OVER-VOLTAGE AKD MOISATOhCIIC HYDROGEN BY WILDER D. BANCROFT
When an attack along any given line has failed for many years to yield any appreciable progress, it is always a good plan to see whether a radically different formulation of the problem may not help matters considerably. For more than fifteen years we have been trying to explain over-voltage on the assumption that the electrolytic decomposition of water is essentially a reversible one. This made it necessary to account for the abnormally large voltages observed a t mercury cathodes and platinum anodes for instance. The total result is anything but gratifying from a theoretical stand-point. It is, therefore, desirable to see whether we can make more progress by starting with the assumption that the electrolytic decomposition of water is essentially an irreversible process, an intermediate product being obtained a t the cathode which is a stronger reducing agent than hydrogen and which gives rise to hydrogen relatively slowly, while an intermediate product is obtained a t the anode which is a stronger oxidizing agent than oxygen and which gives rise to oxygen relatively slowly. We can make the requirements more definite. The intermediate cathode product must be a stronger reducing agent than cadmium, because cadmium can be precipitated electrolytically from a distinctly acid solution. The intermediate anode product must be a stronger oxidizing agent than ozone, hydrogen peroxide, persulphuric acid, or lead peroxide, because these substances may be formed electrolytically a t the anode before oxygen is set free. The potentials of intermediate products having these properties are evidently sufficient to account for the over-voltages and the problem is, therefore, to account for the values which we have hitherto called the normal ones. This is easily done, because the observed potentials will coincide with the values for the reversible evolution of hydrogen and oxygen, if specific electrodes catalyze the reaction to such an extent that the concentra-
tions of the intermediate products do not rise above equilibrium values. This line of attack has been developed by Bennett.‘ The intermediate products are not necessarily compounds of the electrodes, because water is decomposed by radium emanation or by ultra-violet ,light, the reaction products being hydrogen, and hydrogen peroxide with varying amounts of oxygen depending on the conditions of the experiment. Since hydrogen gas does not reduce oxygen gas t o hydrogen peroxide, an active form of hydrogen must have been set free even though no electrodes were present. Any hydrides or oxides which may be formed during electrolysis with metal electrodes are, therefore, substances having lower potentials than the intermediate products and these latter must be special modifications of hydrogen and of oxygen. Bennett shows that Langmuir’s electrically neutral monatomic hydrogen has all the properties which we need to ascribe a t present to the intermediate cathode product and he, therefore, concludes that this substance is electrically neutral monatomic hydrogen while the corresponding anode product is probably electrically neutral monatomic oxygen. n‘hen the reaction 2H -+ H2 takes place relatively slowl)-, we get high over-voltage. a‘hen the reaction takes place relatively slowly, we get low over-voltage. This point of view is not new; but it is timely. Ostwald3 referred to it as a possible hydrothesis and so did E. hZtiller.4 I t was brought forward seriously by Tafel” and was urged again by Lewis and Jackson6 and by Brunner.‘ These men advanced the theory only to meet special cases and it seems not to have been accepted by anybody else. Bennett has emphasized the irreversibility and has generalized the application. The work 1 2
4
Jour. Phys. Chem., 20, 296 (1916). Soddy: “The Chemistry of Radio-Elements,” 11 (1915). Zeit. Elektrochemie, 6, 40 (1889).
Zeit. anorg. Chem., 26, J J (1901). Zeit. phys. Chem., 34, 2 0 0 (1900); 50, 641, 713 (19oj). Proc. Am. Alead.,41, 399; Zeit. phys. Chem., 56, 207 (1906). Zeit. phys. Chem., 56, 331 (1906).
Wilder D . Bancrojt of Langmuir gives an independent experimental confirmation which was lacking when Tafel and when Lewis and Jackson wrote. The test of such a theory is its usefulness in accounting for other phenomena. The rapid breaking down of sodium amalgam when certain impurities such as iron, cobalt, or nickel are present' is due unquestionably to the decrease in the over-voltage, or, in other words, to the increased rate of formation of ordinary hydrogen from monatomic hydrogen. It is for this reason that sodium amalgam must be made in porcelain vessels and not in iron pots or enamelled ware.3 It is interesting to note that Aschan' attributes the slight reducing power of certain sodium amalgams to the fact that hydrogen is given off in the molecular form and is, therefore, inactive. It has taken us a long time to get back to the view that nascent hydrogen is monatomic hydrogen; but we are there at last. Bamberger" accounted for the enormous variations in the reducing power of different samples of zinc dust by saying that some give molecular and, therefore, inactive hydrogen. Kowadays we say that, owing to the increased catalysis of the reaction, 2H + H?,the concentration of active hydrogen is less in certain cases than in others. The essential difference between the two points of view is that we know that the zinc dust was contaminated in one case by a metal having a lower over-voltage for hydrogen6 so that we are not reducing a t a zinc cathode, while Bamberger did not bother about the intermediate steps. I t is not solely the organic chemists who have to change their formulation. Peters' showed that platinum accelerated the evolution of hydrogen from a chromous chloride solution and Jablczynskis showed that mercury had no such 1
e 3
'
Walker and Paterson: Trans. Am. Electrochem. SOC., 3, 185 (1903). Cf. Lewis and Jackson: Proc. Am. Acad., 41,403 (1906). Baeyer: Ber. deutsch. chem. Ges., 2 5 , 1255 (1892). Ber. deutsch. chem. Ges., 24, 1865 (1891). Ibid., 27, 1548 (1894). Frederiksen: Jour. Phvs. Chem., 19,696 (191j). Zeit. phys. Chem., 26, 2 1 7 (1898). Ibid., 64, 750 (1908).
Oyer- I‘oltage aiid M o n a t o m i c Hydrogen
399
action. It was considered that platinum catalyzed the reaction and that mercury did not. To-day we say that hydrogen is set free a t the platinum because of the low over-voltage for hydrogen and is not set free at the mercury because of the high over-voltage. The advantage of the latter point of view is that it shows the desirability of trying other metals with chromous chloride solution so as to find out a t what degree of over-voltage the hydrogen ceases to be evolved rapidly. It seems probable that an explanation can now be suggested for the remarkable results obtained by Fernekesl on the action of sodium amalgam on solutions. Fernekes found that alcohol and many other organic substances increased the rate of reaction between sodium amalgam and water. He accounts for the phenomenon by assuming the intermediate formation of hypothetical compounds between solvent and solute which are extremely instable towards sodium amalgam and, therefore, react very rapidly with it. While this explanation may be right, i t has not proved helpful and is. therefore, useless, at any rate for the present. It seems probable that certain organic substances lower the over-voltage at mercury and consequently make the sodium amalgam instable. This hypothesis is susceptible of proof by direct experiment. While there are no measurements as yet made under conditions strictly comparable to those in Fernekes’ experiments, Carrara? has shown that the over-voltages are quite different in methyl alcohol and in ethyl alcohol from what they are in water. I have often wondered whether the reason that nobody has ever prepared electrolytically a sodium alloy using a cathode of fused Wood’s alloy, might not be because the overvoltage is not sufficient in this case. While a high hydrogen over-voltage and high reducing power generally run parallel, it does not necessarily follow that all reductions will take place more readily a t the cathode showing the higher over-voltage. Other factors may come in, as in the case of nitrates and nitrites. If the over-voltage Jour. Phys. Chem., 7, 611 (1903). Chem., 69, 75 (1909).
* Zeit. phys.
V r d d e r D . Bancrojt
400
were the only factor, i t would not be possible for nitrates to be reduced more readily than nitrites a t one cathode and less readily a t another. This is the case, however.’ At cathodes of zinc, iron, lead, platinum, and gold, nitrite is reduced more readily than nitrate. At cathodes of spongy copper or spongy silver, nitra e is the more readily reduced. Boehringer and Sons’ claim that with hot solutions and a mercury cathode sodium nitrate is reduced much more readily than sodium nitrite. Caffeine is reduced more readily a t a mercury cathode than a t a lead one,”while the reverse is true with succinimide. Muller4 has called attention t o a number of apparently abnormal cases and Chilesotti” has pointed out the peculiar behavior of molybdic acid a t different cathodes. There are three distinct factors which may mask the relation between over-voltage and reducing (or oxidizing) power. One of the substances to be reduced may be adsorbed much more than the other, so that the effective concentration a t the surface of the electrode may be \-ery much higher than the mean concentration in the solution. The electrode may catalyze other reaction besides the one, zH + H?. This possibility was foreseen by Ostwald“ a dozen years ago. A third possibility was pointed out to me b\- A h - . Bennett, that particular substances may catalyze the reaction. 2H --+ H?, so that the over-voltage at a given cathode ma>- vary with the nature of the apparently inert substances in the solution. lye know that this happens when the sol\-ent changes’ to methyl alcohol or ethyl alcohol and we know that i t happens when we add something which “poisons” the electrode? I n this paper it is not necessary to consider these points in detail. I merely 1
lliiller: Zeit. anorg. Chem., 26,
I
(1901); Zeit. Elektrochemie, 9, 9jj
(1903); 1 1 , jog (190j). 2
a 4
6 7 8
Zeit. Elektrochemie, 12,7 4 j 11906). Tafel and S e u m a n n : Zeit. phys. Chem., 50, 7 1 3 (1915). Zeit. Elektrochemie, 13,681 (1907,; 14,429 (1908). Ibid., 12, 146, 173, 197 l(1906). Electrochem. SOC.,G 11, 187 (1904) Trans. Carrara: Zeit. phys. Chem., 69, 75 (1909). Cf. Reichinstein: Zeit. Elektrochemie, 16,927 (1910).
Over- Voltage a i d Monatomic Hj’drogeii
401
wish to emphasize that the theory of over-voltage, as expounded by Bennett, does not require that high over-voltage and high reducing (or oxidizing) power necessarily go together, though this is usually the case. In this paper it has been shown that the theory of the irreversible electrolytic decomposition of water gives us a very plausible explanation for some of the peculiarities of sodium amalgam, chromous chloride, and zinc dust. The normal relation between high over-voltage and high reducing power or oxidizing power may be obscured or made t o disappear entirely in case of special adsorption of the reacting substance by the electrode, in case of a specific catalytic action of the electrode on the reaction, or in case of a catalytic action of the dissolved substance on the reaction, zH -+ H?, either directlJ7 or indirectly by affecting the electrode catalysis. The assumption of monatomic hydrogen as intermediate product a t the cathode has proxTed useful as a working hypothesis. U-hile we now consider nascent hydrogen as consisting in part of electricallj- neutral monatomic hydrogen, we recognize that the percentage of monatomic hydrogen ma\- x-ary enormously and that nascent hydrogen from one source is not necessarily equivalent to nascent hydrogen from another source. Come11
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