What is happening to the [OH-] when the solution turns from pink to violet?

the hydroxide ions are undergoing ionization and the particles are gaining / losing electrons .

Explanation:

i think it is because ::

the phenolphthalein of the solution has gone from having a pH of around 8 (pink) to being alkaline at above 10 which causes it to burn purple / violet .

hydroxyl -oh

alcohols, always polar, can form hydrogen bonds with h2o molecules

carboxyl -cooh  

cargoxylic acids, has acidic properties which is a source of hydrogen ions, found in cells in ionized for with a charge of oh-1

carbonyl > co

ketone if within skeleton, aldehyde if group is at the end of skeleton, k & a may be structural isomers, it determines 2 sugars (ketose & aldose)

amino -nh2

amines, acts as a base, it can pick up h+ ions from surrounding solution, ionized with charge of 1+

methyl ch2

addition to rna/dna affects and gene expression

phosphate -opo3 ²⁻

organic phosphates, contributes to negative charge to, molecule that it is a part of, and potential to react with h20

sulfhydryl -sh

thiols, cross-linking stabilize the protein, and it is structure

your spelling is a little off : ]