sulfonephthalein

Sulfonephthalein is a class of pH indicators derived from phthalein dyes that contain sulfonic acid groups. The sulfonate substituents confer water solubility, enabling use in aqueous solutions without organic solvents. Like other phthaleins, sulfonephthaleins exhibit pH-dependent structural changes between a colorless or pale lactone (closed) form and a colored open form. The balance between these forms shifts with proton activity, producing a characteristic color change that can indicate pH or mark the endpoint of titrations. The exact color and the pH range over which the transition occurs depend on the specific compound.

Common examples of sulfonephthalein indicators include phenol red, cresol red, cresolphthalein, and cresol purple. These dyes are widely used in laboratory analysis and clinical chemistry due to their water solubility and predictable color transitions in moderately wide pH ranges. They are frequently employed in aqueous acid–base titrations, buffered pH measurements, and as components of indicator systems in diagnostic tests.

In practice, the choice of a sulfonephthalein indicator depends on the desired transition range and the medium being tested. Because many of these dyes operate across near-neutral to basic pH, they are common in routine analytical procedures and in educational demonstrations of acid–base chemistry. Handling follows standard laboratory safety practices for chemical indicators.