[Twigg]

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?

[furo]

Because of CO2's low boiling point (-57), you can simply place it in fresh air.
So CO2(aqua) CO2(gas).

I'm i missing something?

[Borek]

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.

[Twigg]

@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.

[DevaDevil]

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?