Question

(a) which voltage,${\mathrm{V}}_1$or${\mathrm{V}}_2$in the diagram is constant in controlled-potential electrolysis? Which are the working, auxiliary, and reference electrodes in the diagram?

(b) Explain how the Luggin capillary in Figure 17-4 measures the electric potential at the opening of the capillary.

Short answer

• The voltage ${\mathit{V}}_{\mathbf{2}}$in the diagram is constant in controlled – potential electrolysis.

Step-by-step solution

Definition of Controlled – potential electrolysis:

• Bulk electrolysis is also known as potentiostatic coulometry or controlled potential coulometry.
• The experiment is a form of coulometry which generally employs a three electrode system controlled by a potentiostat.

Illustration of electrodes: 

• The electrodes are: Auxiliary, reference, working

 Measuring electric potential by using Luggin capillary: 

• The potential of the electrode we are testing is measured so that along the surface of this electrode at a distance of 2-3 mm put the top of the so-called Luggin's capillary that is over the vessel and / or of the corresponding electrolyte bridges connected to some reference electrode with which forms a separate cell.
• With the help of a known value of the reference electrode potential and of the measured cell voltage the electrode potential is calculated.
• In Figure 17-4, the potentiometer measures the voltage between the Cu electrode and a reference electrode immersed in 1.0 ${\mathit{V}}_{\mathbf{2}}$and connected to the bulk solution via a Luggin capillary, AgCl.