# 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.

• Use aqua regia. Safe enough IMO. :) – M.A.R. Feb 19 '15 at 20:27
• To get more of an acid shift and thus more dramatic change in the indicator color, create and dissolve more $CO_2$ into solution. Good old baking soda and vinegar will do the trick. – docscience Feb 19 '15 at 20:52
• This is a really interesting idea, so props for coming up with it! However, looking at this table, it's evident that you'll need to get a massive pressure difference to shift the pH enough. Any indicator will require a pH shift of 1-2 units for a complete colour change, and it will take a factor of 100 difference in the $\ce{CO2}$ partial pressure to alter the pH by just 1 unit. I don't know exactly what indicator will work, but I think you'll need one with a $pK_a$ around 4. – Nicolau Saker Neto Feb 19 '15 at 21:42

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

Also check this wikipage for other options.

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.

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.

• I'm going to try pheolphthalein. I will adjust pH to about 8 and hopefully the pressure change will allow a color change. – Brinn Belyea Feb 20 '15 at 1:05
• Good luck. I had a thought when you mentioned "pressure" because that's important. Assume a student can exert a force of 7lb on a syringe of 1 square inch cross section. As 1atm=14lb/sq in, that means they can go from 1atm to 1.5atm by pushing, and 1atm to 0.5atm by sucking, not a big difference. And we're only there's only a few grams of gas, so liquid volume needs to be small as possible. I'm sure there's something you can do with CO2-Alkali-Phenolphthalein, but all the examples I have given involve irreversible (at room temp) absorbtion of CO2, not release of CO2. Boiling should reverse! – Level River St Feb 20 '15 at 2:11
• This may be of interest: nuffieldfoundation.org/practical-chemistry/… . Here we have a slow reduction reaction which can be temporarily reversed by atmospheric oxygen (by shaking) – Level River St Feb 20 '15 at 2:16