N,N-Diethyl-p-phenylenediamine oxalate salt (1:1), commonly known as DPD oxalate, is a key reagent in analytical chemistry, particularly for water quality testing. It serves as an indicator in the DPD colorimetric method to measure free and total chlorine levels in water, enabling accurate assessment of disinfection efficacy in drinking water, swimming pools, and wastewater. The compound produces a distinctive red color upon reaction with chlorine in the presence of a buffer, allowing for both visual and photometric determination. Additionally, it finds applications in environmental monitoring and some industrial processes, such as in the formulation of dyes for textiles, where it contributes to vibrant and durable coloration.