DEMONSTRATION of the PERIODIC PASSIVITY of IRON ROBERT DUBOIS
AND
WILLIAM A. KAYE
Stanford University, California
A
LTHOUGH the periodically alternating passivity and activity of iron which is the anode of a sulfuric acid cell has been known for many years (I),the authors had difficulty in finding in the literature an entirely trustworthy experimental arrangement for demonstration of this phenomenon. A satisfactory apparatus has been constructed by adaptation and modification of former cells (2) A 100-cc. beaker is used in constructing the cell, and in this is placed a cathode consisting of a piece of platinum gauze or foil 1 cm. X 2 cm., fastened to a platinum wire which is sealed into the end of a glass tube filled with mercury and provided with a lead-in wire. The anode is a loop of electrolytic standardizing iron wire consisting of fifteen strands of wire; the loop is 4 cm. long. To support the anode and to provide electrical connection a copper wire with the end bent into a hook is pushed through a glass tube and the loop of iron wire is pulled back into the tuhe just far enough to secure it. The electrolyte is sulfuric acid of specific gravity 1.185 (81 cc. of C.P. acid made up with distilled water to 500 cc.). The cell is also equipped with a thermometer. The electrical arrangement, as shown in the figure,
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acid slightly. The switch is now closed and the current through the ammeter is adjusted to 1 ampere by varying the depth of immersion of the anode and its distance from the cathode. After a short interval the current through the cell decreases gradually and somewhat irregularly and reaches a minimum, constant value in a few seconds. The passivity persists for some time, after which the iron becomes active rapidly and the current through the cell simnltaneously rises to its earlier, high value. The iron remains active for a short time (much shorter than the duration of the passive phase), and the cycle of changes just described is repeated periodically. The experiments of earlier workers (3), and our own experience have shown that not all kinds of iron are capable of showing the phenomenon and that the occurrence of the periodicity and the length of the periods depend especially on (a) the current density, (b) the concentration of the acid, and (c) the temperature. With the conditions described above and a temperature of 20°C. the average period is about 20 seconds. If it is desired to elaborate the demonstration, an interesting addition is to show the periodic rate of evolution of gas from the cell (a), corresponding to the periodic current through the cell. In this case the electrical arrangement is the same, but the cell is made with an air-tight flask, the only outlet being through a tuhe which branches outside the flask, one branch leading to a capillary tube and the other to an open arm manometer of small bore filled with colored water. An image of the manometer arm may he projected onto a screen, where the colored line will advance and retreat in phase with the movements of the ammeter needle. LITERATURE CITED
(1) C. F. SCH~NBEIN,"Einige weitere Beobachtungen iiber
Verhalten des Eisens gegen die Saltpetersiiure." Pogg. Ann., consists of a 110-volt source of direct current con- das 38, 444 (1836); J. P. JOULE, "On the Intermittent Character of nected through a rheostat, R, of 45 ohms resistance the Voltaic C m n t in Certain Cases of Electrolysis." Phil. and 5 amperes current capacity, and a key to the termi- Mag. [31,24,106 (1844). (2) J. L. BUCHAN AND J. G . N. GASKIN,Nature. 1.79, 209 nals of a voltmeter, V, of 10 volts range. Across the (1932); and references (I), (3), and (4). terminals of the voltmeter are connected, first, the (3) W. OSTWALD."Periodische Erscheinungen bei der cell in series with an ammeter, A, reading to 1 am- AuflBsung des Chroms in Singen. 11," 2. physik. Chem., 35,204 (1900); C. FREDENHAOEN, "Uber die Passivitit des Eisens und pere, and, second, a resistance of 0.7 ohm, made of iiber an Eisenelektroden beobacht.de periodische Erscheinungen," resistance wire, W. Care must be taken that the ibid., 43, 1 (1903); C. FREDENHAGEN, "Zusammenfassende Darstellung iiher der verschiedenen Passjvitatstheorien und neue iron electrode is the anode of the cell. Beabachtungen iiber die Passivitit von Eisen, Nickel, und To start the periods the key switch is closed with Chrom." ibid.. 63, 1 (1908); ALFRED ADLER. "Untersuchungen the iron removed from the cell, and the rheostat R iiber die an passivierbaren Elektroden auftretende periodische ibid., 80, 385 (1912). is adjusted so that the voltmeter reads 2 volts. The Erscheinungen," "Periodische Erscheinungen bei der Auflb(4) W. OSTWALD, switch is then opened, and the iron is dipped into the sung des Chroms in SBuren," ibid.,35,33 (1900). I108
