The Raman spectrum of DCP powder (Figure 1)  exhibited characteristic peaks consistent with previously reported values [4]. Upon dissolution in 0.5 mol/L HCl, the solution displayed Raman peaks at 889 and 1189 cm⁻¹ corresponding to fully protonated phosphoric acid (H₃PO₄) and a peak at 1076 cm⁻¹ from the dihydrogen phosphate ion (H₂PO₄⁻). These features closely matched those of a reference H₃PO₄ solution, confirming the successful dissolution of DCP and the coexistence of multiple phosphate species under strongly acidic conditions.

After adding sulfuric acid, a distinct Raman peak corresponding to SO₄²⁻ appeared at 983 cm⁻¹. In aqueous solution, sulfuric acid dissociates in two steps (Equation 4), SO₄²⁻ being the predominant species under conditions of low pH and dilution (Figure 2).

Phosphate peak intensities changed after H₂SO₄ addition, indicating protonation of H₂PO₄⁻ to H₃PO₄ (Equation 5) and increased ionic strength. These results demonstrate Raman spectroscopy’s ability to detect phosphates and sulfates simultaneously, and its sensitivity to protonation state changes, consistent with prior findings.

Stepwise addition of 1.0 mol/L NaOH raised the pH, leading to visible precipitation. At pH 2.95, the 889 cm⁻¹ peak shifted to 879 cm⁻¹, the 1189 cm⁻¹ peak dropped to baseline, and the 1076 cm⁻¹ peak increased—confirming H₃PO₄ deprotonation (Figure 2). Concurrently, SO₄²⁻ intensity fell by 52%, well above the expected 16%, suggesting sulfate removal via precipitation (likely as gypsum, Equation 6), a known step in phosphorus fertilizer production to eliminate excess Ca²⁺ and SO₄²⁻ [4].

To verify this, the solid precipitate was isolated, washed, dried, and analyzed with Raman spectroscopy (Figure 3). The dried sample exhibited a dominant Raman peak at 1001 cm⁻¹, slightly downshifted from the 1008 cm⁻¹ peak typical of pure gypsum (Figure 4). This shift may reflect the co-precipitation of gypsum, brushite, and ardealite (Ca₂(HPO₄)(SO₄)·4H₂O) [5]. Ardealite exhibits a SO₄²⁻ peak near 1001 cm⁻¹. These mixed precipitates may be the result of elevated local pH (3.5–4) as NaOH makes contact with the reaction mix.

These results underscore the utility of Raman spectroscopy as a selective tool for monitoring reaction progress and product composition in real time. It has potential applications in both process optimization and quality assurance.