| Summary: | Sodium percarbonate (SPC) concentration can be determined spectrophotometrically by using N, N-diethyl-p-phenylenediamine (DPD) as an indicator for the first time. The ultraviolet–visible spectrophotometry absorbance of DPD<sup>•+</sup> measured at 551 nm was used to indicate SPC concentration. The method had good linearity (R<sup>2</sup> = 0.9995) under the optimized experimental conditions (pH value = 3.50, DPD = 4 mM, Fe<sup>2+</sup> = 0.5 mM, and t = 4 min) when the concentration of SPC was in the range of 0–50 μM. The blank spiked recovery of SPC was 95–105%. The detection limit and quantitative limit were 0.7–1.0 μM and 2.5–3.3 μM, respectively. The absorbance values of DPD<sup>•+</sup> remained stable within 4–20 min. The method was tolerant to natural water matrix and low concentration of hydroxylamine (<0.8 mM). The reaction stoichiometric efficiency of SPC-based advanced oxidation processes in the degradation of ibuprofen was assessed by the utilization rate of SPC. The DPD and the wastewater from the reaction were non-toxic to <i>Escherichia coli</i>. Therefore, the novel Fe<sup>2+</sup>/SPC-DPD spectrophotometry proposed in this work can be used for accurate and safe measurement of SPC in water.
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