233 – By adding a few drops of indicator .................. to the solution around the electrode .................. in an electrochemical cell, during operation (with graphite electrodes and the electrolyte being a $1\,\text{mol/L}$ potassium iodide solution), the color of the solution around this electrode turns .................. .
$$K^+(\text{aq}) + e^- \to K(\text{s}) \qquad E^\circ = -2.92\,\text{V}$$ $$2H_2O(\text{l}) + 2e^- \to H_2(\text{g}) + 2OH^-(\text{aq}) \quad E^\circ = -0.83\,\text{V}$$ $$I_2(\text{s}) + 2e^- \to 2I^-(\text{aq}) \qquad E^\circ = +0.54\,\text{V}$$ $$O_2(\text{g}) + 4H^+(\text{aq}) + 4e^- \to 2H_2O(\text{l}) \quad E^\circ = +1.23\,\text{V}$$
- methyl red – cathode – yellow
- methyl red – anode – purple
- phenolphthalein – cathode – red
- phenolphthalein – anode – purple