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Discussion Starter · #61 ·
All this Henry's law talk gave me a new idea for the CO2 sensor design. I don't have to put it in water and fear there would be a leak. I can just place it above the water level and use Henry's law to calculate... My only fear would be water splashing into the electronics.
 

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Discussion Starter · #62 ·
That can't be right. CO2 concentration in air is around 400 mg/L. Fish would all be dead at 1/10 of that concentration in water.
You're forgetting the different units. In air it is ppm by volume. In water it's measured by weight.. like 400ppmv = 0.7ppmw something like that.
 

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You're forgetting the different units. In air it is ppm by volume. In water it's measured by weight.. like 400ppmv = 0.7ppmw something like that.
Yup I did. I noticed that error about a minute after I posted, and went back to fix it, but you caught me before I got it fixed! [smilie=b:
 

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Discussion Starter · #64 ·
Yup I did. I noticed that error about a minute after I posted, and went back to fix it, but you caught me before I got it fixed! [smilie=b:[/QUOTE]

If you want to know the Co2 of a glass of water, that ph&kh chart works great.no need for a sensor.
 

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All this Henry's law talk gave me a new idea for the CO2 sensor design. I don't have to put it in water and fear there would be a leak. I can just place it above the water level and use Henry's law to calculate... My only fear would be water splashing into the electronics.
It's a common environmental sampling technique called head space analysis. It proceeds with collection of a water sample in a capped jar, let it sit and reach equilibrium, then punch through the cap with a Syringe to draw an air sample that feed into an air monitoring device. It's analogous to a drop checker where the test liquid is separated from the tank water with an air space in which co2 reach equilibrium by Henry Law in all three media.

Unless your tank top air is sealed, your measurement won't be accurate. You need to collect the tank water sample in a jar and sealed it with a plastic wrap, wait for equilibrium, and find a way to insert the sensor.
 

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Discussion Starter · #67 ·
Ok, using Henry's Law to find the C of CO2 in water

P is Partial pressure of 400ppm of CO2 in ATM
C=P/K
C = 0.0004/ 29.4
C = 0.0000136M (m/L)
grams = 0.0000136 * 44.01 g/m
grams = 0.0006g/L or .6mg/L or 0.6ppmw

We found earlier the air has .775mg/L

So it's pretty close theoretically... I'll turn on the sensor and measure CO2 on top of water to verify.... I have to do it outside since CO2 in the house is significantly higher.
 

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Ok, using Henry's Law to find the C of CO2 in water

P is Partial pressure of 400ppm of CO2 in ATM
C=P/K
C = 0.0004/ 29.4
C = 0.0000136M (m/L)
grams = 0.0000136 * 44.01 g/m
grams = 0.0006g/L or .6mg/L or 0.6ppmw

We found earlier the air has .775mg/L

So it's pretty close theoretically... I'll turn on the sensor and measure CO2 on top of water to verify.... I have to do it outside since CO2 in the house is significantly higher.
Yup those look like numbers I found too, LATE last night. You're right that the mass concentration of CO2 gas dissolved in water (0.6 mg/L @ equilibrium with 20°C air) is very close to the mass concentration of CO2 in air (0.7 mg/L). It turns out that's just a coincidence due to CO2's high solubility (low K value), not a general principal of gas physics. N2 and O2 mass concentrations in air are MUCH higher than their mass conc's dissolved in water:

N2 gas in air: 75% (by mass) x 1.2 g/L = 900 mg/L in air (20°C)
N2 gas in water: .78 atm / 1639 (K) = .00048 mol/L = 13.4 mg/L in water
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O2 gas in air: 23% (by mass) x 1.2 g/L = 280 mg/L in air (20°C)
O2 gas in water: .21 atm / 789 (K) = .00027 mol/L = 8.7 mg/L in water

And of course the other weird & important feature of CO2 is that it REACTS chemically with water, which N2 and O2 don't do. The dissolved CO2 gas molecules may be only 0.6 mg/L (at air equilib), but lots more CO2 can be held in water as carbonic acid and bicarbonate, so the total amount of CO2 that will dissolve in water (much of it converted to carbonic acid and bicarbonate after dissolving) is a good bit more than the Henry equation shows us.

I'm beginning to understand why my simple question doesn't have a simple answer. If I've botched any of this analysis (again) please correct me.
 

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Discussion Starter · #69 ·
yup, it's because CO2's high solubility that it's close to water & air.
The Henry's Law constant for it is LOW, 29.4 L*atm/mol. Meaning it really doesn't want to leave water.

Yeah, it's pretty complicated. The concentration is influenced by temperature too.
 

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Yup

And of course the other weird & important feature of CO2 is that it REACTS chemically with water, which N2 and O2 don't do. The dissolved CO2 gas molecules may be only 0.6 mg/L (at air equilib), but lots more CO2 can be held in water as carbonic acid and bicarbonate, so the total amount of CO2 that will dissolve in water (much of it converted to carbonic acid and bicarbonate after dissolving) is a good bit more than the Henry equation shows us.
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I don't know if your undertanding is correct. I think Hernry coefficient is derived experimentally by comparing air concentration of co2 to total aqueous concentration of co2 that is sum of ionized (carbonic acid) and free co2. This is similar to total ammonia determination which is the sum of free and ionized NH3.
 

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Discussion Starter · #71 ·
The CO2 in carbonic acid will eventually break loose, in addition to the free CO2, as we all know the pH in CO2 filled water will eventually rise left alone. It's the carbonic acid that affect the pH.
 

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I don't know if your undertanding is correct. I think Hernry coefficient is derived experimentally by comparing air concentration of co2 to total aqueous concentration of co2 that is sum of ionized (carbonic acid) and free co2. This is similar to total ammonia determination which is the sum of free and ionized NH3.
OK thanks tiger - so you're saying the K value for CO2 includes all the dissolved forms: CO2 gas, carbonic acid, and bicarbonate, right? I guess that explains why its K is so much lower than other gases.
 

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OK thanks tiger - so you're saying the K value for CO2 includes all the dissolved forms: CO2 gas, carbonic acid, and bicarbonate, right? I guess that explains why its K is so much lower than other gases.
Actually, I am not certain, but can only assume that K is derived from total dissolved CO2 that by definition includes free and ionized CO2. The two exist simultaneously and interchangeably. Free dissolved CO2 in equilibrium is not the same as CO2 mist created by artificially injection.

Ionized CO2 is carbonic acid. Carbonate and bicarbonate are dissolved salt of cations, not CO2. The only way to ascertain what is included in K is to review the lab procedure that derived K.

The reason CO2 has much lower K than other atmospheric gases is that it is a polar molecule that has great affinity for polar H2O, and can ionize. O2 and N2 are non-polar and dissolve in free phase only, no comparison.
 

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Discussion Starter · #74 ·
I'm finally back in the CO2 game after these many years. I plugged in this contraption into my 75G.
It doesn't look good. The bubble rate is less that 1/sec for the CO2 to dissolve or the the CO2 will overflow out the chamber. The amount of CO2 in the 75G is only around 3ppm. Going back to the regular PVC reactor.
 
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