2.4.3. Measurement of redox potential

The RP of SELF was measured following the 150 min incubation time. The measurement was performed using a platinum oxidation–reduction potential sensor with ARGENTHAL (Ag/AgCl) reference system (InLab Redox Micro, Mettler Toledo, Giessen, Germany), connected to a SevenCompact pH meter (Mettler Toledo). The details related to handling procedures and use of redox sensors with aqueous solutions, which we followed in this work, have been previously described (Nordstrom and Wilde, 2005, Copeland and Lytle, 2014, Striggow, B.: Field measurement of oxidation-reduction potential (ORP), U.S. Environmental Protection Agency, Athens, GA., 2017). Prior to the sample measurements, the electrode performance was verified using a redox buffer (220 mV, pH 7; Mettler Toledo) and values of 220 ± 10 mV were considered acceptable. Prior to each measurement, the outside wall of the glass sensor was rinsed with copious amount of LC-MS grade water. 450 μL of SELF (pH 7.4) was transferred to a 750 μL plastic vial, immediately following the incubation of samples. The redox sensor was placed inside the vial, making sure that the tip of the sensor with the platinum ring was completely submerged without touching the bottom or the inside wall of the vial. The measurement (n = 3) was carried out after 30 sec under standard stabilization criteria at room temperature (21 °C). The reading typically took up to 2 min to stabilize. This depended on the magnitude of redox potential – i.e. the reading for samples with relatively high potential stabilized within seconds.