Acetylcysteine EP impurity D is primarily used in the pharmaceutical industry as a reference standard for quality control and analytical purposes. It plays a critical role in ensuring the purity and consistency of acetylcysteine, a medication widely used for its mucolytic and antioxidant properties. By identifying and quantifying this impurity during the manufacturing process, pharmaceutical companies can maintain compliance with regulatory standards and ensure the safety and efficacy of acetylcysteine-based products. Additionally, it aids in research and development efforts to optimize production processes and minimize the presence of impurities in final drug formulations.