A NEW LABORATORY PROCEDURE FOR THE ELECTROLYTIC DETERMINATION OF THE COPPER CONTENT OF BRASS
Frequently in a small laboratory a teacher of quantitative analysis finds himself unable to give his students any exercises in electroanalysis because he has no source of direct current. It is the purpose of this report to show how such a teacher may give his students practice in this very fundamental phase of quantitative analysis even though he has only the customary 110 volt A. C. current a t his disposal. Apparatus A very simple form of "chemical rectifier" may easily be made from apparatus common to every chemical storeroom. The following materials are needed: four 250-cc. wide-month bottles (gas collecting bottles) ; four two-hole rubber stoppers to fit; four pieces of sheet aluminum, 1" X 6"; four pieces of sheet lead, same size as the sheets of aluminum; a little copper wire; about a liter of saturated NaHC03 solution; two cleat base receptacles; and two sixty-watt lamps. The metal plates should be cut down a t the top so that they may go through the holes of the stoppers, and protrude far enough on top to afford opportunity to make the connections. It is best to drill a third hole in each stopper to allow the gases liberated within the bottles to escape. The wire connections are made according to the wiring diagram, Figs. 1 and 2. The quantity of current desired can be regulated somewhat by the size of the lamps used in the hookup. The larger the lamp, the greater the quantity of current. It was found that two sixty-watt lamps gave a current that was correct for the electrodepositions studied in this report. In this work, as indicated by the diagrams, Figs. 1 and 2, a voltmeter, an ammeter, and a rheostat were available, and therefore used. It was found, however, that when conditions were standardized as specified, that these instruments were not necessary unless a more detailed check on current conditions was desired than that ordinarily necessary in beginning wrrk. A platinum gauze electrode of approximately 103 cm.= area was employed as the cathode. The anode was an ordinary platinum wire about 3 cm. long.
Experimental The experimental work is divided into two parts as follows: (1) An experimental demonstration that this source of direct current gives an analytical result in the hands of beginning students in quantitative analysis
which is sufficiently accurate to be within the limits of experimental error. (2) The development of a curve for the rate of deposition of copper-from a 0.25% solution of copper sulfate.
Part One Twelve average students in beginning quantitative analysis were required to run electrodepositions on the copper content of brass using the "chemical rectifier" as a source of current, and then to repeat the determination using a bank of storage batteries for the source of current.
I = Pb plate W = Decomposition cell A = Ammeter
0 = A1 plate S = Switch
R = Rheostat
L = Lamp V = Voltmeter C = Auxiliary cell
The Foulk method (Foulk, "Notes on Quantitative Chemical Analysis," page 233) was employed, except that the gauze electrode mentioned above was substituted for the flag electrode specified by Foulk. Since the experiments conducted demonstrated the errors between the two methods of analysis to be in the main smaller than the limit of experimental error allowed beginning students, it was concluded that the apparatus gave sufficiently accurate results to be used in beginning quantitative analysis.
Part Two We were interested in finding out something concerning the rate of deposition of copper under the conditions of the experiment. In using the
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CHEMICAL EDUCATION
MARCH, 1927
"chemical rectifier" it was noted that some few minutes were required before the current conditions would settle down and become constant. Therefore, it was necessary t o place an auxiliary cell, C (figure two) in the circuit until the rectifier had attained a constant rate of current delivery. The switch Szwas then thrown so as t o send the current through the decomposition cell, W , and the measurements made. Time mtn.
15 30 60
0 120 150 180 225 240 339
Wt. of Cu deposit
0.0107 g. 0.0259 g. 0.0524 g. 0.0765 g. 0.0909 g. 0.1058 g. 0.1087 g. 0.1169g. 0.1186g. 0.1189 g.
I TABLE Amps.
0.05 0.06 0.055 0.06 0.00 0.06 11. OD 0.085 0.09 0.06
Volts
5.5 5.5 5.5 5.5 5.0 5.5 6.0 6.0 6.0 5.0
Tmp.
21 20 25.5 24 20.5 19.6 20 22 21 21
Conclusions 1. The apparatus described above is very satisfactory for student use in practice determinations of the copper content of brass. 2. Under the experimental conditions used in part two, the copper content of 200 cc. of approximately 0.25% copper sulfate solution may he deposited in 240 minutes.
