CO2 measurement - when all things are taken into account

[Bradders]

I would just declare victory right here.

Yes, but that was at 08:30 in the morning, 5.5 hours before the photo period! In summary, it was olive green over night and was that way at 08:30.

Not sure if that is saying the new tank is having a gass exchange issue......

 

[dw1305]

Hi all,

... saying the new tank is having a gas exchange issue......

Yes.

Is it a deep tank in relation to its length and breadth?

Cheers Darrel

 

[Bradders]

Is it a deep tank in relation to its length and breadth?

100x50x50 cm.

 

[sparkyweasel]

Is it important to do both?

Yes.. 🙂

 

[MrClockOff]

Yeah, all sounds correct except shaking bottle for degassing the water? We tend to leave it alone for 24h to degass as shaking water would mean you mixing it with air and air contains atmospheric CO2.

Shaking the water to degas works, but I prefer leaving a glass of tank water 24 hrs. If using the shaking method you have to keep shaking it until there is no further change in pH ( then you could always leave the sample over night and check again)

Oops wasn't aware of this method. Apologies for misleading info.

 

[Zeus.]

Oops wasn't aware of this method. Apologies for misleading info.

Shaking the sample is the method @X3NiTH uses, and if good enough for 'The Half Blood Prince ' its a valid method for me.
The shaking just speeds up the equilibrium with the atmosphere, just need to take sure the air gets refreshed if using test tube.

 

[hax47]

Degassing needs some time, I usually had to aerate the water for 30-60 minutes to get close to equilibrium.

Also, the equilibrium is a moving target, there is no constant CO2 level indoors. The outdoor CO2 is more stable but the equilibrium with that CO2 is around 0.6 ppm in water. A drop of 1 pH compared to that would mean ~6 ppm in water. If we expect 3 ppm of CO2 in water after equilibration indoors, the air-CO2 should be around 2100 ppm, which many would consider unhealthy. Maybe the equilibration combined with the 1pH drop method works because people do not get full equilibration with air either with shaking (not enough time) or 24-hour in glass (diffusion too slow). The lower the CO2 concentration in the water, the slower the diffusion will be.

I would trust more the DC method or a good pH/KH measurement.

 

[nijat11]

I stopped drop checks a while ago. Get cup of water from aquarium, wait 24 hours, -1 pH point from that. Drop checker will show you that it is yellow, but I dont notice any bad behaviour from fish. I even tried -1.2 points on my nano garden where dropchecke goes almost colorless 😀, all shrimps and fish fine and reproduce well.

 

[dw1305]

Hi all,

The outdoor CO2 is more stable but the equilibrium with that CO2 is around 0.6 ppm in water.

We don't know where the "3 ppm CO2", (and <"30 ppm CO2"> extrapolated from it) came from. Do you know @hax47 ?

All the references go back to <"George and Karla Booth"> <"Estimating CO2 injection rates in ml per minute">

The "three ppm" figure is widely used, even in the scientific literature - <Co2 drop checker vs ph measure>.

There are plenty of papers on natural CO2 levels in freshwater, but they nearly all rely on the carbonate equilibrium (and temperature) to estimate the CO2 level - <"A 30-year dataset of CO2 in flowing freshwaters in the United States - Scientific Data">, rather than actual CO2 measurements. This really just reflects the difficulty of <"measuring dissolved CO2 directly">.

The log10 nature of the pH scale, and how that relates to CO2 content, is described by @hax47 recently <"Confusion concerning 0dKH and nitrification"> and @Jose in this older post <"Question about pressurised CO2 and water disturbance">.

cheers Darrel

 

[hax47]

Do you know @hax47 ?

No, I don't. The discrepancy between the equilibrium with the atmospheric CO2 and the 3 ppm was one of the reasons I started running some gas-exchange experiments.
If you look at my first experiment, the water left in the air without aeration reached 2.7 ppm after 24 hours, although the equilibrated one was already at 1.3 ppm. Since the diffusion slows with decreasing water-to-air CO2 partial pressure difference, there is probably a very slow, long-lasting decline at a few ppm (below ~4) until equilibrium is reached. Maybe most folks measure CO2 in this relatively long period and conclude that the equilibrated CO2 is in this range.