[mithrilhack]

Hi peeps. My (harebrained) biology teacher wanted to use brothymol blue to test for the concentration of carbon dioxide in our breath. Long story short, I persuaded him to let me find a better method. I've googled  around and found one way which involved sodium hydroxide. Without any expensive equipment, do any of you know another way of doing it, or can offer any pointers in making the sodium hydroxide method work?

Big thanks!

[kkrizka]

I know that limewater can be used as an indicator of CO2. Not sure how well it can be used for quantitative observations.

http://en.wikipedia.org/wiki/Limewater

[mithrilhack]

Yeah, the brothymol blue works the same way, but it'll be useless for this experiment because i already know there'll be C02 but the question is how much.

[Alberto_Kravina]

but the question is how much.

You could make a gravimetric analysis.

[mithrilhack]

What's that? and what equipment will i need?

[Alberto_Kravina]

What's that? and what equipment will i need?

The idea is to precipitate the insoluble ppt, separate it from the solution, dry it and weight it. If you have the mass of the precipitation you can calculate the concentration of CO₂. But in this case it could be kind of difficult because if you roast the ppt. it could partially decompose in Barium/Calcium Oxide and CO₂(CaCO₃-----HEAT---> CaO + CO₂). That is not convenient because if you perform a gravimetric analysis the substance that you weight must have a CLEARLY DEFINED STOICHIOMETRIC COMPOSITION.
In that case a volumetric analysis could be better. I suggest a back titration....

My idea would be this:

Precipitate the Barium carbonate, separate it from the solution by filtration. After washing the precipitate with water (that is  VERY IMPORTANT because so you remove ions that could interfer the analysis) dissolve it in excess HCl with an exact concentration.
Also, Beware that you have to add an exact voulme of HCl, this is important for the calculations afterwards.
After the Barium carbonate is dissolved in HCl there is still an excess acid in the solution. This excess HCl is titrated with NaOH.
It's quite complicated, and I don't know if this works, I improvised.

[Borek]

With correctly chosen roasting/drying temperature you may have full control on the composition of the compound weighted.

But I would go for a simpler solution: start with known amount of strong hydroxide solution, absorb CO₂, then titrate excess hydroxide. Look for Warder method.

[Alberto_Kravina]

But I would go for a simpler solution: start with known amount of strong hydroxide solution, absorb CO2, then titrate excess hydroxide. Look for Warder method.

Oh yes! I know good old Warder Method....
...but I'm not sure if 100% of CO₂ absorbs to the NaOH

Winkler method would also work. I'd try both