National Chemistry Week - Experiments - Electrolysis of a water solution

Summary

A water solution is electrolysed using a 9V battery. Electrode phenomena can be observed. The kinetic of the reactions can be changed. Such electrolysis give rise to many industrial applications.

Materials

Commercial battery (9V), 2 conducting wires such as copper wires, (aligator clips or aluminium foil are optional), coffee filter, 2 glass or plastic jars.

Solution A is made by adding approximately eight teaspoons of table salt (sodium chloride) and few drops of a phenolphthalein solution to 500 mL of water. When needed (see tip 1), vinegar (dilute acetic acid) must also be added. Solution B must contain sodium chloride, but the presence of phenolphthalein and vinegar (in other terms, the use of a solution similar to solution A) will not jeopardize the experiment.

Procedure

Version 1

1. Fill the two jars to approximately three quarters full with respectively the cathode solution (solution A) and the anode solution (solution B). Put both solutions beside each other.
2. Insert one wire into one jar and another wire into the second jar. Connect the wire in solution A to the negative battery end and the wire in solution B to the positive battery end.
3. Fold the coffee filter paper and immerse the ends in the two jars, so as to form a bridge between the two solutions.
4. Observe what happens after the dry coffee filter is put in place (see tips 2 and 3).

Version 2

1. Place two metal spoons or folded aluminium foils (see tip 4) in opposite sides of one empty jar, taking care that these metal pieces do not touch each other. Using conductive wires, connect one of these metal pieces to the negative end and the other to the positive end of a 9V battery.
2. Fill the jar to a maximum of three quarters with water (see tip 5). Observe if something happens.
3. Then shake some salt into the water. Observe what happens (see tip 6).

Tips

1. The vinegar is used to keep the solution A slightly acidic and to prevent the red color to appear prematurely (because the solution is too basic). Therefore, if the solution is red, add vinegar drop by drop to the well stirred solution until the red color just disappears.
2. After the dry coffee filter has been put in place, both solutions will begin to wet the paper. The electrolysis will not start before the paper bridging the two solutions becomes continuously wet.
3. The red color will first appear around the cathode wire (solution A) and will gain in intensity as the electrolysis proceeds. Later, solution A may be shaken to distribute color in the entire solution. This red color indicates that the solution is becoming basic (formation of hydroxide). The intensity of this red color can be adjusted by adding more or less of phenolphthalein.
4. By using alligator clips to make the contacts, it should also be easy to fix metal pieces such as aluminium foils around the border of the jar. Alternatively, any holder/spacer can also be used.
5. A slow addition of water should avoid to get any bubble sticking on the surface of the metal pieces. When distilled or deionized water is used, no reaction (no bubbles formation) should be first observed. It is possible to grade this first observation by using other types of water such as tap water. Note also that, in version 1, any type of water will give the same result.
6. After generous bubbling has been observed, this phenomena can be stopped and started again by breaking and remaking a contact at any location in the circuit.
7. Because both versions complement each other, these could be performed by the demonstrator during the same session. In version 2, the addition of table salt (an electrolyte) start the electrolysis at a significant rate. This is equivalent to the wetting of the filter paper in version 1 (see Tip 2).
8. At the cathode, reduction of water produce hydrogen (H₂, the bubbles) and sodium hydroxide (NaOH, the red color). At the anode, oxidation of chloride produces chlorine (Cl₂, the greenish color after a while). These products are formed when cathode and anode compartments are kept separated (version 1). Another product, sodium hypochlorite (NaOCl), is formed by reaction between sodium hydroxide and chlorine, when the cathode and anode are not separated (version 2).
9. The reactions described in tip 8 are used in industry to make these products: draino (sodium hydroxide), bleach (sodium hypochlorite), water desinfectant (chlorine), spacecraft fuel (hydrogen). These products are also starting reactants to make many other products.

Reference

This experiment was first established in the middle of the 1980s by Dr. Roger N. Renaud, now retired from National Research Council Canada. Since, many different versions of this experiment have been presented in primary schools by himself and other members of the CIC Ottawa section. Similar electrolysis experiments are described in Exploring Chemistry, Canadian Society for Chemistry, Vol 1, 9-10 (1996), or in Discover Canadian Chemistry, Canadian Society for Chemistry, Vol 6, 6 (1996).