[da329]

Hello,
I am working on a take-home research chem lab. I had to research pH values for various different chemicals and list whether they were basic or acidic (that was the easy part). Now I need to find expected color/pH range with methyl orange, with bromthymol blue, and with phenophthalein for each of the chemicals. How do I estimate/calculate this?

For example, we were given Pepper with a pH range of 5.5-7.0. Now I need to calculate:
1. Expected color/pH range with methyl orange.
2. Expected color/pH range with bromthymol blue.
3. Expected color/pH range with phenophthalein.

Thanks!

[Borek]

These are not things that you can calculate, color changes are tabulated.

http://www.titrations.info/acid-base-titration-indicators-preparation

[da329]

So I can't even estimate these? This is odd...shouldn't there be SOME way to guess/estimate??

[Borek]

So I can't even estimate these? This is odd...shouldn't there be SOME way to guess/estimate??

On a very general level - there is a method. It requires quantum mechanics, which is way above your head (and I am not sure it is guaranteed to give the correct answer - solutions we are capable of finding are still only approximate).

Such things are easy to be checked experimentally, but there is no need for everyone to repeat the same experiment, when the data is easily available.

[da329]

Could you direct me to where I can find data to answer the lab? Looking up "Pepper and methyl orange" is not getting me anywhere. I will keep looking.
Thanks a lot.

[Borek]

Have you visited the page I linked to? If you know pH, you can check the color there.

[da329]

OK, I wasn't sure if that was the pH of the indicator...sorry I'm not too great at chem! Thanks a ton for all your he lp!