# Calculating the amount of oxygen in a glass using candle burning time

I would like to calculate the amount of oxygen or concentration of oxygen in a glass or conical flask using candle burning time.

I am thinking of a simple way to calculate this based on the fact that the atmosphere contains $$21\,\%$$ oxygen. But I am not sure if this would give correct values.

Here is what I am thinking about: I could do a candle and glass experiment and count the time for burning of candle. First I will take a glass with atmosphere air and then keep it inverted on burning candle and count the time for burning. Next I will take a glass with oxygen-enriched air (with unknown concentration) and then keep it inverted on burning candle and count the time for burning.

Here is a simple formula using cross multiplication, I could use

$$x = \frac{t_\mathrm{OEA}}{t_\mathrm{air}}\cdot 21\,\%,$$

where $$x$$ is the concentration of oxygen-enriched air, $$t_\mathrm{OEA}$$ is the time taken to burn in oxygen-enriched air, $$t_\mathrm{air}$$ is the time taken to burn in atmospheric air.

Is this possible or is there a way to do this without having to buy oxygen sensors?

• Your idea implies the rate of burning is independent on oxygen content, what is not true. – Poutnik Jul 22 '20 at 21:18
• @Poutnik , Thanks for reply, I am implying that the time of burning will depend on oxygen concentration in glass, as other ingredients such as fuel(candle wick) is in sufficient quantity – Fawaz Ahmed Jul 22 '20 at 22:16
• You might want to think about the horrible Apollo 1 tragedy. As @Poutnik says, the burn rate is not independent of oxygen content and a linear relationship cannot simply be assumed. – Ed V Jul 22 '20 at 22:35
• @EdV , Thanks for explaining, so my assumption was wrong, a linear relationship cannot be made. If you could help me with a formula using which concentration of oxygen could be calculated, that would be great help. I just want to somehow get the concentration of oxygen without buying expensive oxygen sensor – Fawaz Ahmed Jul 22 '20 at 22:56
• I would help if I could, but I do not know a good alternative that does not involve titration, etc. But there are many excellent chemists here, so maybe one of them will post a solid answer. I hope that happens! – Ed V Jul 22 '20 at 23:04

I think you will be better off measuring the volume change after complete burning.

A common demonstration was to place a candle in a dish of shallow water and light it, then put a clear cup over it and see how much water was sucked into the bottom as the oxygen is consumed. utilizing the same effect in a more controlled way perhaps would be a straightforward way to get a rough oxygen concentration.

When calculating it remember that 2 oxygen molecules are replaced by a single co2 molecule and you will want your water deep enough so that when the heated air expands it doesn't escape out the bottom and account for temperature differences either by doing the math or waiting for the temp to be the same.

Fully burning steel wool and measuring the gain in weight due to the oxygen getting incorporated into solid iron oxides might also work.

• One–not two–oxygen molecules produce one carbon dioxide molecule. But that can be absorbed in suitable water solutions. – matt_black Jul 23 '20 at 18:51
• For burning hydrocarbons like candles it's 2 to one. It is 2*O2 + CH4 (or other hydrocarbon) -> CO2 + 2*H2O – John Meacham Jul 23 '20 at 22:12
• Actually for methane it is 3 oxygens in total but only one gives a gaseous product. For wax, that will be closer to 2 because the overall formula of the burning compound is closer to CH2, and one of the products is water, a liquid. You are right that you have to account for this to get the loss of oxygen gas, but the fact one product is also a gas is a complication (often ignored, though). – matt_black Jul 23 '20 at 22:20
• Yeah, you would have to work it out for your specific fuel, or just do a batch of atmospheric air and reverse engineer the ratio. It sounds like for their purposes of an oxygen concentrator they will only need some fairly course measurements as they would expect fairly large changes in oxygen concentration and this method should work fine. – John Meacham Jul 25 '20 at 7:20

You might get better results using titration. Pick a substance that reacts fairly completely with atmospheric oxygen, and measure how much needs to be added to remove all the $$\ce{O2}$$ from a known volume.

For example, you might use washed, fine, steel wool (washing is needed o remove processing oils) to remove oxygen, and methylene blue with alkaline glucose to detect any remaining $$\ce{O2}$$.

You might also try the Winkler test to measure dissolved oxygen in water.