Electrolysis in Two Colors
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The following article was originally published in the journal for educators Chemia w Szkole (eng. Chemistry in School) (5/2022):
Electrolysis is a set of chemical transformations in which the structure of a substance changes under the influence of an external electrical potential. The term is often used more narrowly to describe decomposition processes that occur when an electric current passes through a dissociating electrolyte. Electrolysis is also accompanied by additional phenomena, such as the migration of ions toward the electrodes, secondary transformations of ions at the electrode surfaces, and other related reactions. In technological practice, electrolysis encompasses all of these processes together [1].
Electrolysis therefore occurs in systems containing substances capable of dissociating into ions, meaning they can undergo ionization.
In electrolysis the negatively charged electrode, where reduction takes place, is called the cathode, while the positively charged electrode, where oxidation occurs, is called the anode. Each electrode attracts oppositely charged ions. Positively charged cations migrate toward the cathode, while negatively charged anions move toward the anode. Upon reaching the electrodes, ions transfer their charges and, in some cases, also react chemically with the electrode material, forming electrically neutral compounds or elements [2] [3].
Electrolysis has many practical applications, including the production of metals such as aluminum Al, the generation of gases such as hydrogen H2, oxygen O2, and chlorine Cl2, as well as electroplating, which involves the electrolytic deposition of metallic coatings on objects.
Electrochemical phenomena are often perceived by students as complex and abstract. Yet electrolysis can be demonstrated in visually compelling ways. In what follows, I present one of the most strikingly colorful electrolytic reactions.
Experiment
Only a few easily accessible materials are required:
- starch,
- an alcoholic solution of phenolphthalein C20H14O4,
- potassium iodide.
Potato starch, is obtained from the tubers of Solanum tuberosum. It contains about 84% starch. The main additional component is water, the amount of which depends on the relative humidity of the air during storage [4]. Potato starch is a loose, matte, pure white powder free of foreign odors or flavors (Photo.1). When squeezed in the hand, it produces a characteristic crunching sound.
Phenolphthalein is an organic compound most commonly used as a pH indicator. Under normal conditions it appears as a white crystalline solid. Its solubility in water is limited, although it dissolves more readily in alcohols. Phenolphthalein is an acid–base indicator that shifts from colorless in slightly acidic or neutral environments to crimson red in moderately basic solutions. It is most useful in titrations of weak acids with strong bases. The transition range extends from pH 8.3 to 10.0. In strongly acidic solutions it turns orange or yellow due to the formation of a trityl cation, while in strongly alkaline solutions it becomes colorless again. Phenolphthalein was once used in medicine as a laxative, but it should not be ingested unnecessarily because in larger doses it can be harmful.
The next reagent is potassium iodide, the potassium salt of hydroiodic acid HI. It forms colorless crystals that dissolve readily in water (Photo.2). Electrolysis of molten potassium iodide yields elemental iodine I and potassium K.
Although no highly toxic chemicals are used in this experiment, appropriate caution is required as with any laboratory work.
The first step is to prepare a starch solution. Starch does not dissolve in cold water. The simplest approach is to add about 0.5 g (0.018 oz) of potato starch to 6-15 cm3 (≈0.2–0.5 fl oz) of cold water, then fill the volume to 250 cm3 (≈8.5 fl oz) with near-boiling water while stirring. After cooling, the solution should be diluted two to five times with water at room temperature. This solution, along with the other reagents, is shown in Photo.3.
To prepare the electrolyte, add a small pinch of potassium iodide to the starch solution, adjusting the amount experimentally, and stir until fully dissolved. Then add a few drops of a 1% ethanolic solution of phenolphthalein. Distilled water is preferable, but tap water will also produce the desired effect, although the solution may appear slightly more turbid.
Next, assemble a simple electrolytic cell. While the reaction can be performed in a single vessel, the color zones may mix. For better visualization, use a divided cell constructed from test tubes. Its schematic is shown in Fig.1.
The electrolyte bridge can be made from a strip of gauze, paper towel, or tissue soaked with the solution, which enables ionic conduction between the compartments. Electrodes are ideally made of chemically inert materials such as graphite or platinum, but experiments have shown that bare copper wire also performs adequately in this setup. To increase the surface area, the wire can be coiled into a spiral. The completed cell is shown in Photo.4.
Time for electrolysis!
Connect the electrodes to a low-voltage direct current source, for example 4.5 V. It is advisable to place a resistor of a few ohms or a small light bulb in series with the circuit. Almost immediately the solutions begin to change color: pink appears around the cathode and deep blue around the anode. After several minutes the colors become more intense (Photo.5).
If coloration is visible only in the immediate vicinity of the electrodes, gently shake the tubes to mix the solution.
Explanation
In aqueous solution, potassium iodide dissociates into ions according to the equation:
This process produces potassium cations K+ and iodide anions I-, with possible formation of polyiodide species.
When voltage is applied, the ions migrate under electrostatic forces: cations move toward the cathode and anions toward the anode.
Because the standard reduction potential of potassium is much lower than that of hydrogen (-2.9 V measured against the standard hydrogen electrode), water is reduced at the cathode. This releases hydrogen H2 gas and generates hydroxide ions OH-. To maintain charge balance, iodide ions I- are oxidized at the anode to form elemental iodine I2. The local increase in pH at the cathode causes phenolphthalein to turn pink, while iodine forms a complex with starch in the anode compartment, producing a deep blue coloration.
References:
- [1] Carmo M., Fritz D., Mergel J., Stolten D., A comprehensive review on PEM water electrolysis, International Journal of Hydrogen Energy, 2013, vol. 38, Iss. 12, pp. 4901-4934 back
- [2] Laidler K. J., The world of physical chemistry, Oxford University Press, 1995, pp. 219-220 back
- [3] Tilley R. J. D., Understanding solids: the science of materials, John Wiley and Sons, 2004, p. 281 back
- [4] Masewicz Ł. et al., Water activity anf hydrated starch aerogles, Proceedings of the 14th International Conference on Polysaccharides-Glycosicence, Prague, 2018, pp. 87-89 back
All photographs and illustrations were created by the author.
Marek Ples