CONDUCTIVITY MEASUREMENTS OF THORIUM AND OTHER JELLY-FORMING SYSTEMS SATYA PRAKASH Chemical Laboratories, University of Allahabnd, Allahabad, I n d i a Received J u l y 96, 1951
In an important communication by Laing and McBain (l), it has been shown that sol and gel of one and the same substance are identical in their electrical conductivity, lowering of vapor pressure, refractive index, and some other minor properties. However, in the case of curds they found a marked drop in conductivity and osmotic activity. I n previous publications from this laboratory, an attempt has been made to investigate various properties of inorganic jellies, for example, the viscosity and hydrogen-ion concentrations during the course of jelly formation (2), the syneresis of various jellies (3), the thixotropy (4), the changes in extinction coefficients (5), and the influence of various factors on the formation of jellies (6). In this communication, some observations have been made regarding the conductivity of some typical jellies such as thorium arsenate, thorium phosphate, thorium molybdate, ferric arsenate, and aluminum hydroxide. EXPERIMENTAL
The jellies of thorium arsenate, thorium phosphate, and thorium molybdate were prepared by the method described by Prakash and Dhar (7). To thorium nitrate solution containinp 12.5 g. of the salt in 250 cc. (23.54 g. of T h o z per liter), suitable concentrations of potassium arsenate, phosphate, or molybdate were added, and the changes in the conductivities during the course of gelation of these systems were investigated from time to time by the Hartmann and Braun roller bridge arrangement. The temperature was kept constant a t 35OC. The observations have been recorded in table 1. The results recorded in this table show that in the case of thorium arsenate, the mixture of thorium nitrate and potassium arsenate attains an equilibrium within thirty minutes, and after that the conductivity remains fairly constant up to the point of setting. As the opalescence increases, there is a slight decrease in the conductivity, the maximum decrease being obtained in the course of twenty-four hours when the jelly becomes almost opaque. The jelly on further aging, however, shows again an increase in the conductivity. 907
908
SATYA PRAKASH
I n the case of transparent jellies like thorium phosphate and thorium molybdate, when the mixture has once attained the equilibrium, the conductivity becomes constant. Even after the setting of the jellies, the conductivity does not change for a few hours. However, if the aging is allowed to continue, the conductivity also increases. The aging influence is less marked in the case of molybdate jelly. TABLE 1
Conductivities of thorium jellies Thorium nitrate solution = 10 cc. Total volume = 12.5 cc.
Time of setting... , . . . .
. ..
Nature of the jelly. , . . . .{
THORIUM ARSENATB
THORIUM PHOSPEATEI
THORIUM MOLYBDATE
1 cc. of 18 per cent KHz.4504
0 . 7 5 cc. of 20 per cent
1 . 5 cc. of 6 per cent
60 minutes
60 minutes
40 minutes
Opalescent jelly is obtained from clear solution. The opalescence increases continuously with time even after setting.
Opalescent mixture givin clear solution and inally a transparent jelly.
Opalescent mixture giving clear solution and finally a transparent jelly.
Time
0 5 min. 10 min. 15 min. 20 min. 25 min. 30 min. 40 min. 60 min. 100 min. 120 min. 180 min. 200 min. 260 min. 24 hrs. 48 hrs. 72 hrs. 96 hrs.
KHzPOI
Conductivity of the jellies in moh
4.255
-
4.264
-
4.297 4.297 4.297 4.285 -
4.142 4.174
-
4.272
3.710
-
3.635
-
3.616 3.616 3.616 3,615
-
3.654 3.698 3.855 3.946
-
KzMoOi
-
-
< 10-2 2.385 2.398
-
2.395
-
2.390 2.385 2,385
-
2 385
I
2.385 2.482 2.451
-
The influence of temperature on the conductivity of the above aged jellies has also been investigated. As the jellies were obtained by mixing the solutions of thorium nitrate with solutions of potassium arsenate, phosphate, or molybdate, the conductivities of these component solutions have also been measured at various temperatures. The concentrations of the solutions taken were the same as were necessary to give the above jellies. The results are given in table 2.
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CONDUCTIVITY O F JELLY-FORMING SYSTEMS
The results recorded in table 2 show that the temperature coefficients of the conductivities of thorium nitrate, potassium arsenate, potassium phosphate, and potassium molybdate are more regular, amounting to 2 per cent of the original conductivities at 35"C., as is usually found when Kohlrausch's expression is applied to electrolytes. The temperature coefficient of thorium arsenate jelly varies irregularly from 0.9 to 1.4 per cent of the TABLE 2 Conductivities of component solutions Thorium nitrate solution, 10 cc. diluted to 12.5 cc.; potassium arsenate, 18 per cent solution, 1 cc. in 12.5 cc. of water; potassium phosphate, 0.75 cc. of 20 percent solution in 12.5 cc. water; potassium molybdate, 1.5 cc. of 6 per cent solution i n 12.5 cc. water.
x
CONDUCTIVITIES I N MHOS
TEMPERATURE
I'horium nitrate
Potassium arsenate
rhorium arsenate jelly
Potassium phosphate
Thoriun phmphate jelly
Potassium molybdate
Thorium molybdate jelly
-degrees C.
35 40 45 50 55 60
2.433 2.673 2.913 3.132 3.370 3.616
0.8463 0.9263 1.011 1.074 1.165 1,254
4.272 4.522 4.739 4.932 5.217 5.508
0.1471 0.1595 0.1731 0.1879 0.2032 0.2153
3,946 4.238 4.484 4.834 5.168 5.467
0.8245 0.9095 0.9766 1.066 1.137 1.226
2.450 2.713 2.925 3.178 3.418 3.659
Temperature coefficient (approximate). . . . . .
0.048
0,0163
0.049
0.0028
0.061
0,0166
0.048
TABLE 3 Changes i n conductivity of ferric arsenate jelly o n aging TIME
days
1 3 4 5
I
CONDUCTIVITY
mhos X 10-3
4.601 4.853 4.952 5.056
original conductance; that of thorium phosphate is 1.54 per cent, and that of thorium molybdate 2 per cent of the original. One thing which appears to be of interest is that the conductivities of the jellies of thorium arsenate and thorium phosphate are much greater than the total conductivities of the initial components. In the case of thorium molybdate, the cond'uctivity of the jelly is slightly less than the additive
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SATYA PRAKASH
conductivities of thorium nitrate and potassium molybdate. sults are discussed later on.
These re-
Ferric arsenate jelly
A positively charged sol of ferric arsenate, prepared by Holmes’ method
(€9,containing 51.06 g. of ferric arsenate per liter and 0.028 N
in chloride ions showed the following changes in conductivity on aging as it gained in the viscosity. TABLE 4 V a r i a t i o n in conductivity with time Conductivity of 10 cc. of solution with 2 cc. of water at 35°C. = 4.031 X
I
TIME
CONDUCTIVITY
mhos X 10-8
5.865 5.865 5.865 5.865 6.163
0 10 min. 30 min. 2 hrs. 1 day
TABLE 5 InJluence of temperature o n the Conductivity of ferric arsenate sol and j e l l y
~
degrees C.
35 40 45 50 55
I
Temperature coefficient (approximate). . . . . . . . . .
Sol
Jelly
4.056 4.404 4.814 5.223 5.454
6.163 6.665 7.235 7.852 8.170
0.07 to 0.08
0.063 to 0.10
To 10 cc. of the sol of the first day were added 1 cc. of N/5 potassium chloride and 1 cc. of water. The jelly set in 30 minutes. The conductivity showed the variation with time given in table 4. Thus there is no variation in the conductance during and after gelation. However, the jelly undergoes marked aging and gains in cbnductivity on keeping for a longer time. The influence of temperature on the conductivities of 2 days old sol (10 cc. of sol wit,h 2 cc. of water) and the above one day aged jelly is given in table 5.
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CONDUCTIVITY O F JELLY-FORMING SYSTEMS
On account of the presence of potassium chloride, the jelly is more conducting than the sol. The temperature coefficient of the sol is a little less than 2 per cent of the original conductance. per degree, while in the case of the jelly, it is 1to 1.4 per cent of the original. Aluminum hydroxide jelly An aluminum hydroxide sol was prepared by Crum’s method, The sol dialyzed for seven days contained 10.48 g. of A1203per liter. The jelly was TABLE 6 Changes of conductivity with time during and after gelation period Temperature = 35°C.
I
TIME
CONDUCTIVITY
mhos X 10-3
1.477 1.456 1.456 1.456 1.460 1.492 1.505
0
10 min. 20 min. 60 min. 1 day 2 days 3 days
TABLE 7 Injluence of temperature o n the conductivity of a l u m i n u m hydroxide sol and jelly and of potassium chloride CONDUCTIVITIES I N MHOS
x
10-8
TEMPERATURE
so1
Potrtseium ohloride
Jelly
0.1120 0.1247 0.1343 0.1455 0.1549 0.1675
1.409 1.539 1.663 1.794 1.871
1.506 1.655 1.778 1.914 2,061 2.213
0.00215
0.026
degrees C.
35 40 45 50 55 60 Temperature coefficient (approximate)
-
1
0.03
obtained by adding 1 cc. of N/10 potassium chloride and 1 cc. of water to 10 cc. of the sol. From the clear sol, a transparent jelly accompanied with slight opalescence was obtained in the course of one hour. The changes with time during the gelation period and after are given in table 6. The conductivity of the sol previous to the addition of potassium chloride (10 cc. of sol 2 cc. of water) was 1.120 X lo-*. During the course of gela-
+
992
SATYA PRAKASH
t,ion the conductivity remains fairly constant. However an increase is observed on aging. The temperature influence on the conductivity of the following systems has also been investigated (see table 7) : 1. 10 cc. of aluminum hydroxide sol mixed with 2 cc. of water. 2. 1 cc. of N/10 potassium chloride with 11 cc. of water. 3. 10 cc. of sol mixed with 1 cc. of N/lO potassium chloride and 1 cc. of water to give a jelly in 60 minutes. The jelly was allowed to age for three days before the temperature influence was studied. Thus the conductivity of the jelly is almost additive of the conductivities of the sol and of the electrolyte. On the whole, the jelly is slightly less conducting than the components taken together. The temperature coefficient in all the three cases is per degree about 2 per cent of bhe original at 35°C. DISCUSSION
The results recorded in the foregoing pages show the following facts: The conductivity of thorium arsenate goes on decreasing with time as the opalescence increases up to a limit, when the jelly becomes completely opaque. Afterwards the jelly undergoes aging and the conductivity increases. The transparent jellies of thorium phosphate and thorium molybdate, once the equilibrium has been attained between the reactants, do not show any marked variation in the conductivities during the course of jelly formation. After a day or so, however, their conductivities begin to increase on aging. A similar behavior is observed with aluminum hydroxide and ferric arsenate jellies. The conductivities of thorium arsenate and thorium phosphate jellies are greater than the conductivities of their initial components of the same concentrations, taken together. In the case of thorium molybdate, the conductance of the jelly is less than the additive conductances of thorium nitrate and potassium molybdate. The conductivity of aluminum hydroxide jelly is also slightly less than the additive values of aluminum hydroxide sol and the coagulant, potassium chloride. The temperature coefficients per degree for the conductances of thorium nitrate, potassium arsenate, potassium phosphate, and potassium molybdate are about 2 per cent of the initial conductivities a t 35°C. But the temperature coefficients of thorium arsenate and thorium phosphate jellies are considerably less. The graphs obtained by plotting conductivities against temperature are straight lines within the observed range (35°C. to 60°C.). The temperature coefficients of ferric arsenate and aluminum hydroxide sols are also about 2 per cent of the initial conductances. The temperature coefficient of ferric arsenate jelly is less than 2 per cent,- it is only 1 to 1.5 per cent of the conductance at 35°C. For aluminum hydroxide jelly, the temperature coefficient is about 2 per cent.
CONDUCTIVITY O F JELLY-FORMING SYSTEMS
913
It has been observed by Dhar and coworkers (9) that the sols of ferric oxide, copper ferrocyanide, Congo red, arsenious trisulfide, silicic acid, etc., show a marked increase in the electric conductivity on aging. Gessner (10) found a decrease in the case of vanadium pentoxide sol. Roy and Dhar (11) observed that in most cases the light effect on colloids is merely an accentuation of the time effect, and consequently, on exposure to light, the conductivity increases. The stability of a sol is due to the stabilizing influence of similarly charged ions, Precipitates as well as sols, when aged, are found to have much less capacity of binding ions by adsorptive forces, so much so, that on aging some of the absorbed ions are set free. These free ions go to account for the increase in the observed conductivity. The process of gelation is a specific case of coagulation where the uncharged particles instead of settling down after the formation of conglomerates, imbibe the dispersion medium. The coagulant adsorbs some of the ions from the free electrolytes of the medium, and consequently the total conductivity of a jelly system is a little less than the additive conductivities of the components, as has been found in the cases of aluminum hydroxide, ferric arsenate, and thorium molybdate jellies. When thorium nitrate is mixed with potassium molybdate to form the jelly, the reaction appears to proceed as follows: Th(N0s)c
+ 2&MOO4
=
Th(M0Oi)n
+ 4KNOa
During the reaction, thorium molybdate comes down as a gelatinous mass, and thus some of the thorium and molybdate ions are removed from the field. In order to form the colloidal phase of thorium molybdate, some of the stabilizing ions are further adsorbed from the system. This accounts for the 25 per cent diminution in the conductance with respect to the additive values of the components in the case of thorium molybdate jelly. The conductivities of thorium arsenate and thorium phosphate show a reverse effect. The conductivity of the jelly is much greater than the conductivities of the jelly-forming components taken together. This is due to the fact that while the normal potassium molybdate, K2Mo04, was used for the formation of molybdate jellies, the dihydrogen salts, KH2AsOJ and KH2POI, were employed in the case of arsenate or phosphate jellies, where the reaction may be assumed to take place as follows: Th(N0a)r
+ 2KHzAs04 = Th(HAsO4)z + 2KN03 + 2HNOs
From this reaction, it is clear that, owing to the formation of nitric acid, more ions are available after the reaction and consequently, in spite of the diminution due to the usual reasons, there is an increase in the conductivity of the system, Though the conductivity of sols has been investigated by many workers, the temperature influence appears to have been neglected. The influence
914
SATYA PRAKASH
of temperature on the conductances of simple salts, according to Kohlrausch’s classical expression, is linear within a sufficiently wide range and the temperature coefficients per degree approximate to about 2 per cent. The rise in the conductivity on the increase of the temperature is due to an increase in the mobility of the ions. The temperature coefficients of the
Temperature FIG. 1. THE INFLUENCE OF TEMPERATURE ON JELLIES
THE
CONDUCTIVITY OF THORIUM
jellies show an interesting feature. It appears that the increase in mobility of ions in jellies with rise of temperature is somewhat less than in the case of sols or solutions, and therefore the temperature coefficients are less than 2 per cent of the initial in the jellies. This point is exhibited in the case of thorium arsenate, thorium phosphate, and ferric arsenate jellies. One thing interesting t o note is the deviation from the usual parallelism
915
CONDUCTIVITY OF JELLY-FORMING SYSTEMS
between temperature coefficients of conductance and the viscosity of the solvent medium, Ordinarily, it has been found that the conductivity of solutions becomes zero a t temperatures near about -39°C. The jellies which we have investigated would have zero conductance at the temperatures given in table 8, as found by extrapolation (the relation between conductivity and temperature has been supposed to be linear down to the zero conductance). The temperature at which water is supposed to possess an infinite viscosity is -34"C., a temperature a t which it has almost no conductivity. When a sol undergoes setting, it gradually begins to gain in viscosity, and finally, it loses all its fluidity. From this, we might have expected a gradual decrease in conductivity too during the course of jelly formation,a fact contrary to the observations. It shows that the decrease in the fluidity with the fall of temperature is materially very much different from the decrease during the course of jelly formation. It is an interesting fact TABLE 8 Temperatures of zero conductance of jellies JELLY
I
TENPERATVRE OF ZERO CONDVCTANCE
degrees C.
Thorium arsenate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thorium phosphate, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thorium molybdate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ferric arsenate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Aluminum hydroxide, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
- 56 -34 - 15 - 25 -20
that the water bound in a jelly offers no resistance to the ions present in the system. The conductivities of the jellies so far studied are mainly due to the electrolytic impurities, and in their comparison the effect of actual colloidal micelles is negligible. It is impossible to get an inorganic jelly without the free ions in the system, and as such, the conductivity is not a good property to throw much light on the nature of jellies. Laing and McBain got a difference of about one-fourth in conductivities of soap gels and curds. Even our completely opaque jellies, which may be called curds, do noi show much marked decrease in the conductivity during the development of opalescence. SUMMARY
1. The electrical conductivities of jelly-forming systems of thorium arsenate, thorium phosphate, thorium molybdate, ferric arsenate, and aluminum hydroxide have been investigated
916
SATYA PRAKASH
2. It has been shown that once the equilibrium has been established between the jelly-forming components, the electrical conductivity becomes constant and no change is observed during the course of jelly formation or after the setting of the jelly. There is a slight decrease in the conductivity of thorium arsenate along with the development of opalescence until the jelly becomes completely opaque. 3. When the jellies are allowed to age for a number of days, the conductivity increases in all the cases. 4. The conductivities of thorium arsenate and thorium phosphate jellies are greater than the additive values for the conductivities of their components, because in their preparation the dihydrogen salts were used, which give rise to free nitric acid. In the case of thorium molybdate jelly, where a normal salt was used, the conductivity was less than that of the components. This is also the case with aluminum hydroxide. 5. The temperature influence on the conductivity of the jellies has also been investigated between 35°C.and 60°C. The relation between temperature and conductivity has been found to be linear within a wide range. The temperatures of zero conductance have been extrapolated. 6. The temperature coefficient for thorium molybdate and aluminum hydroxide jellies is about 2 per cent of the original a t 35"C.,while in the other cases, it is markedly less than 2 per cent (about 1to 1.5 per cent).
The author expresses his indebtedness to Professor N. R. Dhar for his kind interest. REFERENCES AND MCBAIN: J. Chem. SOC.117,1506 (1920). (1) LAINQ (2) PRAKASH, S., AND DHAR,N. R . : J. Indian Chem. SOC.6,391 (1929); Kolloid-Z. 60, 184 (1932). (3) PRAKASH, S.,AND DHAR,N. R . : J. Indian Chem. SOC.7, 417 (1930). S.,AND BISWAS, N. N . : J. Indian Chem. SOC.8,549 (1931). (4) PRAKASH, (5) PRAKASH, S.:J. Phys. Chem. 36, 2483 (1932). (6) PRAKASH, S.:Z. anorg. allgem. Chem. 201,301 (1931); 208, 163 (1932). (7) PRAKASH AND DHAR:J. Indian Chem. SOC. 6, 587 (1929). (8) HOLMES, H. N., AND ARNOLD, R.: J. Am. Chem. SOC.40, 1014 (1918). (9) DHARAND COWORKERS: Kolloid-Z. 42, 120 (1927); Z. anorg. Chem. 168, 209 (1927). (10) GESSNER:Kolloidchem. Beihefte 19, 283 (1924). (11) ROYAND DHAR:J. Phys. Chem. 34,122 (1930).
