Chapter 6 - The 7th Element
That would be nitrogen, more specifically, this chapter will deal with the three nitrogen species with which we are all familiar (or are soon to be): ammonia (NH3), nitrite (NO2), and nitrate (NO3). The reason that this chapter, on an undeniably central concept to aquarists, was pushed so far down was because to best explain some of the concepts, it was first necessary to talk about concentrations and measurement techniques.
Specifically, I do not intend to talk about toxicity and cycling so much, these are discussed exhaustively in numerous places.
The first subtopic is of measurement and exchange of information. This will extend my gripe with units of ppm. Although I previously mentioned that the key to units is to have a consistent form in which to present our data to each other, complete with relevant benchmarks to tell us what's a healthy level vs. a dangerous one, ppm is especially cumbersome when it comes to discussing the nitrogen cycle.
Why? Because there are now two ways of looking at things, we measure the compound directly, but do we want to talk about the concentration of ammonia in our tank or the amount of nitrogen in the form of ammonia?
Why, you ask, is the latter relevant? Well, let us take the problem apart piece by piece. First, know that there are two ways to report concentrations of each compound:
ammonia vs. ammonia-nitrogen (NH3-N) or total ammonia nitrogen (TAN)
nitrite vs. nitrite nitrogen (NO2-N)
nitrate vs. nitrate nitrogen (NO3-N)
Back to the question of relevance. Well, with the exception of benchmarks for tank health, the first of each pair tells us nothing about nitrogen chemistry. This is back to the problem of ppm being a mass based unit, 1ppm of NH3 does not produce 1ppm of NO2!
To get around this problem, the second set of terminologies was developed. These strip the compounds down to the element of interest, nitrogen. This looks at the issue from the point of view of the nitrogen atom being oxidized and allows each to be compared directly:
1ppm NH3-N = 1ppm NO2-N = 1ppm NO3-N
But how do we arrive at these values? Well, we go back to molecular mass (these are approximate values):
N = 14g/mol
O = 16g/mol
H = 1g/mol
So:
NH3 = 17g/mol
NO2 = 46g/mol
NO3 = 62g/mol
So, what is the weight ratio between each species and nitrogen?
NH3/N = 17/14 = 1.2
NO2/N = 46/14 = 3.3
NO3/N = 62/14 = 4.4
So to convert your measured concentrations to nitrogen equivalents:
NH3-N = [NH3]/1.2
NO2-N = [NO2]/3.3
NO3-N = [NO3]/4.4
Lets try this out on the example from Chapter 4 where I demonstrated that 1ppm of NH3 would produce about 2.7ppm of NO2.
1ppm NH3/1.2 = 0.83ppm NH3-N = 0.83ppm NO2-N
0.83ppm NO2-N x 3.3 = 2.75ppm NO2 and 3.67ppm NO3.
voila!
Well, that makes it all very simple doesn't it? But of course, if we all used molarity, or even better, equivalents, as our base unit of measurement, we wouldn't have to go through all this conversion to start with.
So, if we are going to stick with ppm as our system of units, why don't the kits simply report in units of Nspecies-N instead of having to make the conversion?
Well, there are a few things that come to mind, but most notably is that with the exception of interrelations between the species this is not a very useful unit of measurement. For example, the standards to show the colour change, are prepared gravimetrically. This means that the amount of NH3 (or NO2 or NO3) is weighed on a very precise balance and added to a precisely known mass or volume of water. So simply recording mg/L is convenient, although they would still have to use molecular mass to factor out the Cl in the NH4Cl likely used to prepare the standard. So in short, I'm stumped, I can't really think of a good reason to simply use compound specific ppm, but then, I can't think of a good reason to use ppm instead of molarity to begin with, so there it is.
While 'cycling' is a convenient term, it's derived from a process which does not happen in the aquarium, the nitrogen cycle. In the wild, such as the ocean, there are organisms which cycle nitrogen, both fixing N2 (nitrogen gas) from the atmosphere, converting NH3 to NO3 and others to convert it back to NH3. This doesn't really occur in the aquarium, in facts it's a unidirectional process, fish excrete NH3 (there are other sources as well, like decomposing vegitation and fish scales) and bacteria convert it to NO3. The NO3 is removed through water changes, but is not cycled back to N2.
However, this is advantageous to those of us who do not wish to spend millions of dollars to set up an analytical laboratory to analyse our water.
Fish are not just NH3 factories, their wastes contain all the breakdown products of their food which they did not or could not absorb for their own metabolism, these can be sulfur compounds and organics, neither or which we can measure easily or cheaply. They secrete hormones which while in nature would be flushed out simply remain in the water and build up. None of these are good for the fish but sadly we cannot measure them. But all is not lost. We can measure the NO3 that is produced from the NH3 that they secrete and based on this, we can make a hand-wavey generalisation about the other pollutants in the tank. To my knowledge, no one has actually measured the precise relationship between meausured NO3 from fish secreted NH3 and concentration of fish derived pollutants, but it doesn't matter, we can choose a benchmark NO3 level, like 20ppm and adjust our maintenance and water changes accordingly.
So NO3 is not just the non-toxic (in the short term) end product of 'cycling', it also allows us to have some idea of the level of pollutants in the tank.
That would be nitrogen, more specifically, this chapter will deal with the three nitrogen species with which we are all familiar (or are soon to be): ammonia (NH3), nitrite (NO2), and nitrate (NO3). The reason that this chapter, on an undeniably central concept to aquarists, was pushed so far down was because to best explain some of the concepts, it was first necessary to talk about concentrations and measurement techniques.
Specifically, I do not intend to talk about toxicity and cycling so much, these are discussed exhaustively in numerous places.
The first subtopic is of measurement and exchange of information. This will extend my gripe with units of ppm. Although I previously mentioned that the key to units is to have a consistent form in which to present our data to each other, complete with relevant benchmarks to tell us what's a healthy level vs. a dangerous one, ppm is especially cumbersome when it comes to discussing the nitrogen cycle.
Why? Because there are now two ways of looking at things, we measure the compound directly, but do we want to talk about the concentration of ammonia in our tank or the amount of nitrogen in the form of ammonia?
Why, you ask, is the latter relevant? Well, let us take the problem apart piece by piece. First, know that there are two ways to report concentrations of each compound:
ammonia vs. ammonia-nitrogen (NH3-N) or total ammonia nitrogen (TAN)
nitrite vs. nitrite nitrogen (NO2-N)
nitrate vs. nitrate nitrogen (NO3-N)
Back to the question of relevance. Well, with the exception of benchmarks for tank health, the first of each pair tells us nothing about nitrogen chemistry. This is back to the problem of ppm being a mass based unit, 1ppm of NH3 does not produce 1ppm of NO2!
To get around this problem, the second set of terminologies was developed. These strip the compounds down to the element of interest, nitrogen. This looks at the issue from the point of view of the nitrogen atom being oxidized and allows each to be compared directly:
1ppm NH3-N = 1ppm NO2-N = 1ppm NO3-N
But how do we arrive at these values? Well, we go back to molecular mass (these are approximate values):
N = 14g/mol
O = 16g/mol
H = 1g/mol
So:
NH3 = 17g/mol
NO2 = 46g/mol
NO3 = 62g/mol
So, what is the weight ratio between each species and nitrogen?
NH3/N = 17/14 = 1.2
NO2/N = 46/14 = 3.3
NO3/N = 62/14 = 4.4
So to convert your measured concentrations to nitrogen equivalents:
NH3-N = [NH3]/1.2
NO2-N = [NO2]/3.3
NO3-N = [NO3]/4.4
Lets try this out on the example from Chapter 4 where I demonstrated that 1ppm of NH3 would produce about 2.7ppm of NO2.
1ppm NH3/1.2 = 0.83ppm NH3-N = 0.83ppm NO2-N
0.83ppm NO2-N x 3.3 = 2.75ppm NO2 and 3.67ppm NO3.
voila!
Well, that makes it all very simple doesn't it? But of course, if we all used molarity, or even better, equivalents, as our base unit of measurement, we wouldn't have to go through all this conversion to start with.
So, if we are going to stick with ppm as our system of units, why don't the kits simply report in units of Nspecies-N instead of having to make the conversion?
Well, there are a few things that come to mind, but most notably is that with the exception of interrelations between the species this is not a very useful unit of measurement. For example, the standards to show the colour change, are prepared gravimetrically. This means that the amount of NH3 (or NO2 or NO3) is weighed on a very precise balance and added to a precisely known mass or volume of water. So simply recording mg/L is convenient, although they would still have to use molecular mass to factor out the Cl in the NH4Cl likely used to prepare the standard. So in short, I'm stumped, I can't really think of a good reason to simply use compound specific ppm, but then, I can't think of a good reason to use ppm instead of molarity to begin with, so there it is.
While 'cycling' is a convenient term, it's derived from a process which does not happen in the aquarium, the nitrogen cycle. In the wild, such as the ocean, there are organisms which cycle nitrogen, both fixing N2 (nitrogen gas) from the atmosphere, converting NH3 to NO3 and others to convert it back to NH3. This doesn't really occur in the aquarium, in facts it's a unidirectional process, fish excrete NH3 (there are other sources as well, like decomposing vegitation and fish scales) and bacteria convert it to NO3. The NO3 is removed through water changes, but is not cycled back to N2.
However, this is advantageous to those of us who do not wish to spend millions of dollars to set up an analytical laboratory to analyse our water.
Fish are not just NH3 factories, their wastes contain all the breakdown products of their food which they did not or could not absorb for their own metabolism, these can be sulfur compounds and organics, neither or which we can measure easily or cheaply. They secrete hormones which while in nature would be flushed out simply remain in the water and build up. None of these are good for the fish but sadly we cannot measure them. But all is not lost. We can measure the NO3 that is produced from the NH3 that they secrete and based on this, we can make a hand-wavey generalisation about the other pollutants in the tank. To my knowledge, no one has actually measured the precise relationship between meausured NO3 from fish secreted NH3 and concentration of fish derived pollutants, but it doesn't matter, we can choose a benchmark NO3 level, like 20ppm and adjust our maintenance and water changes accordingly.
So NO3 is not just the non-toxic (in the short term) end product of 'cycling', it also allows us to have some idea of the level of pollutants in the tank.