Australia's Science Forum

Hi all,

Today and last week I helped out with a year 9 science experiment which analysed the carbon dioxide content of a breath. We used limewater, which of course turns cloudy when you breathe in to it, and we also used bromocresol purple in distilled water. The kids had to run various distances (0m, 100m, 800m, 1500m etc) and breathe in to a conical flask of either solution (or both at the same time) through straws.

Now, the theory was that the longer the kids ran, the more CO2 they'd have in their exhalations, and the faster the limewater and bromocresol purple solution would change. This wasn't really the case, with many varying results for each of the two classes.

Why could this be? The limewater was freshly made and distilled before use, and the bromocresol purple changed from purple to clear but seemed to be the wrong indicator to use since it changed in a matter of seconds. The text book (and websites I subsequently searched later) had suggested using bromothymol blue, which is meant to turn from blue to yellow/green. However I tried this out beforehand and again today by breathing in to it through a straw for quite a long time and got no result - it didn't change from purple/blue Why would that be? Are there any other good indicators?

Hi Miranda,

I have had a good long think about this topic you submitted.

It looks as if you are using Indicators not suitable in the pH range you want.
Try using Thymol Blue rather than Bromothymol blue.

The Bromothymol Blue will only change when the pH is quite high, CO[sub]2[/sub] in breath when it dissolves in Water just isn't strong enough to change the indicator.

Thymol Blue should get you out of trouble, otherwise use Universal Indicator with 2 Drops (no more no less) of 0.1M Sodium hydroxide solution to hold it at a Lower pH before breathing into it. Then compare results.

Hope this has helped,

Cheers,

Robb.......

Hi Maranda

I've looked at it too now I'm back, both Lime water and the indicators are gross change indicators, by this I mean they will flip from one state to another very quickly as a rule,

You are better to use a data logger and pH probe for something like this and you can overlay results on a chart and print out

Kim

SOrry guys, I haven't been on this website for a long time and I just found this. Thankyou so much for your help, it's really appreciated

Anyone have any idea of how long it takes to turn cloudy soulution clear when more co2 is bubbled through, obviuosly it dosnt work by breathing in to it more.

Hi Mojo,

I have always tested my limewater solution beforehand to make sure it works for the kids. Breathing through a straw will turn limewater milky in about 30 seconds. It will become clear again if you continue to breath through the straw and will take between 5 and 10 minutes for the change to occur and for your solution to become clear.
This prac keeps the kids breathless and occupied for a reasonable time.

Lyn