Methylthymol blue sodium is widely used as a complexometric indicator in analytical chemistry, particularly in titrations involving metal ions. It is especially effective in the determination of calcium, magnesium, and other alkaline earth metals in water hardness testing. The compound forms distinct color changes when it complexes with these metals, making it a reliable tool for endpoint detection in EDTA titrations. Additionally, it is employed in biochemical research for studying metal ion interactions in biological systems due to its sensitivity and specificity. Its applications also extend to environmental monitoring, where it helps in assessing metal contamination in various samples.