A novel Orange IV-based spectrophotometric method for uranium (VI) determination in phosphate rock and phosphoric acid

An innovative spectrophotometric method was developed for the sensitive and selective determination of uranium (VI) using Orange IV dye as a chromogenic complexing agent. The maximum absorption was observed at λmax 459 nm. The U (VI)-Orange IV complex forms instantaneously at pH 3.3 in an acetate-buffered medium (0.5 mL of 10−2 M H2SO4), exhibiting maximum absorbance at 459 nm. The complex is stable for 2 hours. The method demonstrates excellent analytical performance: a linear range of 0.05–0.5 μg mL−1 (R2 = 0.9993), high molar absorptivity (ε = 2.37 × 105 L mol−1 cm−1), and low calculated detection and quantification limits (LOD = 0.0165 μg mL−1 and LOQ = 0.05 μg mL−1, respectively). Both FTIR and XPS analyses confirmed the formation of the complex and supported a 1:2 (U:dye) stoichiometry, indicating that the reactive groups of the dye interact with U(VI) ions through a proposed complexation mechanism. The tolerance limits for common interfering ions were established. The method was successfully applied to determine U(VI) in complex real-world samples, including wet-process phosphoric acid and phosphate rock (e.g. Abu-Tartur ore), yielding precise results with a relative standard deviation (RSD) of less than 0.8% for accuracy and precision.