ELECTRODE POTENTIALS O F PLATINUM, GOLD, AND SILVER I N VARIOUS SOLUTIONS O F ELECTROLYTES I. M. KOLTHOFF AND C H I N WAhTGl School of Chemistry, Institute of Technology, Gniversity of Minnesota, Minneapolis, Minnesota Received September 81 , 1936
Essin and Lozmanova (2) studied the potentials of gold electrodes in solutions of copper sulfate, copper nitrate, and silver nitrate in the absence of air. It was found that the potential of the gold electrode in copper sulfate solutions depends upon the copper concentration in a way somewhat similar to that of the copper electrode in the same solutions, although the potential of the gold electrode changed about three times as much as ' that of the copper electrode for a given change in the copper-ion concentration. The following relation was found between the potential of the gold electrode ( E A ~ I c ~ S O , )and that of the copper electrode in the same solution ( E C , , ~ ~: ~ ~ O , ) EAu/CuSOd
=
A
+
bECu/CuSOc
in which A and b are constants. Similar equations were found for other combinations of electrolytes and electrodes. Nierstrase and Tendeloo (4) made use of the potential of the system, noble metal-copper oxalate (solid-saturated solution) as an indicator of copper-ion concentration in the presence of air, and they applied these systems to the potentiometric titration of calcium with oxalate. No explanation has been offered for the above behavior of the noble metals; theoretically it seems hardly justifiable to attribute a copper electrode function to the noble metals, since the potential is also dependent upon the nature and concentration of the anions present in the solution. I n the presence of air and in solutions of low oxidation potential, noble metals may be expected to adopt the air potential determined by the net electrode reaction,
O2 + 2Hz0 + 4e
e 40H-
Although, under carefully regulated conditions, air electrode potentials are fairly reproducible (see Richards (6)), the values obtained for the l This article is based upon a thesis submitted by Chin Wang t o the Graduate School of the University of Minnesota in partial fulfillment of the requirements for the degree of Master of Science, June, 1936. 539
540
I. M. KOLTHOFF AND CHIN WANG
oxygen-hydrogen cell are far below the theoretical values calculated from other thermodynamic data. This is partly explained as being due to the formation of oxide films on the noble metal and, as shown by Hoar (3), to irreversible autopolarization of the oxygen electrode. The potentials of gold and platinum electrodes in solutions of copper salts in the presence of air may therefore be expected to depend upon the hydrogen-ion concentration of the solutions. Changing the concentration of the copper salt in the solution also changes the hydrogen-ion concentration (hydrolysis) and hence also the potential. The potential is mainly determined by the hydrogen-ion concentration (activity) and not by the concentration (activity) of the copper ions. In the present study this view was actually shown to be correct. Addition of acid to solutions of copper salts resulted in a marked increase of the potential of gold and platinum electrodes immersed in these solutions. In acetic acid-sodium acetate buffers the potential was found to be determined by the pH and not by the concentration of the copper ions. When, after measuring the potential of gold and platinum electrodes in solutions of copper salts, nitrogen was passed through, the potentials dropped slowly, until after five to ten hours a more or less constant potential was obtained which differed by about 0.25 volt from the “air potentials.” The above results show conclusively that in air the noble metals do not function as copper electrodes. The potential of a silver electrode has also been measured in solutions of copper sulfate and copper nitrate of various concentrations. It was found that in general the potential was not affected by air or by a change in the pH of the solutions. The potentials of platinum and gold in solutions of copper salts, after removal of the air by nitrogen, were not affected, or only very slightly, by the addition of a strong acid. Under these conditions the partial pressure of oxygen is reduced to such a small value that the noble metals no longer function as oxygen electrodes. .Apparently in nitrogen the oxidation potential of the dissolved copper salts is measured with platinum or gold electrodes. These potentials are not very reproducible, because the concentration of the reduced form (cuprous ions or metallic copper) is indefinite and exceedingly small. The case is somewhat comparable to the measurement of the oxidation potential of a pure solution of an oxidizing or reducing agent in which the concentration of one of the electromotively active forms is indeterminate. In a nitrogen atmosphere the potentials measured with platinum and gold electrodes in copper sulfate and nitrate solutions were nearly identical at the same copper concentration, this result lending support to the above interpretation. Erich Muller ( 5 ) , and later Bottger and Schall (l), found that the chloride-silver titration could be carried out using‘ electrodes of gold, platinum, and palladium, instead of silver. Apparently, here again the
’
ELECTRODE POTENTIALS OF XOBLE METALS
541
oxidation potential of the silver solution (Ag+ + e e Ag) is measured with the noble metal electrodes. I n order to substantiate this view experimentally, measurements of the potential have been made in silver nitrate solutions of varying concentrations using platinum, gold, and silver electrodes. In 0.5 molar silver nitrate solutions the potentials were found to be identical, indicating that under these conditions enough metallic silver was formed on the noble metals to enable them to function as silver electrodes. In dilute silver nitrate solutions the potentials of the noble metals were more positive than those of the silver electrode. Under these conditions the amount of metallic silver formed on the noble metals is not sufficient to make them identical with silver electrodes. The activity of the reduced form (silver) is not fixed and the potentials become more or less indeterminate. (Compare with behavior of copper solutions in nitrogen.) A further substantiation of the fact that the noble electrodes do not function as air electrodes in solutions of silver salts of adequate concentration is that the potentials were found to be unaffected, or only slightly affected, by the addition of acid or by the removal of oxygen from the solution. In mixtures of copper and silver nitrate the potential was found to be determined by the silver concentration in the solution. EXPERIMENTAL
Electrodes Gold electrodes: (1) plate electrode, 1 x 2 cm., 1 mm. thick; (2) wire electrode, 1.5 mm. in diameter; (3) a gold-plated platinum gauze electrode. Platinum electrodes: (4) plate electrode, 1 x 2 cm., 1 mm. thick; (5) wire electrode, 1.5 mm. in diameter; (6) a gauze electrode. Silver electrodes: (7) plate electrode, 2.5 x 1 cm., 0.5 mm. thick; (8) silver gauze electrode; (9) a silver-plated platinum gauze electrode. The most reproducible results were obtained with electrodes 3, 6, and 8, which were used in most of the work. The gold and silver electrodes were connected directly with the copper lead-wire. The platinum electrode was sealed into a glass tube and connection made in the ordinary may with mercury. Before the measurements the electrodes were cleaned with a mixture of sulfuric and chromic acids and then washed thoroughly with distilled water.
Electrode cell The cell was made of Pyrex glass and was about 7 cm. in diameter and of 150-ml. capacity. It was closed with a rubber stopper provided with holes for the admission of the electrodes, for the salt bridge filled with sodium sulfate or potassium nitrate making electrolytic contact with the saturated calomel electrode, for the inlet and outlet tubes for air or nitroTHE J O ~ R P A LOF PHYSICAL CHEMISTRY, V O L
41. N O . 4
542
I. M . KOLTHOFF AND CHIN WANG
gen, and for the thermometer (25°C.). The potentials were measured against the saturated calomel electrode.
Potentiometric out$t The ordinary compensation method was used, employing a Leeds and Northrup student potentiometer. Since the electrodes are easily polarizable, measurements have been made also with a vacuum tube potentiometer as a null point instrument. Both methods gave corresponding results if the final data were taken after long enough periods of time. Chemicals and gases used All chemicals were of C.P. quality and were recrystallized before use. Tank nitrogen was purified by washing with sodium hydroxide, by passing over copper turnings heated electrically to about 6OO0C., and finally by washing with the same solution as used in the cell. The air was obtained from the air pressure supply in the laboratory, and was washed through the same solution as used in the cell. EXPERIMENTAL RESULTS
A detailed account of all measurements in air and nitrogen before and after addition of acid are given in the thesis of the junior author (see footnote 1). They are not reported here as the figures have no exact significance, owing to the great polarizability of the electrodes. As an illustration a few figures obtained with 0.05 molar copper sulfate are given in table 1. One hundred ml. of this solution was placed in the cell and the latter closed air-tight after introduction of the electrodes and various tubes. Air was passed through and the potentials were measured after regular intervals. After the potentials had become fairly constant, acid was added (in the case of copper sulfate, sulfuric acid; in the case of copper nitrate, nitric acid), the final normality being known. The potentials were measured again after definite intervals of time until they were constant, or approached constancy. A fresh sample of solution was then placed in the cleaned cell and the entire manipulation repeated in nitrogen. The gold and platinum electrodes give almost identical results. The air electrode function is evident from the great effect of acid upon the potential (increase about 80 mv.) and the effect of removal of air by nitrogen (decrease about 200 mv.). The potential of the silver electrode, on the other hand, is only slightly affected by these changes. In table 2 the final potentials measured with platinum and gold electrodes in neutral and acid solutions of copper sulfate and nitrate of various concentrations in a nitrogen atmosphere are given. The gold and platinum electrodes behave in an almost identical fashion. The results show definitely that under the conditions of these experiments, the noble elec-
5 43
ELECTRODE POTENTIALS O F NOBLE METALS
TABLE 1 Measurements in 0.05 M copper sulfate (against saturated calomel electrode; 25°C.)
b. In nitrogen
a. In air TIME IN HOURS
Au
0.3640 0.3750 0.3890 0.3870
0 6 8 30
0 5
7
I
0 0 0 0
1
TIME IN
1
Pt
1
0.3840 0 3850 0.3920 0.3910
0.2810 0.2570 I 0.2510 1 0.2490
0 0 0 0
0 0 0 0
1 I
4260 4660 4680 4680
Ag
1
2335 2440 2480 2480
0.3670 0.1900 0,1770 0.1800
O 2 5 6
1
4360 4780 4780 4790
I
Au
I-
:t
1,
(1
0 0 0 0
~
3
Pt
0,4320 0.1820 I 0.2520 0.1730 0.2400 0.1780 0.2400
1
1 ~
1770 1920 1930 1940
0 0 0 0
1900 1840 1840 1850
0 0 0 0
225 225 223 225
TABLE 2 Measurements in nitrogen in solutions of copper sulfate and nitrate
1
IN
1
IN
1
1
0.5 M CuSOl
0.5
I
M Cu(N0a)z
1
Au
Neutral . , . , . , . . . . , . . . . . / 0.227 0.05 N " 0 , 0.216
..........,.I
0.219 0.210
~
IN
I
Pt
~
0.05 M CuSO,
1
0.05 M Cu(N0a)z
I
IN
Au
I
Pt
0.182 0.140
1
0.185 0.140
1 I
1
IX
0.005 Y CUSOI
0.005 M Cu(NOa)z
IN
Au
0.137
1 '
Pt
0.132
j
TABLE 3 Measurements in silver nitrate solutions
1 j
IX
Au
0.5 M AgNOa
1
I
1
Pt
I
Ag
IN 0.05
Au
I
M AgNOa Pt
I
Ag
1 1
IN
Au
0.01 A4 AgNOz
1
Pt
1
In air Neutral. . , . . , . . . . . . 0.01 N HXOi . . . . .
0 51051 0 4891 0 4981 0 460; 0 469 0 490' 0 51101 0 4621 0 4901 0 450, 0 4711 0 4801 In nitrogen
Xeutral.. . . . . . , . . . . . 0.01 N "03 . . . . . . .
Ag
544
I. M. KOLTHOFF AND CHIN WANG
trodes do not function as air electrodes (hardly any effect of acid). The potentials measured in neutral copper sulfate and nitrate solutions are practically the same at the same copper concentration, the potential changing 0.045 =t 0.005 volt with a tenfold change of the copper concentration. All measurements were repeated with 0.5,0.05, and 0.005 molar solutions of lead nitrate. The results were still less reproducible than in the copper solutions, but qualitatively the same effects were found. I n table 3 a summary of the measurements in solutions of silver nitrate is given. The figures are interesting in so far as they show that particularly in the more concentrated solution the potentials of gold and platinum are hardly affected by the addition of acid or the removal of air by nitrogen. The results show conclusively that the oxidation potential of the silver solution is measured, which in the more concentrated solutions was found to be identical with the silver-silver ion potential. In 0.01 molar silver nitrate the gold and platinum electrodes apparently become easily polarized. SUMMARY
1. Gold and platinum electrodes in solutions of copper sulfate and nitrate behave as air electrodes. The potential is not affected by the concentration of copper ions. After removal of air by nitrogen, the electrodes no longer behave as air electrodes. Apparently the oxidation potential of the copper solutions is measured. 2. Gold and platinum electrodes in solutions of silver nitrate of a concentration greater than 0.01 M indicate the oxidation potential of the silver solution (Ag+ e e Ag). The potentials are not affected, or only very slightly, by a change of the hydrogen-ion concentration or by the removal of oxygen by nitrogen. 3. In all cases the noble metals are easily polarized and the systems are not suitable for exact measurements.
+
REFERENCES (1) BOTTGER, W.,AXD SCHALL,B. H.: Z. physik. Chem. 166A,398 (1933). (2) ESSIN, O., A N D LOZMASOVA, >I.: Z. physik. Chem. 167A, 209 (1933). (3) HOAR,T. P . : Proc. Roy. SOC.London 142, 628 (1933). (4) NIERSTRASZ, C . A , , A N D TENDELOO, H. J. C.: Rec. trav. chim. 63, 792 (1934). ( 5 ) IIICLLER, E.: Z. Elektrochem. 30, 420 (1924). (6) RICHARDS, W. T.: J. Phys. Chem. 32, 990 (1928).
