Used as a complexometric indicator in EDTA titrations for calcium and certain other metal ions, particularly in the determination of calcium content in water hardness testing. In ammoniacal solution, it changes color from purple-red (metal-indicator complex) to violet (free indicator) at the endpoint, indicating complete chelation of the metal ions by EDTA. The addition of solid potassium sulfate prevents premature decomposition and stabilizes the indicator during storage and use. Commonly applied in environmental labs, water quality control, and industrial analysis for precise detection of calcium and transition metals.