Prepare 250 ml agar agar gel containing 3% NaCl. Dissolve 7.5 gms NaCl in 250 ml distilled water, add 5 gms powdered agar agar and boil the mixture until the agar is dispersed. Then add 5 ml of the Potassium ferricyanide solution and 1 ml of the phenolphthalein solution.

To demonstrate the cathodic and anodic regions, fill a Petri dish with hot gel solution and allow it to cool, until it begins to set. At this point place the plane bare iron nail in the gel and observe the subsequent color developments over a period of several hours. Note the developments of blue and red color at different places on the nail. The blue color indicates the anodic area and the red color shows the accumulation of alkali at the cathodic areas.