Quinaldine red (pronounced /ˈkwɪnəldiːn/, abbreviated QR) is a dark green–red or black solid that does not dissolve easily in water (it is partly miscible). In addition to being used as colored indicator, quinaldine red is also used as a fluorescence probe and an agent in bleaching.

Application

Phenolphthalein is often used as an indicator in acid–base titrations. For this application, it turns colorless in acidic solutions and pink in basic solutions. ... Phenolphthalein is slightly soluble in water and usually is dissolved in alcohols for use in experiments.