Sulfonated Styrene-Acrylic Acid Copolymers with pH-Indicator Property

In this study, a polymer pH-indicator was synthesized by sulfonation of poly(styrene-co-acrylic acid) with concentrated sulfuric acid. The synthesized polymer pH-indicator exhibited pH-responsive color changes in aqueous solutions, being purple in alkaline medium (pH < 4), whereas yellow in acidic medium (pH > 9). The sulfonation process extent and polymer color were affected by time and temperature of the process, as well as acrylic acid (AA) content. The addition of acrylic acid was a crucial point for color change due to the facility of cyclic ketone structure to turn into phenol under acidic condition. Compared with low molecule pH-indicators, the sulfonated copolymer was prepared in form of films being repeatedly used as pH-indicator and easily removed from the acid-base medium.