Instrumentation for Electrogravimetry and Coulometry

The basic apparatus required is a power supply (potentiostat with a DC output voltage), an inert cathode and anode (usually platinum foil, gauze, or mesh), and arrangements for stirring. Sometimes a heater is used to facilitate the processes. 

Electrogravimetry is usually carried out under conditions of controlled potential, so an auxiliary reference electrode is used together with the WE. (The WE can be either the anode or the cathode.) The auxiliary electrode-WE pair is connected to a potentiostat that fixes the potential of the WE by automatically adjusting the applied emf throughout the course of the electrolysis. 

The WE should have a large surface area. Platinum is used because it is inert Pt gauze or mesh electrodes are used to provide a large surface area. The electrode mesh is like wire windowscreening material, with the wire diameter about 0.2 mm. The mesh is welded into an open cylinder to make the electrode. A standard size is a cylinder about 5 mm high and 5 mm in diameter, but Pt mesh electrodes of many sizes are available. 

It is critical for these techniques that the current efficiency be as close to 100% as possible. The percent current efficiency tells us how much of the applied current results in the reaction of interest. The percent current efficiency is defined as being equal to 100 x ( ppn. - 4esiduai)/ appiied. where is the current applied to the cell and 4esiduai is the background current. All real cells have some residual current. To achieve a 99.9% current efficiency, the applied current must be approximately 1000 times the residual current. This is readily achieved for currents in the pA to 100 mA range used in constant-current coulometry. 

---