Eriochrome Black T indicator solution is widely used in complexometric titrations, particularly for the determination of water hardness. It acts as a metal ion indicator, showing a distinct color change when it binds to metal ions such as calcium and magnesium. During titration with EDTA, the solution shifts from wine red to blue at the endpoint, signaling the completion of the reaction. This makes it essential in water quality testing, both in laboratory and field analyses. It is also used in industrial processes where precise control of metal ion concentration is required, such as in boiler water treatment, cooling systems, and in the pharmaceutical and food industries to monitor metal content.