1. Acid-Base Indicator (Most important and classic application)

• Principle: Colorless in acidic or neutral solutions, turns purplish-red in alkaline solutions.

• Specific Applications

• Chemical Experiment Teaching and Analysis: Used in acid-base titration experiments to visually indicate the reaction endpoint (commonly used color change range: pH 8.2-10.0).

• Chemical Production Process Control: For rapid detection of the acidity or alkalinity of liquid products.

• Daily Science Education and Testing: Used to make “invisible ink” or test the acidity or alkalinity of substances (e.g., soil, household liquids, etc.).

2. Pharmaceutical Field (Historically widely used, now strictly prohibited)

• Past Use: Formerly used as a main ingredient in stimulant laxatives to treat constipation.

• Current Status: Due to serious side effects such as potential carcinogenicity, electrolyte imbalance, and intestinal dysfunction from long-term use, many countries including China, the United States, and the European Union have completely banned its use as a pharmaceutical raw material.

3. Industrial and Special Uses

• Cement Accelerating Admixture Component: Used as a component in some cement additives, utilizing its chemical properties.

• Fluorescent Dye Precursor: Its molecular structure can be used to synthesize certain fluorescent dyes.

• Research Reagent: Used as a synthetic intermediate or as a subject of characteristic research in chemical research.