Chlorophenol red pH indicator solution

Chlorophenol red pH indicator solution is an aqueous or alcoholic preparation of chlorophenol red (C₁₉H₁₂Cl₂O₅S), a sulfonphthalein dye used for pH detection in the range of 4.8-6.7, changing from yellow (acidic) to violet or red-purple (basic).​

The solid dye appears as a dark brown to reddish-brown powder with λ_max at 572 nm, pKa around 6.0, melting point of 261°C, and sparing solubility in water but good solubility in ethanol (e.g., 10 mg/mL in 95% ethanol). In solution, it dissociates via loss of a proton from the phenolic OH, shifting the chromophore structure and color due to resonance stabilization.​

Typically prepared as 0.04-0.1% (w/v) in 50% ethanol or water-ethanol mixes for lab use in titrations, chlorine dioxide detection (0.1-1.9 mg/L at pH 7), enzyme assays, and microbial studies; handle as irritant with eye/skin protection.​