A study of reaction between quinones and thiourea: determination of thiourea in orange juice

The study of the reaction between several quinones and thiourea and the determination of a method for determination of thiourea in orange juice were done. After reaction with thiourea (pH 3.0), the UV-Vis spectra showed new bands only for p-benzoquinone (PBQ) (290 nm, 410 nm), 2,5-dichloro-p-benzoquinone (2,5-DC-PBQ) (330 nm, 420 nm) and tetrachloro-p-benzoquinone (TC-PBQ) (255 nm, 320 nm). The results of FT-IR of the reaction of PBQ and 2,5-DC-PBQ/thiourea showed a substitution of the sulfhydryl group on the ring of quinone. The best concentrations of 2,5-DC-PBQ and PBQ, for determination of thiourea, were 5.0 and 10.0 mm, respectively. The products of reaction 2,5-DC-PBQ and PBQ/thiourea were stable in the period of 2 h and 20–120 min after the heating, respectively. The assay of thiourea using 2,5-DC-PBQ was susceptible to interference by iron (III), copper (II) and ascorbic acid. The detection limit for thiourea with PBQ was 0.90 μg/mL. Using filtration method, the recoveries of thiourea with PBQ from the samples were 100%.