Thymol blue

Thymol blue is a reversible pH indicator that responds to the pH of the solution through structural changes of protonation and deprotonation. Thymol blue is red (HI^- form) under acidic conditions and blue (I^2- form) under alkaline conditions. Quantitative detection is achieved through the absorption peak shift (435 nm/596 nm) of the UV-visible spectrum. Thymol blue selectively responds to pH changes, changing from red to yellow at pH 1.2-2.8 and from yellow to blue at pH 8.0-9.6. Thymol blue can be fixed in a silica gel matrix through sol-gel technology to form a solid-state sensor for in-situ pH measurement in the marine environment and acid-base monitoring in the biomedical field[1][2]. In Vitro:Thymol blue acid-base indicator protocol (solution state)^[1]:
1. Solution preparation
Dissolve Thymol blue sodium salt in ethanol (or water/ethanol mixed solvent) to prepare a 0.1% (w/v) stock solution.
2. pH response test
Take the solution to be tested and add 1-2 drops of Thymol blue indicator:
Acidic conditions (pH<1.7) The solution turns red (HI^- form).
Neutral to weakly alkaline (1.7≤pH≤8.9: the solution is yellow (intermediate state.
Strongly alkaline (pH>8.9): the solution is blue (I^2- form).
3. Quantitative detection
Use a spectrophotometer to measure the absorbance at 435 nm (HI^-) and 596 nm (I^2-), calculate the ratio R=A₅₉₆/A₄₃₅, and calculate the pH value using the ionization formula.