What causes a solution to change colors in different pH buffers?

Question

(for example a cyanin solution)

Answer 1

If you're referring to an indicator, they're usually weak acids or bases whose molecular structure changes subtly when ionized. This in turn changes the range of visible light energies they re-emit, giving them a different color.

When phenolphthalein is placed in a solution with a pH above about 9, for example, it loses two hydrogen ions, causing a shift in its structure. It turns from colorless to pink.

I hope that helps. Good luck!

Answer 2

You have an indicator, like methyl orange or phenolphthalein. Suppose the indicator is phenolphthalein. Let's call phenolphthalien HP. HP is colorless. P(-) is deep red. So HP + OH- ===> P- + H2O. The solution turns deep red.