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ct_

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I've read all over the place that at atmospheric equilibrium an aquarium has about 2-4 ppm CO2.

When I do the math with Henry's law I get something like 0.5-0.6 ppm CO2 depending on what I use for the concentration of CO2 in the atmosphere (350-400ppm).

Am I getting something wrong? Everyone states some number around 2-4 as fact but I can't figure out where that number comes from.

Tiger15

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I've read all over the place that at atmospheric equilibrium an aquarium has about 2-4 ppm CO2.

When I do the math with Henry's law I get something like 0.5-0.6 ppm CO2 depending on what I use for the concentration of CO2 in the atmosphere (350-400ppm).

Am I getting something wrong? Everyone states some number around 2-4 as fact but I can't figure out where that number comes from.
Correct. If you do the calculation based on 400 ppm atmospheric CO2 and dimensionless Henry coefficient of 0.83, the equilibrium CO2 in pure water is about 0.6 ppm. Note that ppm in air is expressed by volume and ppm in water is by weight and here is how you convert ppm by volume to mg per m3 in air.

Parts Per Million (ppm) Converter

Accordingly, 400 ppm of CO2 in air by volume is equivalent to 775 mg/m3 or 0.775 mg/l in air by weight, and multiplying Henry coefficient of 0.83 will yield 0.64 ppm CO2 in pure water.

The catch is pure water has kH=0, and natural water has some kH. A tiny increase in kH can lead to quantum jump in solubility or Henry coefficient for CO2. For example, a pH 7 and kH 1 will get you 3 ppm CO2 in the pH-KH CO2 chart.such as this.

ct_

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The catch is pure water has kH=0, and natural water has some kH. A tiny increase in kH can lead to quantum jump in solubility or Henry coefficient for CO2. For example, a pH 7 and kH 1 will get you 3 ppm CO2 in the pH-KH CO2 chart.such as this.
Thanks for the reply! I was with you up until this point. Those ph/kh/co2 tables come from carbonic acid/bicarbonate/carbonate equilibrium equations. Is it really true that the Henry coefficient changes that much? I know I can't just "adjust" ph and kH, wait for it to settle and get any CO2 I want for example.

Tiger15

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Thanks for the reply! I was with you up until this point. Those ph/kh/co2 tables come from carbonic acid/bicarbonate/carbonate equilibrium equations. Is it really true that the Henry coefficient changes that much? I know I can't just "adjust" ph and kH, wait for it to settle and get any CO2 I want for example.
No, you will not increase dissolved CO2 according to the chart by raising kH, but you can reduce degassing of CO2 as kH is binding to CO2. Natural water contains kH and micros that respire and thereby has higher equilibrium CO2 than pure H2O. Henry coefficient is obeyed only under STP and pure H2O conditions that do not exist in nature.

Greggz

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I can tell you from experience that it's about right. If anything, I see degassed more often in the 3 to 4 ppm range.

But much depends on our measurement methods. When my dKH was 2, degassed was always around 7.2, or 3.8 ppm CO2.

In the end, it matters little. The bottom line is what is the pH drop from fully degassed, and even then it is not an exact science. Fine tuning requires keeping a close on fish and plants and finding the sweet spot.

Deanna

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Well, you have excellent scientific and practical application explanations. In tandem with this, you may want to next consider the interrelationship with the carbonate system al la the Bjerrum plot.

The different carbon species are dissolved CO2, a tiny amount (.25%) of carbonic acid (H2CO3), bicarbonate, and carbonate. In acidic conditions, the dominant form is CO2; in basic (alkaline) conditions, the dominant form is CO3; and in between, the dominant form is HCO3.

If water is saturated with CO2, there is a mix of dissolved CO2 in water with a little carbonic acid and some carbonates, depending on the KH of the water and the temperature. Dissolved gasses are not molecularly combined with H2O, just dissolved. Similarly, if water is saturated with oxygen, it does not create hydrogen peroxide, just water with dissolved oxygen.

Between this and @Tiger15's contribution, you'll always know where you stand with CO2 and how to move it around. Then you can measure it as @Greggz showed.

Greggz

ct_

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No, you will not increase dissolved CO2 according to the chart by raising kH, but you can reduce degassing of CO2 as kH is binding to CO2. Natural water contains kH and micros that respire and thereby has higher equilibrium CO2 than pure H2O
I don't quite follow this. do you have a reference, or can you express this more rigorously? By kH do you mean carbonates? My understanding is
CO2+H2O<->H2CO3
H2CO3<->H + HCO3
so, CO2 doesn't bind to carbonates.

Henry coefficient is obeyed only under STP and pure H2O conditions that do not exist in nature.
This is surly untrue because if it only works under standard temperature and pressure then there's no independent variables left. I do get that the the constant is usually defined for pure water, but can you point me to somewhere I can see how the constant changes for not pure water, specifically carbonates? I'm having trouble finding resources.

Tiger15

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kH in water is typically associated with carbonate rock, which reacts with and binds CO2.

CaCO3+ CO2+ H2O → Ca(HCO3)2

Henry coefficient is another way to express solubility of a gas in water which is affected by the presence other solutes in water as discussed in the last paragraph of this link:

The solubility of a gas in water is affected by the presence of other solutes. The most important are electrolytes causing the solubility to decrease (salting-out) or increase (salting-in). Furthermore, some dissolving gases ionize in water, slightly like CO2 or fully like HCl. Then, additional species determine the equilibrium distribution between phases and in particular solubility in the liquid phase.

It’s much easier to let water equilibrate with the atmosphere and measure CO2 than to theoretically calculate it due to interference of other geochemical factors.

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