Eriochrome Black T is widely used as an indicator in complexometric titrations, particularly for determining the concentration of metal ions such as calcium, magnesium, and other divalent cations in water hardness testing. It forms a distinct color change from blue to red when it binds to metal ions, making it a reliable tool for endpoint detection in EDTA titrations. Additionally, it is employed in analytical chemistry for studying metal ion interactions and in qualitative analysis to identify the presence of specific metals in solutions. Its application extends to industrial water treatment processes, where it helps monitor and control water quality by measuring the levels of hardness-causing ions.