"Besides the traditional colorimetric acid/base indicators, there is another class that doesn't use color, the olfactory acid-base indicators! These compounds smell or don't smell based on their pH and a lot of them are plant-derived compounds. For example, vanillin (C8H803), the active flavoring in vanilla flavoring or vanilla extract, is an olfactory acid base indicator. It loses it's characteristic smell when it is basic. The ka for vanillin is 1.79x10^-8.

You want to make a vanilla cake and give it that extra vanilla-ly flavor, you add exactly 1.00 g of pure vanillin (acid form, C3H7O3-) to 500.0 mL of cake batter. But, your roommate, trying to help, adds too much of the rising agent NaHCO3. He adds 5.0 grams. Will you be able to smell the vanillin in your cake? To anser this question, you need to determine how much of the protonated form of vanillin will be in cake batter. IF it is over 90%, then you will not be able to smell it!"

(C3H7O3)- + (HCO3)- = C3H8O3 + (CO3)2- The equal sign stands for the equilibrium arrows.