The Electromotive-force and the Change of Inner. State of the Daniell Cell.

Transcription

1 . 370 SAMESIMA: [Ser. 2, Vol. 8, The Electromotive-force and the Change of Inner State of the Daniell Cell. Zitusaburo SAMESIMA. [READ OCTOBER 16, 1915.] 1. History. The Daniell cells were already studied by many authors since its discovery in 1836 by J. F. Daniell. There were many forms of this cell which served as a standard of e. m. f. The literatures are given in, for example, the paper of E. Cohen, F. D. Chattaway and W. Tombrock. (1) The e. m. f. of this cell remarkably alters according to the states of copper and zinc electrodes. The cells under various conditions of elec trodes were studied by C. R. Alder Wright (2), and J. A. Fleming (3). The results obtained by these authors were as follows: An electrolyti cally deposited copper electrode gives lower e. m. f. (nearly 0 E006 volt) than that of bright copper, and an amalgamated copper gives the lowest value of all. When copper electrode is oxidized, the value is higher than that of unoxidized one, and it increases more and more with the degree of oxidation. As to the zinc electrode, the amalgamated zinc gives a lower value than that of pure one, and an impure zinc also decreases the value. When metallic copper deposits upon the zinc electrode by the diffusion of copper sulphate solution, the voltage is considerably de creased (by Alder Wright 6%). According to the researches by C. R. Alder Wright (4), E. Kittler (5), J. A. Fleming (6), G. Meyer (7), St. c (1) Cohen, F. D. Chattaway & W. Tombrock, Zeit. f. phys. Chem. 60 (1907), S (2) C. R. Alder Wright, Phil. Mag. [5], 13 (1882), p (3) A. Fleming, Phil. Mag. [5], 20 (1885), p c. E. Kittler, Wied. Ann. 17 (1882), S Meyer, Wied. Ann. 33 (1888), S. 276.

2 Feb., 1916.] ON THE ELECTROMOTIVE-FORCE OF DANIELL CELL. Lindeck (1), Streintz (2), J. Chaudier (3), E. Cohen & J. W. Co melin (4) etc., the concentrations of copper sulphate and zinc sulphate solutions also affect the e. m. f. of the cell. It is clear that the concen trated copper sulphate solution gives high value and concentrated zinc sulphate solution gives low value of voltage, because the osmotic pressures of the ions increase with the concentrations of these solutions. There is a formula deduced by W. Nerust representing the relations among e. m. f., osmotic pressure, solutional tension and temperature coefficient of e. m. f. The effects of temperature on the cell were studied by H. Jahn (5) G. Meyer (6), J. Chaudier (7), E. Cohen, F. D. Chattaway & W. To brock (8), The results of these authors show that the temperature coefficient of the e. m. f. varies within a considerably wide range, ex tending from positive to negative values according to the variation of the concentrations of the solutions. It seems from these facts that it is not an easy work to obtain a cell of definite voltage to 1/10 millivolt. A few reserches were carried out on the e. m. f. of the Daniel cell, which containes a porous pot. W. Block (9) measured the variation of e. m. f. of a cell with the time under various external resistances, which served as the source of electric current. But his observation lasted only about a day after constructing the cell. I measured the variations of e. m. f. of Daniel cells and studied the functions of porous pots, during many days. 2. Initial voltages. Cells without porous pot. Zinc electrode: Extra pure sample from Merk was cast in a form of rod and amalgamated with metallic mercury which was previously distilled. Copper electrode: The purest sample of thin plate form was used. It was ascertained by analysis that it contained no impurity in measur able amount. Lindeck, Zeit. f. instr. Kunde. 12 (1892). S. 17. Streintz, Sitzber. Wien Akad. 103, II. a, (1894) S. 98. J. Chaudier, Compt. rend. 134, (1902), p Cohen & J. W. Commelin, Zeit. f. Elektrochem. 9, (1903) S Jahn, Wied. Ann. 28, (1886) S. 21. c. c. c. Bloc k, Ann. d. Phys. 22, (1907), S. 505.

3 Z. SAMESIMA: [Ser. 2, Vol. 8, Copper sulphate solution: Ordinary material was recrystallized, once with water containing some nitric acide and once with alcohol. 124 E9 grams of this salt was dissolved to 500c.c. solution. It was 2 normal solution. Zinc sulphate solution: Ordinary material was recrystallized with water and then treated with lead peroxide to get rid of the trace of iron compounds and again crystallized. 143 E8 grams of this salt was dissolved to 500c.c. solution, i.e. 2 normal solution. A Rubens galvanometer and a potentiometer by K. Feussner (made by Siemens Halske Co.) were used to measure the voltage. To stand ardize the voltage, I used the Weston normal cadmium element which was constructed by myself according to the description given by F. A. Wolff and C. E. Waters. (1) The results obtained are shown in the following table, compared with the date obtained by other authors. TABLE T. J. Chaudier (2) obtained the value-0 E0002 for the temperature coefficient of e. m. f. of Daniell cell, in which the copper sulphate solu tion was saturated at 15 Ž. having the specific gravity 1 E20, and the zinc sulphate solution of the specific gravity 1 E14-1 E25. From this value we can calculate the voltage at other temperatures. At 15 Ž. we F. A. Wolff & C. E. Waters, Bulletin of the Bureau of Standards. Vol. 4, p. 1. c.

4 Feb., 1916.] THE ELECTROMOTIVE-FORCE OF DANIELL CELL. obtain: 1 E1036 volts for electrolytically deposited copper electrode, and 1 E1090 volts for bright copper electrode. Cells having porous pots. Outer vessel: The vessel had a cylindrical form made of glass and the dimensions were:- inner diameter 6cm., depth of liquid 7cm. Porous cylinder: It was of fine structure and white colour, made in Tokyo and the dimensions were outer diameter 3 E5cm., thickness of wall 0 E25cm. This porous pot was boiled with dilute hydrochloric acid and then boiled many times with water. The upper part of the cylinder was covered with paraffin to prevent the evaporation of the salt solutions and from destruction by crystallization of salts. The zinc electrode had the form of rod having the surface area about 15 square centimeters. The copper electrode was a plate, the surface area of which was about 85 square centimeters. The data obtained were as follows:- TABLE U. From these results, it seems that there is no difference in two decimal figures of voltages between the two kinds of cells, the one with porous pot and the other without it. The former is, however, some what greater than the latter in the third decimal figure. In all cases the bright copper electrode gives the higher value than the electro deposited one. It is expected that the worked copper, which has cirtain strain in its inner structure, will give higher voltage than the unworked one, because the former has the higher chemical potential than the latter. In addition to these, I made a number of cells which were con structed with more impure materials. Copper, zinc amalgam, copper

5 374 Z. SAMESIMA: [Ser. 2, Vol. 8, sulphate and zinc sulphate had the degree of purity of so-called com mercially pure substances. There were two sorts of porous cylinders, one of them had the fine structure which was already described, and the other had the coarse structure, of greyish colour and of somewhat larger dimensions than the former one. The results were as follows:- TABLE V. 3. Variations of voltages with lapse of time. If the two solutions of a cell are isotonic, that is of the same normality under the assump tion that the dissociation degrees of zinc sulphate and copper sulphate solutions are equal, then the variation of concentration which is caused by the difference of osmotic pressures does not occur. But if the solu tions are not isotonic, the concentrations of the two solutions vary and

6 Feb., 1916.] ON THE ELECTROMOTIVE-FORCE OF DANIELL CELL. 375 there occurs a difference between the levels of the outer and inner liquids. To avoid from this disturbance, both of the solutions were made to two normals. TABLE W.

7 376 Z. SAMESIMA: [Ser. 2, Vol. 8, The numbers marked with asterisk are the values obtained in reduced pressure (13cm. mercury)

8 Feb., 1916.] ON THE ELECTROMOTIVE-FORCE OF DANIELL CELL. 377

9 378 Z. SAMESIMA: [Ser. 2, Vol. 8, The cells thus constructed were put into closed vessels to keep from the evaporation of water. One of the vessels was connected to a water pump to reduce the pressure, and the effects of accumulating bubbles upon the electrodes were observed. If the pressure in the vessel was lowered, most of the bubbles were removed from the electrodes and then the e. m. f. was measured. The circuits of currents were always opened except when the measurements were done, while in the cell No. 1 it was always closed, a resistance of ohms being inserted in the circuit. The data obtained are shown in TABLE W. and Fig. 1. From these data, it can be seen that there is no noticeable dif ference of e. m. f. between the open circuit and closed circuit including ohms or more resistances in it. Moreover, the bubbles evolved at the zinc electrode have little effect on the e. m. f. Cell No. 5 had the same construction as the others except that the solution on the outside of the porous pot was 2 normal zinc sulphate solution instead of copper sulphate. Therefore, it had the following arrangement, amalg. zinc 2N. ZnSO4 solution bright copper. 4. Changes of inner states of the cells. If we derive a large amount of electric current in a short time from a cell, it is evident that zinc electrode dissolves into zinc sulphate solution, and copper deposits on copper electrode from the copper sulphate solution. When all the copper ions which was present in solution has deposited on electrode as metallic copper, the zinc ion begins to deposit as metallic zinc on copper electrode, and when it is wholly covered with zinc the voltage of the cell will fall to zero. But when the circuit is opened, or a very small current is taken, the course of changes becomes more complex than in the above case. The voltage gradually fell during several days, and after ten days or so it took a minimum value, then passing a slight maximum it took the value which was moderately constant over a month or more. These are shown in Fig. 1 Several hours after setting of the cell, the zinc electrode was cover ed with greyish spongy deposit which was probably copper amalgam deposited from copper sulphate solution coming into the porous pot by diffusion. It was already mentioned that this phenomenon causes a considerable fall of e. m. f. The amount of falling is affected by the form and quantity of deposition, or it is defined by the factors: shape of zinc electrode, porosity and thickness of porous cylinder, diffusion velocity of copper sulphate solution etc. The deposition gradually in

10 Feb., 1916.] ON THE ELECTROMOTIVE-FORCE OF DANIELL CELL. 379 creased in quantity and after several days it became reddish black and at the same time there deposited a small quantity (circ. 1-2c.c. in one day) of gas bubbles. It was tested that these bubbles were hydrogen gas. It was considered that the deposited copper and zinc rod immersed in zinc sulphate solution made a cell of continuously flowing current local current, and therefore, hydrogen which had come from the dis sociation of water deposited on the spongy copper, while hydroxyl ion went to the zinc surface and there made zinkate ion or zinc hydroxide. This consideration was ascertained by the precipitation of zinc hydroxide at the bottom of the vessel. The dissociation degree of water is very small, so the quantity of deposition of hydrogen is also small. The cause of the uncommon falling of voltage in the case of cell No. 2 was probably the covering of zinc with copper or thin hydrogen layer. On three or four days after setting, it was observed that the com act metallic copper deposited on the outer surface of the porous cylinder. The deposition did not occur on the whole surface but on a part of it in the form of globules. The detail of the study on this phenomenon will be given in the next section. After passing the above stages the cell assumes the form Zn, Hg & Cu ZnSO4 solution containing Zn(OH)2 etc. Cu It seemed that the constant value 0 E98 volts attained half a month after setting was the e. m. f. of the above cell. From this consideration, I made a cell amalg. zinc 2N. ZnSO4 solution bright copper, the e. m. f. of which is shown in the Fig. 1, (curve of cell No. 5). The porous cylinder was also used in this case in order to compare with the Daniell cells. Five days after setting, the voltage took a constant value 02 volts. M. B. C. Damien (1) measured the e. m. f. of the cell: Zn ZnSO4 solution, sp. gr. 1 E064 Cu, and got the values shown in Fig. 1. He showed that if the amalgamat ed zinc was used instead of pure zinc, the voltage became a little higher than the above values. 5. The deposition of copper on porous wall. There are a few c ases ever studied about the metal deposition on a porous wall. The one case is that studied by F. Braun (2), H. N. Holmes (3) etc. and M. B. C. Damien, Ann. Chim. et Phys. [6] 6, (1885), p F. Braun, Wied. Ann. 42, (1891), S & 44, (1891), S H. N. Holmes, Journ. Amerc. Chem. Soc. 36, (1914), p. 784.

11 is called "electrostenolysis." The other case is that studied by W. Ostwald (1), and F. Braun (2) using the copper intrate and sodium sulphide solutions. But in the present case, it is different from these phenomena. It was not a capillary action but caused from local current, which was ascertained by the following experiments. At first, I observed that the deposition occured on the porous wall even when there was no copper electrode in the cell. Therefore, the copper plate, which was dipped in copper sulphate solution, was of no service for the occurence of this phenomenon. Also, it was perceived that the deposition occured at the opposite side of such a part of the porous walls where the zinc rod touched. Furthermore, the following experiments were carried out. Fig. 2. The porous cylinder, containing zinc sulphate solution was put into copper sulphate solution, and at the bottom of this cylinder a glass cup was placed which served to hold the copper powder precipitated on zinc rod and to keep from the contact of that powder to the porous walls. The zinc rod was stood on the cup and it was kept by a holder at the central part of the cylinder, so that it touched the porous wall neither directly nor indirectly, (Fig. 2, A). In this case no copper deposition was observed on the porous surface. A copper strip was attached to the inner side of the cylinder as is shown in Fig. 2, B. other parts were constructed in the same manner as above described. The copper did not deposit also.. Ostwald, Zeit. f. Phys. Chem. 6, (1890), S. 75. F. Braun, Wied. Ann. 44, (1891) S. 501.

12 Feb ] ON THE ELECTROMOTIVE-FORCE OF DANIELL CELL The zinc rod and the copper strip which have been described above were now connected by a wire. Then after a few days the copper deposited on the outer surface of cylinder where the strip touched the inner side (Fig. 2, C). From these facts, the phenomenon can be explained as follows: At first, the zinc rod is covered with spongy copper which has come from the diffused copper sulphate solution. It gradually increases in quantity and at last it reaches the porous wall. The growth is never stopped by this wall, but proceeds into the mass of porous wall passing through its narrow canals. We can ascertain this fact by breaking the cylinder. After it has reached the outer side of the wall, it increases continuously in volume. Now it becomes to the same state as a Daniell cell, because the deposited copper on the outside of wall acts as the positive electrode while the zinc rod dipped in zinc sulphate solution acts as the negative one. These two electrodes are connected by the sponges and thin threads of metallic copper and therefore the electric current flows co tinuously. On this account the deposition grows more and more until all the copper ions present in solution are deposited in the metallic state (Fig. 2, D). Thus the phenomenon is explained as a local current in a simple manner and is caused neither by capillary action nor by chemically re ducing action. The present study has been carried out under the guidance of Prof. H. Nagaoka to whom my best thanks are due.