CATHODE POLARIZATION AS A FUNCTION OF T H E CURRENT DENSITY I N FUSED SALTS 8. KARPATSCHOFF AND 0.POLTORATSKA Institute o j Physical Chemistry, Scientific Researches, Sverdlovsk, U.8.S. R . Received December 6, 1936
Cathode polarization in fused salts has been studied very little up to the present time. There is one paper by Aten and others (l),but Aten was interested only in the relation between the cathode potential and the current density in connection with the character of cathode deposits obtained from fused salts. Our method of experimenting was the same in principle as that used by Aten. The fused salt in a porcelain crucible was placed in an electric furnace with the carbon electrodes-cathode and anode-passing through the opening in the cover of the furnace. The potential was measured between an auxiliary electrode and the bathode at various current densities. The auxiliary electrode was constructed in the following way. The fused metal being plated was placed at the bottom of a quartz test tube with capillary jet at the side which was closely pressed to the cathode. The test tube was filled with fused electrolyte through the capillary jet; thus a lead or cadmium electrode was obtained. The contact of liquid metal in the test tube with the wires connected to the potentiometer was made by means of an iron wire. This wire was enclosed in a porcelain capillary in order to prevent contact of the iron wire with the fused electrolyte. We measured the resistance of the cathode in order to make the necessary correction. I n this measurement the cathode was at the same temperature condition as that during the investigation at cathode polarilation. The highest possible error was 0.01-0.02 Q in the determination of cathode resistance. In the case of lead chloride the curve Eoath = f (current density) has the form shown in figure 1. It is seen that the potential of the cathode is more positive than the potential of the auxiliary electrode a t comparatively low current densities. The cathode potential becomes less and less positive by increasing the current density and finally equals the potential of the auxiliary electrode (figure 1, point A ) . The current density a t point A is equal to 0.2-0.15 ampere per square centimeter. Thus electrolysis of fused lead chloride at significant current densities can take place when the difference of the potentials between the electrodes is leas than the E.M.F. of the system Pb(PbCl2 fused) CIS. The effect indicated was previously noticed 763
764
S. KARPATSCHOFF AND 0. POLTORATSKA
by R. Lorenz (3), who explains the phenomenon as a depolarizing action. This is confirmed by the fact that we were able by saturating the fused lead chloride with metallic lead to reduce the current density, a t which the curve E c a t h = f (current density) crosses the axis of the ordinates, from 0.2 amp. to 0.015 amp. It is known that lead chloride can “dissolve” a significant quantity of metallic lead. R. Lorenz supposed that in this case an emulsion of the lead is formed in the fused salt. However, Eitel and Lange (2) showed that the metal is not in colloidal form, but in true solution. The solution of the metal in the fused salt is explained by these authors by the formation of compounds of metallic lead with lead chloride.
0.1
0
07
0.2
a3
FIG.1
The part of the curve that lies in the region of the positive values of potential can be explained by assuming that during the dissolution of metallic lead in the fused salt monovalent lead ions are formed. Consequently it is possible that in the case of absence of metal in fused salt during electrolysis the discharge of Pb++ ions to Pb+ ions took place a t the cathode. The potential of the cathode may be expressed in the following form:
where a is constant, and Cpbt and Cpb++are the concentrations of the monoand bi-valent ions near the cathode.
765
CATHODE POLARIZATION IN FUSED SALTS
Reasoning by analogy from the case of concentration polarization in water solutions, Cpb+and Cpb++,and placing their values in equation 1 we have RT K I D - CQ E=a---ln F CA - KzD Here K 1 and K z are constants at a given temperature; Coand C: are the concentrations of the ions Pb+ and Pb++ in the mass of the electrolyte. CO is insignificant (the fused salt was not in contact with the metal). Dividing the numerator and the denominator of the logarithmic fraction by C: and combining all constants in a, we shall finally have:
RT E=a--1n-
F
D 1-KD
The value of the constant a is determined by the condition that the difference of the potentials between cathode and auxiliary electrode is zero at a certain current density, DO (point A , figure 1).
RT F
a = -1n-
DO 1 - KDQ
Putting the value of a in equation 2 we shall have:
RT E = -In F
Do (1 - KD) (1 - KD,) D
(3)
The results of the experiments with fused lead chloride at 600°C. and the values of the constant K are shown in table 1. The part of the curve E = f (current density) was studied for fused lead chloride in the region of the negative values of potential in experiments 2 , 4 , and 5 at 550°C. The results of these experiments are shown in table 2. In a given case the relation E = f (current density) may follow an equation of the form E = a - b In D. The coefficient b has the following values : exp.. ................................ b . . ..................................
2
4
5
0.0348
0 0391
0.0417
From the figures given it is seen that the coefficient b is approximately equal to the quantity RTISF, which has the value 0.0354 at 550°C. The relation obtained can be explained by supposing that in the region of the negative values of the potential the process Pb++ + 2Q + Pb on the cathode plays the coordinal r61e. In this case we can suppose that atoms of the metal which are formed at the cathode remain near it in the dissolved state and then these atoms condense in droplets with a certain definite
766
S. KARPATSCHOFF AND 0. POLTORATSKA
speed (the fused metal did not wet the carbon cathode). In connection with the above view, we can write the following expression for the potential of the cathode:
TABLE 1 Results of experiments with fused lead chloride at 600°C. EXPERIMENT
1
EXPERIMENT 2
EXPERIMENT
3
EXPERIMENT
__
E D
K
_
D
_
K
D
_
~
D
K
volts
atnp.jsp.
amp./sg.
cm
zmp./sg. cm.
rmp./sq.
Cm.
0.000 0.025 0.050 0,075 0.100 0.1% 0.150 0.175 0 200 0.250
0,150 0.130 0.107 0.090 0,073 0.063 0,050 0.040 0.033 0.020
0.153 0.130 0.110 0.093 0.087 0.067 0.060 0.050 0.040 0.023
0,170 0.150 0.137 0 120 0,107 0.097 0.083 0.067 0.043 0.023
0.200 0.180 0.157 0.140 0.117 0,100 0.087 0.073 0.057 0.030
4.00 3.74 3.96 3.76 4.40 4.40 4.50 4.70 4.90
3.50 3.95 3.97 4.00 4.40 4.80 5.05 5.02 5.05
4
K
cm.
3.84 4.32 4.40 4.74 4.78 4.21 3.40 4.84 4.34
3.54 3.44 2.92 3.64 3.73 3.90 3.99 3.97 3.69
Results of experiments with fused lead chloride at 550'C. EXPERIMENT
2
EXPERIMENT
EXPERIMENT 5
4
E
D
E
D
E
D
no118
amp./sq. em.
VOllS
amp./sq. em.
volts
a m p . / s q . cm.
0.009 0.011 0.017 0.027 0.033 0.044 0.052 0.060
0.160 0.186 0.213 0.264 0.422 0.527 0.580 0.738
0.001 0.014 0,029 0.048 0.052
0.225 0.279 0.446 0.670 0.837
0.015 0.029 0.033 0,035 0,044 0.048 0.052
0.614 0.837 1.000 1.115 1.228 1.396 1.507
c p b is the concentration of the atoms of lead which are in the dissolved state near the cathode. Regarding the value Cpb++as constant, we shall have: RT E = a - - In Cpb (4) 2F
Assuming a t each current density an equality between the speed of discharge of the ions Pb++ and the speed of condensation of the dissolved metallic atoms, we shall have:
767
CATHODE POLARIZATION I N FUSED SALTS
0
KD
=
Cp, - Co
(5)
where Cois the value of a concentration corresponding to equilibrium a t a given temperature. Placing the magnitude Cpb in the expression of potential and making some simplification, we shall finally have:
E
=a
- R-InT
(1
2F
+ KID)
If the constant K 1is great enough, equation 6 may be written thus:
E
= a
T - RIn D 2F
(7)
The curve E = f (current density) in fused cadmium chloride is similar to the corresponding curves for lead chloride, only here the curve intersects Results EXPERIMENT
E
TABLE 3 experiments with cadmium chloride
8
EXPERIMENT
D
D
E
D
!Jolts
arnp.lsq. em.
-0.084 -0.104 -0.117 -0.132 -0.141 -0.150 -0.157
0.044 0.048
volts
omp.lsq. cm.
UOllS
amp./sq. cm.
10,120 +0.113 +O ,081 +O ,060 +O ,033 +O ,026 +O ,004 -0.027
0.006
-0.016 -0.022 -0.033 -0.048 -0.053 -0.067 -0.073
0.002
0.024 0.045 0.084 0.253 0.353 0.446 0.670
7
E
0.006 0.010 0.014 0.018 0.0% 0.034
0.066 0.088 0.110 0.132 0.176
the axis of the ordinates at greater current density (experiment 6). Saturating the fused cadmium chloride with metallic cadmium (experiment 7 ) we succeeded in reducing the value of this current density almost to zero; thus the total curve lies in the region of negative values of the potential. The results of experiments 6 and 7 can be seen in table 3. In cadmium chloride the dependence E = f ( D ) also seems to follow an equation of the form E = a - b In D. The coefficient b is equal to 0.0435 (experiment 6) and to 0.0348 (experiment 7), Le., to R T / 2 F . I n the region of the negative values of the cathode potential (experiment 7 ) the existence of the indicated relation can be explained in a similar way to thLt of chloride. The relation RT E = a - -In D 2F for cadmium chloride, which is not saturated with metallic cadmium (experiment 6 ) , can be explained by supposing that by dissolving the T H E JOURNAL OF PEYBICAL CHBMIWRY, VOL.
40,
NO.
0
768
S. KARPATSCHOFF AND 0. POLTORATSKA
metallic cadmium in its chloride salt the formation of the ions Cd+ does not take place; thus on the cathode the process Cd++ + 2 0 -+Cd takesplace. In connection with this, at all current densities the velocity of formation of the cadmium atoms will equal the velocity of their diffusion in the mass of the electrolyte. Thus we can write:
K D = Cc, - Co where Co is the concentration of the cadmium atoms in the mass of the electrolyte. Determining the concentration of the cadmium atoms near the cathode, CCd, we obtain
If Co is very small (the fused salt was not in contact with the metal), we shall finally have :
E
=a
- RT In D 2F
R$SUM~
The cathode polarization in fused salts of lead chloride and cadmium chloride has been experimentally studied. The part of the curve E = f ( D ) , lying in the region of the positive values of the potential, is explained by the slowness of the diffusion process of Pbf ions or cadmium atoms from the cathode in the mass of the electrolyte. I n the region of the negative values of the potential the relation E = f ( D ) can be expressed by the equation
E
=a
- RT -In 2F
D
We have explained the equation of this form by the slowness of condensation of the lead or cadmium atoms which are formed at the cathode in the liquid. In conclusion w e desire to express our thanks to Prof. A. N. Frumkin for his attention t o this work. REFERENCES (1) ATEX AND OTHERS: Trans. Am. Electrochem. Soo. 47, 265 (1925). (2) EITELAND LANGE:2 . anorg. allgem. Chem. 171, 158 (1928). (3) LORENZ,R. : Die Elektrolyse geschmolzenen Salze (1904).
rt
