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Topic: Simple Way of Removing CO2 from Water?  (Read 32067 times)

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Offline Twigg

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Simple Way of Removing CO2 from Water?
« on: April 02, 2011, 05:02:45 PM »
I'm not sure if this is the right spot for this, but I didn't want to bug people in the undergraduate forums with a question about basic bench methods (again).

I want to experimentally measure pH of a standard limewater (Ca(OH)2) solution. Simple enough, but I'm stuck with a few limitations.

Firstly, I only have tap water, and the tap water is loaded with dissolved CO2. The carbon dioxide-carbonate equilibrium will throw off the pH to begin with, but more importantly it will react with the calcium and precipitate calcium carbonate. It would seem that I just answered my own question, but there's another limitation.
Secondly, I have only 1-2g of powdered Ca(OH)2. I don't know how much CO2 I will need to remove and even if I used the calcium hydroxide to remove the CO2, the pH would be thrown off by the lingering hydroxide ions. I won't know what the concentration of hydroxide ions left over will be because some of them will be neutralized by an unknown amount of hydronium released by the carbon dioxide-carbonate equilibrium shift. I don't know the concentration of CO2 in the tapwater.

I was thinking of heating the water, but I'm not sure how effective that'll be. Any help?

Offline furo

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Re: Simple Way of Removing CO2 from Water?
« Reply #1 on: April 02, 2011, 06:14:00 PM »
Because of CO2's low boiling point (-57), you can simply place it in fresh air.
So CO2(aqua) :rarrow: CO2(gas).

I'm i missing something?

Offline Borek

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Re: Simple Way of Removing CO2 from Water?
« Reply #2 on: April 03, 2011, 04:50:32 AM »
I'm i missing something?

Yes, there is plenty of CO2 present in the air, so water in contact with it is always saturated.

Preparing CO2 free water is not an easy task, but I guess in this case boiling and keeping everything closed - under inert atmosphere if possible - should be enough.
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Offline Twigg

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Re: Simple Way of Removing CO2 from Water?
« Reply #3 on: April 03, 2011, 05:03:15 PM »
@Borek:
I'll try that, but I think there may be a way around.
Would the water definitely be saturated with CO2? If it is then I could just correct for the concentration of CO2 in the equilibria calculations. I could let it sit out for a while, but I'd need to know if it'll reach saturation point in a convenient amount of time, whether or not it might supersaturate if the pressure drops, and how long it takes CO2 to dissolve. My idea is this: if I measure the pH of the solution immediately after adding the Ca(OH)2, then I might get the pH before a significant amount of CO2 dissolved and formed carbonate. I know the precipitation is really quick, so I won't have to wait to take the pH. What I don't know is the rate of CO2 dissolution and acidification. Also, I'd need to know that the solution was saturated in CO2. Any help?

Thanks.

Offline DevaDevil

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Re: Simple Way of Removing CO2 from Water?
« Reply #4 on: April 04, 2011, 11:05:26 AM »
the most reliable way of making a solution CO2-free is to make sure the vessel it is in is sealed, then bubble an inert gas through. (making sure this inert gas can exit the vessel through a water-lock for example)
Do you have access to something like this?

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