Water Chemistry

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happychem

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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.
 

happychem

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Chapter 7 - Ammonia and ammonium

Ammonia (NH3) vs. ammonium (NH4+) deserved its own chapter. NH4+ is much less toxic than NH3.

Ammonia reacts with water to produce ammonium and OH-, so long as the pH is low enough:

NH3 + H2O <=> (NH4+) + (OH-)

This is an equilibrium reaction, so we recall our dissociation constant, kd. But since this is a base dissociation, it's written kb.

kb=[products]/reactants]=[NH4+]x[OH-]/[NH3]

Technically, water should be included; however, since all the situations we will see are dilute solutions, the concentration of water can be said to be constant and is therefore lumped in with the constant.

For 25oC kb for NH3 = 1.75x10^-5 which means 0.0000175.

For those interested in a little meat
math alert
Now you can actually determine for yourself how much of each species is present using this information and pH.

Remember that pH = -log[H+], well, for aqueous solutions (a solution where water is the solvent) pH has a range from 0-14.

pOH is just the opposite. pOH = -log[OH-] = 14 - pH.

So, [OH-] = 10^-(14-pH) (the "^" symbol means "raised to the power of" and is commonly used when exponents aren't available in the typeset.)

Now, using a little algebra, we isolate [NH4+] from the kb equation above:
[NH4+] = {kb/10^-(14-pH)} x [NH3]

Your ammonia test kit measures all the ammonia, regardless of ionization. So your test kit result, let's call it x:
x = [NH3] + [NH4+]

Now we can substitute in our equation from the equilibrium constant and pH:

x = [NH3] + {kb/10^-(14-pH)} x [NH3]

And finally, isolating [NH3]:

[NH3] = test kit measure/{1+kb/10^-(14-pH)}
and:
[NH4+] = test kit measure - [NH3]

and remember that kb = 1.75x10^-5 at 25oC, which is pretty close to average tank temp.

I know that nutrafin test kits have a chart that tells you how much NH3 you have based on the test kit measurement and pH, but it's nice to know where it comes from and to know that you can do the calculation yourself!
 

happychem

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Chapter 8 - Filter inserts

Since this is a chemistry article, I'll be brief on the bio/mechanical inserts. This is not a chapter on filter selection

There are numerous choices for filter media. Most sites (that I've seen) will tell you that you need three types of filtration in your filter: mechanical, biological and chemical.

Mechanical filtration refers to the removal of particles from your water. These can be such things as uneaten food, fish wastes, scales, dead plant matter, etc. If it's small enough to fit into your intake tube and small enough to get caught up in your media, it's subject to mechanical filtration. That said, there are a variety of sponges, foams, and finer media like diatomaceous earth and micron filters that are available to you. Strictly speaking, you can filter out as small a particle as you want, as long as you're willing to spend the money. Know that it's not strictly necessary to remove every tiny bit.

Biological filtration is the single most important type of filtration. The development of a biological filter is what we refer to as cycling. In doing so, filter media is colonized with nitrifying bacteria. The key elements to this are water flow, oxygen, and surface area. The first two are supplied by the filter pump and the last by your media. There exists a variety of products from generic filter sponges to bio-balls that can accomplish this end. The only rule here is go big or go home, fill your filters with as much surface area providing media as you can fit in, and don't rely on the manufacturer's maximum rating, it's overly generous.

Chemical filtration is the addition of various media to your filter to remove "undesireables" from your water. As much as I love chemistry, this type of filtration is simply not necessary in the day to day workings of a well maintained and properly stocked tank. There are three types that I'll discuss here: activated carbon, zeolite, and 'others'.

Activated carbon or charcoal is, as the name indicates, little chunks of carbon. The 'activated' part means that each of these little chunks are littered with holes and crevasses, look at a piece of lava rock to get an idea. These holes greatly increase the surface area provided by the carbon, the importance of this will soon be evident.

Activated carbon (AC) works by a phenomenon called adsorption. This differs from absorption in that nothing as actually sucked into the compound. Simply speaking, absorption does not occur at the molecular level, one molecule can not suck up another the way a sponge suck up water.

Adsorption in a result of intermolecular attraction. Think of your favourite building set, when you build a big structure the pieces in the middle are surrounded by other pieces, supported by them, but the pieces on the outside are left open. A similar occurance happens at the molecular level, atoms (or molecules) inside the structure are stabilized by those around them, but those on the outside are not. While they are satisfied as far as bonding is concerned, there's still this side of them that isn't complete and in this void a short force field exists, other molecules that come into contact with this force field stick to the surface, filling the void. This force of attraction does not propagate through the stuck on molecule because it is not a part of the solid, merely associated with it through a relatively weak force of attraction. So once all the surface is covered in stuck on molecules, no more will be adsorbed. Hence the importance of surface area!

This is wonderful! AC can be used to remove medications, organics and all sorts of undesireables from your tank. But if used regularly all the adsorption sites would be filled within a couple of weeks, depending on the stocking and feeding levels of your tank. So you see that AC can be a wonderfully useful tool for post-medication and for a vacation when you'll be unable to do a water change for a couple weeks. But for general purpose filtration, not terribly useful. As an added downside, if you keep a planted tank, AC will also adsorb chelates, which means it will pull trace elements out of your water. It does not discern one organic compound from another.

Zeolite is used for ammonia removal and is sold under brand names like ammo-lock. For day to day use this is one product that could actually be harmful to use. Much like AC adsorbs organics, zeolite adsorbs NH3. This makes it a very attractive product for people who are afraid of ammonia and especially uninformed consumers and/or new hobbyists. The danger lies in that it does exactly what it promises, it removes ammonia, but this means that it starves NH3-nitrifiers, reducing their colony size. Consequently, NO2-nitrifiers are also deprived of their food. But you've got zeolite, so who needs them, right? Wrong! Again in the same way that AC will become expired after a short time depending on stocking and feeding, as will zeolite. Once the adsorption sites are full, there's no more NH3 removal. Your fish are still producing the same levels as before, but your biological filter is unable to support the bio-load, so you get an NH3 spike and a NO2 spike. It is not a bad product to have for an emergency, something happens (like a well-meaning roomate cleans your filter in chlorinated water) and your bio-filter crashes, but it is not something to be relied on.

Others. I've group the remaining types into a general group because there are simply a limitless number of possible options and the rest are generally less prevalent than the above, for example, products that adsorb phosphates (PO4). As I said, there are limitless possibilities, if there's something that someone decides should not be in a tank, it's easy enough to develop something that will adsorb it or chelate it and stick it to a teflon or silica bead. The bottom line that I want to make clear in all this is that these are generally useless products, or to quote one of my favourite expressions: "a solution in search of a problem". As with all the above chemical filtration media any problem these products propose to remedy can be solved by water changes and proper aquarium care. As with any other hobby, there is work involved to be successful in aquaria. Trying to get around tank maintenance, water changes, proper feeding and stocking by adding products such as special (and expensive) filter media to atank will not only not solve your underlying problem, but in the long run will lead to a lack of success in the hobby.
 

OrionGirl

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Asking for clarification: Does the adsorption rate gradually decline, or is it an on/off situation? For example, if zeolite is used, will the adsorption taper off, allowing some ammonia to become available for bacteria, or will the zeolite work at the same rate until it's full?
 

happychem

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It's difficult to quantify without knowing the available adsoption of the zeolite (which would probably be expressed in moles of NH3) and the NH3 production of your tank.

If there's some adsorptive capability remaining, it should grab the NH3, so I believe that it would have a pretty steep drop in activity. Nothing is really on/off, there's always some rate of change.

I believe that the drop in activity would be pretty fast because the adsoptive capability of the amount of zeolite added to a filter is likely to be a great deal higher than the amount of NH3 produced in a day. In other words, initially, everything would be snatched up, and fractions of the zeolite would be filled each day. Eventually there would be a fraction remaining that would be close to the amount of NH3 being produced, so one day it may take it all up, the next there may be a little to show, but the third you'd have the full amount being produced free.

Think of it like filling a cooler with brownies. Every day, someone gives you a brownie and you have to put it in the cooler or eat it. Let's make them poison brownies ;). At first, you have no trouble fitting them in, but after a while, there's room for one and a half. The next day, you break a brownie in half, but you have to eat the remaining half.

Yes, nitrifiers will pick up when this final stage is reached, but I think that the zeolite will fail at a much faster rate than the bacteria can grow to match. Plus the ammonia nitrifiers are limited by the zeolite adsorption in much the same way as NO2 nitrifiers are normally limited by NH3 nitrifiers, so there's another level of restriction placed on nitrifier growth, which I think will be more detrimental to NO2 levels in the tank that NH3, since NH3 nitrifiers can grow more quickly.

But, as I said at the beginning, without some kinetics experiments, this is all just an educated guess.
 

beviking

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Something to consider with bio-media, specifically sinstered glass and other porous materials. Bacteria colonize internally but as the colony grows and some bacteria die off, they subsequently plug the small spaces and starve any remaining internal bacteria. Eventually, you end up with media that is only good for bacterial colonization on the outside. Plus, you have another source of decomposing matter, the bacteria within the media.
 

Gealcath

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Actually there is anerobic bacteria in aquariums that do break down NO3 into nitrogen, so a complete cycle does happen. Since they live in low Oxygen enviornments they take the Oxygen out of NO3 and convert it to Nitrogen gas.
 

RTR

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Biomedia in FW tanks are generally considered to be for the nitrogenous-waste oxidizers, which are aerobic. And the massive surface area claimed by that type of media is just about as meaningful as the massive surface area of activated carbon - as soon as any biofilm develops, the available surface drops precipitously, and you end up with an expensive rock, the equavalent of a piece of gravel.

If you want anaerobic reduction of nitrates (and the question there is really "Why?"), then there are alternate techniques for that which do not rely on the degrdation of aerobic biomedia to do so.
 

aliasaid

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you mentioned moles a few times but never said how much it is. so in case ppl wanna know.. 6.02 X 10^23, known as avagadro's number.

so if pH is 1, then the concentration of H+ is 0.1 mole/ litre, or 60200000000000000000000 atoms in 1 litre. concentration of 1mole/L (M) gives a pH of 0. 2M gives negative pH! donno why im mentioning this.. i feel like i wanna let ppl know i know some stuff about chem.. which i think why most ppl post.
 

happychem

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I did consider it, but the actual magnitude seemed unimportant, or worse, possibly confusing. A deeper knowledge of chemistry does require familiarity with Avagadros number, but unless you're getting into Quantum Chemistry, or some of the trickier Phys. Chem, the number itself becomes less relevant. It's like saying that there are 1,000,000 microns in one metre, if most of what you're measuring or discussing is best described in the scale of kilometres or metres, then talking about the number of microns just adds confusion.

One of the beautiful things about molarity is that it provides a scale for talking about the activity of molecules on a more macroscopic scale. Combining it with the metric system's unit designations: nano-, micro-, milli-, etc. add yet more power to it. The smaller the number used to quantify, the easier to understand and discuss. For example, we know that nanomolar (0.000000001 moles/litre of solvent) is a tiny concentration, but if that's the concentration range of our compound of interest then it's much easier and comprehensive to discuss it in terms of how many nanomoles are present. We clear out the zeros, figure out the chemistry and let our brain remember that we're dealing with very small numbers.

pH is a little trickier because it is an exponential base number sytem instead of a simple base 10 like we're more accustomed to. But as long as we keep in mind that a change of 1pH is actually an order of magnitude change then we're fine.

Besides, no aquarium is going to encounter pH 1 conditions, and negative pH isn't possible in an aqueous system, so keeping my discussion somewhat relevant to the hobby was part of my discussion. So, just finish your Intro. to Inorganic Chem.?
 
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