CO2 pH reference - quick degassing

[Yugang]

CO2 users may measure the pH drop, and use that as an estimation for the CO2 ppm. The reference level should be the pH of fully outgassed water (in equilibrium with ambient air), and is usually checked on a glass of tank water that has been left open for about 24-36 hrs. That's a long time.

I have a faster method that works in perhaps 10 minutes, that I posted on another forum (deleted, then probably soon forgotten so most can go back to waiting for the traditional 24 hrs), where others tested it successfully. The topic came up in a PM conversation today, and I believe it is good to share here so that it is not forgotten and we can all benefit from it.

I use a glass jar, to which I add just enough tank water to later take a pH measurement with a probe. Less water, more air in the jar, speeds up the process.



I go outdoor with the jar (our homes may have elevated CO2 concentrations), let air in, close the lid and shake violently for a few minutes. Open, new fresh air in, shake again and repeat a few times. I can get to the fully outgassed reference in perhaps 5 minutes, but for others 10 minutes may be a safer target without accidentally dropping the jar.

How to double check if it works, how much time is needed, and if indeed the final degassed pH value has been achieved? Just measure pH, shake another few minutes and measure pH again. If pH is stable and not further increasing, then the first measured pH was already good enough and further shaking or waiting does not give a better value.

I hope this saves us all some time, and we have a good solid reference for outgassed pH as well.

 

[BryceM]

I do something similar. Just run an air stone in the container for 30 minutes or so.

 

[Yugang]

Whatever creates a larger surface area for accelerated outgassing.

 

[Yugang]

I am following up here, having had a conversation with @Dennis Wong regarding his experiments with a dedicated CO2 meter.

If pH is stable and not further increasing, then the first measured pH was already good enough and further shaking or waiting does not give a better value.

The last bit of outgassing is the hard part. As the CO2 ppm gets closer to its equilibrium, the process gets slower and slower. But, as the relationship between CO2 ppm and pH is logarithmic the change in pH may still be too significant to be ignored.

The equilibrium between atmospheric CO2, 421 ppm and rising due to climate change, and CO2 dissolved in water follows Henry's law

Henry's law - Wikipedia

en.wikipedia.org

Working out Henry's law, we find that the equilibrium with ambient air gives us 0.6 ppm CO2 dissolved in water. This has also been confirmed experimentally.

Probably due to historic reasons, this goes back to some articles more than 2 decades ago, it is sometimes assumed that outgassed water sits at 2-3 ppm, rather than the 0.6 ppm as per Henry's law.

Aquatic Concepts: CO2 Injection

CO2 Chart

Now what is really important is to correctly use the pH drop method on outgassed water.

If we would assume our water is 3 ppm outgassed, then a 1 pH drop would give us a 30 ppm CO2 following the logarithmic relationship. This may be realistic if our methodology for outgassing is not perfect (not patient enough, or using indoor air with elevated CO2 rather than outdoor air). But if we do perfect outgassing and per Henry's law go down to 0.6 ppm (at current ambient CO2 levels), then indeed a 1 pH drop would only equate to 6 ppm rather than the previous 30 ppm.

Starting from perfectly outgassed water, 0.6 ppm, a 1.5-1.7 pH drop seems a better guess than a 1 pH drop to get to 30 ppm.

The alternative to using pH drop would be to use the famous pH/KH/CO2 tables. These tables go back to Pauli Hopea from Finland, and George and Karla Booth around 1992. As far as I found on the web these were measured data, and so far they seem to work well enough for our hobby as ultimately the plants will tell us what they need.

 

[Koan]

but for others 10 minutes may be a safer target without accidentally dropping the jar.

 

[Yugang]

On a serious note I believe that I should have put less emphasis on speed of the shaking process, and have put more clearly how important it is to do the outgassing well enough and don't take shortcuts for saving a few minutes time.

Let's take an example with the real numbers to illustrate how quickly things go wrong if we don't outgas our water fully, and that we should prioritise quality over speed.

Following Henry's law water will outgas oversaturated CO2 until it reaches equilibrium with ambient air. This equilibrium is calculated from Henry's law, and is the 0.6 ppm CO2 in water as mentioned earlier. It will be obvious that when we shake and shake and get closer to equilibrium the process of outgassing will slow down. So let's take one concrete example what happens shaking down the last 0.6 ppm, so from 1.2 ppm down to 0.6 ppm.

In the below table we can take the line for KH = 1 as an example. When we are too much in a hurry, and shake down to 1.2 ppm we get a pH 7.4 reading. However if we had taken more time, and shake a real long time until the theoretic minimum of 0.6 ppm we would have had pH 7.7. So just the last bit of 0.6 ppm, and as we are close to equilibrium this will be hard to get it done, will make a 0.3 difference in our reading of pH. Now the nature of a table that is based on a logarithmic relationship (or inverse exponential) is that wherever you look up in the table 0.3 pH to the left increases CO2 ppm by 100%, and 0.3 to the right cuts it by 50%. This also applies around our targeted 30 ppm: to the left an error of 0.3 brings it to 60 ppm and to the right we get down to only 15 ppm. So we see that the last bit of 0.6 ppm shaking out while degassing will make a critical difference to the accuracy of the CO2 ppm when applying a pH drop method.

A similar argument can be made for shaking outdoor, where we know that the ambient air is now at 421 ppm, or indoor where we and other living organisms exhale and increase CO2 levels. For indoor CO2 less than 1000 ppm is recommended for good health, but who knows what your family and 2 dogs do in your well insulated home in the winter. When we assume indoor only 850 ppm, we are at the same ratio as in the previous calculation, leading to a 0.3 deviation of our base pH measurement and with that we may in fact target 100% too high with our tanks ppm.

As I said in the original post, always shake outdoor and you know the job is done when pH does not further increase when shaking another few minutes. Small details matter here.

Finally the question that some may ask themselves: why do this complicated outgassing and pH drop method if we could just have applied the table? The answer is that the table assumes pure water with some KH and CO2, but no other acids. It won't work on a tank with a lot of wood and no/small water changes.

 

[FrankZ]

On a serious note I believe that I should have put less emphasis on speed of the shaking process, and have put more clearly how important it is to do the outgassing well enough and don't take shortcuts for saving a few minutes time.

Let's take an example with the real numbers to illustrate how quickly things go wrong if we don't outgas our water fully, and that we should prioritise quality over speed.

Following Henry's law water will outgas oversaturated CO2 until it reaches equilibrium with ambient air. This equilibrium is calculated from Henry's law, and is the 0.6 ppm CO2 in water as mentioned earlier. It will be obvious that when we shake and shake and get closer to equilibrium the process of outgassing will slow down. So let's take one concrete example what happens shaking down the last 0.6 ppm, so from 1.2 ppm down to 0.6 ppm.

In the below table we can take the line for KH = 1 as an example. When we are too much in a hurry, and shake down to 1.2 ppm we get a pH 7.4 reading. However if we had taken more time, and shake a real long time until the theoretic minimum of 0.6 ppm we would have had pH 7.7. So just the last bit of 0.6 ppm, and as we are close to equilibrium this will be hard to get it done, will make a 0.3 difference in our reading of pH. Now the nature of a table that is based on a logarithmic relationship (or inverse exponential) is that wherever you look up in the table 0.3 pH to the left increases CO2 ppm by 100%, and 0.3 to the right cuts it by 50%. This also applies around our targeted 30 ppm: to the left an error of 0.3 brings it to 60 ppm and to the right we get down to only 15 ppm. So we see that the last bit of 0.6 ppm shaking out while degassing will make a critical difference to the accuracy of the CO2 ppm when applying a pH drop method.

A similar argument can be made for shaking outdoor, where we know that the ambient air is now at 421 ppm, or indoor where we and other living organisms exhale and increase CO2 levels. For indoor CO2 less than 1000 ppm is recommended for good health, but who knows what your family and 2 dogs do in your well insulated home in the winter. When we assume indoor only 850 ppm, we are at the same ratio as in the previous calculation, leading to a 0.3 deviation of our base pH measurement and with that we may in fact target 100% too high with our tanks ppm.

As I said in the original post, always shake outdoor and you know the job is done when pH does not further increase when shaking another few minutes. Small details matter here.

Finally the question that some may ask themselves: why do this complicated outgassing and pH drop method if we could just have applied the table? The answer is that the table assumes pure water with some KH and CO2, but no other acids. It won't work on a tank with a lot of wood and no/small water changes.

View attachment 5055

Why not just build a @Yugang reactor sized for overflow mode and save yourself the headache? I’m a chemist/chemical engineer and that stuff makes my brain hurt.

The math for your reactor is so much easier to understand and it just flat out works without constant fiddling.

 

[Pepere]

go outdoor with the jar (our homes may have elevated CO2 concentrations)

I bet people have no clue how high their inside co2 levels may be.

I have an airthings air quality meter that measures co2 levels in addition to other parameters….

Since getting it I no longer close my bedroom door when I sleep… co2 levels can get up to2,500 ppm with a single person in a closed 12 x13 bedroom in a building built in 1936 with no particular air sealing measure originally installed… with all interior doors open in living space, but exterior doors and windows closed with exterior temps between 55 and 80, and a single person in the space, co2 levels can still get to around 1,300 ppm within 5-6 hours. In colder exterior temps, the temp gradient drives more exterior air exchange with the interior due to the chimney effect increasing and co2 levels drop to around 700 ppm inside.

When I run window ac with all exterior windows and doors closedI get to about 1600 ppm co2 levels in living space with just bedroom door open to hallway, kitchen and bathroom as those are the only rooms I cool. When I leave for the day, the levels drop to 650 ppm over about 15 hours. They rise back up to 1,600 ppm in about 3 hours…

With 2 windows open for cross ventilation and asingle fan in one window forcing air flow interior levels right now are 664 ppm. About an hour ago I had the bedroom window only open with drapes in front of it and I was at 885 ppm. Co2 injection into the tanks had been off for an hour…

Air quality reccomendations are to provide enough outside air ventilating in to keep co2 levels below 1,000 ppm. This is largely due to considering co2 levels as a proxy for other airborne contaminants. I have a few hepa filtration units to keep particulate matter and volatile organic contaminants down. I certainly can perceive a difference between mental clarity when I am breathing 2,500 ppm co2 and 1,000 ppm. Not so much between 1,600 ppm and 1,000.

I recall reading where the US submarine fleet works to keep CO2 levels below 4,000 ppm…

The takeaway from this? If your windows are open and fans in a few for forced ventilation you are probably fine outgassing your water in the house… But if all your windows and doors are closed, going outside would make a significant difference….

 

[Yugang]

I recall reading where the US submarine fleet works to keep CO2 levels below 4,000 ppm…

It must have been a submariner applying Henry's law and finding 3 ppm CO2 in an outgassed sample
When reading about absorption of CO2 in water, I found that that research has been funded for submarines. They need effective absorption for combustion engine exhaust CO2 in sea water without leaving bubbles that can be detected.