# Determination of CO2 in combination with an aquasoil



## Cor (14 Apr 2020)

Imagine you have a tank with a gravel bottom.
If you measure the KH and pH, you could theoretically determine the ppm CO2. (the pH / KH Table)
For example: KH: 8.0 and pH 7.9. Then you would have 3ppm CO2 in the water, according to the pH / KH table.

If you have a tank with an aquasoil then you measure for example a KH of 4 and a pH of 6.4.
In theory you should have 48ppm CO2 in the water. But we all know that would be impossible without extra CO2 addition.

How should we then reliably determine the ppm CO2 in a scape with aquasoil?

The pH drop is usefull for 30ppm CO2, but if I only want 20ppm CO2. How do you determine that if you can't use de pH/KH chart? Or are we then only depending on a drop checker?

Cheers, Cor


----------



## dw1305 (14 Apr 2020)

Hi all, 





Cor said:


> The pH drop is usefull for 30ppm CO2, but if I only want 20ppm CO2.


I'm not a CO2 user, but I think that because the drop checker uses the CO2 ~ HCO3- ~ pH equilibrium, you can change the hardness of the dKH solution (the HCO3- part) to give you the same colour change for less CO2 addition. If you have a 2dKH solution, you should get the pH colour change to yellow (protonation of the bromothymol blue pH reagent) at around 15 - 20ppm CO2 





Cor said:


> If you have a tank with an aquasoil then you measure for example a KH of 4 and a pH of 6.4. In theory you should have 48ppm CO2 in the water. But we all know that would be impossible without extra CO2 addition.


You can't ever reliably use the the chart, other than with a drop-checker. That is because it is only with a drop checker, where just CO2 diffuses across an air gap, that the chart is calibrated for.

cheers Darrel


----------



## jaypeecee (14 Apr 2020)

Hi @Cor


dw1305 said:


> You can't ever reliably use the the chart, other than with a drop-checker. That is because it is only with a drop checker, where just CO2 diffuses across an air gap, that the chart is calibrated for.



Or, you could try what I use, which you'll find here:

https://www.ukaps.org/forum/threads/why-advise-a-1ph-drop.58798/#post-574693

JPC


----------



## Dr Mike Oxgreen (14 Apr 2020)

Or you could take a glass of water from your tank and leave it for 24 hours. It will then have reached equilibrium with atmospheric CO₂ giving about 4ppm of CO₂ in the equilibrium sample of tank water.

Then when you measure the pH of your tank water, work out the pH drop between the equilibrium sample and the tank water. Grab a calculator and work out 10 to the power of whatever that drop is. The resulting number is the multiple of how much tank CO₂ there is, relative to the equilibrium sample. Multiply that by the 4ppm CO₂ that is likely to be in the equilibrium sample, and you have an estimate of your tank’s CO₂ ppm.

So for example, let’s say your tank’s pH is 6.8, and you take a sample and leave it for 24 hours, and the pH becomes 7.2. The pH difference between the two is 0.4. Your calculator will tell you that 10 to the power 0.4 is 2.5, meaning that your tank has 2.5 times as much CO₂ as the equilibrium sample. Multiply 2.5 times the 4ppm that we assume is in the equilibrium sample, and our estimate of tank CO₂ is about 10ppm.


----------



## dw1305 (14 Apr 2020)

Hi all, 





Dr Mike Oxgreen said:


> It will then have reached equilibrium with atmospheric CO₂ giving about 4ppm of CO₂ in the equilibrium sample of tank water.


Theoretically it is less than <"1 ppm of dissolved CO2">. I never found out whether there was any empirical research indicating where <"the "3 ppm" of CO2 came from">. 

As alluded to in the link we have a CO2 sensor in the lab., and it usually reads more than the <"~415ppm of CO2"> (April 2020) atmospheric value.

cheers Darrel


----------



## Dr Mike Oxgreen (14 Apr 2020)

dw1305 said:


> Hi all, Theoretically it is less than <"1 ppm of dissolved CO2">. I never found out whether there was any empirical research indicating where <"the "3 ppm" of CO2 came from">.


Yes, it’s the one aspect of all this that I find a bit uncomfortable. I can’t find anything to back it up, either - it’s almost like a piece of fishkeeping folklore. Although I’m sure I read someone saying that the figure for the 21st century would be more like 4ppm in water - but I can’t find any evidence for that either. I don’t entirely like basing things on an apparently random number!



dw1305 said:


> As alluded to in the link we have a CO2 sensor in the lab., and it usually reads more than the <"~415ppm of CO2"> (April 2020) atmospheric value.


What happens if you all stop breathing?


----------



## dw1305 (14 Apr 2020)

Hi all, 





Dr Mike Oxgreen said:


> What happens if you all stop breathing?


It goes down. As CO2 levels rise it changes colour, from green through to red at 2000ppm CO2, at which point it shuts down the gas and electric (but not the students breathing).





Dr Mike Oxgreen said:


> Although I’m sure I read someone saying that the figure for the 21st century would be more like 4ppm in water


I'm not sure, to even get to 3ppm dissolved CO2 you need a huge amount in the atmosphere (@Ed.Junior's post below).





Ed.Junior said:


> 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:
> ...





dw1305 said:


> 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 ), 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.


cheers Darrel


----------



## jaypeecee (14 Apr 2020)

Dr Mike Oxgreen said:


> ...and you have an estimate of your tank’s CO₂ ppm.



Hi @Dr Mike Oxgreen 

At the end of that process, I'd want something better than an estimate. Try the method that I'm suggesting. Much easier and more accurate.

JPC


----------



## jaypeecee (14 Apr 2020)

dw1305 said:


> I never found out whether there was any empirical research indicating where <"the "3 ppm" of CO2 came from">.



Hi @dw1305 

Look no further than that veritable treasure trove of early aquatic plant empirical research - The Krib! Here is possibly where it all started:

https://www.thekrib.com/Plants/CO2/co2-loss.html

George Booth said:

"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)".

JPC


----------



## sparkyweasel (15 Apr 2020)

George Booth also said:

"Note that the LaMotte CO2 test kit has a resolution of 1 ppm (mg/l) and an
error of about +/- 2 ppm."


----------



## Dr Mike Oxgreen (15 Apr 2020)

jaypeecee said:


> At the end of that process, I'd want something better than an estimate. Try the method that I'm suggesting. Much easier and more accurate.


And how much did your equipment and test kits cost? Your method sounds neither cheap nor easy!


----------



## dw1305 (15 Apr 2020)

Hi all, 





jaypeecee said:


> Look no further than that veritable treasure trove of early aquatic plant empirical research - The Krib! Here is possibly where it all started:


I still refer back to the Krib. That is where I saw it, there is actually a link in @xim's post in <"Determination of CO2........">, but as far as I know there isn't a scientific reference. 





sparkyweasel said:


> "Note that the LaMotte CO2 test kit has a resolution of 1 ppm (mg/l) and an error of about +/- 2 ppm."


Back to why the drop-checker might offer some advantages. 





Dr Mike Oxgreen said:


> And how much did your equipment and test kits cost?


Measuring dissolved gases is never cheap. You need a gas permeable membrane, and <"kit with these tends to be expensive">. 

If you have an accurate DO meter, they are easy to calibrate and use, but they still suffer from the same fundamental issue which is that dissolved oxygen is different from everything else, it can be right 99.9% of the time, but any period, however short, where it isn't leads to the death of all your livestock. You need measure DO levels for 100% of the time, a snapshot isn't very useful.  

Freshwater biologists get around this by using a <"biotic index">, where you take an invertebrate sample and find out which <"pollution sensitive invertebrates are missing">.  These indices are constantly being tweaked, and now offer fine scale differentiation in water quality.  

cheers Darrel


----------



## Dr Mike Oxgreen (15 Apr 2020)

sparkyweasel said:


> George Booth also said:
> 
> "Note that the LaMotte CO2 test kit has a resolution of 1 ppm (mg/l) and an
> error of about +/- 2 ppm."


And the experiment was conducted at 5,000 feet above mean sea level. That would surely have had an effect on the partial pressure of CO₂ (although I would expect the sea-level concentration of CO₂ to be greater). They were working at a pressure of around 828 mb, give or take (you lose a millibar for every ~27 feet you climb, and ISA sea level pressure is 1013).

So if our “2-3 ppm” equilibrium CO₂ is based on flawed measurements, I’m now wondering what other “knowledge” is derived from that. Certainly it seems that the 1 pH drop is based on the requirement to reach 10 times the equilibrium CO₂ level, when actually you’d need more like 50 times 0.6 to reach 30ppm, implying a pH drop of about 1.7!

For that matter, where do we get 30 ppm from? Is that based on plant requirement, or fish toxicity?


----------



## dw1305 (15 Apr 2020)

Hi all,





Dr Mike Oxgreen said:


> And the experiment was conducted at 5,000 feet above mean sea level. That would surely have had an effect on the partial pressure of CO₂ (although I would expect the sea-level concentration of CO₂ to be greater). They were working at a pressure of around 828 mb, give or take (you lose a millibar for every ~27 feet you climb, and ISA sea level pressure is 1013).


That is right, a lot less dissolved CO2 (and all other gases), because of Henry's Law.





Dr Mike Oxgreen said:


> So if our “2-3 ppm” equilibrium CO₂ is based on flawed measurements, I’m now wondering what other “knowledge” is derived from that.


The question would be has any-one actually measured the amount of CO2 using an <"ion selective electrode">, rather than extrapolating from an acid base titration (which is how both the lamotte test and drop checker work).  I've never seen any analytical equipment specifically for measuring CO2, although <"I know it exists">. 





Dr Mike Oxgreen said:


> For that matter, where do we get 30 ppm from? Is that based on plant requirement, or fish toxicity?


Fish toxicity, air has 400 ppm CO2 and in Tomato glasshouses etc they <"burn propane"> to get up to about 1200ppm CO2.




 

cheers Darrel


----------



## jaypeecee (15 Apr 2020)

Dr Mike Oxgreen said:


> And how much did your equipment and test kits cost? Your method sounds neither cheap nor easy!



Hi @Dr Mike Oxgreen 

Ah, in that case, you are in for a _big_ pleasant surprise! On the basis that an aquarist may already own a pH meter, the total cost of my proposed method is trivial - less than a tenner! Assuming that you already have a KH4 DC solution, all that's needed are a very short length of perspex tube, some self-amalgamating tape, some very thin silicone (PDMS) membrane and a tiny amount of silicone sealant. I got the membrane material completely free as a sample. I got enough to make a few of these gizmos.

That's all there is to it.

JPC


----------



## Dr Mike Oxgreen (15 Apr 2020)

... and my method costs, er... nothing whatsoever!

But anyway, regardless of the method, the problem is the same: if you’re trying to estimate CO₂ ppm from pH, we are extrapolating from what seems a very dubious starting point. The fact that “2-3ppm” doesn’t fit with well-established scientific theory is a big problem.

Do we even know that a green drop checker corresponds to 30ppm CO₂ or is that based on the same extrapolation from the dubious 2-3ppm figure?


----------



## jaypeecee (15 Apr 2020)

dw1305 said:


> The question would be has any-one actually measured the amount of CO2 using an <"ion selective electrode">, rather than extrapolating from an acid base titration (which is how both the lamotte test and drop checker work).  I've never seen any analytical equipment specifically for measuring CO2, although <"I know it exists">.



Hi @dw1305 

You may find this to be of interest:

https://atrium.lib.uoguelph.ca/xmlu.../9149/Thompson_Cody_201509_PhD.pdf?sequence=7

JPC


----------



## jaypeecee (15 Apr 2020)

Dr Mike Oxgreen said:


> Do we even know that a green drop checker corresponds to 30ppm CO₂ or is that based on the same extrapolation from the dubious 2-3ppm figure?



Hi @Dr Mike Oxgreen

I'm trying to get a better handle on this. We infer that a green DC indicates 30 ppm CO2 because of the pH/KH/CO2 relationship. In what way is this _extrapolated_ from the 2-3 ppm figure? You may well be correct but I see the 30 ppm and 2-3 ppm figures derived from one and the same source - the pH/KH/CO2 relationship. Isn't this known as the Henderson-Hasselbach equation? Yikes, my brain is now too old for some of this. My days of seriously-heavy maths are long gone. 

JPC


----------



## jaypeecee (15 Apr 2020)

Hi Folks,

In order to break away from the pH/KH/CO2 relationship, perhaps a better way to measure dissolved CO2 is by absorption spectrometry. Take a look at this:

https://www.capnography.com/physics/physical-methods-of-co2-measurement

JPC


----------



## jaypeecee (15 Apr 2020)

Me again!

My suggestion above may well be within the realms of hobbyist aquatics. I'm thinking of _Seneye_ with their approach to measuring free ammonia.

JPC


----------



## dw1305 (15 Apr 2020)

Hi all, 





Dr Mike Oxgreen said:


> Do we even know that a green drop checker corresponds to 30ppm CO₂ or is that based on the same extrapolation from the dubious 2-3ppm figure?


My guess is extrapolation, and that "30 ppm CO2" doesn't necessarily mean "30 ppm of CO2".

The normal CO2 sensor for _in situ_ measurement  is a "Severinghaus probe", so I'll see what I can find out using that as a search term. I think I've got it  (from <"The measurement of dissolved and gaseous carbon dioxide concentration">. It looks to have all the answers.





> 2.1. Analytical determination of CO2in liquids
> 
> Unlike other gases such as oxygen, the determination of dissolved CO2 is more difﬁcult due to its chemical reactions with water. This is important for interpreting analytical results as well as for understanding the measurement principle applied e.g. in the Severinghaus electrode. Therefore, the interaction of CO2 with water and the acid–base equilibrium have to be discussed in more detail. The concentration of CO2 is only reasonable with the knowledge of the pH of the analysed medium


This one <"Dissolved CO2 in freshwater systems">, has some figures, that suggest that lakes and rivers typically have between four and eight times more CO2 than they would at equilibrium (0.6 ppm CO2), so we are back somewhere near the "3ppm CO2" figure. The other issue in vegetated water would be that you would get big diel variations in CO2 & O2 levels (back to <"Canford Park again">)





jaypeecee said:


> My suggestion above may well be within the realms of hobbyist aquatics. I'm thinking of _Seneye_ with their approach to measuring free ammonia.


I'm pretty sure that isn't going to work, because of the pH issues, when you change the pH you change the amount of CO2. This means that techniques like converting all the NH3 to NH4+ (by acidification), or measuring oxygen levels by reduction aren't going to work for CO2.





jaypeecee said:


> You may find this to be of interest:


We use Ion Selective Electrodes for NO3 and NH3/NH4+ measurement (and also for DO measurement). Because we have scanning spectrophotometers, HPLC, GC, and AAS we tend to use them for most other elements. We don't have MS or ICP, which a big analytical lab. would have.

Other than CO2, I'd be reasonably happy that I can get fairly accurate results for a water sample in the lab. but it would be quite time consuming, and I'd need access to all the appropriate reagents and analytical equipment.

cheers Darrel


----------



## jaypeecee (15 Apr 2020)

jaypeecee said:


> On the basis that an aquarist may already own a pH meter, the total cost of my proposed method is trivial - less than a tenner!



I was being ultra-cautious here. Too cautious. For many aquarists, they will already have the items I listed. The only thing that is not likely to be in one's aquarium cupboard or toolbox is the very thin silicone membrane. And I even got that free of charge.

JPC


----------

