• You are viewing the forum as a Guest, please login (you can use your Facebook, Twitter, Google or Microsoft account to login) or register using this link: Log in or Sign Up

Estimating CO2 injection rates in ml per minute

I think Ian means for the pH ranges that's common for aquariums, a 1 unit drop in pH roughly indicates about 30ppm of CO2 is around.
Still dependent upon the systems buffering capacity...injecting CO2 also consumes kh.

We use 4 dkh in a drop checker for a reason. The indicator shows ph not ppm CO2, kh of the water impacts the ph reading. Same thing in our tanks

sent from tapatalk on my phone so auto correct and other errors are bound to happen
 
Still dependent upon the systems buffering capacity...injecting CO2 also consumes kh.

We use 4 dkh in a drop checker for a reason. The indicator shows ph not ppm CO2, kh of the water impacts the ph reading. Same thing in our tanks
Agree about the drop checker comment.

Agree about the buffering capacity.

Don't agree about injecting CO2 'consuming' kH. I don't it will meaningfully/practically change the kH at all.

Agree that pH drop will dependent on kH BUT for the ranges of pH,kH for our tanks, 1 pH drop is quite a reasonable starting point for about 30ppm of CO2.
 
Agree about the drop checker comment.

Agree about the buffering capacity.

Don't agree about injecting CO2 'consuming' kH. I don't it will meaningfully/practically change the kH at all.

Agree that pH drop will dependent on kH BUT for the ranges of pH,kH for our tanks, 1 pH drop is quite a reasonable starting point for about 30ppm of CO2.


I agree, that it is a reasonable place from which to make an estimate. My point was that pH and KH are not in fact independent of each other. I cannot say what degree of pH change would be expected for a given amount of CO2 at a given KH and if that change is negligible within the average operating range of our tanks, it very well could be. I also don't remember quite enough of my stoichiometry to calculate it, it is possible to do though. I can say that my tank's KH fluctuates over time, due to the seiryu stone leaching carbonate. Goes from 7 dKH to about 14 dKH over 10 days. From memory, that alters the pH reading by about +0.3-0.4, which is enough to throw off our target...however an increase in buffering leads to an overdose not an under dose if using pH to set your target ppm. Water changes are our way of rectifying these fluctuations and maintaining normal ranges.

KH is "consumed", but like you mentioned it may be negligible. I cannot say. I still think it important to understand how the process works (the only times I really wish I'd remember more of my chemistry from college). I've used Seachem's great article as a good refresher. http://www.seachem.com/downloads/articles/General-Chemistry-of-the-Planted-Aquarium.pdf

All in all, estimating CO2 in our systems is an inexact science and we should accept some degree of inaccuracy while at the same time not over simplifying the relationships of the elements involved. We also can't lose the forest for the trees and should use our judgement and experience when targeting CO2.
 
Even if you can, and I still doubt the accuracy of the taps. BPS is not useful because there is too much variation between systems. A more standardized method makes comparison better, although still not perfect.

The ph drop part is even more of a shot in the dark than the bubble tapping. Without knowing the kh value there is not an accurate way to estimate ph drop, it's just a wild guess.

sent from tapatalk on my phone so auto correct and other errors are bound to happen
The accuracy of the taps becomes more tuned with the amount accounted for. If you only tap 6 times, the average will be poor. Tap 100 times accounting for every bubble will give you a super accurate indication of average.

I also failed to indicate that this method is only for keeping an eye on your own setup or for when changing a canister etc, not for comparing bubble size with others. This is important when running larger tanks with a cranked needle valve.

Sent from my E5823 using Tapatalk
 
I agree, that it is a reasonable place from which to make an estimate. My point was that pH and KH are not in fact independent of each other. I cannot say what degree of pH change would be expected for a given amount of CO2 at a given KH and if that change is negligible within the average operating range of our tanks, it very well could be. I also don't remember quite enough of my stoichiometry to calculate it, it is possible to do though. I can say that my tank's KH fluctuates over time, due to the seiryu stone leaching carbonate. Goes from 7 dKH to about 14 dKH over 10 days. From memory, that alters the pH reading by about +0.3-0.4, which is enough to throw off our target...however an increase in buffering leads to an overdose not an under dose if using pH to set your target ppm. Water changes are our way of rectifying these fluctuations and maintaining normal ranges.

KH is "consumed", but like you mentioned it may be negligible. I cannot say. I still think it important to understand how the process works (the only times I really wish I'd remember more of my chemistry from college). I've used Seachem's great article as a good refresher. http://www.seachem.com/downloads/articles/General-Chemistry-of-the-Planted-Aquarium.pdf

All in all, estimating CO2 in our systems is an inexact science and we should accept some degree of inaccuracy while at the same time not over simplifying the relationships of the elements involved. We also can't lose the forest for the trees and should use our judgement and experience when targeting CO2.
Shawn you make good points.

It's important to remember that in the Co2 > Carbonic acid equilibrium, most of the CO2 remains in water as CO2(dissolved). Only 1:400 (I think) go to carbonic acid. Even less so to the right. So kH is not affected. This is my basic understanding. This is missed by most beginners.
 
I believe it was MarcelG who experimented a lot with this, he does some kind of horticultural study (never found out what his excact study is) and did al sort of lab tests on aquatic plants. He controversialy decided to go down with co2 to 15ppm and achieved equaly good results.. Anybody intrested find his posts, they are worth reading.. It doesn't answer the TS question, but it's more an answer to what and why and how much are you estimating and what for.
http://www.ukaps.org/forum/threads/...hing-to-alternatives.37923/page-5#post-410277
 
You don't need to know your kH, pH drop is independent of kH, which is why you measure a change rather than an absolute value. Thus a 1 point drop in pH is about 30ppm CO2 regardless of your waters kH.

Uh, not at all! This statement is quite misleading and totally wrong. Shawn is right: without kH information, drop into pH is just telling you that there is injection of CO2 but nothing else. And there is no reason to tell that 1 unit of dropping in pH is equal to 30ppm concentration. Different kH values will have different buffer effect, masking the effect of CO2 injection. It does not matter the pH ranges in which we are talking about; smaller jumps of pH at high kHs are just the result of large amounts of CO2. However, at low kH values, a bit of CO2 injection can cause a large drop of pH. The only situation in which Ian is correct is when the kH is equal to 4, that can be common in planted tanks, but not necessarily the case of most. Just check it in the table below, for example.

co2_ph_kH_chart.png


Cheers,
Manuel
 
It's important to remember that in the Co2 > Carbonic acid equilibrium, most of the CO2 remains in water as CO2(dissolved). Only 1:400 (I think) go to carbonic acid. Even less so to the right. So kH is not affected. This is my basic understanding. This is missed by most beginners.

Be careful, Rebel. This is only true in high-tech tanks, As Seachme says:

"In a planted aquarium employing a properly set up CO2 injection system the KH and pH should remain stable because one adds only as much CO2 as the plants need, thereby maintaining a constant level of CO2. In practice this is what you get when you use a CO2 injection system with a pH feedback metering system."

kH is only preserved when you inject as much CO2 as being consumed by the plants. Your statement is correct in high-tech tanks with lot of control, but will not work in low-tech tanks, when the plants will actually consume kH to obtain the inorganic carbon they need. From Seachem, again:

"In a well planted tank without CO2 injection the plants will use the CO2 available, causing the carbonates present to re equilibrate thereby producing more CO2. The acidifying agents mentioned above will also tend to drive the bicarbonate equilibrium to produce more CO2. In this type of setup the presence of acidifying agents can have a beneficial effect; it helps to maintain a higher level of CO2 than would be present if utilization were the only driving force for the formation of additional CO2. With this system one must actively maintain the KH by periodically adding bicarbonate based buffer. The plants are in essence 'eating' the buffer."

Besides, even at high-tech, people aim to get a stable concentration of CO2, which is not the same than adding the amounts plants are consuming as there is also a degasification process. The error here is to assume that losses of CO2 in the system, when injection CO2, are only due to plants consuming it, but this is not true. Degasification process will have also an effect into the CO2 concentrations. And in fact, by Henry's law, the higher the concentration water respect to the equilibrium point, the larger the degasification process will be, meaning that more CO2 is required to keep a stable concentration (i.e. CO2 injection should be equal to plant consumption + degasification). At high pH this is not much an issue, as inorganic carbon is mainly in bicarbonate form, but below pH=8.2 fraction of gas increases, and degasification will start. The lower the pH the larger the degasification will be.

Nonetheless, kH will remain unchanged so far you cover the assumption of adding more than being used by the plants.
 
I doubt you will ever got 120ppm CO2 @ 4 dKH when your pH is 6.

Make your numbers: http://www.chem1.com/acad/webtext/pdf/c3carb.pdf

You can doubt it but you are wrong. There are many information around showing similar results.

For instance: http://aquadaily.com/2009/01/29/co2-ph-kh-chart/

Or this one: http://aquariuminfo.org/co2calculator.html

Or this other reading, if you prefer: http://www.thekrib.com/Plants/CO2/kh-ph-co2-chart.html

Obviously these are theoretical values, which are not considering, for example, the role of alkalinity (assuming all alkalinity is associated to carbonates). There can be some deviation between reality and these tables and results, but they are quite close to be useful.

Cheers,
Manuel
 
Uh, not at all! This statement is quite misleading and totally wrong. Shawn is right: without kH information, drop into pH is just telling you that there is injection of CO2 but nothing else. And there is no reason to tell that 1 unit of dropping in pH is equal to 30ppm concentration. Different kH values will have different buffer effect, masking the effect of CO2 injection. It does not matter the pH ranges in which we are talking about; smaller jumps of pH at high kHs are just the result of large amounts of CO2. However, at low kH values, a bit of CO2 injection can cause a large drop of pH. The only situation in which Ian is correct is when the kH is equal to 4, that can be common in planted tanks, but not necessarily the case of most. Just check it in the table below, for example.

co2_ph_kH_chart.png


Cheers,
Manuel
Sorry but one pH drop in tank water equates to approximately 30ppm regardless of dkH your chart confirms this. I say appoximate as there can often be interfering other chemicals affecting a pH reading, but for plant keeping is fine.

Normal tank water will have about 3ppm CO2 from the atmosphere. So for instance with a dkH 10 the pH at 3ppm will be 8. Drop the pH by one to 7 by injecting CO2 and you will find the ppm is now 30ppm. Same for 5dKH, pH is 7.7 drop to 6.7 and ppm is 29.9. If you had a table going higher than pH 8 you will find exactly the same is true, 30ppm corresponds to pH drop of 1. It is a physical chemistry fact going from 3ppm to 30ppm (changing by one unit of power of 10) will drop pH by 1.
 
Sorry but one pH drop in tank water equates to approximately 30ppm regardless of dkH your chart confirms this. I say appoximate as there can often be interfering other chemicals affecting a pH reading, but for plant keeping is fine.

Normal tank water will have about 3ppm CO2 from the atmosphere. So for instance with a dkH 10 the pH at 3ppm will be 8. Drop the pH by one to 7 by injecting CO2 and you will find the ppm is now 30ppm. Same for 5dKH, pH is 7.7 drop to 6.7 and ppm is 29.9. If you had a table going higher than pH 8 you will find exactly the same is true, 30ppm corresponds to pH drop of 1. It is a physical chemistry fact going from 3ppm to 30ppm (changing by one unit of power of 10) will drop pH by 1.

You are right, but only if we assume that the system reached equilibrium, i.e. your starting pH is equal to the one you get considering a pure bicarbonate solution at equilibrium with atmosphere. Plants and CO2 injection means that starting point in a running tank, everyday, is not necessarily that one, and dropping pH for one unit can mean higher concentration of CO2. In other words, if you have kH 4 and you have already 6ppm of CO2, a drop of pH in one unit leads you to 60 ppm. This is why I mean is misleading, because you assume your tank gets fully in equilibrium everyday and that is not necessarily the case.
 
The real problem with this chart: it assumes we can properly measure carbonate hardness aka KH. But all available KH tests on any market can't measure real KH. They all measure alcalinity.
 
The real problem with this chart: it assumes we can properly measure carbonate hardness aka KH. But all available KH tests on any market can't measure real KH. They all measure alcalinity.

Having a problem reading kH does not invalidate the values in the chart, which was your original point. It just means that we cannot estimate with accuracy CO2 levels, whether you use a pH meter and looking for the 1 unit drop (which as I have pointed out, just works at certain circumstances) or you take your kH/pH measurements. Due to this is why using drop checkers is better, because they are not affected by other types of alkalinity, but then you have the issue of the color determination, but there are ways to improve that. The above chart links the color of the drop checker with the CO2 concentration. Indicator solution only changes color as a function of pH inside the drop checker, which usually comes in a solution with 4 dkH obtained by dissolving bicarbonates, i.e. no side effect of other substances into alkalinity, so it really works, so far you can determine the right color. However, it is enough for most people.

Cheers,
Manuel
 
Sorry but one pH drop in tank water equates to approximately 30ppm regardless of dkH your chart confirms this. Normal tank water will have about 3ppm CO2 from the atmosphere.

I am sorry for resurrecting an old thread but I really need help here.

I agree with the 10x progression, but where did you find this "3ppm CO2 from the atmosphere"?
I have been going through this over and over, and what I've found out is that our tanks should have around 0,55ppm of CO2 from the atmosphere.
I would love to be wrong and learn something...

I played around with Henry's Law, mostly with the solubility being defined via concentration, so that I can finally find it. That can be calculated using [CO2(aq)] = KH x pCO2

To calculate the CO2 partial pressure I considered that the air had 0,037% CO2 and temperature is 25C, and to be a bit precise, we discard the water vapour pressure by using this equation: pCO2 = (P° - pH2O) * XCO2
The result is:
pCO2 = (P° - pH2O) * XCO2
pCO2 = (1,0000 atm - 0,0313 atm) * 3,7 x 10-4
pCO2 = 3,69 x 10-4 atm

I dont think removing the water vapour is relevant, but I did anyways.
From there we proceed with the rest.

[CO2(aq)] = KH * pCO2
[CO2(aq)] = 3,38 x 10-2 * 3,69 x 10-4 mol x L-1 x atm-1 x atm
[CO2(aq)] = 1,25 x 10-5 mol x L-1

and then convert it to mg/l:

[CO2(aq)] = 1,25 x 10-5 * 44 x 103 mg x L-1
[CO2(aq)] = 0,55 mg x L-1

So, in the end I am looking at this 0,55ppm value and wondering, is that it, really?
A 1 point pH drop would add 5,5ppm. Considering that the water is not completely degassed from one day to the other, it would be possible to higher levels after a couple days.
 
Hi all,
I played around with Henry's Law, mostly with the solubility being defined via concentration, so that I can finally find it. That can be calculated using [CO2(aq)] = KH x pCO2
I'm not a CO2 user, but I think you are right, the level of dissolved CO2 depends upon Henry's law, and if you assume 400 ppm CO2 in the atmosphere, standard pressure (1013mb) and a temperature of 20oC, then you have 1.35 x 10-5 mols l-1 of CO2 dissolved.

The RMM of CO2 is 12 + (16*2) = 44 (44g of CO2 in one Kg ), and if you work that out as ppm, it comes to ~0.6ppm (0.594ppm). The reason that that level is higher than the quoted 0.55ppm is just because the level of atmospheric CO2 has risen.

We have a CO2 monitor in the lab. and that usually sits at about 600ppm, but it is still along way from there to the dissolved 3 ppm quoted.

The only reference I could find for the 3ppm datum was the experimental work that George and Karla Booth did in the 1990's <"CO2 loss in large aquariums">.

cheers Darrel
 
Last edited:
Every claim I've found about this seems to be from George and Karla Booth. All the links here are their writings.

http://www.hallman.org/plant/booth2.html
"After letting the water equilibrate for one day we measured dissolved CO2 at 2-3 ppm. We then set up a large powerhead to circulate the water (Project RS-500, ~500 gph) and let it run for a day. The CO2 remained about 2-3 ppm. At the end of most of the tests, CO2 again measured about 2-3 ppm, indicating that this was the equilibrium value for the experimental conditions (note that the altitude was 5000 feet above sea level)."


http://aquaticconcepts.thekrib.com/Co2/
"This is good for the fish. However, fish won't produce enough CO2 to raise the level above normal equilibrium levels (2-3 ppm) unless you are massively overcrowding the fish. It will diffuse into the air as fast as the fish produce it."


http://aquaticconcepts.thekrib.com/Co2/co2_faq.htm
CO2 has a natural equilibrium in water of about 0.5 mg/l. In an aquarium with a moderate fish load, the fish may raise the concentration to 2 to 3 mg/l.
 
Back
Top