Novel and practical spectrophotometric method for urea determination using p-dimethylaminobenzaldehyde reagent system augmented with chromotropic acid

The reliable determination of urea is essential for quality control, diagnostics, and process monitoring in clinical, industrial, and environmental applications. Conventional spectrophotometric methods based on p-dimethylaminobenzaldehyde (PDAB) often suffer from poor reagent stability and limited reproducibility, compromising analytical reliability. This study describes a practical spectrophotometric procedure employing a novel dual-reagent system composed of PDAB and the disodium salt of chromotropic acid. Both reagents were prepared using defined volumes of concentrated sulfuric acid and mixed in equal volumes to yield a stable color-forming solution. The reagent composition was optimized through Central Composite Design within a Response Surface Methodology framework to ensure maximum absorbance and chromogen stability. Method validation adhered to Part 1040 of the Standard Methods for the Examination of Water and Wastewater, confirming strong linearity up to 33.30 mM (R² > 0.99), with detection and quantitation limits of 0.34 and 0.83 mM, respectively. Recovery rates at 0.83, 16.65, and 33.30 mM ranged from 90 % to 110 %, while precision, expressed as inter-laboratory relative standard deviation values, did not exceed 6 %. The chromogen formed rapidly and exhibited prolonged stability under the optimized conditions. Interference from ammonia, ammonium salts, amines, nitrate, formaldehyde, thiosemicarbazide, and hydrazine was examined, with the latter effectively accounted for using a standard procedure. This method overcomes the long-standing limitations of conventional PDAB assays, simplifying reagent preparation, reducing analysis time, and ensuring reliable spectrophotometric quantification of urea in diverse matrices.