This study introduces a new, highly sensitive, and reliable method for detecting and measuring orthophosphate in environmental water samples. This method combines cetyltrimethylammonium bromide (CTAB)-mediated coacervation extraction with digital image-based colorimetry, providing a robust and efficient approach for orthophosphate analysis. In this system, CTAB, a cationic surfactant, serves a dual role as both an ion-pairing agent and an extraction medium. Tetrahydrofuran (THF) plays a critical dual function as well, acting as a dispersive solvent to facilitate the homogeneous distribution of CTAB in the aqueous phase and as a coacervation-inducing agent to promote surfactant self-assembly. The addition of CTAB and THF to the sample creates a cloudy solution, enabling the extraction of the β-phosphomolybdenum blue complex, formed by the reaction of orthophosphate with ammonium molybdate and ascorbic acid. The surfactant-rich phase is isolated via centrifugation, dissolved in dimethylformamide (DMF) and nitric acid, and analyzed using a smartphone-based colorimeter and UV-Vis spectrophotometer. The method achieves a linear detection range of 25–700 μg L⁻¹ (LOD: 9.8 μg L⁻¹) with colorimetry and dual ranges of 10–150 μg L⁻¹ and 150–700 μg L⁻¹ (LOD: 3.9 μg L⁻¹) with spectrophotometry. Recovery rates of 90–110% in various aqueous samples confirm its reliability for environmental monitoring. The proposed technique was successfully applied to the determination of orthophosphate in various aqueous samples, yielding recovery rates between 90% and 110%, demonstrating its reliability and effectiveness for environmental monitoring.

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