# How to determine amount of CO₂ in soda water

I've got a problem in determining the amount of $\ce{CO2}$ in soda water. Basically, I have boiled some purified water to remove the dissolved gas. By using a SodaStream machine, I made carbonated water.

After that, I have tried to determine the amount of carbon dioxide through titration with phenolphthalein and also $c(\ce{NaOH})=0.1~\mathrm{mol/L}$ sodium hydroxide.

The problem is I am not sure if this is the correct method to determine the amount of dissolved $\ce{CO2}$ in water.

• I'm leaving this as a comment since it is not directly answering your question, but you could check your titration (which should work fine as described) by the difference in mass of the bottle. Weigh before carbonation and after, and account for any splashing by finding the difference in mass of a dry paper towel and the mass of the same paper towel after drying off every drop of water you can find. It should be fairly accurate and will give you a better idea of the mass of $\ce{CO2}$ in the system immediately after carbonation. – Jason Patterson Nov 13 '14 at 18:25

First of all, after degasing by heating you have to be sure that no $\ce{CaCO_3}$ is formed, so you can minimise this problem by transfering the water in another flask.

• More accurate would be to start directly with demineralised water which you can buy cheap at filling stations.

Secondly, you shouldn't wait to much while titrating, otherwise the water is taking up $\ce{CO_2}$ from the air.

• You should use a bulb which doesn't contain much air (which contains $\ce{CO_2}$ and can interact as well. You can use a round flask and directly connect it with a stopper.

Third, Phenolphtalein is a good indicator because you titrate a weak acid with a strong base.

• Make sure, you don't use too much Phenolphtalein.

Last but not least be sure that the $\ce{CO_2}$ from flask doesn't go away in the air or into the NaoH of the burette.

• To cool will help, because $\ce{CO_2}$ is more soluble in cold water.

If you want to be a super-purist, do a potentiometric measurement and plot the point of change using a simple voltmeter.

In the brewing industry this problem is solved using a device made by Zahm and Nagel. Basically, carbonated beer in admitted into a chamber of known volume. The chamber is shaken which causes the CO2 to come out of solution moving a piston. The temperature, pressure and volume of gas are measured and a table consulted.

Don't use NaOH for titration here. It grabs CO2 forming sodium carbonate. If you want to titrate I guess you could add an excess (but known) amount of NaOH and then back titrate.