A Simple Apparatus for Experiments in Electrochemistry OTTO F . STEINBACH City College of New York, New York HE apparatus which is described here is a modi- ratio of hydrogen to oxygen as illustrated in Figure 1. ficatlon of the Hoffman apparatus, and has been A delivery tube, connected to the top of each limb, found to be very satisfactory for individual student permits the collection of gas by displacement of water. experiments in electrochemistry. It is easily con- Using a graphite cathode and a platinum wire anode, verted from one use to another, a variety of diierent the electrolysis of a 10 per cent solution of sulfuric acid electrodes can be used, and it is readily cleaned. It will produce a full 30-ml. test tube of hydrogen in about may be used for experiments on primary cells, electroly- five minutes. Meanwhile, approximately half this amount of oxygen is obtained. I t is quite obvious that sis, conductivity, and even for cataphoresis. The apparatus can be made from pyrex test tubes or the volume of oxygen obtained is less than half that of from 12- to 15-mm. stock tubing by anyone with an the hydrogen obtained. The reason for this discrepelementary knowledge of glass blowing. The design ancy is due in part to the greater solubility of oxygen and approximate dimensions are shown in the ac- in water than hydrogen, and in part to the formation companying figure, which incidentally illustrates the of persulfuric acid and ozone. The odor of ozone is setup for determining the relative volumes of hydrogen distinct and may be detected by its effect on starch iodide paper. The formation of ozone may be exand oxygen formed by the electrolysis of water. When this apparatus is used to house a primary cell pected when the current density is high. I t probably and it is desirable to keep the anolyte and catholyte results from the decomposition of persulfate ion which from mixing, a plug of cotton or glass wool can he in- is formed by the electrolysis of disulfate and sulfate serted into the connecting tube. The two solutions ions. This is a good illustration of several different should be poured into the side arms almost simultane- electrode reactions taking place simultaneously. I t ously to avoid hydrostatic seepage through the plug. suggests a reason why sodium hydroxide is used as the The plug can later be removed by blowing into the electrolyte in the commercial electrolysis of water when emptied H tube. The apparatus is most convenient for studying the chemical and physical effects produced during electrolysis. When solutions of chlorides or bromides are electrolyzed, the odor of free halogen can readily be detected and a good test for the latter can be obtained with starch iodide paper held above the anolyte. When sodium chloride is electrolyzed using graphite electrodes, good tests for hydrogen, chlorine, and sodium hydroxide are obtained. A cotton plug may be inserted to prevent undue diffusionof catholyte and anolyte and will serve to draw a closer analogy to the function of the asbestos diaphragm in the Nelson cell. The setup provides a satisfactory illustration of the reactions taking place in the Nelson cell or similar type cell used for the manufacture of sodium hydroxide. The side arms may be loosely stoppered to prevent diffusion of gaseous products. The electrolysis of copper bromide solution with graphite electrodes provides an excellent experiment. The average student obtains a good test for free hromine and observes the plating out of copper as well as the change in concentration around the cathode as cupric ions are removed from the solution. The electrolysis of water gives some very interesting results which can be made semiquantitative. The student can arrange the apparatus to determine the 3(
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hydrogen and oxygen are the desired products. When a 10 per cent solution of sodium hydroxide is used as the electrolyte instead of sulfuric acid, the ratio of the volumes of hydrogen to oxygen is nearer to 2:l. In any case, more accurate results are obtained if the solution is electrolyzed for about a minute before collecting the gases. This will saturate the catholyte and anolyte with hydrogen and oxygen, respectively. If a graphite anode is substituted for the one of platinum, the volume of oxygen obtained is considerably less than half that of the hydrogen when sulfuric acid is used as the electrolyte, and the student cannot help but wonder what causes the discrepancy. The reason is that a considerable amount of the oxygen produced is consumed in oxidizing the graphite anode to carbon dioxide. The attention of the student can then be drawn to the analogous process that takes place in the Hall process for the manufacture of aluminum. Using sodium hydroxide as the electrolyte in place of sulfuric acid, oxygen and sodium carbonate are produced in the anolyte and the latter can be detected by withdrawing a portion and adding hydrochloric acid. If it is desired only to identify the hydrogen and oxygen, tests for these gases may be made directly in the limbs of the apparatus by loosely stoppering them and allowing the electrolysis to proceed for a few minutes before inserting glowing or burning splints directly in the limbs. The presence of ozone may be shown by its odor and the starch iodide paper test. The physical effects of an electrical current can readily be ascertained by the student. The most noticeable is the heating effect due to the ohmic resistance of the electrolyte. The attention of the student can be drawn to the fact that both Ohm's law and Joule's law apply to electrolytic solutions as well as metallic conductors, and the student is thus led to consider the similarity and difference between metallic and electrolytic conduction.
When a flow of electrons takes place in a metallic conductor, a magnetic field is set up a t right angles to the direction of flow. In a similar fashion, the migration of positive and negative ions also sets up a magnetic field. This effect, discovered by Oersted, can be demonstrated with a small compass. Place the compass on the table and hold one of the lead wires directly over it so as to be parallel to the needle. Upon completing the circuit, the needle is deflected. Placing the compass over the wire reverses the direction of deflection as well as does reversing the current. To show moving ions, create a magnetic field, place the compass on the horizontal limb of the IT tube and orient the tube so that it is parallel to the needle, that is, it is parallel to the earth's magnetic field. Upon completing the circuit the needle deflects. Likewise, holding the compass on the under side reverses the deflection as does reversing the current. It may be pointed out that the magnetic fields established by both the moving positive and negative ions are in the same direction. The apparatus also serves nicely for qualitative observations of relative conductivity, as the volume of the apparatus is small and it is easily rinsed out and the electrodes are fixed. The distinction between strong and weak electrolytes and non-electrolytes can easily be demonstrated, using suitable solutions. The relation of ionic concentration to conductivity can be shown by starting with water in the H tube and adding small amounts of electrolyte in separate portions. Finally, the apparatus can be used in cataphoresis experiments in order to determine the sign of the charge of the colloidal particle. Good results were obtained with a colloidal electrolyte such as basic fuchsin, and with the sols of hydrous ferric oxide and arsenious sulfide. Electro-osmosis was observed incidentally during the experiment with basic fuchsin. The author wishes to thank Professor R. A. Baker for his many suggestions and for his kindness in constructing a variety of H tubes.
