Methyl red

Methyl red is a pH-sensitive acid-base indicator and colorimetric reagent with a pKa of 5.1. Methyl red achieves visual judgment of the titration endpoint through reversible structural changes of protonation (red, pH <4.4) and deprotonation (yellow, pH >6.2). Methyl red is widely used in acid-base titration in chemical analysis and buffer pH monitoring in biochemistry^[1][2]. In Vitro:Methyl red is used as an acid-base indicator in titration experiments. When the pH changes from 4.4 to 6.2, the color of the solution changes from red to yellow, which can be used to determine the titration endpoint^[1].
Methyl red can be dissolved in ethanol to make a 0.1% solution. A few drops are added to the solution to be tested. During the titration process, the color change is observed to determine the pH endpoint^[1].