Thymolphthalein complexon is primarily used as a metallochromic indicator in complexometric titrations, particularly for the determination of calcium and magnesium ions in water hardness analysis. It changes color in the presence of these metal ions, providing a clear visual endpoint during titration. This makes it a valuable tool in environmental testing and quality control processes for water treatment facilities. Additionally, it is employed in analytical chemistry for studying metal ion interactions and in research applications where precise metal ion detection is required. Its stability and sensitivity to specific metal ions make it a reliable choice for such applications.