Sulfate radical formation by Cr(III) activation of peroxydisulfate – Diphenylcarbazide spectrophotometric determination of sulfate radical and its scavenging activity

Abstract

Even though sulfate anion radical (SO4−) is a very reactive oxidant used in advanced oxidation processes, a reliably selective and simple colorimetric method for determining this radical can hardly be found. Peroxydisulfate (S2O82−) or peroxymonosulfate (HSO5−) can be activated with transition metal ions to produce SO4−. We have discovered that Cr(III) can be an activator for persulfate, generating Cr(VI) along with SO4−. By measuring the emerging chromate with diphenyl carbazide (DPC) spectrophotometry at 542 nm, we could measure both the formation of SO4− and its scavenging with antioxidant compounds. We could also investigate a number of UV-absorbing SO4− scavengers which could not be measured with other UV spectrometric methods. In addition to conventional antioxidants (phenolics such as quercetin, catechin, epicatechin, caffeic acid, thiols like cysteine and N-acetyl cysteine, and ascorbid acid), nitro-aromatics (represented by 2,4,6-trinitrophenol and 2,4-dinitrophenol) used in ammunition formulations could also be measured as scavengers. The presence of scavengers caused a reduction in the amount of Cr(VI) generated, where the difference in absorbance (ΔA) of chromate – with respect to the DPC method – in the absence and presence of scavengers was a linear function of SO4− scavenging capacity. Ethanol and tert-butanol were tested as solvents to show the selectivity of the method for SO4−. The method was statistically compared to a suitably modified ABTS/persulfate assay. The efficiency order of sulfate radical scavengers was determined and ranked (Spearman’s test) using both the proposed method and modified ABTS/persulfate method to reveal a moderate correlation.