Chlorophosphonazo Ⅲ is widely used as a sensitive and selective chromogenic reagent in analytical chemistry. It is particularly effective in the spectrophotometric determination of rare earth elements, such as lanthanum, cerium, and yttrium, due to its ability to form stable, colored complexes with these metals. This property makes it valuable in environmental monitoring, where it helps detect trace amounts of rare earth elements in water and soil samples. Additionally, it is employed in metallurgical industries for quality control and analysis of metal alloys. Its high sensitivity and specificity also make it useful in biochemical research for studying metal ion interactions in biological systems.