Simultaneous determination of phosphite and phosphate in fertilizers by Raman spectroscopy

Raman spectroscopy is emerging and powerful technique for identifying oxyanions of phosphorus, but it still is not straightforward applied to identify and quantify simultaneously phosphite and phosphate species in solution. Herein, simultaneous determination of phosphate (PO₄) and phosphite (PO₃) in fertilizer were evaluated by Raman spectroscopy. The influence of pH on Raman spectra of species was evaluated at various solution pH values, and the results showed accurate and selective analysis for phosphate and phosphite by using pH = 10.0 and bands located at 874 cm^-1 and 2321 cm^-1, respectively. Linear working range in the 0.15%-6.20% (w/v) P concentration was consistently obtained with regression coefficients (r) of 0.9953 (PO₄) and 0.9945 (PO₃). The limits of detection were 0.10% (w/v) P (PO₄) and 0.05% (w/v) P (PO₃). Relative standard deviations were lower than 6% for samples containing 10% P₂O₅ (PO₄) and 31% P₂O₅ (PO₃). Commercial fertilizers were analyzed by the proposed method using external calibration and found concentrations of P were in agreement with those obtained by the comparative spectrophotometric method at the 95% confidence level (paired t-test). Recoveries of spiked samples were in the 92-110% range.