. It would be extremely unlikely to add 1ppm ammonia and then at any given time after that to ever see 2.7ppm nitrite – for that to happen, all the ammonia would have had to be converted to nitrite, but not any of the nitrite to nitrate.

It may be worth adding a warning note: For hobby test kits, the zero reading is not a true zero. It simply means that by this specific test protocol, any material below the lowest stated positive concentration cannot be reliably detected. For my Tetra test kit for ammonia, the lowest positive reading is 0.25mg/l (or ppm) ammonia. It does not mean any lighter color is zero, only that any lighter color cannot be reliably read by eye. You could have 0.12ppm but you could not easily quantify that reading, so for test purposes it is zero, but would be better recorded as less than 0.25ppm or as undetectable instead of zero. After you have done a few of these tests you can easily recognize positive but below minimum scale readings. I think it is better to just record these as less than 0.25ppm(<0.25ppm). Tests where you can detect no color shift from the scale 'zero' can be recorded as 'undetectable'.

I hope you can now see that (1) there is valid reason for two separate measurement scales in nitrogen measurement, (2) that you can convert from one to another by good approximation conversion factors if you should need to do so, and (3) that you cannot expect fishless cycling results or any other nitrogen oxidation balance to look clean and even at first glance, especially if routine hobby scales are used. That is really all you need to carry away from this note (there will be no final exam and no grade). If you need the conversion factors you could derive them, but there is no penalty for using this or other reference material for retrieval. All aquarium maintenance and monitoring is an open-book practical examination.

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