# How to capture the gas produced by combusting alcohols

I am burning various alcohols and am attempting to capture the carbon dioxide produced. I could burn the alcohols in a conical flask with a cork on the top, with a tube directing the gas into a gas jar. However, due to the small volume of a conical flask, there would be almost no oxygen to burn and the flame would extinguish quickly. Is there any experimental setup I might have access to with high-school lab equipment that allows me to burn alcohols in an environment with plenty of oxygen, while capturing the gas produced? Thanks

If you look at the overall reaction, there is actually no gas produced, but there is a consumption of gas:

$$\ce{CH3(CH2)_n-OH + \frac{3}{2} (n + 1)O2 -> (n + 1)CO2 +(n + 2)H2O}$$

So 1.5 mol of oxygen are consumed, while only 1 mol of carbon dioxide is formed, so the volume will be less if the temperature is maintained.

To perform this experiment, you must collect the carbon dioxide in a solution of alkali and then titrate the solution. The carbon dioxide will be converted to carbonate:

$$\ce{2NaOH + CO2 -> Na2CO3 +H2O}$$

A constant flow of air must be put into the system and the gas that comes out must be bubbled with a diffuser through the alkaline solution. The air flow must be slow, so that all the carbon dioxide is washed out of the gas stream.

In the titration there will be two equivalence points, the first one of the excess of alkali and the second one of the carbon dioxide. This one has a much smaller pH change.

Be sure to make a careful blank of the alkali, because it normally has some carbonate that will add to the value of carbon dioxide.

• Hi, thanks for the help, just a few questions: 1. Why does the solution that the CO2 is bubbled through need to be alkaline? 2. How could I pump a constant flow of air into the system, realistically, in a lab setting? Thank you again. – Caspar B May 23 at 11:49
• Look at the second reaction. The carbon dioxide is converted to carbonate which is not a gas and will stay into the solution. In our lab we have a line of compressed air. There are also small pumps wich give low flows of air. They are simple and cheap – Raoul Kessels May 23 at 12:03
• Is there a reason you can think of that could justify this experiment to determine the CO2 emissions of alcohols over just using their mole ratio in the reaction you provided to find out? Possibly to find the environmental emissions per litre of fuel used as opposed to per mol? Thanks – Caspar B May 23 at 13:11
• If you have an unknown alcohol, you could determine the chain length by doing this experiment, and if an alcohol is mixed with water you could, in principle, determine the content. However, the precision of the results will probably be very low, so it will be kind of orientative only. To analyze water in alcohol it is better to use Karl-Fisher – Raoul Kessels May 23 at 13:55