RESULTS AND DISCUSSION
Sodium polyacrylate is basic and as such it forms alkaline solution in which Cu2+ ions precipitate as Cu(OH)2. This does not happen in the commercial process, which is carried out in acidic solutions. In this demonstration, ammonia solution is added in the sodium polyacrylate gel. Applying voltage to the electrodes, the copper atoms in the copper electrode release electrons and dissolve into the electrolyte solution as Cu2+ ions. If ammonia solution is not added, Cu2+ ions form copper(II) hydroxide (Cu(OH)2), which is not soluble in water and is precipitated on the anode electrode.7 Cu2+ ions react with ammonia to form deep-blue colored [Cu(NH3)4]2+ ions that are soluble in water and move away from the anode toward the cathode under the applied voltage (Figure 1B). At the cathode, hydrogen gas is evolved at first until the [Cu(NH3)4]2+ ions reach the cathode. At this point, the blue color disappears and metallic copper begins to grow (Figure 1C). In the parts of the cathode that are not reached by [Cu(NH3)4]2+ ions, evolution of the gas still continues (Figure 1D).
In this demonstration, the students can see not only the dissolution of copper ions from the copper electrode, but also simultaneously the migration of copper ions from the anode to the cathode in the electrolyte solution,6 and the deposition of copper metal on the cathode7 (Figure 1D). They can also understand that the electrons produced in the anode flow via a battery to the cathode where they are used for reduction of Cu2+ to Cu metal. On the basis of this demonstration, students can also understand other electrolysis process.