LABORATORY PREPARATION of OXYGEN and of CHLORINE FRANCIS C. KRAUSKOPF The University of Wisconsin, Madison, Wisconsin
New methods for the student laboratory prepzrations of oxygen and chlorine are given, and the advantuges of these methods over those ordinarily used are discussed.
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OST laboratory manuals of general chemistry include as standard exercises: (1) the preparation and properties of oxygen; and (2) the preparation and properties of chlorine.
If to a suspension and partial solution of calcium hypochlorite* in water there is added cobalt oxide (or any salt of cobalt, which the free base in the calcium hypochlorite solution converts to cobalt oxide) the calcium hypochlorite is catalytically decomposed with the rapid evolution of oxygen. The reactions may be represented as follows:
THE PREPARATION OF OXYGEN
Almost without exception the method used for the preparation of oxygen consists in beating a mixture of manganese dioxide and potassium chlorate in a testtube fitted with a stopper and delivery tube. The materials used are abundant and cheap. The reaction involved can be presented to the student as a case of simple decomposition by heat of an oxygen-containing compound. The catalytic effect of the manganese dioxide can be easily demonstrated. If desired the substances remaining after the reaction are readily separated and identified. For these reasons the method is well suited to the beginning student. However, certain disadvantages are apparent to those who have watched its use by students. (1) The fusion of the chlorate mixture may cause a stoppage in the test-tube, or the sudden evolution of the oxygen may blow some of the loose material into the entrance to the delivery tube. Either of these eventualities may cause the breaking of the test-tube, some*' times with explosive violence. (2) If, when the tube is heated, the end of the rubber stopper becomes too warm, i t will ignite in the atmosphere of pure oxygen and burn with disconcerting violence. (3) The possibility of the potassium chlorate's becoming contaminated with some reducing agent, such as carbon, sulfur, sugar, etc., makes it a dangerous substance to heat. (4) Once the mixture is heated to the temperature of rapid evolution of oxygen the reaction does not slow down as the heat is removed. In this way much of the evolved oxygen is lost in the process of changing the bottles used for collecting the gas, and if more oxygen is desired another charge must be made up. These considerations and the desire to furnish a new exercise for those students who were already familiar with this method led us to seek another reaction by which a student, can conveniently prepare oxygen for use in the laboratory.
The detailed directions for the student are as follows: Use a generating flask with a thistle-tube and delivery tube similar to that used for the preparation of hydrogen from zinc and sulfuric acid. Add about 150 cc. of water and 10 cc. of a 2% cobalt chloride solution. Prepare a suspension of calcium hypochlorite as follows. Place 20 g. of bleaching powder in a beaker, add 100 cc. of water, and stir until no lumps remain. Heat the water in the generator to 80-85" (almost to boiling) and pour through the thistle-tube 20 ec. of the S U ~ p ~ s i oofn calcium hypochlorite. Discard the first 100 cc. or so of the evolved gas, for it is largely air, and then collect the amount of oxygen needed for further experiments. When the evolution of the gas slackens, add more of the calcium hypochlorite suspension in 10-cc. portions as needed, keeping the contents of the flask just below boiling...
This provides an economical and convenient method of preparing oxygen. The speed of evolution of the oxygen can be governed by the temperature and the rate of addition of the calcium hypochlorite. The generator even after standing can be put into.use immediately by the addition of more calcium hypochlorite and warming. THE PREPARATION OF CHLORINE
In the preparation of chloriue a large majority of the manuals use the time-honored combination of manganese dioxide and hydrochloric acid. A few use potassium permanganate in place 'of manganese dioxide. As in-the traditional oxygen preparation, the materials are abundant and cheap and a simple oxidation-reduction reaction is employed. The disadvantages are: (1) The reaction is often slow in starting. (2) The formation of water in the reaction and the exhaustion of the hydrochloric acid dilutes any additional HC1 added. (3) It is difficult to impress upon the student the necessity of keeping the concentration of HCI high,
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Calcium hypochlorite is sold under the trade name of HTH by the Mathieson Alkali Works, Inc., New York, or under the trade name of Perchloron by the Pennsylvania Salt Mfg. Co., Philadelphia. Pa.
and of keeping the temperature below boiling so as to minimize the distillation of the water from the flask into the bottle used for collecting the chlorine. Here again a method was sought that would obviate some of these disadvantages. Bleaching powder or calcium hypochlorite is substituted for manganese dioxide as the oxidizing agent, and instead of adding KC1 solution to the solid bleaching powder in the generating flask, which gives a sudden and discontinuous stream of chlorine, a suspension of bleaching powder is added gradually to the hydrochloric acid. The directions to the student are as follows: T o avoid a too-rapid evolution of chlorine the bleaching powder must be added t o the concentrated HC1 in small amounts. Further, the experiment should he carried out in a hood or with other arrangement for taking care of escaping chlorine. Arrange a generator as for the preparation of oxygen above except that the delivery tube should be arranged t o collect the gas by displacement of air. Place in the flask 25 cc. of concentrated HCl (sp. gr., 1.12). Place 15 g. of bleaching powder or calcium hypochlorite in a beaker, add 50 ec. of water, and stir until no lumps remain. Pour about 10 cc. of this suspension slowly through the thistle-tube, then as needed for the evolution of chlorine, add 5 cc. portions.
Collect the chlorine gas in bottles by displacement of air and use a perforated piece of cardboard t o cover each bottle during collection of the gas. Place a white paper behind the bottle as a background, and consider the bottle filled as soon as a greenish-yellow color appears. Remove the bottle from the delivery tube, cover with a vaselined plate, and quickly replace with another bottle. If needed, more chlorine can be obtained from the generator by the addition of bleaching powder as before. When not collecting the gas in the bottles, place the end of the delivery tube in a test-tube of sodium hydroxide solution. To empty the flask, proceed as follows. Insert the delivery tube in the test-tube of sodium hydroxide solution and pour water through the thistle-tube until the flask is completely filled. This drives the chlorine into the absorbing solution. Remove the stopper and empty the flask into the sink.
By this method the chlorine is evolved as rapidly as desired and in a practically continuous stream as long as fresh bleaching powder is added. The generation of the chlorine can be regulated a t will by regulating the addition of the bleaching powder. As long as the acid lasts, additional amounts of chlorine are always available by merely adding more bleaching powder. It is not necessary to heat the contents of the generating flask, so water is not distilled over with the chlorine.
