Hydroxynaphthol blue is widely used as a metallochromic indicator in complexometric titrations, particularly for the determination of calcium and magnesium ions in water hardness testing. It forms distinct color changes in the presence of these metal ions, making it a valuable tool in analytical chemistry. Additionally, it is employed in biochemical research for studying metal ion interactions with proteins and other biomolecules. Its ability to chelate metal ions also finds applications in industrial processes, such as in the treatment of wastewater to monitor and control metal ion concentrations.