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Old 05-01-2005, 08:13 PM   #1 (permalink)
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Default Aqueous Carbon Chemistry: or Organic Chemistry Finally Paid Off!

I read an interesting little bit of information the other day while working in the lab. According to William Schlesinger, one of the best biogeochemists in the world (ranking up there with Tom's old prof, K.R. Reddy) at the pH levels we commonly keep in our tanks Carbon would be available as Carbonate in nature. This got me thinking on our CO2 supplimentation habits and the chemistry going on inside the glass boxes we keep.

After scratching down some chemical reactions I came to the conclusion that Dr. Schlesinger is right. Since chemistry is thirsty work I opened a bottle of soda and the evidence sprayed me in the face. Unless kept under artifically high pressures CO2 gas is not present in aqueous solution. Instead, it is absorbed and converted into Carbonic Acid, which then undergoes some pretty spiffy chemistry.

In our tanks with CO2 gas supplimentation the CO2 is quickly converted to Carbonic Acid by interaction with water which then reacts with other free water molecules to form Bicarbonate and Hydronium ions. These steps are what happens in our reaction chambers and is how CO2 is absorbed and held in aqueous solution.

1. CO2 + H20 --> H2CO3 (Carbonic Acid)
2. H2CO3 + H20 --> H3O+ and HCO3- (Hydronium and Bicarbonate)

3. HCO3- + OH- --> CO3(2-) (Carbonate)

Due to water's property of auto-ionization [where water molecules interact to form OH- (Hydroxide) and H+ (Hydride), OH- being basic and H+ being acidic] the Bicarbonate can react with the strongly basic Hydroxide to form Carbonate and H20. This is pretty unstable stuff, but is more easily accessible by plants than Bicarbonate since it's chemically similar to CO2 and doesn't have a lot of side groups to remove in order to be useful in photosynthesis.

Further evidence is the mechanism we use to test for CO2 levels in water, pH. The definition of an acid is a molecule that has at least one Proton (Hydrogen atom without the electron) available to be donated to or removed by another molecule/atom/ion. Since CO2 has no Hydrogens to provide the protons it can't be an acid. Therefore, CO2 is not available in our aquariums.

If there were nothing adding or removing one or more of the elements/atoms/ions involved in these steps the reactions would go on and on in equilibrium forever. In the cases of our tanks though, we have plants which do a smashing job of removing what carbon they can from the water, throwing a wrench into the lovely equilibrium we've established. That's why we add CO2 or another source of carbon to our tanks.

If you're going by Diana Walstad's tests then it stands to reason that the plants would do better in a tank with harder water because the elements are there to provide/produce higher amounts of carbonate/bicarbonate in the absence of CO2 supplimentation. Decomposition of organic matter produces CO2 but also the harder water is higher in Calcium Bicarbonate, which the plants can use as a source of carbon through biogenic decalcification. The plant absorbs the Calcium Carbonate (CaCO3) and breaks down the molecule to get at the Carbon. This is why we often see Ca deposits on plants that are getting their Carbon by this mechanism.

Also worth mentioning is the high solubility of CaCO3 (Calcite/Limestone) in water. Again, due to the auto-ionization of water the acidic Hydronium (H3O+) interacts with the CaCO3 to form calcium, water, and CO2.

1. CaCO3 + H3O(+) --> Ca + H2O + CO2
2. CO2 + H20 --> H2CO3 etc.

This is why plants can do so well in areas where limestone is available in close contact with surface water or where groundwater flows through limestone, ie: Florida.

Last edited by Phil Edwards; 05-01-2005 at 08:17 PM..
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Old 05-01-2005, 10:31 PM   #2 (permalink)
 
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This again brings up my question of carbonate hardness (KH) test kits. When testing the KH (with Aquarium Pharm, Tropic Marin kits), I assume you are basically adding an acid to a base and see how many drops it takes to cause a color change.

For example, when water goes through a water softener, cations such as Ca+ and Mg+ are are removed and replaced with another cation: Na+.

Baking Soda (Sodium bicarbonate) will also raise the carbonate hardness of the water, thus creating NaCO3?

"Decomposition of organic matter produces CO2 but also the harder water is higher in Calcium Bicarbonate, which the plants can use as a source of carbon through biogenic decalcification. The plant absorbs the Calcium Carbonate (CaCO3) and breaks down the molecule to get at the Carbon. This is why we often see Ca deposits on plants that are getting their Carbon by this mechanism"

So mainly when referring to 'hard' water, CaCO3 isn't the only product increasing the alkalinity. If someone wanted to use RO/DI water mixed proportionately with water through a water softner it is possible to obtain an appropriate dKH', and adding the calcium and magnesium with appropriate buffers to raise the general hardness. But the KH reading is coming from sodium bicarbonates.
Therefore, when using a KH test kit how can we be sure that we have the 'right' kind of carbonates/bicarbonates in the water? And what would happen to the Na+?

My favorite way of having a successful planted tank is to start from scratch (RO/DI) and adding my personal favorite buffers from SeaChem to adjust GH and alkalinity. But when using well water diluted with RO/DI water I am not sure I feel comfortable with what my KH test kit is actually testing............
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Old 05-01-2005, 11:14 PM   #3 (permalink)
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Quote:
Originally Posted by Phil Edwards
Unless kept under artifically high pressures CO2 gas is not present in aqueous solution. Instead, it is absorbed and converted into Carbonic Acid, which then undergoes some pretty spiffy chemistry.

In our tanks with CO2 gas supplimentation the CO2 is quickly converted to Carbonic Acid by interaction with water which then reacts with other free water molecules to form Bicarbonate and Hydronium ions. These steps are what happens in our reaction chambers and is how CO2 is absorbed and held in aqueous solution.
Phil. Not all of the carbon dioxide (CO2) we inject into our tanks become carbonic acid (H2CO3). When gaseous CO2 is added to water, it becomes CO2(aq), a hydrated species.

CO2(g) => CO2(aq)

This aqueous carbon dioxide is what plants uptake. Only a very small amount of CO2(aq) goes on to react with water (H2O) to become carbonic acid (H2CO3).

CO2(aq) + H2O(l) => H2CO3(aq)

H2CO3 is a weak acid so it will disassociate as follow:

H2CO3(aq) <=> H+(aq) + HCO3-(aq) <=> H+(aq) + CO3(2-)(aq)

But at the pH range relevant to the typical planted tank, HCO3- is the dominant species. So, it's basically:

H2CO3(aq) <=> H+(aq) + HCO3-(aq)

The liberation of the hydrogen proton (H+) lowers pH, which is why we see a decrease in pH as a result of CO2 injection. Furthermore, the equilibrium between H2CO3 and HCO3- (bicarbonate) is then employed by hobbyists to calculate the concentration of CO2(aq) in the water (pH/KH/CO2 chart).

---

In nature, three main sources of carbon are derived from (1) the atmosphere, (2) organic decomposition by bacteria, and (3) KH. (1) and (2) account for only a few ppm CO2 so plants in nature are usually limited by CO2. Mother nature, in her infinite wisdom, gifts many plants with the ability to utilize the water's KH (the measure of the concentration of HCO3- [ok, CO3(2-) (carbonate) as well ]). This extraction is energetically expensive so plants do not resort to this unless CO2 concentration is low. THEREFORE, for plants possessing this ability, high KH water is beneficial.

---

What I do not understand is that why some plants (soft water plants) are adversely affected by the presence of HCO3-. It doesn't make much sense in my head; but at least we are no longer blaming high Ca/Mg as the cause for these plants' poor health.
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Old 05-02-2005, 04:09 AM   #4 (permalink)
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cS,

Seeing as you're actually a chemist would you please tell me how CO2 would dissolve into H2O without becoming Carbonic Acid? Seriously, I always thought that to be soluble there had to be an exchange of e- between compounds or a "taking" of e- (ie: van der Waal's) that created some sort of bonding between the two. The only way I can think of is a hydrogen bond between the two unless somehow there are London Dispersion forces holding them together at an H-C interaction. However, the H bonding seems the much more plausible and stable interaction to me.

The only practical stuff I have on this type of chemistry is out of my environmental science books and they all only mention the CO2 + H2O --> HCO3 reaction. I just happened to try writing out the reactions step-wise to see how all the stuff moved around, which I blame my Org. prof for getting me into the habit of doing.

By the way, I LOVED the EluciDate forward you posted in "Faces..." way back when.
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Old 05-02-2005, 04:43 AM   #5 (permalink)
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Quote:
Originally Posted by cS

This aqueous carbon dioxide is what plants uptake. Only a very small amount of CO2(aq) goes on to react with water (H2O) to become carbonic acid (H2CO3).

CO2(aq) + H2O(l) => H2CO3(aq)
That would be the normal situation, but we are pumping CO2 into the tank. We are overloading the CO2(aq) side of the equation and pushing the CO2 to dissolve into carbonic acid and we see that in the pH shift.
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Old 05-02-2005, 07:56 AM   #6 (permalink)
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Originally Posted by cS
What I do not understand is that why some plants (soft water plants) are adversely affected by the presence of HCO3-. It doesn't make much sense in my head; but at least we are no longer blaming high Ca/Mg as the cause for these plants' poor health.
I donít think itís the HCO3 presence as much as the absence of Ca and Mg in other forms, as sulfates and chlorides. Soft water plants seem to prefer those more.

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Old 05-02-2005, 05:07 PM   #7 (permalink)
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Quote:
Originally Posted by Phil Edwards
cS,

Seeing as you're actually a chemist would you please tell me how CO2 would dissolve into H2O without becoming Carbonic Acid? Seriously, I always thought that to be soluble there had to be an exchange of e- between compounds or a "taking" of e- (ie: van der Waal's) that created some sort of bonding between the two. The only way I can think of is a hydrogen bond between the two unless somehow there are London Dispersion forces holding them together at an H-C interaction. However, the H bonding seems the much more plausible and stable interaction to me.

The only practical stuff I have on this type of chemistry is out of my environmental science books and they all only mention the CO2 + H2O --> HCO3 reaction. I just happened to try writing out the reactions step-wise to see how all the stuff moved around, which I blame my Org. prof for getting me into the habit of doing.

By the way, I LOVED the EluciDate forward you posted in "Faces..." way back when.
Phil,

I don't know if this is relevent to your question, but think about other gases than C02. Take Nitrogen for example. Atmospheric nitrogen is relatively inert. but it disssolves relatively easily in our blood. At 1Atm our blood holds a certain amount. If you increase the pressure (i.e. through diving) more N2 will dissolve.

Don't always think of dissolving as a dissociation (NaCl -> Na+ Cl-). That only happens mainly to salts. Think of dissolving as a single molecule of whatever being surrounded by water molecules on all sides. Take sugar for instance. In solid state, sugar is next to sugar which forms a crystal structure. If you place this crystal sugar in water the individual sugar molecules break apart because water surrounds it. The crystal (which is more structured) turns into moving chaos (dissolved). It is basically entropy at work. Entropy is also why there is a loss of heat (energy) through evaporation to get back to the crystal structure (you have to keep the laws of physics happy).

Sorry to go off on a tangent. I though it might help you understand it better

Ken T.

Last edited by cousinkenni; 05-02-2005 at 05:11 PM.. Reason: nonsense sentence
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Old 05-02-2005, 05:09 PM   #8 (permalink)
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Quote:
Originally Posted by SCMurphy
That would be the normal situation, but we are pumping CO2 into the tank. We are overloading the CO2(aq) side of the equation and pushing the CO2 to dissolve into carbonic acid and we see that in the pH shift.
SCmurphy,

Cs actually makes note of this in her statment.

Ken T.
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Old 05-03-2005, 11:05 AM   #9 (permalink)
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Quote:
Originally Posted by timdz
This again brings up my question of carbonate hardness (KH) test kits. When testing the KH (with Aquarium Pharm, Tropic Marin kits), I assume you are basically adding an acid to a base and see how many drops it takes to cause a color change.
Timdz. Alkilinity test kits measure the concentration of the anion of weak acids. In our carbonate buffered system at the operative pH, H2CO3 is the weak acid whose anion is HCO3- (see above post). These test kits use some kind of strong acid to do so -- represented below as HX.

HCO3- + HX => H2CO3

Then they use some fancy mathematics to convert how much HX you used into ppm CaCO3, which is then converted further into degrees KH.

It's important to take note that "ppm CaCO3" is used here as an "unit of measurement". We are NOT actually testing for CaCO3. Think of it as a conversion factor: how much CaCO3 would we need to add to produce the same KH reading as what you have just measured.

Quote:
For example, when water goes through a water softener, cations such as Ca+ and Mg+ are are removed and replaced with another cation: Na+.
Water softeners are not recommended for the planted tank since it replaces Ca/Mg with Na. Effectively, it reduces the GH and raises the salinity to toxic levels. If you want to use "pure" water, then use RO or RO/DI water.

Quote:
Baking Soda (Sodium bicarbonate) will also raise the carbonate hardness of the water, thus creating NaCO3?
No. Sodium bicarbonate is NaHCO3. It dissolves in water into sodium ions (Na+) and bicarbonate ions (HCO3-). The HCO3- is what actually raises carbonate hardness (KH). The Na+ just tags along. For most people, the Na+ coming from baking soda is neglible. Such is not the case with the use of water softeners, which have the potential to raise Na+ quite high.

Quote:
So mainly when referring to 'hard' water, CaCO3 isn't the only product increasing the alkalinity. If someone wanted to use RO/DI water mixed proportionately with water through a water softner it is possible to obtain an appropriate dKH', and adding the calcium and magnesium with appropriate buffers to raise the general hardness. But the KH reading is coming from sodium bicarbonates. Therefore, when using a KH test kit how can we be sure that we have the 'right' kind of carbonates/bicarbonates in the water?
I am not sure that I understand what you're asking but I'll try to clarify. Think of GH and KH as separate entities. For all intents and purposes, they do not concern one another, unless you adding a substance that raises both the GH and KH, such as CaCO3 & CaMg(CO3)2.

GH is the measure of the concentration of primarily Ca2+ and Mg2+ ions.
To raise GH, one can use CaCO3, CaCl2.xH2O, CaSO4 (gypsum), MgSO4.7H2O (Epsom salt), CaMg(CO3)2 (dolomite), or any other products containing "Ca" and "Mg" in the formula.

KH is the measure of the concentration of HCO3- and CO3(2-) ions.
To raise KH, one can use CaCO3, CaMg(CO3)2 (dolomite), NaHCO3 (baking soda), KHCO3, or any other products containing "CO3" or "HCO3" in the formula.

Now, if you look at the above listing of chemicals, you'll notice that CaCO3 and CaMg(CO3)2 appear in both places. That's because if you look carefully at their chemical formulas, you'll notice that they contain elements that affect both the KH and GH.

CaCO3: the Ca raises the GH while the CO3 raises the KH.
CaMg(CO3)2: the Ca and Mg raise the GH while the CO3 raises the KH.

Quote:
And what would happen to the Na+?
The Na+ goes on to increase the salinity of the water. For most folks, this increase is neglible, so we don't talk about it.

Quote:
My favorite way of having a successful planted tank is to start from scratch (RO/DI) and adding my personal favorite buffers from SeaChem to adjust GH and alkalinity. But when using well water diluted with RO/DI water I am not sure I feel comfortable with what my KH test kit is actually testing.
I am assuming that the Seachem buffer you are using is Seachem Equilibrium? If so, then you can trust your KH test kit. Just don't start adding any "pH Up" and "pH Down" products and you'll be fine. Use your CO2 injection to adjust that.

I hope that things are a bit clearer?
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Old 05-03-2005, 12:44 PM   #10 (permalink)
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Quote:
Originally Posted by cousinkenni
SCmurphy,

Cs actually makes note of this in her statment.

Ken T.
Yes, I know, that's what I'm getting at. Her statement that not much CO2 dissolves is a common one based on what happens in nature where most dissolved CO2 is in the CO2(aq) form. When we overload the equation the way we do, a lot more CO2 dissolves that what would be the norm. You know, the old 'overload one side of a chemical equation and watch the shift to the other side...'
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