Quinaldine Red is a colorimetric phosphate detection reagent which is known to be a cationic fluorescent probe for proteins. It is also a pH indicator with a transition interval from 1.4 - 3.2 (colorless - rose).A cationic fluorescent probe. Quinaldine Red is a colorimetric phosphate detection reagent which is known to be a cationic fluorescent probe for proteins. It is also a pH indicator with a transition interval from 1.4 - 3.2 (colorless - rose).