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†
W
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.
992
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
A
B
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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