In the Laboratory edited by
The Microscale Laboratory
R. David Crouch Dickinson College Carlisle, PA 17013-2896
Microscale Chemistry in a Plastic Petri Dish: Preparation and Chemical Properties of Chlorine Gas†
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Martin M. F. Choi Department of Chemistry, Hong Kong Baptist University, Kowloon Tong, Hong Kong, SAR, PRC
The study of chlorine chemistry is a significant and important part of the curriculum in secondary schools. However, the toxicity and hazards of chlorine gas preclude handson experience in the preparation of chlorine gas for students. Although halogens can be prepared and their visual properties can be presented safely in laboratory glassware (1), the glassware used is bulky and the experimental setup is cumbersome and inconvenient. With the introduction of microscale chemistry, it is now possible for students to perform and study chlorine chemistry safely. Microscale chemistry is popular in laboratory practice because it uses only small quantities of chemicals and simple apparatus. The advantages of microscale chemistry are its cost saving, use of smaller amounts of chemicals, safety, speed, and environmental friendliness (2). Some toxic gases can be generated safely and their chemical properties studied on a microscale in a petri dish (3). In our recent studies, we could prepare chlorine gas safely and study its chemical properties in situ on a microscale. The aim of this demonstration is to present some reactions of chlorine (4) in a volume about the size of a water droplet. This experiment provides suitable handson experience for students at secondary-school level. It is also a quick, simple, and safe method for preparing chlorine gas, and can be used for visual study of chlorine chemistry. Principles Common bleach solutions contain sodium hypochlorite (~5% by mass) as the active ingredient. Sodium hypochlorite is prepared by reacting chlorine gas with a cold solution of sodium hydroxide. Sodium chloride is produced as a byproduct in the bleach solutions (5): Cl2 + 2NaOH → NaCl + NaClO + H2O Thus, chlorine gas can easily be generated via the reaction between a bleach solution and sulfuric acid enclosed in a petri dish:
Student information recording sheet One piece of white paper Two 9-cm plastic petri dishes (base plus lid) Nine plastic disposable pipets Tissue Commercial bleach solution ~5% NaClO Ammonium iron(II) sulfate solution (freshly prepared) 1% (w/v) Sodium sulfite solution (freshly prepared) 2% (w/v) Potassium iodide solution 0.05 M Sulfuric acid 1 M Potassium thiocyanate solution 1% (w/v) Barium chloride in 0.1 M HCl 1% (w/v) Grape juice from commercial sample
Procedure The base of a plastic petri dish was directly placed on a piece of white paper. The test solutions were added to the plastic petri dish in the positions and quantities indicated in Figure 1. A drop of bleach solution was then dropped into the center of the dish; this was followed by a drop of sulfuric acid and the dish was quickly covered with a lid. After about 10 minutes, color changes in the droplets could be observed and recorded. The lid was then removed and one drop of potassium thiocyanate solution was added to the iron(II) solution to prove the presence of iron(III). Similarly, a drop of acidified barium chloride solution was added to the sodium sulfite solution to confirm the formation of sulfate. A control experiment can be performed by putting a drop of deionized water (instead of bleach solution) in the center of another petri dish and repeating the procedure described above. The color change of the droplets in the first petri dish should be easier to observe than those in the control dish.
ClO᎑ + Cl᎑ + 2H+ → Cl2 + H2O
1 drop Fe2+ solution
The chlorine gas then diffuses into and reacts with reagents placed in the dish. The products of these reactions will visually demonstrate the chemical properties of chlorine gas. Experimental Procedure Supplies and Chemicals The following materials and chemicals were provided for each group of students before the start of the experiment. †
An oral presentation on this topic was given at the Microscale Chemistry Workshop at the Hong Kong Baptist University, Hong Kong SAR China, on 35–5 July 2000.
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1 drop Na2SO3 solution
1 drop bleach and 1 drop H2SO4 solution 1 drop KI solution
1 drop grape juice Figure 1. Diagram of experimental setup. Test solutions were added to the petri dish in the positions and quantities indicated.
Journal of Chemical Education • Vol. 79 No. 8 August 2002 • JChemEd.chem.wisc.edu
In the Laboratory
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Oxidation of Iron(II) to Iron(III) Chlorine turned iron(II) from pale green to pale yellow: 2Fe2+ + Cl2 → 2Fe3+ + 2Cl᎑ On the addition of potassium thiocyanate to iron(III), the color changed to reddish brown: Fe3+ + SCN᎑ → FeSCN2+
Figure 2. Preparation and study on the chemical reactions of chlorine gas in a plastic petri dish. (A) Before chemical reactions: 1, a drop of Fe(II) solution; 2, a drop of Na2SO3 solution; 3, a drop of grape juice; 4, a drop of KI solution; 5, a drop of bleach solution. (B) Ten minutes after addition of sulfuric acid, and then other reagents:1, drops of Fe(II) and KSCN solutions; 2, drops of Na2SO3 and BaCl2 solutions; 3, a drop of grape juice; 4, a drop of KI solution; 5, drops of bleach and H2SO4 solutions.
Hazards Liquid and mists of bleach solution may severely irritate or damage the eyes. Contact with bleach solutions will irritate the skin, causing possible inflammation. Chlorine gas has a pungent odor. It is highly toxic to fish and other aquatic organisms. It can cause severe eye irritation with corneal injury, which may result in permanent impairment of vision, even blindness. Excessive exposure causes severe irritation of the upper respiratory tract and lungs and may cause lung injury. It can also cause severe skin burns. Ammonium iron(II) sulfate solution may cause severe irritation or burns to the upper respiratory tract, with coughing and shortness of breath. Repeated exposure to dilute solutions may cause irritation, redness, pain, and drying and cracking of the skin. It may cause severe irritation or burns to eye tissue, esophagus, and gastrointestinal tract, with nausea, vomiting, diarrhea, and black stool. Contact with sodium sulfite solution may cause slight irritation to the skin, eyes, and mucous membranes. Repeated exposure may result in respiratory sensitization. It may also cause slight gastrointestinal irritation. Contact with potassium iodide solution may cause allergic respiratory and skin reactions. Repeated ingestion may cause iodism and reproductive toxicity. Contact with potassium thiocyanate solution causes irritation to skin, eyes, and respiratory tract, redness, and pain. Sulfuric acid may cause redness or itching of skin and irritation and tearing of eyes. Mists and aerosols cause irritation of the upper respiratory tract. Contact with barium chloride solution may cause slight irritation to the skin and moderate irritation to the eyes. Mists and aerosols may cause irritation to the upper respiratory tract. Barium chloride may cause slight gastrointestinal irritation with nausea, vomiting, diarrhea, incoordination, mental confusion, dizziness, and lethargy. Students must wear splash goggles and gloves to prevent contact with bleach solution and all chemicals, even though the reagents are used in very small quantities. Results The color change of the droplets could be observed after 10 minutes and chemical reagents were added to some drops as shown in Figure 2. The chemical reactions of chlorine being studied are described in detail below.
Conversion of Sulfite to Sulfate Sodium sulfite was oxidized to sodium sulfate in contact with chlorine gas: SO32᎑ + Cl2 + H2O → SO42᎑ + 2HCl The presence of sulfate was confirmed by the addition of acidified barium chloride, producing a white precipitate: Ba2+ + SO42᎑ → BaSO4↓ Bleaching Action of Chlorine on Dyes In the presence of chlorine, grape juice containing natural dye was decolorized.
Conversion of Iodide to Iodine Iodide was quickly oxidized to brownish-yellow iodine on exposure to chlorine gas: 2I᎑ + Cl2 → I2 + 2Cl᎑ Discussion One of the special features of these experiments is that the spontaneous diffusion of chlorine gas replaces all mixing of reactants, making it easier for younger students to perform the experiments. The reactions are quick and the color change can be observed within 10 min. In the past, teachers usually performed these experiments on a macroscopic scale and students could only observe the demonstration because of the hazards of chlorine gas. Our demonstration can provide suitable hands-on experience for students and certainly help improve their learning attitude and incentive. It is simple and safe to perform, as only very small quantities of chemicals are required. Moreover, secondary school teachers can modify these experiments for other hands-on experiments on other gases. For example, they can demonstrate the preparation and chemical properties of gases such as sulfur dioxide and hydrogen sulfide if relevant chemical reactions are available. Supplemental Material Notes for students and instructors are available in this issue of JCE Online. W
Literature Cited 1. Liprandi, D. A.; Reinheimer, O. R.; Paredes, J. F.; L’Argentière, P. C. J. Chem. Educ. 1999, 76, 532–534. 2. Singh, M. M.; Szafran, Z.; Pike, R. M. J. Chem. Educ. 1999, 76, 1684–1686. 3. Skinner, J. Microscale Chemistry; The Royal Society of Chemistry: London, 1997; pp 54, 141. 4. Ramsden, E. N. A-Level Chemistry, 3rd ed.; Stanley Thornes: Cheltenham, UK, 1994; pp 396–397. 5. Chang, R. Chemistry, 4th ed.; McGraw-Hill: Hightstown, NJ, 1991; p 907.
JChemEd.chem.wisc.edu • Vol. 79 No. 8 August 2002 • Journal of Chemical Education
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