Phenylhydrazine-4-sulfonic acid (PHPA) is primarily utilized in the field of analytical chemistry as a reagent for the detection and quantification of aldehydes and ketones. It forms colored derivatives with these carbonyl compounds, making it useful in spectrophotometric analysis. PHPA is also employed in the synthesis of various organic compounds, particularly in the preparation of heterocyclic structures, which are important in pharmaceutical and agrochemical industries. Additionally, it serves as a key intermediate in the production of dyes and pigments, contributing to the vibrant coloration in textiles and other materials. Its role in biochemical research includes the study of enzyme inhibition and metabolic pathways, providing insights into biological processes.