The Chemistry Student
I'
AN EXPERIMENT ILLUSTRATING VOLTAIC POLARIZATION* H. P. CADYAND ROBERT TART,UNIVERS~TY O F KANSAS, LAWRENCE, KANSAS
One of the most difficult 'ideas to convey to the student of electrochemistry is that polarization occurring in voltaic cells is not due primarily to the increased resistance caused by the formation of gas a t the electrodes. The reason for the difficulty is no doubt of historical origin. The early investigators on Volta's cells recognized that the drop in voltage, whicb occurred when a cell was used, was accompanied by the formation of gas at the electrode (or electrodes) and stated that the polarization, i. e., the drop in potential across the cell with use, was due to the high resistance built up by the insulating layer of gas bubbles. This explanation has been continued, until it is found a t the present day in the majority of textbooks on general physics and even in chemical texts treating this topic. If polarization be defined as a decrease in the potential of a voltaic cell due to changes produced during the passage of the current, we can, for purposes of arriving at a satisfactory explanation of polarization, consider that the phenomenon is due to a t least'three effects: (1) concentration changes around the electrodes, i. e., concentration polarization; (2) a change in the nature of the electrode (or electrodes), i. e., chemical polarization; (3) an increase in the internal resistance of the cell due to a film of gas, other non-conducting material, etc., i. e., mechanical polarization. In addition to these three, another effect which opposes the passage of the current is recognized and is classed by most students of electrochemistry under the heading overvoltage.' The polarization effects, particularly the first and second, may be clearly shown by a simple study of one of Volta's original cells. In Volta's cell, copper serves as the positive electrode, zinc as the negative, and the electrolyte consists of dilute sulfuric acid. If a small current be taken from these cells and voltage-time readings be made and plotted, results such as those shown in Figure I will be obtained. If the copper electrode is cleaned before use, i. e., freed from copper
* Presented before the Division of Chemical Education at the St. Louis Meeting of the American Chemical Society. April 18, 1928. Overvoltage may be defined ss a deviation from the equilibrium electrode potential due to incomplete reversihility of the electrode reaction. Cf. Creighton, "Electrochemistry," John Wiley & Sons, Inc., New York City, 1928, 2nd ed., p. 247.
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compounds a t its surface, data as shown in Figure 1-4 will be obtained. If the electrode contains some copper compounds initially and these are not removed, results as shown in Figure 1-1 may be obtained. If the compounds (oxidized copper) are present in still greater quantity, then data as represented by the curve in Figure 1-3 may be obtained. Finally, if copper sulfate (i. e., a large supply of Cu++) is actually added to the electrolyte, the voltage of the cell remains a t its high Q 1, ! ! ! ! ! ! ! ! initial value forlonrrueriods at! , -. of time. (See Figure 1-21 0 4 8 I2 16 20 24 28 32 36 40 On the basis of the definition Ttma (mwwtas) of polarization given these F I G ~ RI E could be explained as follows: The cell C U ~ H ~ S O ~ has ~ZU its initial value (about 1 volt) due to the fact that the positive electrode is actually CulCu+" E and the negative
,
I
I u
l
ZnlZn++
The initial copper-ion concentration in those cases where it is not actually added, is produced as a result of the solution of the copper compounds on the surface of the electrode. When the cell is used then the reaction a t the positive electrode c u t + + 2 c = Cu" takes place, tending to remove Cu++. When these are used up the next available ion discharges, which is in this case H+, but the potential H2IHC
is much lower than CU~CU++ and consequently the cell will have a smaller potential due to a change in the nature of the electrode. The original slow slope is due therefore to a change in the concentration of the ions around the electrodes, the copper-ion concentration diminishing, the zincion concentration increasing; that is, to concentration polarization. The sudden break, however, is due to chemical polarization for the cell becomes
Of course, a t this lower potential hydrogen is liberated but it is not the increased electrical resistance of the hydrogen bubbles that produces the drop in potential across the cell. Actual measurements of the cell to be described have shown that within the range of experimental error the resistance of the cell remained the same, i. e., 0.5 of an ohm. To produce a diierence of 0.5 of a volt (the measured value in the case mentioned) the internal resistance should have increased by 50 or more ohms if the polarization were due solely to this cause. The variation in the time necessary to reduce the cell potential to its final value in the cases 4, 1, 3, and 2 (Figure I) are, of course, due to the increasing amounts of copper ion present in each case. When copper compounds are actually added to the cell (2, Figure I) the voltage maintains a high value for a considerable period of time (depending, of course, on the amount of copper compound originally added). It has been the custom to speak of those substances which tend to maintain the cell a t values I close to their original values Aas depolarizers. Copper ion is 8- Level~ngbulb therefore a depolarizer for this c-C,U ,,,,7, r simple voltaic cell as it prevents chemical polarization, that is, a change in the nature of the positive electrode. On the older view, depolarizers FIG- 11 were substances which removed the gas from the electrodes as fast as it was formed. From the modern view, depolarizers are substances which produce higher electrode potentials a t the positive electrode than hydrogen; a t the negative electrode a depolarizer would be any substance producing a lower electrode potential than oxygen. Or, to put it another way, a depolarizer is a more powerful oxidizing agent than hydrogen ion, or a more powerful reducing agent than hydroxyl ion. To show these results as a laboratory experiment, a cell may be made from ordinary laboratory apparatus as shown in Figure 11. In this figure (11), A is a Gooch filter funnel connected by rubber tubing to the leveling bulb B. The copper electrode C is a piece of copper gauze bent to fit the inside of the funnel, one strand of the gauze being carried through the stopper;(; to make electrical contact. The copper gauze should not be cleaned, i. e., there should be a good oxide coating on its
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surface to insure the presence of copper ions when brought in contact with the electrolyte. D is the zinc electrode which is amalgamated before use. E and F are glass tubes for introducing and removing natural gas. The voltmeters we have used for the voltage measurements are Model 280 Weston instruments. These are particularly satisfactory for this purpose as they draw on closed circuit an amount of current sufficient to show the polarization effects in a reasonable length of time. Of course other types of voltmeters could be used hut if they are of higher resistance than the Model 280 instruments2 some means of drawing a small current (0.01 to 0.02 ampere) should be provided. The electrolyte is placed in the leveling bulb closed by a screw clip, and upon closing the voltmeter circuit thesolution is allowed to flow around the electrodes. We have used 6 N sulfuric acid & the electrolyte as this was the most convenient to use. Other concentrations of acid might of course be used. When the copper gauze is completely immersed (save for the single strand) voltage readings are taken a t one- or twwminute intervals. After the voltage reaches a constant value for some minutes the readings are discontinued. In order to vary the amount of .coppercompound presentj.bhe.eledro7 ... lyte is removed and the electrodes -allowed to .stand exposed. .to..ainfer,. two or three minutes, the reaction 2Cu
+ 2H2SO4 + O1 = ?CuSO. + 2Hz0
taking place upon the copper as i t is still wet by the acid. The electrolyte may be returned and another set of time-voltage data obtained. To obtain the results with the "clean" electrode, the cell is discharged to its lowest voltage value and natural gas introduced as the electrolyte is slowly removed by lowering the leveling bulb. The function of the gas is to provide an oxygen-free atmosphere. The electrolyte is removed from the leveling bulb and gas bubbled through it in order to free it as much as possible from dissolved oxygen. This dissolved oxygen would oxidize some of the copper to copper compound. Upon returning the oxygenfree electrolyte and taking another set of time-voltage data results such as are shown diagrammatically in Figure 1-4 are obtained. The initial reading of about 1 volt followed by the almost immediate drop indicates that, despite the precautions observed, a trace of copper had been oxidized. In order to obtain data as shown in Figure 1-2, enough copper sulfate should be added to color the solution distinctly blue. It is also instructive to use the same pair of electrodes in alkaline solution (2N NaOH) thus forming a Lalande type cell. Results such as are shown in Figure 1-5, -6, and -7 can be obtained. l Aceording to the manufacturer, the resistance of the Model 280 voltmeter is approximately 62 ohms per volt.
Figure 1-5shows the initial electrode with its copper oxide coating having the initial value of approximately 1.2 volts. This is rapidly used up but soon reaches a new level in the neighborhood of 0.3 volt and is due evidently to the electrode O*IOH-
the oxygen being that present dissolved in the electrolyte; for if the electrolyte is removed and freed of oxygen, the copper electrode in the meanwhile being allowed to stand exposed to air, data such as are shown in Figure 1-6 will be obtained. ' The "hump" a t 0.3 volt is clearly missing. It will also be noticed that the initial value is not as high as the original value, the reason lying in the fact that, in the presence of alkali, copper is oxidized to cuprous oxide, so that the positive electrode is cuicu+
rather than cu1cu++
The very rapid drop to the final value, Figure 1-7, is obtained in the same way as described for Figure 1-4. It should also be noted that the final value of the cell, which is of the same type as that finally obtained in the acid solution H3IHflNaOHIZn++lZn
is much lower than that in the acid solution. This is due, of course, to the smaller hydrogen-ion concentration in the alkali cell, corresponding to a lower positive electrode potential. The two sets of final as results. however. are not altogether comparable as the electrode potential of the negative electrode is certainly not the same in the two solutions, although they cannot be greatly different. The special polarization effect which is spoken of as I 1 -02 -\ CPbC overvoltage may also be illustrated by a modification of 01 I the same experiment. If other 1 1 0.0 o 4 s 12 16 20 z+ z8 32 36 40 metals are substituted for Tme (pnutes) copper in either cell, the final FIGURE111 value, that is, the hydrogen value, will be found to be different from that obtained with copper. The results obtained by using six different metals as the positive electrodein dilute sulfuric acid are shown
lob~. I I I I
,
I l I I
*?I
in Figure 111. Thin sheet metals were used, the area of each being approximately 36 square cm. A comparison of the finale. m. f. of the six different cells will show the order of overvoltage a t approximately the same current density. As the current was less than 0.01 of an ampere in any case, the overvoltages should correspond to those values determined by static methods.3 Using Caspari's value of 0.09 volt as the overvoltage of hydrogen against smooth platinum, the overvoltages of the other five metals by difference are as follows: '
positive eleetrde of cell
Overvoltage of hydrogen
Final
e. m. f.
Pt Cu
0.71 0.48 0.41 0.28 0.18 0.02
Ag
Zn Pb Hg
0.09 (assumed) 0.32 0.39 . 0.52 0.62 0.78
The overvoltages were calculated by subtracting the final e. m. f. of the cell from that of platinum and adding 0.09 to the difference. A comparison of these values with those of Caspari's are given below: HYDROGEN OVI~RVOLTAGE Metal
Pt (smooth) Cu Ag Sn Pb Hg
C. and T.
Canpari
0.09 0.23 0.15 0.53 0.64 0.78
,
... 0.32 r 0.39 0.52 0.62 0.78
Fair agreement is shown except in the case of silver, which is not in the same order as in our list. A number of determinations have been made for this particular cell, giving results showing agreement within a few hundredths of a volt. The silver used is that furnished to the jewelry trade and marked 9995 fine. I t should be borne in mind that the discrepancies such as are shown in the case of silver and copper could be attributedto either the purity of the metal or the condition of the surface, as i t is well known that both of these factors, as well as the method used, determine the actual value of hydrogen overvoltages obtained by different investigators. J
Such as that used by Caspari, Zeil. Phys. Chem., 30,89 (1899).
