Which pH indicator will give a dramatic color change with small changes in the amount of carbon dioxide dissolved in water?

I want to do a demonstration for students where they take a capped syringe and change the volume above a solution of seltzer water. The amount of carbon dioxide dissolved will vary, hopefully changing pH enough to cause an indicator to change color, the more dramatic the color change, the better.


I have tried universal, bromcresol green, and methyl red with little success. I can add substances to the seltzer water or change the temperature of the water. I'd like a SAFE system I can create that will change color upon moving the plunger.




Try *neutral red*. It's transition region is from 6.8 to 8.0 > red to yellow


Also check this [wikipage](http://en.wikipedia.org/wiki/PH_indicator) for other options.


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I once worked in a QC lab where we tested the acidity of organic esters to see if they had any free acid in them. The method was based on titration with strong base in methanol solution, using phenolphtalein.


To do this you would need an aqueous solution of sodium acetate or bicarbonate to get the phenolphthalein into its pink range, but I can tell you from experience that if you have a titration right at its endpoint, atmospheric CO2 and a bit of swirling is all it takes to turn the phenolphtalein back to colourless. Whether you can get the CO2 back out again by sucking with a syringe is another matter. Heating would help.


If you do it this way, you're going to need to change the experiment somewhat, but you should have something that will work. Here's a simple experiment (not exactly what you want but it uses the reagents I mentioned.) <http://www.sciencegeek.net/Chemistry/chempdfs/QualitativeAnalysis.pdf>


A great thing about phenolpthalein is that when it's in its colourless range it is *absolutely* colourless (due to the lack of conjugation in the molecule) so you can put lots in for dramatic effect. According to <http://en.wikipedia.org/wiki/Phenolphthalein> Phenolphtalein is used to test the curing of concrete by CO2, and the reaction and indicator are used in various toys.




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Another thought: We used to add solid CO2 pellets to water with indicator in it for fun. That's certainly a way of getting a lot of CO2 into water. If I remember rightly the indicator was methyl orange. The colour changes only while the CO2 pellet is subliming. Once the pellet has completely disappeared, the colour changes back.


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