The absorption spectra of OTA solutions were acquired in the range 250–450 nm with a Jasco V-550 UV-Vis double-beam spectrophotometer, provided with Hellma quartz cuvettes for flow-through measurement (10.00 mm optical path length). The pH of the solutions was evaluated by a Metrohm potentiometer (model 713, resolution of 0.001 pH units) equipped with a Metrohm combined glass electrode (mod. 6.0259.100). The combined electrode was daily calibrated in terms of H+ concentration. The slope and the formal potential E0 of the Nernst equation were elaborated by the ESAB2M program.33 The experiments were conducted in oxygen-free atmosphere by flowing nitrogen in the solution, in continuous stirring, at 25 °C (thermocryostat mod. D1-G Haake). Initially, the OTA solution (20 mL) was adjusted to pH 3 with HCl. Subsequent aliquots of NaOH solution (10 μL) were added by a Metrohm 765 Dosimat burette (resolution 1 μL) until the OTA solution reached a pH value of about 7.5. Each addition corresponded to an increase of 0.1/0.2 pH units.
A total of 45 UV-Vis spectra were elaborated with HypSpec® software34 (link) that operates with stoichiometric principles by applying mass and charge balance equations and Lambert–Beer's law.
A total of 45 UV-Vis spectra were elaborated with HypSpec® software34 (link) that operates with stoichiometric principles by applying mass and charge balance equations and Lambert–Beer's law.