THE SIGN O F THE ZINC ELECTRODE BY WILDER D. BANCROFT
The German physical chemists have changed their position several times as t o the sign to be applied t o the zinc electrode. I n 1887, Ostwaldl used the minus sign. “Zinc and cadmium are negative in all the acids tried and copper, antimony, bismuth, mercury, and silver are positive in all, while tin, lead, and iron show positive and negative values of 0 . 1 0 . 2 volt. The average potential for zinc is -0.7 volt, for cadmium -0.3, for tin, lead, and iron =to,for copper +0.30.4, for bismuth +0.4, for antimony + 0 . 3 , for silver +0.5, and for mercury +0.8.” Ostwald takes the potential of the solution as zero, p. 600. In 1893, Ostwald2 uses the plus sign with zinc, probably because Nernst had developed the idea of solution pressure and zinc had a positive solution pressure. “The values in volts for the single potentials of a number of the more important metals in solutions containing one gram-atom of the metal per liter are: Mg + 1 . 2 2 , Zn +0.51, A1 + o . z z , Cd + o . ~ g , Fe +0.06, Pb -0.10, Cu -0.60, Hg -0.99, Ag -1.01. The values are given from the metal t o the electrolyte, the potential of the metal being called zero. The plus sign with zinc denotes that the electrolyte is charged positively with respect to zinc. If one counted from the electrolyte t o the metal, one would call the zinc potential negative.” L,eBlanc3 adopts the same point of view in 1896. “We will call the potential of the metal or the electrode zero, in which case the plus or minus sign shows whether the electrolyte is positive or negative with respect t o the electrode. We must, therefore, say Hg 1 Hg2C12in N/I KC1 = 0.56 v o l t . . . . and Zn j N / I ZnSOe = +0.52 volt.” I n 1900, 1
2
Zeit. phys. Chem., I , 604 (1887). Lehrbuch allgem. Chemie, 2 11, 946 (1893). Lehrbuch der Elektrochemie, I 7 7 ( I 896).
W i l d e r D . Bancroft
374
Neriist,l Wilsmore2 and Ostwald3 all use a plus sign with the zinc electrode and a minus one with the calomel electrode. Another change was foreshadowed in 1905 when Lutheri wrote : “I consider it very advisable t o put a plus sign before the potential difference when the electrode in question is positive to the zero electrode, whether that be the calomel electrode, the hypothetical flowing electrode, or the extrapolated hydrogen electrode.” None of the books now accessible show when the Bunsen Society adopted this suggestion ; but in I 9 I 5, Foerster writes : “To show the direction of the electrical polarity between metal and solution we place before the value of the difference of the two potentials, the so-called potential of the metal, the sign of the charge on the metal. When dipped in solutions of their own simple salts or of acids, the readily oxidizable metals such as Al, Mg, Mn, Zn, Cd, “1, Fe, acquire a negative potential, while the less readily oxidized, or noble metals such as Cu, Hg, Ag, acquire a positive charge.6 Volta arranged his first cell, Cu, Zn, H2S04,Cu, Zn, and consequently the copper carried off the negative charges and the zinc the positive charges. Because of this, Berzelius called the readily oxidized metals electropositive. This nomenclature remained unchanged even though the error of Volta’s arrangement was soon corrected. In consequence, for a long time-and also in the first edition of this book-the sign of the potential of the solution was made the same as the charge on the solution and the potential of the zinc was, therefore, called positive.7 Especially for students, the difficulties were so great in a point of view whereby the negative current flowed from a positive electrode, that it became necessary t o adopt the recommendaZeit. Elektrochemie, 7, 2 5 4 (1900). Zeit. phys. Chem., 35, 291 (1900). Ibid.7 35, 333 (1900). 4 Zeit. Elektrochemie, I I, 780 (1905). 6 Elektrochemie der wasserigen Losungen, I 25 (I 9 I 5 ) . 6 [The rest of this paragraph is a foot-note in the original, but t h a t seems unnecessary here.-W. D. B.] 7 [Foerster has overlooked Ostwald’s position in I 887 .-W. D. J3.] 1
The Sigvl of the Z i m c Electrode
375
tion of the Bunsen Society to denote the potential in accordance with the charge on the electrode.” All the Germans now use the minus sign for the potential of zinc in zinc sulphate solution, and so do the English. In this country the practice is by no means uniform as may be seen from the rather acrimonious discussion a t Detroit. Lewis2 came out strongly some years ago in favor of using the plus sign for the zinc electrode. “On account of the importance in free energy calculations of those chemical reactions which occur in a voltaic cell during the passage of a current, it is very desirable that suitable conventions be adopted for the sign of an electromotive force, or of a single potential difference. It is not sufficient to state that a cell composed of cadmium amalgam, saturated cadmium sulphate solution, mercurous sulphate, and mercury gives an electromotive force of 1.0183 volts at 2 0 ° ; for a person unacquainted with the cadmium cell would not know whether the cell would do work through the consumption of cadmium and the deposition of mercury or vice versa. We may, however, in this as in other cases, eliminate any ambiguity by writing the chemical equation,
+
+
Cd(12.5% amalg.) Hg,SOs(s) 8/3HzO (saturated with CdS04 8/3H20) = CdS04 8/3H20 (5) 2Hg (I)
+
and now if we say that a t 2 0 ° the E. M. F. is 1.0183volts, the positive sign will indicate that the cell is capable of doing work when the reaction runs from left to right. So also we write for the reaction I/zHi@f(g) l/zCl~(g)= HCl (0.1%!); E = +1.4881 at 25” meaning that work is done by a cell in which hydrogen and chlorine combine t o form hydrochloric acid in 0.1&! solution. Instead of writing the chemical reaction in such a case, we may express the same facts in another way, which shows better the actual construction of the cell in question. The form H2(g), HCl(o.1 M ) , C12(g); E = +1.4881
+
Trans. Am. Electrochem. SOC.,31, 249-255 ( 1 9 1 7 ) ?.Jour. Am. Chem. SOC.,35, I , 25 (1913)
376
W i l d e r D . Bancroft
expresses the same facts. In such an expression the positive E. M . F . indicates the texdency of the positive currext to ~ u x through the given cell f r o m left to right.. . . . . . “The committee of the Bunsen Gesellschaft has adopted two symbols E , and Eh for electrode potentials measured against the two standards, the normal calomel electrode and the normal hydrogen electrode. The potentials, as given in the following papers, based upon the standard above defined, are approximately equal to E h except for sign. The Bunsen Gesellschaft makes the potential of sodium negative, and that of chlorine positive. I have adopted the opposite sign in conformity with the older and more common usage. actly what the committee means by the normal hydrogen electrode they have not stated. The potential of hydrogen against a normal solution of hydrochloric or of sulphuric acid is a perfectly definite thing, but the potential of hydrogen against a solution normal in hydrogen ion must depend upon some assumption regarding the degree of dissociation, and different views will lead to values differing by 5 or I O millivolts.’’ We are all agreed that electricity tends to flow from a place of higher potential to a place of lower potential, but that is not sufficient to enable us t o settle the problem. In the Daniel1 cell the so-called positive current flows from the copper to the zinc outside the cell, from which we may deduce that the potential of copper is higher than zinc. On the other hand, the positive current flows from zinc t o copper inside the cell, from which we may deduce that the potential of zinc is higher than that of copper, and we are no further ahead than before. The physicists have been interested in the current outside the cell and consequently they call the copper pole the positive one. The engineers have followed the lead of the physicists and call the positive pole of the dynamo the one where the positive current comes out of the machine. This brings the dynamo and the primary cell into line when considered as sources of electrical energy. It is also the point of view adopted officially by the Bunsen Society,
ax-
The S i p of the Zizc Electrode
377
and unofficially by the English electrochemists. It is apparently the only point of view which can possibly be adopted universally. On the other hand, it is one that is repugnant to chemists a t first sight. Zinc precipitates copper from a copper sulphate solution, and, theref ore, has a higher potential than copper. This is the side of the problem which appeals to Lewis, for instance. He knows that the free energy of zinc is greater than that of copper. I sympathize fully with Lewis’ point of view and if there were no way out of the dilemma, I should be inclined to break with the physicists rather than t o sacrifice a fundamental tenet of chemistry. As I see it now the difficulty is not a real one. We knowthat the electromotive force of a reversible cell is a measure of the chemical potential, or chemical affinity, and I have made the mistake in the past of assuming that since the values were the same the signs must also be. The whole difficulty disappears if we say that the chemical potential of zinc in the Daniel1 cell is higher than that of copper while the electrical potential of copper is higher than that of zinc. When a piece of zinc is dipped into a zinc sulphate solution, some of the zinc goes into solution as ion, charging the solution positively and leaving the zinc charged negatively. A positive current tends t o flow from the solution, the place of higher electrical potential, to the zinc, the place of lower electrical potential; but no current does flow because the electrical potential from the solution to the metal is balanced by the chemical potential from the metal to the solution. If we dip a piece of copper into a copper sulphate solution, we usually consider that soine copper ions are precipitated on the copper charging the copper positively, and leaving the solution charged negatively. KO current flows after the charging current because, by hypothesis, the positive electrical potential difference from metal to solution is balanced by the chemical potential difference from the copper sulphate to the copper. When the zinc and copper electrodes are joined metallically, a current flows from the copper t o the zinc and the chemical reaction regenerates the electrical potential difference continually. Neglecting a possi-
378
WiZder D . B a m r o f t
ble potential difference between the zinc sulphate and copper sulphate solutions, the electrical potential in a Daniell cell on closed circuit drops from the zinc sulphate solution next the zinc electrode t o the copper sulphate solution a t the copper electrode. Here it is pumped up by the chemical energy if the copper is positive to the solution on open circuit. From the copper electrode the electrical potential drops through the metallic circuit to the zinc electrode which is the place of lowest electrical potential in the whole circuit. Here the chemical energy of the zinc pumps up the electrical potential to the old level and so forth. While the zinc electrode is always the place of lowest electrical potential in the circuit, the place of highest electrical potential will vary with the conditions under which the cell is operating. If the internal resistance is low and the external resistance is high, the drop of electrical potential through the solutions will be less than the rise of potential a t the copper electrode and consequently the copper electrode will be the point of highest electrical potential in the system. The limiting case is, of course, when the cell is on open circuit. If the internal resistance is very large and the external resistance practically negligible, the place of highest electrical potential will be the zinc sulphate solution in immediate contact with the zinc electrode, because the drop of potential through the solution is greater than the rise of potential at the copper electrode. If we have a cell of the Daniell type, Zn I ZnS04 j CdS04 ] Cd, where cadmium is also charged negatively and the solution positively by definition there will be a fall of electrical potential when passing from the cadmium sulphate solution to the cadmium electrode, and consequently the place of highest electrical potential will always be in the zinc sulphate solution right next the zinc electrode, though the drop of potential from here to the cadmium sulphate solution in contact with the cadmium electrode will be practically zero when the cell is on open circuit. When the electrodes form positive ions, the metal with the higher chemical potential, or the greater free energy, becomes the anode. With electrodes which form anions, the
The Sip of the Ziizc Electyode
379
reverse will, of course, be true and the substance with the greater free energy will form the cathode. Bromine sets free iodine from iodide solutions and, therefore, has the greater free energy. In a cell with iodine and bromine electrodes, the bromine electrode is the cathode. It is interesting to note that this whole argument was given in a very condensed form by Gibbs.l “When all the conditions of equilibrium are fulfilled in a galvanic or electrolytic cell, the electromotive force is equal t o the difference in the values of the [chemical] potential of any ion or apparent ion at the surface of the electrodes multiplied by the electrochemical equivalent of that ion, the greater [chemical] potential of an anion being at the same electrode as the greater electrical potential, and the reverse being true of a cation.” The general results of this paper are as follows : I . The chemical potential of the zinc iii the Daniel1 cell is greater than the chemical potential of the copper, but the electrical potential is less. 2. With electrodes which form cations the electromotive force is a measure of the difference of the chemical potentials, but has the opposite sign; with electrodes which form anions the two potentials have the same sign. 3 . Since the electrical potential of the copper in the Daiiiell cell is higher than that of the zinc, and since the zinc electrode is the place of lowest electrical potential, though highest chemical potential, we ought t o use the minus sign when speaking of the electrical potential difference Z n I ZnS04. 4. Calling the zinc potential minus is in line with the practice of all physicists, of the Bunsen Society, and of the Faraday Society. 5. The distinction between chemical potential and electrical potential was stated concisely by Gibbs. Cornell Uwiversity
Scientific Papers,
I, 333 (1906)
