STUDIES ON ORGANIC DEPOLARIZERS’ W. H. HUNTER AND L. F. STONE School of Chemistry, University of Minnesota, Minneapolis, Minnesota Received December 7, 1934
Determinations of single potentials of cathodes against organic depolarizers made by Hunter and Wernlund (8) as a preliminary to comparing electrolytic reductions of organic compounds with those carried out by chemical reagents showed that the order of the cathodes used, based on their single potentials, was essentially the same with different depolarizers. The present work extended these measurements using the electrodes platinum, nickel, gold, silver, and tin. The results obtained agree with those of Hunter and Wernlund and have led to a further simplification of the theory of depolarization values by including the idea of the “electron affinity” of the depolarizer. Physicists have made a very thorough study of the emission of electrons into gases in their work on the photoelectric effect, contact potentials, resonance and ionization potentiaIs, and electron emission from hot bodies. They have made it clear that the emission of electrons by metals into gases involves the performance of work, and that the work required is different for each metal and is greatly influenced by the nature of the gas surrounding the emitting surface. It is thus certain that their values found for the work function can not be transferred without change to aqueous solutions, but it is equally certain that the idea of the work function should be used in a study on organic depolarizers. When an electrode with no current flowing is placed in a solution containing a depolarizer, which may be defined as anything whose presence causes a cathode to become more positive against a solution than it was before the material was added, electrons will be removed from the electrode by the depolarizer. This will result in a continuous and usually rapid increase in the positiveness of the electrode until equilibrium is reached. The single potential of the cathode with the current turned on passes through a minimum “positive” value, owing to the fact that when a depolarizer molecule takes an electron from the metal it then unites with a hydrogen This article is based upon the thesis of L.F.Stone, submitted t o the Graduate School of the University of Minnesota in partial fulfilment of the requirements for the degree of Doctor of Philosophy, June, 1927. The manuscript was prepared by the junior author after the death of Dr. Hunter in 1931.-L. I. Smith.
1139 THE JOURNAL OF PHYSICAL CHEMISTRY, VOL. XXXIX, NO.
8
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W. H. HUNTER AND L. F. BTONE
ion, thus producing a change in the ratio of the depolarizer to its reduction product and resulting in a gradual increase in the potential. The magnitude of the increased positiveness obtained with any one depolarizer will depend to a large extent on the work required to remove electrons from that cathode or on the work function under the conditions of the experiment. Other effects will have an influence on the actual value of the single potential, but in general their magnitude will be small and will be of the same order with any one depolarizer except insofar as the
FIQ.1.
SINQLE
POTENTIALS OF CATHODES AQAINST DIFFERENT DEPOLARIZERS
effects are due to the electrode material. Thus the value of the single potential of a cathode in contact with a given depolarizer will be largely dependent on the work function of that electrode and will therefore vary with different metals. As shown in figure 1 the values of the single potentials of the cathodes in contact with a given depolarizer, such as quinone, become more negative in the following order: platinum, gold, silver, nickel, and tin. The values of the work function of these electrodes, under the conditions of our experiments, increase in the same order.
STUDIES ON ORGANIC DEPOLARIZERS
1141
The values of the single potentials of nickel and tin are often complicated, owing to the fact that in the presence of some depolarizers these cathodes become sufficiently positive to send their ions into solution. When a tin electrode, for example, is placed in a depolarizer solution, some of the electrons will be removed and the force holding the tin ions in the metal therefore becomes less. As the normal potential of the electrode is approached, this force is overcome and metal ions enter the solution. The potential will then remain close to the value of the normal potential for tin, the actual value depending on the number of ions sent into the solution as well as on the depolarizer and its reduction products. Solution of metal ions will occur with any electrode that is in contact with a depolarizer that has a sufficient force of attraction for electrons, or “electron affinity,” to reduce the potential of the electrode to its normal potential against its own ions. The effect of the work function thus appears to be masked to some extent in these cases, owing to the introduction of a new depolarizer,the metal ion. I n determining the single potential of a cathode against various depolarizers it will be found that some of the latter decrease the potential very slightly, while with others the effect is very marked. Every depolarizer will have a definite potential with a given cathode, and that position will be determined by the electron affinity of that depolarizer. It is well known that the potential of a pure substance has an infinite value and will not give the “definite potential” noted above except when a finite quantity of the reduced material is also present, and the value under these conditions will be dependent upon the ratio of the concentration of the depolarizer to its reduction product. The value of the electron affinity of any one depolarizer depends on two parts of the molecule: the “characteristic group,” which is that part of the compound that takes up electrons from the cathode, and the rest of the depolarizer molecule. Different groups bary considerably in their force of attraction for electrons, while the rest of the compound has very little effect on the single potential and probably exerts its influence only through the characteristic group. The difference between the single potentials of nitrobenzene and nitrosobenzene, for example, would be much larger than that between nitrobenzene and m-nitroaniline. Although the actual values of electron affinity can not be determined, we believe that relative values for different depolarizers can be deduced from a study of the data obtained in our experiments. The values of the single potentials obtained in our work with different depolarizers against the cathodes platinum, gold, silver, nickel, and tin are given in figures 1 and 2. The values of the single potentials of several of the depolarizers against platinum, given in figure 2, were obtained from data given by other investigators. The single potential values of formaldehyde, acetaldehyde, pyridine, and acetone were obtained from the work carried out by Hunter
1142
W. H. HUNTER AND L. F. STONE
and Wernlund (8) under conditions identical with those of the present work. The values for I-, IZ (solid) given by Thompson (11) and those for OH-, O z ;Br-, Brz (liquid); C1-, Clz (gas); and F-, Fz (gas) given by MacDougall (9) were obtained in studies in which there was no external electromotive force impressed on the cell, and are therefore slightly more positive than they would be under the conditions of our experiments.
FIQ.2. SINQLEPOTENTIALS OF PLATINUM AQAINST DIFFERENT DEPOLARIZERS
I n figure 1 it may be seen that the depolarizers tested against gold, in the order of decreasing positiveness of single potential, are as follows: quinone; ferro-ferri; m-nitroaniline; azobenzenesulfonic acid; and hydrogen ion. On the basis of our theory the relative values of the electron affinity of these depolarizers decrease in the same order. This order of the depolarizers is the same as that obtained on the other electrodes with the exception
STUDIES ON ORGANIC DEPOLARIZERS
1143
of three points. I n these cases the difference in the values is very small and may well be due to other effects not considered by us. Assuming that all cations present in a catholyte may be considered as depolarizers brings the concept of hydrogen overvoltage into accord with our general picture of cathodic action. Thus, the single potential of quinone against the cathodes tested becomes more negative in the order: platinum, gold, silver, nickel, and tin. This is also the order of the metals against the hydrogen ion. I n the latter case the actual value of the single potential on a given cathode is probably not dependent on the electron affinity of the depolarizer alone but also on other effects, such as adsorption of hydrogen into the metal and gas films. The failure of the hydrogen discharge point to “space” as well as other depolarizers as regards their distances on the scale of potentials given in figure 1may well be due to these latter, acting not as primary causes of overvoltage but as secondary effects superimposed on that of electron affinity. It therefore seems possible to speak of the overvoltage of any depolarizer as well as of hydrogen and with the same meaning,-the increase of potential necessary to discharge it on a given cathode over and above that required on platinum. It thus appears that the “electron affinity” of the depolarizer and the “work function” of the electrode are definite properties of the depolarizer and electrode, respectively, and that it should be possible to estimate the single potential of a depolarizer on a given cathode when its value on platinum is known. Although the present work did not include the formulatjon of an equation for calculating the electromotive force of a cell, we believe that an equation, perhaps of the Nernst type, should also include other terms that are a function of the “electron affinity” of the depolarizer and the “work function” of the electrode. EXPERIMENTAL
Electrodes All electrodes were 3.8 cm. in diameter and 1.5 mm. in thickness. Gold electrodes made from sheet metal, 99.99 per cent pure, were plated using the solution given by Blum and Hogaboom (1). The silver electrode made from sheet silver, 99.99 per cent pure, was plated using the solution given by Blum and Hogaboom (2). Platinum electrodes made from sheet platinum were plated using the bath recommended by Findlay (7). Tin electrodes containing 99.97 per cent tin and 0.03 per cent lead were plated using the solution given by Blum and Hogaboom (3). Nickel electrodes, prepared from electrolytic nickel, were plated using the solution recommended by Blum and Hogaboom (4). Depolarizers Quinone, prepared from hydroquinone by the method given by Vliet (12), was recrystallized from benzene and sublimed twice. A fresh 0.015
1144
W. H. HUNTER AND L. F. STONE
molar solution in 2 N sulfuric acid was made for each run. Twenty-five grams of m-nitroaniline, m.p. 112.5"C. after recrystallization twice from water, was treated with 10 cc. of concentrated sulfuric acid to form the sulfate, and dissolved in 2 N sulfuric acid to give the 0.181 molar solution used in the experiments. The ferrous-ferric solutions, having a ratio of 1: 1, were made from Merck's Blue Label iron sulfates. Azobenzenesulfonic acid, prepared from azobenzene, was recrystallized from water and dissolved in 2 N sulfuric acid to give a 0.01 molar solution. 3,3'-Diaminoazoxybenzene, m.p. 145-147"C., made from m-nitroaniline according to the procedure given by Meldola and Andrews (lo), was recrystallized from alcohol, and a 0.002 molar solution in 2 N sulfuric acid was made for use in the runs on platinum.
FIG. 3. THEAPPARATUS
Apparatus (see j i g . 3) The cathode chamber of the cell was 10 cm. in height and 4.5 cm. in diameter, and was separated from the anode chamber by a stopcock. Current from the storage batteries, used to impress an external electromotive force on the cell, was regulated by a resistance box (R) and'was measured by a milliammeter (A). The cathode (E) was connected to a Leeds and Northrup type K potentiometer (P) and the cell completed by the calomel electrode (C), salt bridge, and capillary tube. The calomel electrodes, prepared from distilled mercury, fresh calomel, and a saturated solution of recrystallized Merck's Blue Label potassium chloride, were
1145
STUDIES ON ORGANIC DEPOLARIZERS
frequently checked against each other and showed variations of the order of 0.1 to 0.05 millivolt. Typical runs I n method A 75 cc. of 2 N sulfuric acid was added to the cell after all connections were made. A constant current of 50 milliamperes was passed through the cell and readings taken, constant stirring being maintained by means of an electrically driven glass stirrer. The stopcock TABLE 1 Results obtained with the depolarizers m-nitroaniline, 3,S’-diaminoazoxybenzene,quinhydrone, and hydrogen i o n AVERAGE
MINUTES AT MILLIAMPEREB
Platinum .......................... Gold .............................. Gold .............................. Silver.. ...........................
AVERAGE V A L U E OF E h AT
vAiz&F5Eh 50
ELECTRODE MATERIAL
M.N.A. M.N.A. M.N.A. M.N.A.
Nickel ............................ M.N.A. Nickel.. . . . . . . . . . . . . . . . . . . . . . . M.N.A. Platinum. ........................ OXY
A
B A B A B A B A B B
’
0.3012+ 0.2889+ 0.0467+ 0.0162+ 0.0404+ 0.0272+ 0.24410.24510.1020+ 0.0964+ 0.0116-
0.21010.2276+ 0.6970+
B B B B B
0.02120.47520.48890.86420.4738-
* Saturated solution. t Data given are the average of the final, nearly consta t values determined over a period of three t o four hours.
separating the chambers was then closed and 50 cc. of the 2 N sulfuric acid in the cathode chamber was replaced with 50 cc. of a depolarizer solution. The stopcock was then opened and readings again taken. I n method B the depolarizer solution was added to the cell, and readings taken before and after turning on the current of 50 milliamperes. Preliminary runs indicated that the values of the single potentials of the electrodes in contact with the various depolarizers passed through the minimum “positive” value noted above, and were becoming more negative very slowly, approximately five minutes after the addition of the depolarizer, in the case of method A, and five minutes after the current of 50
TABLE 2 Results obtained with the d Nolarizers quinone, jerro-ferri, and Sn++ DEPOLARIZER
FLECTRODE MATERIAL
IN 2
N H2S04
B B B B B
Platinum. . . . . . . . . . . . . . . , . . . . . . . . . Gold. . . . . . . . . . . . . . Silyer. . . . . . . . . . . . . . . . . . . . . . . . . , . . Tin*. ... . . . , . , . . . . , . , . . , . , , . . , . . . Nickelt. , . . . . . . . . . . . . . . . . . . , . . Platinum. . . . . . . . . ... . . , . , . , . . , . , . . Platinum. . . . . . . . . . . . . , . . . . . . , . . . Gold. . . . . . . . . . . . . . . . . . . . . . . . . . . . Gold., . , . . . . . . . . . . . . . , . , . . . . , . . . Silver. .. . . . . . . . . . . . . . . . . . . . . , , . . . , Silver. . . , . . . , . . . . . . , . . . , . . , , . . , . , Tin*. . . . . . . . . . . . . . . . . . . . , . . . . , . Tin*. . , . . . . . . . . . . , . . . . . . . . , . . . . . . . Nickelt. . . . . . . . . . . . . . . . . . . . . . . . . .
... .
.
.
.. .
. .
.
. .
.
A B A B A B A B
. .
.
IETHO
.
.
A B B
Tin. . . . . . , . . . . . . . . . . . . . . . . . . . . . , . .
AVERAGE V A L U E OF Eh AFTER 6 M I N U T E S AT 5( MILLIAMPEREL
0.6703+ 0.6045+ 0.5925+ 0.23630,3268+ 0.6197+ 0.6244+ 0.5938+ 0.5965+ 0.6073+ 0.6159+ 0.24880.24820.3418+ 0.3491+ 0.2348-
AVERADE V A L U E OF Eh
A T 0 MILLIAMPERES
0.7804+ 0.6481+ 0.20630.3569+ 0.6684+ 0.6666+ 0.6604+ 0.20670.3797+ 0.2036-
* A positive test for tin ions in the solution was obtained a t the end of the run. t A positive test for nickel ions in the solution was obtained at the end of the run. $ Molar solution of tin ions.
TABLE 3 Results obtained with the depolarizers azobenzenesulfonic acid and guinone-guinhydrone AVERAQE V A L U E O F Eh AFTER 5 MINUTES AT 50 MILLIAMPE E 9
DEPOLARIZER
ELECTRODE MATERIAL
IN 2
N HzSO4
-
.
Platinum. . . . . . . . . . . . . . . . . . . . . . . . . . , Gold.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
........................
....
Tin*. . . . . . . . . . Nickelt.. . . . . Platinum.. . . . . . . . . . , . . . . . . . . . . . . . . . . Gold,. . , , . . . , . . . . . . . . . . , . . . . . . . . . . . . . Silver ...... . . . . . . . . . . . . . . . . . . . . . . . . , ............ Nickel.. . . . . , . . . Tin ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.
.
.
.
Azo Azo Azo Azo Azo
Q-Q I Q-Q Q-Q Q-Q Q-Q
B B B B B
AVERAGE V A L U E OF Eh AT 0 MILLIAMPERES
0.0584+ 0.27450.32030.57290.32730.7452+ 0.7433+ 0.6792+ 0.3517+ 0,1994-
* A negative test for tin ions in the solution was obtained a t the end of the run.
t A negative test for nickel ions in the solution was obtained a t the end of the run.
$ Saturated solution quinone-quinhydrone. Constant values for the platinum and gold electrodes were reached in about five minutes. An initial value of 0.6552 was obtained for silver, the final constant value of 0.6792 being reached in about two hours. The value for the nickel electrode was not constant, probably owing t o the complexity of the solution. The value given for the tin electrode was fairly constant. Tests on the solutions in contact with the latter two electrodes showed the presence of nickel and tin ions, respectively. 1146
1147 ,
STUDIES ON ORGANIC DEPOLARIZERS
milliamperes was turned on, in the case of method B. All of the values given in the tables are based on readings taken five minutes after the start of the run. These readings were corrected to 25°C. and 760 mm., using the data given by Fales and Mudge (6). These corrected values were reduced to the normal hydrogen scale (EA)by adding algebraically 0.2464, the value given by Clark ( 5 ) for the saturated calomel electrode, referred to the normal hydrogen electrode a t 0°C. Although readings were taken to four places, the true value is not certain closer than a few millivolts when measurements are made during reduction.
+
Tables The data obtained on the cathodes against the various depolarizers tested are given in tables 1,2, and 3, and in figures 1and 2. Abbreviations used are as follows: azo = azobenzenesulfonic acid; oxy = 3,3’-diaminoazoxybenzene; M.N.A. = m-nitroaniline; Q = quinone; Q-Q = quinonequinhydrone. SUMMARY
1. The single potentials of several depolarizers have been measured against different cathodes. 2. The idea of the “work function” of electrodes has been developed as applied to electrolytic action, and the variation of its value with different metals has been explained, 3. The “electron affinity” of depolarizers has been defined and its function discussed. 4. A new view of “overvoltage” has been developed along the lines indicated by our theory. REFERENCES BLUMAND HOQABOOM: Principles of Electroplating and Electroforming, p. 307. McGraw-Hill Book Co., New York (1924). Reference 1, p. 299. Reference 1, p. 287. Reference 1, p. 237. CLARK:The Determination of Hydrogen Ions,, p. 456. The Williams & Wilkins Co., Baltimore (1923). FALESAND MUDQE:J. Am. Chem. SOC.42, 2434 (1920). FINDLAY: Practical Physical Chemistry, p. 152. Longmans, Green and Co., New York (1923). HUNTER, W. H., AND WERNLUND,C. W.: Unpublished work, University of Minnesota. MACDOUQALL: Thermodynamics and Chemistry, p. 325. John Wiley and Sons, New York (1921). AND ANDREWS: J. Am. Chem. SOC.69, 7 (1896). MELDOLA THOMPBON: Theoretical and Applied Electrochemistry, p. 87. The Macmillan Co., New York (1925). VLIET: Organic Synthesis, Vol. 2, p. 85. John Wiley and Sons, New York (1922).
