Effect of Selected Organic Solvents on Hydroxyl Radical-Dependent Light Emission in the Fe²⁺-EGTA-H₂O₂ System

Numerous compounds that are scavengers of hydroxyl radicals (•OH) in Fenton systems have low solubility in water. Therefore, they are dissolved in organic solvents to reach suitable concentrations in the reaction milieu of the Fenton system. However, these solvents may react with •OH and iron, leading to significant errors in the results. We evaluated 11 solvents (4 alcohols, acetone, 4 esters, dimethyl-sulfoxide, and acetonitrile) at concentrations ranging from 0.105 µmol/L to 0.42 µmol/L to assess their effects on light emission, a recognized measure of •OH radical activity, in the Fe²⁺-EGTA-H₂O₂ system. Six solvents inhibited and four solvents enhanced light emission at all tested concentrations. Acetonitrile, which initially suppressed light emission, lost this effect at a concentration of 0.105 µmol/L, (-1 ± 13 (2; 0) %, p > 0.05). Methanol, at the lowest tested concentration, inhibited light emission by 62 ± 4% (p < 0.05), while butyl butyrate enhanced it by 93 ± 16% (p < 0.05). These effects may be explained by solvent-driven •OH-scavenging, inhibition or acceleration of Fe2+ regeneration, or photon emission from excited solvent molecules. Our findings suggest that acetonitrile seems suitable for preparing stock solutions to evaluate antioxidant activity in the Fe²⁺-EGTA-H₂O₂ system, provided that the final concentration of this solvent in the reaction milieu is kept below 0.105 µmol/L.