The Effects of Ultrasound on the Electro‐Oxidation of Sulfate Solutions at Low pH

Abstract The electro‐oxidation of sulfate solutions is a well‐established route for the generation of powerful oxidants such as persulfate. Despite this, the effects of simultaneous ultrasound irradiation during this process has attracted little attention. Herein, we investigate the effects of a low‐intensity ultrasonic field on the generation of solution‐phase oxidants during the electro‐oxidation of sulfate solutions. Our results show that at high current densities and high sulfate concentrations, ultrasound has little effect on the Faradaic and absolute yields of solution‐phase oxidants. However, at lower current densities and sulfate concentrations, the amount of these oxidants in solution appears to decrease under ultrasonic irradiation. A mechanism explaining these results is proposed (and validated), whereby anodically‐generated sulfate and hydroxyl radicals are more effectively transported into bulk solution (where they are quenched) during sonication, whereas in the absence of an ultrasonic field these radicals combine with one another to form more persistent species (such as persulfate) that can be detected by iodometry.


SI-3: Sonochemical methods S4
SI-4: Colorimetric tests S4 Figure S1: calibration with ferricyanide S6 Figure S2: cyclic voltammetry S7 . The effect of the observed temperature rise during sonication was also probed. This data is summarized in Table 1 in the main text (entries 1 and 2).

ARTICLE
Controlled current electrolysis at low current density (23 mA cm −2 , Table 1, entries 3 and 4) was carried out in a similar manner, except that the working electrode was now a boron doped diamond foil (surface area = 0.88 cm 2 ) and the electrolyte was 0.5 M ammonium sulfate solution in 1 M H2SO4.
All potentials are reported uncorrected for cell resistances, which were found to be around 10 Ω.
Controlled Current Electrolysis in the Presence of Naphthol Blue-Black: 25 mL of 0.5 M ammonium sulfate and 3.2 µM naphthol blue-black in 1 M H2SO4 was subjected to a current density of 23 mA cm −2 for 5 minutes. Temperatures were initially 18 °C in all cases. When samples were stirred, they were stirred with a magnetic stir bar at 500 rpm. Some of the solutions subjected to controlled current electrolysis were also sonicated at 37 kHz, whilst others were not as described in the main text.
Naphthol Blue-Black Oxidation using Ammonium Persulfate: To 25 mL of 0.5 M ammonium sulfate in 1 M H2SO4 was added 0.106 g (0.466 mmol) of ammonium persulfate. The ammonium persulfate fully dissolved. The solution was then made to a concentration of 3.2 µM in naphthol blueblack and left to stir at room temperature for 5 minutes.

SI-3: Sonochemical Methods:
A Fisher Scientific FB15050 ultrasonic bath (frequency = 37 kHz) was employed, always filled with 2.2 L of water. A 100 mL beaker was used as the reaction vessel, and this was always submerged to the same depth (1 cm) and clamped in exactly the same position in the bath for each experiment. The volume of solution submerged was 12.5 cm 3 and the total surface area of the beaker exposed to the bath was 25 cm 2 . Using this set-up, the consistent temperature rise during sonication of 25 mL pure water over 30 minutes (7 °C) could be used to gauge the acoustic power dissipated during sonication as 443 ± 83 mW.

SI-4: Total Oxidation Determination and Colorimetric Tests
To determine the total amount of oxidants made during electrolysis, iodometric titration was used To produce the calibration curve, known amounts of KMnO4 were added to the VOSO4 solution to produce known amounts of pervanadyl, and then the absorption at 360 nm was measured.

Determination of Hydrogen Peroxide
To determine the concentration of hydrogen peroxide present, the analyte solution was mixed with titanium oxysulfate (TiOSO4) (a 1 mole equivalent relative to the theoretical maximum amount of hydrogen peroxide that could be made based on the charge passed). The TiOSO4 solution was made by dissolving TiOSO4 in 2 M H2SO4 (aided by sonication). When hydrogen peroxide reacts with TiOSO4, titanic acid is formed, turning the solution yellow. Absorption at 407 nm was taken and compared with a calibration curve to determine the amount of hydrogen peroxide made. To produce the calibration curve, known amounts of H2O2 were added to the TiOSO4 solution and then absorption at 407 nm was measured.   For dosimetry, 25 mL of a 2 mM solution of terephthalic acid in 5 mM NaOH / 10 mM phosphate buffer was then sonicated for 15 minutes in our set-up, and the yield of 2-hydroxyterephthalic acid thus generated by reaction of terephthalate with hydroxyl radicals was measured by fluorimetry as shown below. Figure S4: The average (of three repeats) of the fluorescence evident using the dosimetry protocol above. This translates to a concentration of 0.27 ± 0.02 μM 2-hydroxyterephthalic acid, suggesting that at least 6 nmol of hydroxyl radicals are generated during 15 minutes of sonication under our standard conditions. Figure S5: Schematic of the cell set-up with a cellulose membrane preventing the probe molecule from accessing the working electrode directly. Figure S6: The average (of three repeats) of the absorbance of naphthol blue black when subjected to sonoelectrochemistry as specified in the main text in the absence (red line) and presence of a cellulose membrane (green line). The cellulose membrane prevents the dye from directly accessing the working electrode. Also shown for comparison is a dye-electrolyte solution that was not subjected to any sonoelectrochemical treatment. The sonoelectrochemically-treated samples both with and without a membrane return dye discoloration levels that are the same as each other within error.