Eriochrome® Black T is widely used as a metallochromic indicator in complexometric titrations, particularly for determining the concentration of metal ions such as calcium, magnesium, and other divalent cations in water hardness analysis. It forms colored complexes with these metal ions, typically producing a wine-red color, which transitions to blue when the metal ions are chelated by EDTA, signaling the titration endpoint. This makes it a crucial tool in analytical chemistry for water quality assessment and industrial processes where monitoring metal ion concentrations is essential. Additionally, it is employed in educational laboratories to teach principles of complexometric titrations due to its clear and distinct color change.