SPECIAL EXERCISES for STUDENTS in GENERAL CHEMISTRY* 11.
Electrochemistry
G. BRYANT BACHMAN
AND
J. K. FARRELL
The Ohio State University, Columbus, Ohio
An experiment and the accom@nying afifiaratus for a laboratory study of the dichromate battery and the processes of electroplating and electroforming are described. Results obtained lead the student to a calculation of Faraday's electrochemical equiwalent.
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W 0 of the more interesting electrochemical laboratory exercises which may be readily adapted to a course in general chemistry are (1) the assembly of a battery and (2) the verfication of Faraday's laws relating to the electrodeposition of metals from solutions of their salts. Experiments in both of these subjects may be readily carried out in the laboratory with the aid of the home-made apparatus shown in the photo-
left is the battery container. In it are immersed a carbon rod and two zinc rods which have been amalgamated by dipping them into a mercuric chloride solution for a few minutes. To complete the battery the student is directed to prepare a solution containing 50 g. potassium dichromate, 100 g. concentrated sulfuric acid, and 400 cc. water. When the beaker is filled with this solution and the electrodes are immersed in it, the battery is ready for use. The voltage-about two volts-may be determined by making a connection through the push-button to the voltmeter. ELECTRODEPOSITION OF COPPER (QUANTITATIVE) As soon as the student has satisfied himself that the battery is in good working order and that i t has attained the correct voltage, he is ready to deposit copper electrolytically. The deposition takes place in the right-hand beaker shown in the photograph. The plating bath is made by dissolving 200 g. of copper sulfate pentahydrate and 80 g. sulfuric acid in 1 liter of water. A part of this solution is placed in the beaker and a copper strip anode hung over the positive pole into the solution. The cathode consists of a copper
graph. For students in whom the science of chemistry has awakened an especial interest and who are eager to perform extra experiments over and above those required in the first-year course, this exercise is especially suited. The cost of the materials from which the apparatus was constructed amounts to about $2.50.
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DIAGRAM
gauze (4 em. square), soldered to a copper wire, in turn soldered to a strip of copper which makes contact with the negative pole. The gauze cathode is rinsed with The construction of the apparatus is evident from the distilled water, dried in an air blast or high over a Bunphotograph and the wiring diagram. The beaker on the sen flame, and carefully weighed when cool to the near* The first of this series appeared in J. C H E MEouc., 10,241-2 est hundredth eram. It is then sus~endedfrom the (Apr., 1933). negative electrode in the solution. If directions have 689 CONSTRUCTION OF A DICHROMATE CELL
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been followed carefully the ammeter will immediately show a flow of current. The student should adjust this flow to 0.25 ampere by means of the controlling rheostat and from time to time correct any variation. The current is allowed to flow for a definite length of t i e , say 30 minutes; the cathode is then removed, rinsed, dried, and again weighed. The difference in weight represents the amount of copper deposited.
in nickel-plating some small personal article such as a key. The article should be carefully cleaned or burnished with steel wool, in order to obtain a durable coating of nickel. It may be necessary to renew the battery solution if this has become exhausted. Both battery and plating solutions are stable and may be made up in larger quantities than are needed for one student. ELECTROFORMING
CALCULATION OF PARADAY'S EQUIVALENT
Quantity of electricity is measured in units called coulombs. To determine the number of coulombs which were necessary to deposit the copper in the previous experiment, the time in seconds, during which the current flowed (30 X 60 seconds) is multiplied by the rate a t which i t flowed (0.25 ampere). The result (450 coulombs) expresses the number of coulombs of electricity consumed in depositing the copper. From this may be calculated the number of coulombs necessary to deposit one equivalent weight of copper. VERIFICATION OF FARADAY'S LAW
If Faraday's law is correct, it should require the same number of coulombs of electricity to deposit one equivalent of nickel or any other metal from its solution, as it took to deposit one equivalent of copper, provided, of course, that nothing else occurs to consume the electrical energy besides the deposition of the metal. This may be verified by replacing the copper-plating bath with a nickel bath and the copper anode with a nickel anode and electroplating nickel onto the gauze cathode. In this case as with the copper almost all of the electrical energy is used in depositing the metal; that is, the "current efficiency" is high. Unfortunately this is not true for all electrolytic processes. Often large amounts of heat are evolved, gases are given off, or secondary oxidation or reduction takes place and the current efficiency becomes much smaller than 100 per cent. The plating bath consists of a solution of 120 g. nickel sulfate heptahydrate, 15 g. ammonium chloride, and 15 g. boric acid in a liter of distilled water. It will be found in the case of nickel electroplating that a current of 0.25 ampere probably cannot be maintained for thirty minutes, and that a current of 0.15 ampere is more satisfactory. In this event, a longer time may be allowed to deposit sufficient nickel to weigh accurately. At the end of the experiment, the student should compare his results for nickel with those for copper. NICKEL-PLATING
If further time remains the student may be interested
Further use of the apparatus described may be made in electroforming one or more small objects. For this purpose a wax mold of the object is made in a small porcelain crucible. The wax is melted in the aucible, a wire or narrow strip of copper is immersed in it, and finally the object to be formed (lightly greased with vaseline or mineral oil) is supported in the surface of the melted wax in such a manner that i t will leave the desired impression in the wax. When the melt has cooled and solidified,the object is carefully removed and the im~ressiondusted with finelv ~owderedpra~hiteor, better -still, bronze powder. ~ h 'bronze pgwier must make good contact with all parts of the mold as well as with the copper strip which is imbedded in the wax and which acts as a conductor of the current during the electroplating. In order to form a strong shell the mold must be copper-plated for a t least one hour starting with fresh battery solution. When this has been accomplished, the mold is removed from the bath and dried with an air blast. Sufficient melted lead (or solder) is then poured in to fill the mold. The electroformed object is removed from the crucible as soon as i t is cool and cleaned with a cloth. It will be necessary to file off any excess lead which may have overflowed the mold and formed a projection a t the side. In writing up the exercise the student is directed to tell briefly what he has done. He should diagram the apparatus, tabulate his data, and compare his results in the electrodeposition of copper with those in the deposition of nickel. In these laboratories the values obtained for Faraday's equivalent have been, on the average, as accurate as the weighing permitted, i. e., to the nearest hundredth gram. For the historical derivations of the names of the various units of electrical measurement-volts, amperes, coulombs, ohms, etc.-the student is referred to a dictionary. He is also asked to state Faraday's laws, and Ohm's law, and to define the terms polarization and current efficiency. Other appropriate questions will come to the mind of the individual instructor. For further reading on the subject of electrochemistry, a bibliography is appended.
SELECTED REFERENCES
(1) MCPHEKSON AND HENDERSON, "An ekmeutary study of chemistry." rev. ed., Ginn & Co., New York City, 1932, 729 pp., Chap. 37. (2) BLUMAND HOGABOOM, "Principles of electroplating and
electroforming." McGraw-Hill Book Co., Inc., New York City, 1924, 356 pp. (3) TIMM."An introduction to chemistry," McGraw-Hill Book Co., Inc., New York City, 1930, 561 pp., Chaps. 34, 35.
