Generating and Detecting Ozone

Introduction

Ozone is being used in an increasing number of industrial applications, such as a disinfectant in hospitals, food factories and nursing care facilities, for municipal water treatment as well as for a host of other uses that take advantage of ozone’s powerful oxidizing ability. Build an ozone mini-generator to show the oxidizing effect this unique allotrope of oxygen has on different substances.

Concepts

• Electrolysis
• Oxidation–reduction reactions

Materials

Safety Precautions

Sulfuric acid solution is corrosive to eyes, skin, mucous membrane and other body tissue. The ozone gas produced is a respiratory irritant. Place generator in a fume hood or operate in a well ventilated room. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Follow all laboratory safety guidelines. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The reaction mixtures in each test tube may be flushed down the drain with excess water according to Flinn Suggested Disposal Method #26b. The leftover acidic solution in the pipet bulb may be neutralized and disposed of according to Flinn Suggested Disposal Method #24b.

Prelab Preparation

Reaction Chamber
The reaction chamber consists of a plastic transfer pipet filled with 3 M H₂SO₄. The stem is stretched into a thin capillary delivery tube prior to filling the bulb with acid. The reaction chamber is constructed using a plastic thin-stem Beral pipet, a piece of mechanical pencil lead and a length of thin platinum wire at least 3 cm long.

1. Stretch the pipet stem to make a delivery tube that is 30 cm long. To do so, grip the pipet stem with one hand just at the point where the stem joins the bulb, and hold the open end of the tube with the other hand (wrap the open end of the pipet around a finger for a better grip). Pull the pipet stem outward with a firm, steady pull. As the pipet stem is being drawn, the diameter will vary from its original diameter to capillary diameter at certain points along the length; that is, the stem will alternate thin or thick regions, as shown in Figure 1b. Continue pulling the stem with a steady motion until all of the thick parts are gone. It is possible to achieve a capillary length of 30 cm or more. A longer capillary length will allow the ozone generated to be delivered some distance away. Cut off the end of the pipet stem that was being gripped and did not stretch. It is usually necessary to practice on a few empty pipets as this is somewhat of a learned skill. Figure 1 shows before, during, and after views of the pipet stem being stretched.
   {12685_Preparation_Figure_1}
2. Next the two electrodes must be inserted. Both must form an airtight fit through the pipet’s bulb. A T-handle dissecting needle makes a suitable pilot hole for the mechanical pencil lead. For the thinner diameter Pt wire, use just the tip of the dissecting needle.
3. The reaction chamber is completed by filling the pipet bulb ¾ full with 3 M sulfuric acid. This is done using a 10 mL syringe equipped with a hypodermic needle. Remove the graphite electrode, which has a diameter similar to that of the hypodermic needle and inject the acid through the hole. Reinstall the graphite electrode and the reaction chamber is complete.

The reaction chamber will deliver approximately 10 mL of gas (H₂ + O₂ + O₃) per minute. At this rate, 0.5 mL of water is consumed per hour of use. After just a few hours of use, the level of solution in the bulb will be noticeably lower. Since sulfuric acid is not consumed during the electrolysis of water, only distilled water needs to be added to the reaction chamber to replenish the liquid level. To do this, remove the graphite electrode and inject more distilled water via the syringe, but never exceed ¾ full. Remove the needle and reinsert the graphite electrode.

Ice Bath
The beaker shown in Figure 2 is for an ice bath necessary to keep the system cool. The pipet bulb is suspended in the ice bath with a dowel rod and two rubber bands. Without the ice bath, the electrodes will heat up and may enlarge the holes through the pipet. If this occurs, the gases will no longer be delivered through the capillary tube but rather will leak out the enlarged hole(s) around the electrode(s). The ice bath should contain more water than ice so that the entire surface of the pipet bulb is in contact with ice water. The ice bath also improves the yield. The power supply is connected as shown in Figure 3. The 6-volt setting is optimal and a 6-volt battery can be used as the power supply, although the latter has a short lifetime. Power supplies come with a variety of connectors. The one shown in Figure 3 is a push-pin style. We fashioned a U-shaped wire from part of a paperclip to slip inside the connector where the pin would go. The positive (+) lead from the power supply or battery is connected to the platinum electrode (anode) and the negative (–) lead is connected to the graphite electrode (cathode). Two wires with alligator clips on both ends are used to connect the power supply or battery to the electrodes.

Procedure

1. Using a 10-mL graduated cylinder filled with water and inverted in a beaker of water, determine the flow rate for gas production using water displacement.
2. Label six clean test tubes 1–6 and place them in a test tube rack.
3. Add two drops of blue food coloring, followed by 5 mL of water, to each test tube 1 and 2. Swirl to mix.
4. Repeat step 3 using the green food dye in test tubes 3 and 4.
5. Place a rubber band in each test tube 5 and 6.
6. Connect the leads of the 6-volt power supply or battery to the electrodes in the mini-ozone generator. Make sure the positive lead is connected to the platinum wire and the negative lead to the pencil lead.
7. Place the end of the delivery tube in the second test tube filled with diluted blue food dye. Push the tip down near the bottom of the test tube. The ozone will start to change the solution from blue to a light green color. This will take from 15 to 20 minutes. This is a good time to review at the board the chemistry of the production of ozone in the generator.
8. Repeat step 7 using the green food dye solution in test tube 4. The ozone will change the solution from green to light yellow.
9. Place the delivery tube in test tube 6 containing a rubber band. After fifteen minutes, the rubber band will have been cleaved in several spots.
10. Blow up the balloon. Tie the inflated balloon to the test tube rack.
11. Place the delivery tube approximately 2 mm away from the middle of the balloon. Hold the tube in place until the balloon “pops” (about 20 to 40 seconds).

Student Worksheet PDF

12685_Student1.pdf

Teacher Tips

• This kit contains enough materials to perform the demonstration seven times: 1.5-inch piece of platinum wire, 15 pencil leads, 2 connector cords with alligator clips, 15 mL of both green and blue food dye, 30 mL of 3 M sulfuric acid solution, 15 thin stem pipets, 15 small rubber bands, 15 large rubber bands, 15 pencil leads and a dowel rod.
• Ozone levels can build up in the air through a series of complex reactions involving ultraviolet light, hydrocarbons, nitrogen oxides, hydroxyl radicals, and water vapor. When breathed in, ozone can react with the carbon-carbon double bonds of fatty acids in the lungs, producing very reactive radicals. These, in turn, cause inflammation and stressful breathing, particularly in people with asthma and COPD. The EPA has set the normal ozone standard to 75 parts per billion (ppb). For people at higher risk of being adversely affected, the recommended level is as low as 40 ppb.
• To show that the green food coloring is a combination of yellow and blue, add the non-reacted blue dye solution to the yellow solution created in step 8. This will reproduce the green color.

Answers to Questions

1. Describe what happened in this demonstration. A gas was generated in the reaction bulb. When the gas mixture was bubbled through a blue solution, the solution decolorized. When the gas was bubbled through a green solution, the solution became yellow. When the gas was blown on a balloon, the balloon burst. When a rubber band was exposed to the gas, the rubber band split into several pieces.
2. Ozone has a structure that is bent, like that of SO₂. Sketch the Lewis dot structures for O₃ and SO₂. Show the two resonance structures for each molecule.
   {12685_Answers_Figure_8}
3. Ozone is an oxidant and is capable of oxidizing metals that molecular oxygen (O₂) does not attack. Write and balance the reaction that takes place when ozone reacts with silver metal to form silver oxide and oxygen (O₂).

   aAg(s) + bO₃(g) → cAg₂O(s) + dO₂(g)
   for Ag atoms: a = 2c
   for O atoms: 3b = c + 2d
   Let c = 1 a = 2
   3b = 1 + 2d; if b = 1, then d = 1
   2Ag(s) + O₃(g) → Ag₂O(s) + O₂(g)

Discussion

Ozone is an allotrope of oxygen with the molecular formula of O₃. Ozone is a strong oxidant and highly reactive. Naturally occurring ozone levels in the atmosphere range from 0.001 μg O₃/m³ (0.001 ppm) to 0.125 ppm, depending on altitude, atmospheric conditions, and locale. In this lab, oxygen and ozone are generated at the anode (platinum electrode) and the reactions are:

The cathode (graphite) reaction is: The overall major reaction is: whereas the overall minor reaction is: Thus, the gas mixture that is collected is about ⅔ hydrogen and ⅓ oxygen. We have found that ozone represents 0.25–0.38% of the total gas produced. Typically, 10 mL of gas are generated per minute. Note: If the electrodes are connected backwards, the electrolysis reaction does not work and the sulfuric acid solution becomes black from suspended graphite as the graphite electrode slowly disintegrates.

The reaction of ozone with an alkene, that is, a molecule containing a carbon-carbon double bond, is called ozonolysis. Natural rubber is a polymer of isoprene. When this rubber is exposed to a stream of ozone, the following reaction takes place at many of the alkene sites: In the case of the rubber band, ozonolysis results in the breaking of the band in numerous places. For the balloon, the ozone creates a small hole, resulting in the “popping” of the balloon.

Food dyes are highly conjugated molecules, that is, they have long chains of alternating single and double bonds. The pi system of bonds and resonance structures creates excited electronic states that allow the molecules to absorb light in visible region.

Blue food dye has one alkene site. Ozone cleaves this bond, destroying the long string of conjugation and the blue dye from the solution. Green food dye is a combination of yellow and blue food dyes. The yellow dye, while conjugated, does not have an alkene carbon-carbon double bond (the aromatic ring does not react as an alkene). The yellow dye is an azo dye containing a N=N double bond. This group is quite stable and is only cleaved by a strong reducing agent. When green food dye is exposed to ozone, the blue dye is decolorized, leaving the solution yellow.

References

Flinn Scientific would like to thank Bruce Mattson, Department of Chemistry, Creighton University, Omaha, NE, for sharing his original idea and procedure with us.

J. G. Ibanez, B. Mattson, et al., J. Chem. Ed., 2005, 82, pp 546–548.