POLARIZATION VOLTAGES OF SILVER NITRATE SOLUTIONS BY J. A. WILKINSON AND H. W. GII,LETT
Some qualitative experiments having shown that the decomposition voltage for acidified silver nitrate solutions varied with the concentration of the acid, experiments were made t o determine the various factors. The electrodes consisted of two smooth stiff platinum electrodes, each having a surface of 14.64 em2. These were placed 3 cm apart in a beaker which required 150 cc of solution t o fill it t o the upper edge of the electrodes. This same amount of solution was used in each case. Instead of determining the decomposition voltage or point at which the current begins to increase beyond the residual current it was found better t o measure the polarization voltage or counter-electromotive force given by the cell after it had been polarized by a charging current. To do this, the charging circuit was broken and the cell shortcircuited on a high-resistance voltmeter. Readings were taken every quarter minute and plotted with voltages and time as the co-ordinates. The horizontal portion of the curve is taken as the polarization voltage,. In cases where there is no good horizontal portion, the apparent inversion point of the curve was taken. In Table I is given a good example. The charging current was 6.j milliamperes for 1.2j minutes, and the polarization voltage is clearly 1.02 volts. The polarization voltage, as thus determined, is identical with the decomposition voltage when the reaction is reversible. When there is no acid a t all in the solution, the readings are not congruent. When there is a large amount of acid in the solution or when the temperature is high, it is hard to get a satisfactory polarization voltage because the chemical action is increased and the deposit dissolves too quickly. The charging or polarization current was varied to meet the exigencies of each particular case, being larger when the concentration of acid or when the temperature was high. When
I
Polarizntion Voltages o f Silver Nitrate Solutions
383
TABLE I Temperature 18.5' 3.75 g AgNO, and 7 g HNO, per 150 cc Voltage readings every quarter minute -
1.08 1.05 I .04 I .03
1.025 1.02 I .02 I .02 1.02
1.02 1.02 1.02 I .02 1.02 I .02 1.02 1.02
1.015
1 ' ~
~
1.015 1.01
;:a; 1.00
0.99 I
1 ~
0.97 0.94 0.90
0.84 0.75 0.67 0.59 0.51 0.40 0.35 0.32
0.37
,
0.27 0.25 0.24 0.22 0.21 0.20
0.16
0.19
0. I2
0.18
0.12 0.11
0.16
0.15 0.14 0.14 0.13 0.13
difficulties were encountered, readings were taken every five seconds. In every case the observations were checked by repeating them under the same conditions. A calibration curve for the Weston voltmeter was obtained by checking against a standard one belonging to the Department of Physics. There are four variables to be considered : temperature, silver nitrate, acid, and water. In our experiments we have let each one of these vary by itself, keeping the other three constant. Thus, keeping the acid, water and silver nitrate constant, we varied the temperature. Keeping the temperature and the ratio of acid t o water constant, we varied the concentration of the silver nitrate. Keeping the temperature and the ratio of silver nitrate to water constant, we varied the concentration of the acid. Keeping the temperature and the ratio of acid to silver nitrate constant, we varied the concentration of the water. In this way a pretty fair idea can be obtained of the effect due to each of the variables although it was not attempted to make the treatment exhaustive. The silver nitrate was from the stock room and was not purified further. The acid was so-called, pure, concentrated nitric acid from the stock room and its specific gravity was 1.41. In Table I1 are two runs t o show effect of temperature,
_
_
TABLEI1 EFFECT. O F TEMPERATURE ~
~
~
~
p
p
_
_
Solution A = 3.75 g AgNO, _ _
~
Temp.
Temp.
I
~
Volt
__-O0
I 1.5 15.0
1
.____--.A
Volt
0.91 0.913 0.918
~
Temp.
~p.
_
~
Volt
Temp. Volt ____~____ ~
20'
27 31
1 ~
~-
~
+ 0.7 g HNO, per 150 cc
0.924 0.928 0.929
~~-
~
1
Temp.
1
Volt
1
Temp.
1
Volt
:::: i 39.5'
~
0.938 0.g44 0.952
~
PoZarizatioiz VoZtages o f Sidvev Nitrate Solutions
385
TABLE I11 EFFECT O F SILVER N I T R A T E
1
g AgNO, per 150 cc g HN0,per
ISOCC
I
Volts
1
Temperature
0.91 0.928 0.948 0.967 0.985 I no5 1.025
5.473 2.923 1.928 1.069 0.623 0.427 0.201 0. I O 0
I .040
Fig.
I
that this extrapolation is justified !but it does not seem an impossible value. The change of 0.13 volt, while the silver nitrate concentration varies in the ratio of I : 55, is unexpectedly large. In Table IV are the data obtained when nitric acid was the variable. A correction for temperature must be applied before the results are strictly comparable. The results are shown graphically in Fig. 2. Increasing the nitric acid concentration increases the polarization voltage, at first very much but afterwards less. With no nitric acid this concentration of silver nitrate gives a polarization voltage of about 0.65. Addition of 0.14gram HNO, per 150 cc raises the value to 0.818 and apparently causes the formation of the peroxide or peroxynitrate a t t h e . anode. The increase in concentration from 0.14gram H?:O, to 10.5 grams raises the polarization voltage by 0.24 volt. In the experiments to determine the effect of water as a
TABLE IV EFFECT O F HNO, 3 E AgNO, 3er 15p cc g HNO, per 150 cc
0.14 0.28 0.42 0.84 1.12
1.36 I .40 1.68 1.82 I .96 2.16
Volt
0.818 0.86 I 0.915 0.927 0.947 0.948 0.952 0.965 0.966 0.972 0.981
Temp.
;"Os
per 150 cc
2 So
2.24
25
2.80
25
25 20
3.08 3-36 3.64 4.20 4-90 6.16
25
7-70
2.5 25
19.5
.
1.015 1.025
1.033 1.034 ' 1.044 I .os8
-
-
20
0.986 0.995 I .005 1.005
10.50
19.5
Volts
Temp.
i940 21 20 20 20 21 21 25
21 21
-
1.0
0.9
0.8
0.7
HNO3 2
6
4
Fig.
8
IO
2
variable, the ratio of acid to silver nitrate was kept constant. The results are given in Table V and in Fig. 3. With increasing dilution the values for the polarization voltage fall off slowly at first, but decrease very rapidly for the very dilute solutions. It is not clear what would be the significance of an extrapolation to infinite dilution. It must also be kept in mind that the product formed at the anode probably changes when the polarization voltage falls dis-
Podarization Voltages of Sidver Nitrate Sodzitions
387
tinctly below 0.8 volt though this is not a point on which we have any evidence t o offer. TABLE V EFFECT OF WATER _-~
-~
~~
~
~
I
g*gNOs per 150 cc -
-
~-
Volts
I
~~~~~
_____
~
I
g"O3 per 150 cc -
__~__
Temp.
I
-
Ratio of AgNO .o HNO, = 4-13 ~~
I
.
1.296 7.527 5.090 2.545 1.273 0.636 0.3 I 8 0.159
.
2.735 1.823 1.234 0.617 0.30 8 0.154 0.077 0.038
~~
zoo
0.924 0.91 0.90 0.89 0.866 0.832 0,745 0.620
20
22 22.5
24
.
25
~
-
24-5 24.5 .
~~
Ratio of AgNO, to " 0 , ~~
~~
13.030 4.340
~
~~
~
0.700 0.233 0. I 16 0.058 ,0.046 0.029 0.435
2.170
1.085 0.870 0.543 0.435
= 18.7
0.793 0.755 0.655 0.630 0.639
21 21
24
24 24
0.9
0.8
0.7
0.6
2
4
6
8
IO
12
Fig. 3
From the last three tables we see that the polarization voltage rises when the concentration of silver nitrate decreases and that of nitric acid remains constant; that it falls
/. A. Wilki~~soia ami H. W. Gillett
388
when the concentration of nitric acid decreases and that of silver nitrate remains constant; and that i t falls when the concentrations of silver nitrate and nitric acid decrease in the same ratio. Owing to the marked effect thus due to the nitric acid, a few experiments were made to determine the effect of nitrid acid at each electrode. The same two platinum electrodes and the acidified silver nitrate solution were placed inside a porous cup which dipped into a beaker containing H,SO, ( I : 5), Hg,SO,Hg. The porous cup had previously been boiled out with a potassiuni nitrate solution to prevent the precipitation of silver sulphate in the pores. The mercurous sulphate was prepared by the method of Hulett'. The connections were so arranged that i t was possible t o polarize the platinum electrodes and then t o connect either one of them through the voltmeter with the mercury electrode. Using a voltmeter one measures potential differences instead of electromotive forces ; but this is immaterial so far as studying the relative effect of acid on the two polarized electrodes. The mercury electrode is the anode in all cases. The data are given in Table V I and are plotted in Fig. 4.
iQ2a
0.1
c
Silver
HNO 2
4
6
8
Fig. 4 1 Trans. Am.
Electrochem. SOC.,6 , 109 (1904).
IO
I2
14
PoZaYization Voltages of Silver Nitrate Solzitiom
389
From Table VI it is clear that addition of nitric acid decreases slightly the potential difference between silver and the electrolyte and increases largely the potential difference be~~
~
~
I9O 20 20 20 20 20
20.5
-
23 24
1
TABLE VI -
Volts Hg-peroxide
g &NO, : cc : : : ; per c150
Temp. .-
-~ -~
I
3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00
~.
0.00
0.07 0.14 0.28
0.42
0.77 1.47 2.94 6.37 9.80 13.3
0.16 0.16
0.15 0.145 0 . I4+ 0. I35
0.89 0.948 0.987 1.015 1.035 I.055
0.12
I .09
0.115
1.107 1.148 1.154
0.085 0.08
0.06+
1.16
~
22
22.5 22.5 22.5
26
1
3.00 3.00 3.00 3.00 3.oo 3.00 3.00 3.00 3.00
0.00
3.00 3.00
0.07
0.15
0.I35
0. I45
0.28
0.49 0.77 1.47 2.80 6.30
0.14 0.135 0.125
1.01 5
1.03 1.045 1.058
0.12
1.078 I .og8
9.80
0.105 0.08 0.07+
13.3
0.06 f
1.150
1.120 1.135
tween the peroxide electrode and the electrolyte. The result is not what one might have expected. The addition of nitric acid might reasonably have been expected to force back the dissociation of silver nitrate to some extent and thus to have increased the potential difference. On the other hand it must be remembered that the addition of nitric acid has an effect on the potential difference between the solutions. If we ignore the potassium sulphate in the walls of the porous cup and the silver nitrate in the cathode chamber, the effect due to the nitric acid is of the same general order as that required by Nernst’s formula.
390
J. A. WiZkiiisoiz and H. W. Gillett
It is an interesting question whether one is justified in assuming that the solution pressure of silver in aqueous silver nitrate is the same as in that of a solution of silver nitrate in aqueous nitric acid. Kahlenberg' has shown that the addition of pyridine to aqueous silver nitrate changes the solution pressure. Pyridine is an organic liquid, miscible in all proportions with water. Nitric acid is an inorganic liquid, miscible in all proportions with water. It would be very interestingthough entirely outside the scope of these experiments-to determine the so-called single potentials for silver and silver nitrate in a solvent varying continuously from pure water to practically pure nitric acid. The effect of acid at the peroxide electrode is qualitatively what one would expect. It is impossible to discuss the quantitative side until more is known about the real composition of what is called silver peroxide. The general results are as follows: I , The polarization voltage of acidified silver nitrate solutions may be made to vary from 0.625 volt to 1.05 volts. 2, The polarization voltage varies with variations of temperature, silver nitrate, nitric acid and water. 3. The temperature coefficient is approximately 0.8 millivolt per degree over a fair range of compositions and temperatures. 4. A decrease in the concentration of silver nitrate increases the polarization voltage if the concentration of nitric acid be kept constant. 5. A decrease in the concentration of nitric acid decreases the polarization voltage if the concentration of silver nitrate be kept constant. 6. The polarization voltage decreases if the concentration of silver nitrate and nitric acid are decreased in the same ratio. 7. When measured against a sulphuric acid, mercurous sulphate, mercury electrode, the potential difference between silver and the electrolyte is decreased slightly by the addition
* Jour. Phys. Chem.,
3,403 (1899).
Polarization Voltages o f Silver Nitrate Solutions
391
of nitric acid, while the potential difference between peroxide and electrolyte is apparently increased a good deal. 8. The abnormal result with the silver electrode may be due to a change in the potential difference between the s o h tions. 9. Therc is nothing in the data obtained to show whether or when the product formed at the anode changes from peroxide t o oxygen. This work was suggested by Professor Bancroft and has been carried on under his supervision. Cornel1 Univemity
