Color changes in chemical reactions often serve as vital visual signals, guiding scientists through complex processes. Among the most fundamental of these signals is the pH indicator, a substance that dramatically alters its color at specific pH levels. Cresol Red (CAS 62625-29-0) is a prime example of such a compound, renowned for its distinct transitions that make it a staple in analytical chemistry laboratories worldwide. Understanding the chemical principles behind its function is key to appreciating its utility.

Cresol Red belongs to the sulfonphthalein class of dyes, which are characterized by their pH-sensitive chromophores. The color change observed in Cresol Red is a direct consequence of alterations in its molecular structure, specifically involving protonation and deprotonation of its phenolic groups. In highly acidic solutions (pH below 0.2), Cresol Red exists in a protonated form, exhibiting a red hue. As the pH increases, it undergoes a deprotonation, leading to a structural rearrangement that shifts its absorption spectrum, causing the color to change to yellow in the pH range of 1.0 to 1.8. This transition is crucial for pinpointing the endpoint in titrations involving strong acids.

A second significant color change occurs in the neutral to slightly alkaline range. Here, Cresol Red transitions from yellow at pH 7.2 to a vibrant red by pH 8.8. This second transition makes it particularly useful for titrations involving weak acids and strong bases, or weak bases and strong acids where the equivalence point falls within this pH range. The ability to purchase Cresol Red from reliable manufacturers like NINGBO INNO PHARMCHEM CO.,LTD. ensures that students and professionals have access to a consistent and accurate indicator.

The physical properties of Cresol Red, such as its form as a brown powder and its solubility in water and ethanol, simplify its preparation into standard indicator solutions. The price point of Cresol Red also makes it an economical choice for widespread use in educational institutions and research facilities. By understanding the underlying chemistry of how Cresol Red functions, scientists can confidently employ it to achieve precise measurements and further their understanding of chemical reactions and equilibria. The continuous demand for Cresol Red underscores its irreplaceable role in analytical chemistry.