A simple and cost-effective distance-based paper analytical device for phosphate ion determination.

Phosphate ion (PO₄^3-) monitoring in water sources is crucial due to its significant role in living systems and environmental impact, particularly in eutrophication. Conventional phosphate detection methods require sophisticated instruments and trained personnel. We report the development of a simple and cost-effective distance-based paper analytical device for phosphate ion detection. The device utilizes a complex of ferric ion (Fe³⁺), salicylic acid (SA), and polyethyleneimine (PEI) as the colorimetric indicator. Upon reaction with phosphate ions, Fe³⁺ forms a complex with phosphate, replacing the Fe³⁺-SA complex, leading to a visible color change from orange to colorless. The phosphate concentration is quantified by the distance of color change, which can be observed with the naked eye. Under the optimal conditions, the linearity was found in the range 5 to 75 mg/L (R² = 0.9954), and the naked-eye detection limit was found at 2 mg/L. Furthermore, by applying a preconcentration method, the detection limit can be improved tenfold, achieving an LOD of 0.2 mg/L. Our proposed device for water sample application was also validated with the standard molybdenum blue method. A paired t-test was performed to compare the results obtained from both methods, and the statistical analysis indicated no significant difference between the two methods (p > 0.05). The method validation studies also showed acceptable performance, with spiked sample recoveries ranging from 85.05% to 117.97%, within the internationally recognized acceptable criteria of 80-120%. This method offers an alternative approach for accurate, rapid, on-site phosphate monitoring.