# The Effects of Acidic, Alkaline, and Neutral Anolytes on Electrochemical Seawater Deoxygenation
## 2 Materials and Methods  
The tests were performed in a two-compartment electrochemical cell. Two cells, Cell A and Cell B, were constructed as shown in Figure 1. Both cells were flat plate rectangular structures made of polycarbonate. Cell A was a closed cell with a possibility for measuring oxygen concentration at the catholyte outlet, whereas Cell B was an open cell with the facility of reversible hydrogen electrode (RHE). Titanium plated with iridium oxide and silver-plated Monel mesh were used as the anode and cathode, respectively. A cation exchange membrane (CEM) (Ionac MC-3470) was used between the anode and the cathode compartments. Figure 1: (a) Side view of Cell A and Cell B; (b) front view of Cell A; (c) front view of Cell B. Sodium chloride solution (0.5 M) was used as the catholyte in all experiments. Three different solutions were used as analytes for both cells: 0.1 M nitric acid as an acidic solution, 0.1 M sodium nitrate as a neutral solution, or 0.1 M sodium hydroxide as an alkaline solution. The acidic, basic, and neutral buffer solutions with compositions given in Table 1 were tested as analytes in Cell B. All solutions were prepared in MilliQ water. The level of oxygen removal from water from each electrolyte was measured in Cell A. Figure 2 shows the experimental set-up for the test. The peristaltic pump (Watson Marlow 520S) was used for pumping the anolyte and catholyte. The power supply (Manson HCS 3300) was used for providing the current to the cell. The initial and final oxygen concentrations in the electrolytes were measured in Cell A by an optical oxygen sensor (Visiferm DO 120, Hamilton, accuracy 1 \(\pm\) 0.05 vol%). The conductivity of the electrolytes was measured by a conductivity meter (WTW 350i). The hydrogen peroxide concentration was measured in the catholyte at the outlet using a hydrogen peroxide test kit (CHEMets(r) Visual Kit, CHEMetrices, Washington, D.C., US) containing ferric thiocyanate. The ranges of measurement were 0-0.8 ppm and 1-10 ppm. The analysis time was <1 min. The catholyte and anolyte were pumped through the cell at constant flow rates of 10 mL-min\({}^{-1}\). For each test, a new membrane, anode, and cathode were installed. The theoretical required current was calculated for the deoxygenation of 0.5 M NaCl solution passing through the cell at a flow rate of 10 mL-min\({}^{-1}\). The output current for the deoxygenation of water was measured and compared with the calculated theoretical current. The current difference was used for calculation of the amount of hydrogen gas evolved. The oxygen concentration, pH, and conductivity were continuously monitored at the outlet, and the applied voltage with resulting current were recorded. The experiments were performed minimum 2-3 times to ensure their reproducibility. After each test, the cell was dismantled, and the anode, cathode, and CEM were stored for scanning electron microscope (SEM) analyses. Cell B rigged with an RHE, was used for measuring the voltage distribution at the anode and the cathode. Voltages in the range of 1.9 V to 2.3 V between the electrodes in the cell and the corresponding \begin{table}
\begin{tabular}{l l l} \hline \hline  & **Buffer Solutions** & **pH** \\ \hline Acidic: 10 mM H\({}_{3}\)PO\({}_{4}\)/H\({}_{2}\)PO\({}_{4}\)\({}^{-}\) buffer & 0.78 g NaH\({}_{2}\)PO\({}_{4}\) (2H\({}_{2}\)O (5 mmol) & \\ Acidic: 10 mM H\({}_{3}\)PO\({}_{4}\)/H\({}_{2}\)PO\({}_{4}\)\({}^{-}\) buffer & 0.34 mL H\({}_{3}\)PO\({}_{4}\) (85\%, 14.7 M) (5 mmol) & 2.6 \\  & Diluted with water to 1 L & \\  & 500 mL 0.1 M KH\({}_{2}\)PO\({}_{4}\)\({}^{-}\)2H\({}_{2}\)O & \\ Neutral: 100 mM H\({}_{2}\)PO\({}_{4}\)\({}^{-}\)/HPO\({}_{4}\)\({}^{2-}\) buffer & 291 mL 0.1 M NaOH & 7.0 \\  & Diluted with water to 1 L & \\  & 6.18 g H\({}_{3}\)BO\({}_{3}\) (100 mmol) & \\ Alkaline: 100 mM H\({}_{3}\)BO\({}_{3}\)/H\({}_{2}\)BO\({}_{3}\)\({}^{-}\) buffer & 2.81 g KOH (50 mmol) & 9.1 \\  & Diluted with water to 1 L & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Buffer solutions used as analytes.