Hydrochloric acid, c = 1 mol/L - WARNING
Potassium bromide - WARNING
Potassium nitrate - WARNING
A photogalvanic two-pot cell is assembled from a flat cuvette, a beaker and a salt bridge. In the flat cell there is potassium bromide solution, c = 1 mol/L, and a photoelectrode (see note), in the beaker hydrochloric acid, c = 1 mol/L, and a platinum electrode. A filter paper strip soaked with a saturated potassium nitrate solution can be used as a simple salt bridge between the two half cells. The photoelectrode is connected to the negative pole of a digital voltmeter, the platinum electrode to the positive pole. The photoelectrode can be irradiated with a 300 Watt Ultravitalux lamp from a distance of approx. 15 cm at 5 intervals (approx. 20 s light each, approx. 20 s darkened) or LED light sources of different wavelengths, and the voltage U and current intensity I are measured.
- When irradiated with the Ultravitalux lamp, voltages of up to U = 450 mV are measured, but only very low currents I in the microampere range. When darkening the photoelectrode with e.g. cardboard, the voltage collapses but does not drop to zero.
- When irradiated with an LED light source, the resulting voltage depends on the primary wavelength of the LED lamp. A wavelength of λ = 365 nm gives comparable results to the ultravitalux lamp. Longer wavelength result in lower voltages, with wavelength of the green spectra (λ ≈ 530 nm) or longer resulting in no voltage at all.
To investigate the chemical processes in the photogalvanic two-pot cell, the two electrodes in the outer circuit are short-circuited using a cable and the photoelectrode is continuously irradiated for about 20 minutes.
- In the half cell with the photoelectrode, elemental bromine can then be detected using potassium iodide/starch solution (blue coloration). At the same time, elemental hydrogen has formed on the platinum electrode. It can sometimes be recognized as tiny bubbles. However, it can always be indirectly identified using a half cell made of copper and copper sulphate solution. If the half cell with the hydrochloric acid and the platinum electrode with the half cell of copper and copper sulfate solution are combined to form a galvanic cell with the platinum electrode as minus pole, a value is measured that is close to U = 0.35 V, i.e. the standard potential of the redox pair Cu(s)/Cu2+(aq). This means that the platinum electrode from a photogalvanic cell has become a standard hydrogen half cell with the redox pair H2(g)/2H+ after 20 minutes of irradiation.
ITO or FTO glass plates coated with titanium dioxide (anatase) are used as photoelectrodes in this and other experiments with photogalvanic and photoelectrochemical cells. Details on the preparation of photoelectrodes can be found on the Webpages of the Wuppertaler Chemiedidaktik.