A classic demonstration (From the RSC) to show an indicator at work.
A long tube such as a burette is filled with acid, alkali and universal indicator.
Inverting the tube creates a pH gradient along the length of the tune which show up as different colours thanks to the universal indicator.
We also have a different (foolproof) version that can be carried out in any container that uses concentrated sodium carbonate solution to make a density/concentration gradient.
Rainbow reaction – SSERC Concentration gradient version
Rainbow Reaction – Risk Assessment