Different CO2 forms under low vs. high pH

[Marcel G]

I have a question as I don't fully understand the correlation between the CO2 form at different pH/KH.

You all probably know the following chart:
View attachment 3672

According to this chart we know that if we want to have good amount of CO2 (in the form of H2CO3, which should be best for aquatic plants), then we need to ensure pH < 7 (best 6.5 or so). If we have pH > 7, then nearly all CO2 in out tank will be in the form of HCO3 (which for plants is harder to digest). BUT what to do if I have very high KH (let's say KH 15)?

See this chart:
View attachment 3673

According to the above chart, if I have KH 15, then I have around 35 mg/L CO2 at pH 7.2. But at pH 7.2 nearly all the CO2 is in the form of HCO3. So if I would like to serve my plants with "better digestible" H2CO3, then I would need to lower the pH, BUT if I do this, then I would have extreme levels of CO2 in my tank (given such high KH)!

So is there any solution in this situation? As I understand it right now, that's the reason why is better to use soft water with low KH (with low KH we can have low pH, and thus higher % of CO2 as H2CO3). Is this correct assumption?

 

[R_barber001]

Subscribed.

 

[Yo-han]

Love this question although I don't know the answer. A solution I have: use softer water, but this doesn't answer the question. I think it doesn't work in the way you think in practice. Because, by adding CO2 with high pH, this would assume it converts to CO3 and this would raise the KH, something I've never heard of. But I really don't know what does happen, so I stay tuned as well!

 

[Marcel G]

Recently I'm making a test with my 60L (~15G) nano tank. I used a layer of Dennerle DeponitMix soil, and caped it with a layer of Dennerle Shrimp Gravel (Ø 1mm). I planted it quite dense with eleocharis parvula, vallisneria, ludwigia, pogostemon ... etc. Then I tried to pump inside a lot of CO2 (the drop checker is yellow permanently). I don't have any critters inside (although some unasked snails live there; I don't know if they like it). I use Estimative Index targeting the following levels of nutrients: 30 mg/L NO3, 30 mg/L K, 3-5 mg/L PO4, 0.5 mg/L Fe + micro (on Fe+micro I use EasyLife ProFito). Also I dose EasyLife EasyCarbo (twice the recommended dosage each day). The tank is old approx 1 month. Although I inoculated some algae in there (mainly BBA + Staghorn), after I trimmed the plants at the end of week, the algae did not come back. So now it seems tha tank is algae free (in a sense). I have 50-60 umol PAR at the substrate (measured by Apogee MQ-200 PAR meter). I have good flow, and I'm using a glass diffusor for CO2 distribution. All seems to work perfectly for now.

Now, I'm a little afraid that because I have very high KH in my tap water (around 15), and I pump inside the tank a lot of CO2, so the pH is being pushed down by the CO2, so actually the CO2 could be in more digestible form for my plants (as H2CO3). The pH is about 6.5-7. BUT I'm afraid that when I reduce CO2 (so that I can put critters there), then less CO2 will bring higher pH again, which gives me more CO2 as HCO3 (80%) and less as H2CO3 (20%). So this is my concern to find out if this could have some negative impact on my plants? I have read that the plants can adapt to different forms of CO2 (H2CO3 vs. HCO3), but with HCO3 they have to make more effort to digest it (more rubisco enzyms?). And I think that the ballancing on the edge of pH < 7 or pH > 7 (H2CO3 vs. HCO3) is maybe not good for plants as they need to switch every while. Also it seems to me that if I add pressurized CO2, then its much more effective if the pH is low (6.5), because in this environment the plants can use H2CO3. In the environment of pH > 7 they must switch the metabolism to adapt to HCO3. I would like to know if pressurized CO2 (as micro-bubbles) is of any use to the plants in the environment of pH > 7, where there is nearly all CO2 in the form of HCO3? I would be glad if someone can shed light on it (how the CO2 behave under different pH/KH).

 

[mi5haha]

KH can be brought down in using oxalic acid to treat hard water. So the water with hardness of GH24, after the treatment with oxalic acid, can be turned into a water around GH12, with KH3-4.

It is not as handy as using RO water. But it is economic.

 

[Tom Barr]

In the graphs...............you are not adding CO2 gas, if you stop adding CO2 gas, then the pH will rise up and follow those graphs based on the amount of KH/bicarbonate/-HCO3 you have.

So a soft water, say 1 KH will have a low pH and more CO2 relative to KH..........and harder water will have more HCO3, less CO2.

But if we add CO2 gas to say 1 KH and to 10 KH aquarium water, you still.....can add however much cO2 you wish, say 30 or 50 ppm etc.
Thus the pH is used to measure the CO2 enrichment relative to a KH value(HCO3), but that's it.

Plants will use HCO3 either direct or mostly indirect, algae as well............they will both go after CO2, then HCO3 once the CO2 is gone and drops to a level where it's better for the plant/algae to use the bicarbonate.
If we enrich the tank with CO2, then none of this applies.

Perhaps what happens to some species of aquatic plants that prefer low KH: they take up the CO2, but the HCO3 gets in the way and interferes with the uptake of CO2 somehow. This may be due to pH alone.
Say you have 30ppm CO2 in a KH of 10 and a 30ppm CO2 in a KH of 1.

The pH difference is? 1.0 pH units.

6.0 vs 7.0 pH.

Without adding CO2?
About 7.2 and 8.2

Enzymes that take up CO2, HCO3 are optimized for specific pH's for many plant species, but by and large, most aquatic plants do not care.
KH, CO2 and GH are some of the hardest issues for hobbyist to grasp.

Good discussions are on the TheKrib.com

 

[Marcel G]

1. Question: When the CO2 dissolves in which form is it most found : CO2, H2CO3 or HCO3-?
Answer: If CO2 is added to water, it's still CO2. If you add baking soda, it's still HCO3 and Na.

2. Question: Which of this forms is easely assimilated by the plants?
Answer: CO2, then a few might use HCO3 if there is no CO2 available, there are two methods for that, direct uptake and indirect uptake.

H2CO3, is a weak acid, it is very weakly disassociated. So you have very little H2CO3 relative to CO2, we just assume it's all CO2. If you want a precise amount, at least for seawater, it's about 400 CO2's for every 1 H2CO3 for a ratio of dissociation.

Some people think they can change the KH/HCO3 using CO2, that's rubbish and they need to get that monkey business out of their heads asap.

If you want more CO2 in the water, ..........well.........add more CO2, it's so obvious folks miss the simple solution.

That's what plants want, not a KH/HCO3 really.

Cited from: http://www.aquariacentral.com/forums/showthread.php?125277-CO2-vs-H2CO3-vs-HCO3&p=1074508&viewfull=1#post1074508

Hi Tom,

I have found one thread on aquariacentral.com which partially answers my questions (see the above citation).
Still I have some unresolved questions in my mind. As I understand it, there is something we can call "carbonate equilibrium" in the water. According to this basic idea the carbon is present in the water in 3 forms (CO2/H2CO3, HCO3, CO3), and the mutual ratio between them depends on the pH.
So this means that although the total amount of carbon remains the same, the ratio changes based on the actual pH. For example, if I have pH 7.0, the carbon in the water is in the ratio of 1:4 (CO2 : HCO3, i.e. 25% of CO2 + 75% of HCO3). If I have 12 mg/L of total carbon in the water, then I have acctually 3 mg/L CO2 (25%) + 9 mg/L HCO3 (75%). But if I decrease the pH to 6.0, the total carbon is still the same (12 mg/L), but now I have 9 mg/L CO2 (75%) + 3 mg/L HCO3 (25%), because of the change of carbonate equilibrium based on pH.

So as I understand it, if we add more CO2 into the water, it will remain in the free (molecular) form [i.e. as CO2 or H2CO3] ONLY if we keep low pH. BUT if I have pH 8, then nearly all the CO2 I pump into the water will change its form (mainly in the favor of HCO3). Under pH 8 the ratio of carbon in the water should be 97:3 (97% HCO3 + 3% of CO2).

This is a theory. Acctually I don't know how LONG it takes for this change to happen (= for CO2 [we add] to change into HCO3 under higher pH). If it takes long (let's say "days"), then it can be that the plants can use molecular CO2 even in the water with higher pH/hardness, so the supply of CO2 is of good use to them.

Please, can you shed some light on it?
1) How long it takes for CO2 to change into HCO3 under higher pH?
2) Also I would like to know, what the drop checker actually measures? I know that it measures pH/KH, and then we derive the amount of CO2. But does it really mean CO2? Because KH means actually only HCO3 + CO3 ions (not CO2).

Thank you beforehand for your patience in answering these questions.

 

[Tom Barr]

So this means that although the total amount of carbon remains the same, the ratio changes based on the actual pH.

You are confusing yourself.

We add more Carbon in the form of CO2, which also influences pH, so the total Carbon is NOT the same when you enrich CO2 into water.
It is now higher/more than before you add CO2.

HCO3 or KH, will not degas, you add baking soda, the KH/pH will go up if you go from say 1 KH to 10KH.

If you enrich your aquarium with CO2 and the KH is 10 dKH, does the KH go down?
No.

Does the total carbon go up?
Yes.

Does the fraction of CO2 relative to HCO3 and CO3 increase? Obviously..........you are adding more CO2...........
This is not equilibrium, we are adding CO2 continuously to keep up with degassing rates.

Now if you added say acetic acid, or HCL to the water, then this would destroy the KH and the pH would go down, the acid would break the HCO3 into H2O+CO2.
CO2 enrichment will not do that however.

Your pH has to drop if you add more and more CO2 gas into your aquarium water.

Plants want CO2, that is why we add it, not for pH control etc.
KH is more relevant to use than pH for us/our purposes.

Stick with KH, leave the pH out of this unless it's to measure relative CO2ppm.

 

[Marcel G]

we add more Carbon in the form of CO2 (which also influences pH), the total Carbon is NOT the same when you enrich CO2 into water. It is now higher/more than before you add CO2 ... Does the fraction of CO2 relative to HCO3 and CO3 increase? Obviously..........you are adding more CO2. This is not equilibrium, we are adding CO2 continuously to keep up with degassing rates ... Stick with KH, leave the pH out of this unless it's to measure relative CO2ppm.

Tom, just to be clear, so if I understand it correctly now, the KH (which consist of HCO3 + CO3) is not changing when we add more CO2. Right?

So strictly speaking:
CO2 + H2O => HCO3- + H+
Since HCO3- level is our KH, adding CO2 WILL increase the KH ... BUT the amount of HCO3 produced by the additional CO2 is NEGLIGIBLE in relation to our natural HCO3 level. So in practice, we can say the KH doesn't change at all with added CO2. Do I understand it correctly?

So actually even if there IS a chemical reaction in which CO2 gas changes into HCO3 under higher pH, the amount is too small to have any notable impact on KH. Right? This is very important to know for me. That would explain a lot.

 

[Tom Barr]

Tom, just to be clear, so if I understand it correctly now, the KH (which consist of HCO3 + CO3) is not changing when we add more CO2. Right?

So strictly speaking:
CO2 + H2O => HCO3- + H+
Since HCO3- level is our KH, adding CO2 WILL increase the KH ... BUT the amount of HCO3 produced by the additional CO2 is NEGLIGIBLE in relation to our natural HCO3 level. So in practice, we can say the KH doesn't change at all with added CO2. Do I understand it correctly?

So actually even if there IS a chemical reaction in which CO2 gas changes into HCO3 under higher pH, the amount is too small to have any notable impact on KH. Right? This is very important to know for me. That would explain a lot.

Tell you what, rather than reading and trying to find something that is specific and applied to our case where we enrich CO2 gas for plants..................let's try a common sense approach and see what we can find and understand through observation and a simple test:

Step 1: measure the KH using say a good KH test kit. Write this degree or ppm down.
Add CO2: Drop pH say 0.5pH units, then 1.0 pH unit and measure the KH at each point.
If you have no fish etc or livestock, try 1.5 and 2.0 pH units and measure the KH.

The KH should be the same in each case.

Now you can add a little HCL or acetic acid without livestock, and measure the pH/KH, the pH and the KH will both drop.
And after you stop adding the acid, the pH will remain lower than the starting point.

This simple test/observation is all you need to understand what is really going on.

So in practice, we can say the KH doesn't change at all with added CO2

Do this test above and tell me what you find and what you conclude from it.

 

[Marcel G]

This simple test/observation is all you need to understand what is really going on.

I think the main problem lies in the fact, that people are confused by the equation of "carbonic equilibrium", from which they deduce (probably mistakenly) that if the water has alkaline/basic pH (i.e. pH=8), then nearly all added CO2 should dissociate to HCO3- and H+, which should further lead to lowering of pH (in the water arises more H+ ions), and at the same time to increasing of KH (in the water should arise more HCO3-). This is the root of the trouble.

Unfortunately people, who understand it, instead of explain how it works in reality, they hang us out, and are just telling us not to try to understand it and manage (make do) with the explanation that when we add more CO2 into the water, we will have more CO2 in the water (that's it).

This simple conclusion is in fact under many controversies:
When we add more CO2 into the water the pH drops (usually by 1 degree), which must be caused by increase of H+ ions. But H+ ions arise only in dissociation of H2CO3 to HCO3+ and H+, which in turn must inevitable bring everybody the the conclusion that if there are increasing H+ ions (witnessed by the drop of pH), then logically there must come up more HCO3- also (which should lead to the increase of KH!). Then if there is no KH increase (and there's none as we know), it shouldn't surprise you that the people are so confused, and they just don't know what to belive or what's going on in the water.

I don't understand, why someone who understand these things can not explain us in clear words what is going on in our tanks when we add more CO2 into the water ... and how should we understand the equation of "carbonic equilibrium"? Instead of this we find out elsewhere a lot of incomplete informations, which only contribute to a greater confusion.
For example:
- CO2 is very good soluble in the water ... vs. the ratio of CO2 to H2CO3 in the water is about 400:1 (so for each 400 parts of dissolved CO2 there is just 1 part of H2CO3) => this leads to a question: what is the CO2 dissolved to, if not H2CO3?
- H2CO3 is very weak acid, which dissociate very poorly ... vs. how could adding more CO2 lead to lowering pH, if there arise only very few/little H2CO3, and moreover this weak acid dissociate to HCO3- + H+ so bad?
- When CO2 dissolves in the water, there doesn't arise any HCO3- (= no KH increase).
- At pH=8 the carbon is present in the water in the following forms and ratio: 3% CO2, 96% HCO3, 1% CO3 ... vs. when we add CO2 into the water, there will be CO2 in the water (no increase of HCO3 or CO3).

You are right that we can live with the fact that when we add CO2 into the water, there is more CO2 in the water (which is our goal). But then it could be useful to explain us why the "carbonic equilibrium" doesn't apply in this case. Otherwise the confusion will continue.

 

[Marcel G]

Finally I've got the answer

After consulting this issue with one chemist, I have figured out how it works:

CO2 gas which we add to our tanks becomes CO2 solution (the CO2 soluted in the water is there mainly as free hydrated molecules (the solubility of CO2 in the water is about 1500 mg/l at normal pressure and room temperature). Small part of CO2 (about 0.2-1%) reacts with water and forms H2CO3 (pH of H2CO3 = 3.6), and a small part of this H2CO3 dissociates further to HCO3- and H+. (The concentration of H2CO3 is proportional to the concentration of soluted CO2 ... in the ratio of 1:100 to 1:500, i.e. from each 100-500 parts of soluted CO2 just 1 part dissociates to H2CO3.) However even the relatively big/high increase of CO2 concentratin (let's say to 30-40 mg/l) does not lead to significant drop of pH (usually the pH decreases by about 0.5 to 1.0 degree => more if the pH is alkaline/basic, less if the pH is acidic). Just for comparison: The content of CO2 in the mineral/sparkling water is about 4000-6000 mg/l (soft sparkling waters has about 1500-4000 mg/l CO2), yet still this water has not so much lower pH (usually 5.0 to 6.0). Similarly also the insignificant amount of HCO3- will have almost no impact on the KH. Only thing which can higher concentration of CO2 affect, is the plant's grow rate and the health of our critters (concentration over +-50 mg CO2/l can be poisonous to them).

I hope this helps.

 

[Tom Barr]

I generally tell folks not get confused and use the CO2, to simply add CO2, you can use the pH/KH chart as a relative measure.
If you add more CO2, it does NOT affect the KH.

The dissociation is generally about 1:400, not enough to make much difference.
Some seem to think that is NOT the CASE and I've had some serious flame Wars over this in the past.

CO2 of 50ppm is not harmful FYI for many aquariums.
My 180 Gal tank has 60-70ppm and has for nearly 8 years, I've bred a lot of fish and shrimp in that aquarium.

When you discuss animal health with respiration, it MUST include both O2 and CO2 concentrations.
If you have higher O2, then there is more room for higher CO2 ppm and less stress to the fish.
This has limits, but in general, a wet/dry filter and good plant growth = the best O2 levels, thus you can also add higher levels of CO2.
That range where the health of the livestock and where plants thrive is a much larger effective range if the O2 is higher.

 

[Tom Barr]

So as you can see and have learned, either by discussion or by trying and testing it yourself..............
KH is the variable that matters if you want certain soft water plants.

Otherwise CO2 is the only variable of real concern.
I do not even need to discuss the equation of "carbonic equilibrium".
I do not think it is particularly helpful unless you are talking about a non CO2 planted tank.

Most hobbyist that use the non CO2 do not test and are often more into the low tech aspects. So a large hard to understand theory can be, well, not very interesting or applied to them or their ways of thinking. There are exceptions, but they are very very few and far in between.
I had the misfortune of having to plot and make the carbonic equilibrium by hand... in a class I took years ago.

The key is the weak dissociation due to being a weak acid. Which is why I suggested adding acetic acid or HCL.
Once you understand that, then can test kit with the KH and add CO2 and realize the KH does not "magically" drop, now you have a good grasp of the concepts.
Technically the KH does move... but... a tiny tiny insignificant amount.

 

[Marcel G]

Agree

CO2 is the only variable of real concern ... I do not even need to discuss the equation of "carbonic equilibrium". I do not think it is particularly helpful unless you are talking about a non CO2 planted tank ... Technically the KH does move but a tiny tiny insignificant amount.

This I appreciate the most: I have find out that the "carbonic equilibirum" equation (although valid) apply mostly to non CO2 tanks (as Tom said), because when we add more CO2 to he tank, then it's VERY IMPORTANT to know that just a SMALL FRACTION of CO2 is acctually dissolved to H2CO3, and further to HCO3- and H+. If we don't know this fact (i.e. just "small fraction") ... then we could be really confused and lost in it (as I was). Because if we mistakenly think that ALL CO2 changes to H2CO3 ... and ALL H2CO3 changes to HCO3- and H+ ... then we make WRONG conclusions as to the KH change/increase. Once we know it's just small fraction which plays a role here ... then everything seem to be CLEAR (= no notable KH change + just relativelly small pH change/drop).

 

[detlef]

Hi Tom and kwisatz,

I have to read a couple more times to fully grasp what you are talking about here. But thank you so much for discussing the matter in detail. This has been confusing me for years also because I wasn't able to understand what was really going on in the water when we add CO2 gas. Thanks again for making things more clear!
Being dumb though did not affect the well being of my tank at all, thank you. In fact plants have been doing great since I managed to kill spiro using easy carbo!

Best regards, Detlef

 

[Marcel G]

Trying to be more clear ...

I have to read a couple more times to fully grasp what you are talking about here ...

I'll try to make it more clear (just for better/easier understanding):
When we add CO2 into our tank, 99% of it remains in the form of CO2, and just about 1% (or less) reacts with water. From this reaction is created H2CO3 (but because of so small fraction of CO2 reacts [only 1%], there is very little H2CO3 there). Then this H2CO3 further dissociates either to HCO3 + H ... or to CO3 + 2H (whether it will dissolve to HCO3 or CO3 depends on the pH ... if the pH is around 8 it dissolves to HCO3, if the pH is around 10 it dissolves to CO3). Because this H2CO3 is acid ... and because this acid dissolves to H+ ions, these H+ ions make water more acidic ... so in this process the pH goes a little bit down (in most cases by 0.5-1 degree). Also because there is so little HCO3/CO3 created in this process (and just a small part further reacts with Ca or Mg to create carbonates) ... it has nearly no impact on KH (as we know KH consists mainly of HCO3 and CO3, so if there would be high increase of these compounds, it should raise KH ... but because there is just very, very small increase of these compounds, the KH remains practically the same).

So the main lesson: The equation of carbonic equilibrium don't apply much to tanks where we add more CO2 ... and 99% of added CO2 remains in the form of CO2 (wheather completely soluted or as not yet soluted gas). Also it follows that when we try to lower pH in assumption that the HCO3 magically changes into CO2 (as suggests the carbonic equilibrium equation), we gain nothing ... again because this applies just to small fraction [

 

[detlef]

OK, helping me to get it even better. Since I'm using fill-water with zero KH for > one year and consistantly measure 3dKH for in-tank water I would assume, thanks to your explanation, that HCO3 comes by far through the addition of ADA's Brighty K as the main K+ source. Two year old ADA Amazonia soil by the way therefore no lowering of ions of any kind.

Thanks again, Detlef

 

[Tom Barr]

Hi Tom and kwisatz,

I have to read a couple more times to fully grasp what you are talking about here. But thank you so much for discussing the matter in detail. This has been confusing me for years also because I wasn't able to understand what was really going on in the water when we add CO2 gas. Thanks again for making things more clear!
Being dumb though did not affect the well being of my tank at all, thank you. In fact plants have been doing great since I managed to kill spiro using easy carbo!

Best regards, Detlef

It was a good discussion, many people who have been in this hobby for decades fail on this topic. I got into a rather big spat with some on some forums elsewhere. They chose not to test their KH before and after. Simply said I did not know what I was talking about.
Wanted to debate theory, but had no clue on application. You do not get to debate theory without some basic understand of what it is you are talking about and doing some basic test to see if it makes sense.

I think on that matter of fact level, (I tested the KH, it did not move), it's far more telling to the newbie or neophyte or even the folks who never bothered to get into the thick of this issue.
Detlef, the algaefix is even better than Easy Carbo. But Easy Carbo is doing something for the plants also, not just killing algae and shrimp.

CO2(added/not added) is still the critical factor in all of this.

It's interesting to find correlation in non CO2 tanks, but I've seen the tanks move towards high KH=> low KH over time without water changes.
1. Indirect Bicarbonate use by plants
2. HNO3 production in the sediments perhaps=> destroys KH
3. Perhaps some other processes and export by trimming plant biomass.

So most of the Carbon is as CO2 for the first hour or two, until the plant biomass removes most all of it and the pH rises.
In very soft water pools with lots of plants, pH change can go from 6 up to 10 every day.
In Tide pools in CA with lots of marine plants/algae, we measured changes around 8pH up to 10 pH in 3-4 hours' time.
You can track the species of dissolved inorganic Carbon during these changes.

Basic take away from this, adding CO2 does NOT change the KH(at least not a significant amount).

 

[Tom Barr]

Put another way: if the claim was true about CO2 removing KH...........no one would use or need RO water in their hard water tanks ever again

Wouldn't that be nice?