Hydroxy naphthol blue disodium salt is widely used as a metallochromic indicator in complexometric titrations, particularly for the determination of calcium and magnesium ions in water hardness analysis. It forms distinct color changes in the presence of these metal ions, transitioning from blue to red, which helps in accurately identifying the endpoint of the titration. Additionally, it is employed in biochemical research for studying metal ion interactions and in analytical chemistry for detecting trace metals in various samples. Its stability and sensitivity make it a reliable choice for precise quantitative analysis in laboratories.