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Among the many demonstrations of ionic migration in an electric field, the ones showing the migration of colored Cu2+ and CrO42- ions are popular. The demonstration described here introduces some modifications to allow a fast displacement of these ions. Copper(II) chromate is dissolved in concentrated ammonia, where Cu2+ forms the complex ion [Cu(NH3)4]2+. The medium is a strip of filter paper soaked in an electrolyte prepared with ammonia and ammonium chloride. Owing to the low viscosity of this medium compared to a gel, the movement of the ions is very fast and they can do a complete forward-backward trip in minutes.
Two Petri dishes are filled with the electrolyte, and two paper clips are used as electrodes. A strip of filter paper is soaked with the electrolyte, and a line of copper chromate dissolved in ammonia is drawn on it. The strip is put as a bridge about 3 cm long hanging with its ends immersed in the Petri dishes. Using a voltage of 30 V, the separation of the green line into two narrow bands, yellow and blue, moving toward the opposite electrodes, can be seen in one minute. The voltage can be inverted to make the ions move backward.
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