1024
L. A. MUNRO AND J. A. PEARCE
REFERENCES (1) MCBAIN,J. W.:Chapter V in Alexander’s Colloid Chemistry, Vol. I. The Chemical Catalog Company, Ino., New York (1926). (2) MCBAIN,J. W., BROCK, G. C., VOLD,R. D., AND VOLD,M. J.: J. Am. Chem. Soc. 60,1870 (1938). (3) MCBAIN,J. W.,LAZARUS, L. H., AND PITTER,A. V.: 2. physik. Chem. A147, 87 (1930). (4) MACBAIN,J. W., VOLD,R. D., AND JAMEBON, W. T . : J. Am. Chem. Soc. 61, 30 (1939). (5) MCBAIN,J. W., VOLD,R. D., AND VOLD,M. J. : J. Am. Chem. SOC.60,1866(1938) (6) VOLD,M.J.: J. Am. Chem. SOC.62, December (1940). (7) VOLD,M.J., MACOMBER, iM., AND VOLD,R. D.: J. Am. Chem. SOC.62, December (1940). (8) VOLD,R. D . : J. Phys. Chem. 4S, 1213 (1939). (9) VOLD,R. D . , AND FERGUSON, R. H.: J. Am. Chem. SOC.60,2066 (1938). (10) VOLD,R. D., AND VOLD,M. J.: J. Am. Chem. SOC.61,37 (1939). (11) VOLD,R.n.,ANDVOLD, hf. J.: J. Am. Chem. SOC.61,808(1939).
T H E T I M E OF SET OF SILICA GELS. V THE EFFECT
OF
ALCOHOL8
AND
PH
I,. .\, MUNRO
AND
ON THE
“HEAT O F ACTIVATION”
J. .4. PEARCE’
Depnrtment of Chemistry, Queen’s Uniaersity, Kingston, Canada Receaved August 1 , 1959
Hurd and Letteron (7) have discussed the application of the equations of chemical kinetics and the Arrhenius equation to the setting of silicic acid gels. A value of 16,940 cal. was obtained for the “heat of activation.” This was independent of the soda-silica ratio (8) and the weak acid used ( 5 ) but varied when strong acids were used (6). Prasad and Desai (13) have recently estimated the “‘heat of activation” of various inorganic gels of unrecorded pH. Previous investigation (12) has shown that the effect of addition agents on the time of set of silica gels is specific a t all pH values except 7.0 and changes in a regular manner as the acidity decreases. The heterogeneity of the gelating system and the difference in properties of acid and alkaline gels made it seem of interest to determine whether this change in behavior and the specific effect of addition agents would be reflected in the “heat of activation.” Present address: Department of Chemistry, &Gill Canada.
University, Montreal,
1025
TIME OF SET OF SILICA GELS EXPERIMENTAL
A stock silicate solution of sp. gr. 1.1569 containing 6.70 per cent SiOz was prepared from “Crystalline sodium silicate, SasSiOI.9H20” (NazO : SiOz = 1.00:1.02). The standard acetic acid was 1.636 N (100 cc. of glacial acetic acid 1 liter of water). The esperimental technique was the same as previously described (10, 11, 12).
+
RESULTS
The results for one concentration of addition agent (0.5 111per liter) are shown in tables 1, 2, and 3. The relation between concentration of addition agent and time of set \vas found to be similar t o carlier work (11). TABLE 1 &$a/o j oleohols irpon the tzirie o j set o j s i l m gels ’
A D D I T I O N O F 25 CC. OF 3.35 PER CENT si02 S O L U T I O N TO:
~
~
TIME
TEYPERATURE
or SET
ita
+
TROL GEL
~
363 1 22 15 0 8 87
loo0 155 57 0 10 5
930. 132. 49.0 !).75
900. 5.74 5.61 130. 126. 4 8 . 0 47.0 5.76 9.20 9 . I ( 5.93
1906.
I
( h ) 15 cc. of acid* 0 watcr addition i 20 ngrnt ( = 25 rr.) 40 50
+
Mannitolt
“C.
:Iddition agent (= I 30 25 cc.)
OF
CON-
Glycerol
_~
pH
I N MINUTES
~
60 0 185 105 53.0
Coagulatcd Coagu1:itcd 75.0 256 124. Coagulated 215. 400. Coagulated Coagulated
_____
* 100 cr. of glacial acetic acid + 1250 ec. of natei t Mannitol values at 0.5 molc per liter of addition a g r n t ‘irr ol)t.iincd a t
10.59 10.50 (0.60
10.63
tlir lo\vri
temprraturrs hy extrapolation from smaller additions
The pH changes recorded arc the corrected (3) initial pH aiid the pH 011 setting for the control gel In the region of pH 9.5 to 11.0 the corrected glass-electrode readings were between 0.2 and 0 4 pH unit lower than that obtained by indicators (11). Figure 1 (data in table 3(a)) shows the log time of set plotted against the reciprocal of the absolute temperature and is representative of the acid gels. Only limiting ciirves are shown, the remainder lying between and parallel to thcse. The “heats of activation” calculated for these acid gels are, respectively: 16,900; 16,500; 16,780; and 16,300 cal..--essentially thc same as reported by Hurd. Data from table 3(c), for slightly alkaline gels, have been used to construct figure 2. In this figure the curve for methanol crosses, and that for I-propanol approaches, the curve for the control gel (12). This appears
TABLE 2 Effect of alcohols upon the time o j set of silica gels TIME OF BET I N MINUTES
ADDITION OF 25 cc. OF 3.63 PER CENT Si01 SOLUTION TO:
pH OF CON-
PERA-
TROL (IEL
'C.
(a) 22 cc. of standard acid water +addition agent (= 25 cc.)
0 20 30 50
(h) 16 cc. of standard acid water addition agent ( = 25 cc.)
0 20 30 50
+ +
+
-I-/--
-
/-I l-l
717. 765. 91.5 98.8 35.8 , 38.7
2.92 5.00 4.75 2.66
925. 118. 41.0 6.83
Too Too Too Too
1.00 1.90 1.75 1.00
765. 763. 732. 97.0 95.8 93.3 37.8 37.3 37.0 6.32 6.33 6.15
fast fast fast fast
2.25 3.58 3.33 1.75
5.52 5.57 5.55 5.82
4.25 10.7 10.09-10.11 6.00 16.0 10.18-10.49 5.25 12.4 10.26-10.42 2.15 5.45 10.39-10.56
t Mannitol values a t 0.5 mole per liter of addition agent are obtained a t the lower temperatures by extrapolation from smaller additions. TABLE 3 Effect of alcohols upon the time'of set of silica gels
-
-
ADDITION OF 25 cc. OF 2.59 PER CENT 6 1 0 2 SOLUTION (SP. OR. 1.0569) TO:
TEYPERATURE
TIME OF #El' I N MINUTES
Control (1.)
__-
~
Methanol
PA
Mannitol?
1-Pro- Glycol Glycero panol ~
___
~
CHANQES N CONTROL OEL
~
'C.
(a) 15 cc. of standard acid water addition agent (= 25 cc.)
+
0 20 30 50
852. 1955. 2307. 1958. 1910. 276. 298. 320. 296. 292. 111. 105. 117. 124, 113. 16.6 18.0 20.4 18.0 17.5
910. 289. 108. 17.1
(h) 14 cc. of standard acid water addition agent (= 25 cc.)
0 20 30
532. 559. 581. 78. 85.5 94.0 31.1 32.8 36.0 5.25 5.5( 6.1:
550.
(c) 13 cc. of standard acid water addition agent ( = 25 cc.)
0 20 30 50
21.0 7.50 3.75 1.33
22.3 7.2! 3.5( 1.2:
0
26.0 19.3 13.4 6.66
19.0 12.8 9.5( 4.6:
+
+
+
+
+
(d) 12 cc. of standard acid water addition agent ( = 25 cc.)
+
+
50
20 30 50
552. 83.0 33.0 5.6:
553. 82.0 32.5 5.5(
21.0 6.3: 3.1: 0.9:
22.6
6.N 5.N 3.6: 1.9i
5.36 5.49 5.41 5.60
31.8 5.x
.70-5.81 .02-6.35 .02-5.98 .24-6.29
4.a 1.4:
24.8 8.2) 4.2( 1.4(
31 .O 9.8; 4.8: 1.M
.70-7.70 .92-8.12 .99-8.11 .20-8.38
26.5 19.0 11.5 5.5;
32.8 23.0 15.0 7.M
69.0 37.8 24.3 10.9
.86-9.08 .20-9.68 .18-9.55 .57-9.90
8.N
80.0
t Mannitol values a t 0.5 mole per liter of addition agent are obtained a t the lower temperatures by extrapolation from smaller additions. 1026
TIME O F SET O F SILICA GELS
1027
to be due to the change in pH with temperature. Such a shift in pH has been noted by Batchelor (1). The “heat of activation” calculated from the data for the control gel is 9820 cal. This difiers greatly from Hurd’s values.
I To A
FIG. 1. Relation between time of set and absolute temperature for acid gels of different silica concentrations a t different pH with or without addition agents. 0 , control gel; 0 , I-propanol.
Figure 3, from table 3(d), illustrates the same kind of a change in the case of glycol. It is no longer possible to calculate a “heat of activation.” The values for gels containing glycerol approach the values for the control gel very closely a t the higher temperatures.
i
?x
-
FIQ. 2. Change in temperature-time of set relation at pH 7.9 (Si% 1.3 per glycerol; R cent). 0,control gel; A, methanol; 0,I-propanol; 6,glYC01;
FIO. 3. Change in temperature-time of set relation at pH *' 9.2 (Si% 1.3 per cent). 0,control gel; A, methanol; 0,1-propanol; 0 , glYcol; A, glY$erol; a ma"";+nl
1029
TIME OF SET OF SILICA GELS
I
TOA FIB. 4. Change in temperature-time of set relation a t pH 0, control gel; A, methanol; 0 , glycol; A, glycerol;
cent).
FIG. 5. Change in temperature-time of set relation a t pH cent). 0, control gel; 0 , glycol; A,glycerol; . , mannitol.
.,-
-
10.2 (SiO,, 1.8 per mannitol.
10.6 (Sios, 1.7 per
1030
L. A. MUNRO AND J. A. PEARCE
Table 2(b) and figure 4 show the effect at increased silica content and increased pH. It will be seen that the curving is more pronounced. At 50°C. glycerol has actually become an accelerator of gelation. Table l(b) gives the data for figure 5 , a t pH 10.59. As this is just inside the limit of alkalinity in which gels are formed (12, 15)’ these gels are very fragile. The readings, therefore, can only he approximations. The
FIG. 6. Variation in specific effect of the alcohols (100t./t,) with pH. nol; 0 , 1-propanol; 0 , glycol; A, glycerol; ,. mannitol.
A, meths-
important points are that the curving of the lines is still more pronounced and that glycerol joins the accelerators between 30”and 4OOC. Calculations on Batchelor’s (1) data* gave a few straight lines for the “heat of activat,ion” curves, but the slopes obtained were not the same as Hurd’s. This might be explained by Batchelor’s use of a mixture of strong acids and by his method of determining the time of set, which does not give
* Kindly supplied by the author.
TIME OF SET O F SILICA GELS
1031
exact readings in those gels which are definitely thixotropic. His data, in spite of these limitations, support our findings that the “heat of activation” is not constant over a pH range. THE GENERSLIZATION FOR SILICA GELS WITH BLCOHOLS AS ADDITION AGENTS
When the time of set of a gel containing addition agent (ta) expressed as a percentage of the time of set of the original or control gel ( t c ) is plotted against the p H of the control gel, a system of smooth curves is obtained (12). In the present work this generalization is extended t o other temperatures. A series of similar curves is obtained (figure 6). T o avoid congestion experimental points have been omitted on the acid side. The deviations of the present 20°C. curves from those reported elsewhere are doubtless due to the p H discrepancy mentioned above and the difficulty of preparing silicate solutions of exactly the same colloidal properties. Those addition agents that have the greatest specific action on the time of set are affected most by change in temperature. On the alkaline side, as the temperature is increased all reagents become less specific in their effect. For those alcohols which change from accelerators t o retarders the pH of reversal is practically constant and is the same as previously noted (12). The effect of temperature on the glycol curve is astonishingly small. The lack of specificity a t p H 7.0 seems to indicate that the dielectric constant of the addition agent plays a minor rBle in the setting phenomenon. This is in opposition t o the suggestion of Davis and Hay (2) that the most important variable of colloidal silica is the sign and magnitude of the electrokinetic potential. DISCUSSION
Hurd, Raymond, and Miller (9) report that there is a straight-line relationship between the log time of set and the pH of acid gels. The former quantity plotted against the reciprocal of the absolute temperature also gives a straight-line relationship. .4ny changes in pH therefore that occur with temperature would not cause any curvature of the activation line for acid gels. Hurd (4) considers that there is a similar relationship for alkaline gels. It has been shown (12) that this is only true for a limited range, and that the curve goes through a pronounced minimum. Hence the change in pH with temperature may here be an important factor in causing 3, change in slope for alkaline gels. Davis and Hap (2) appear to have confused “the heat evolved during the formation of the gel,” which they refer t o as the “heat of formation,” with the “heat of activation.” They suggest six possible sources of the heat evolved. Two of these, e.g., heat of neutralization and heat of dilution, are not determinative of the relatively slow gelation process. The term “heat of activation” is descriptive of the rate of the determining
1032
L. A. MUNRO AND J. A. PEARCE
reactions which immobilize the sol and produce a standard rigidity, defining the time of set. Sorption of the dispersion medium, displacement of water from the silicic acid complex, surface energies, and loss of kinetic energy of the molecules are all factors that determine the so-called “heat of activation.” If the importance of one or the other of these changes with temperature, a straight line will not be obtained. In the presence of addition agents the importance of these factors might be expected to be altered and other factors introduced. The specific nature of the effect of alcohols on the alkaline side would point to the increasing importance of adsorption. This factor alone would tend to cause greater deviation from ‘a theoretical “heat of activation.” On the acid side the addition agents apparently affect but little the mechanism of gelation, although the time of set is altered. Differences between acid and alkaline silica gels are shown in other respects than in their behavior towards addition agents. For acid silica gels syneresis is slow; for alkaline silica gels of similar concentration syneresis is rapid. Acid silica gels are more transparent, more rigid, and more elastic. Alkaline gels appear to be less stable and tend to be granular. It would seem that the setting of acid and of alkaline gels is governed by different factors. It is even doubtful if a theory of gelation for acid gels can be applied to alkaline systems. The difference in the curves for the Arrhenius equation, described above, supports this view. SUMMARY
.
1. Calculations of the so-called “heat of activation” have been made for silica gels over a pH range. 2. The presence of addition agents does not essentially change the slope of the curve for acid gels. 3. For acid gels the values for the “heat of activation” were the same as those obtained by Hurd. 4. For alkaline gels the slope changes and finally a straight line is no longer obtained. 5. The generalized curve relating pH and per cent effect on the .set for a series of alcohols is given for the temperatures studied. 6. It is concluded that tbe gelations of acid and alksline gels are fundamentally different. REFEREKCES (1) BATCHELOR, H. W . : J. Phys. Chem. 12, 575 (1938).
(2) DAVIS,H.L., AND HAY,K . D.: J. Am. Chem. Soc. 61,1020 (1939). (3) DOLE,M.:Measuring p H with the Gloss Electrode, p. 12. Coleman Electric Maywood, Illinois (1937). (4) HURD,C. B.: Chem. Rev. 31,403 (1938). (5) HURD,C. €3.: J. Phys. Chem. 40,21 (1936).
co.,
AN .4NOEMLOU6 CONCENTRATION CELL
1033
(6) HURD,C. B., FREDERICK, K. J.,
A N D HAYNES, C. R.: J. Phys. Chem. 41, &5 (1938). (7) HURD,C.B.,AND LETTERON, H. A . : J. Phys. Chem. 36,604 (1932). '(8) HURD,C. B., AND MILLER,P. S.: J. Phys. Chem. 36,2194 (1932). (9) HURD,C. B.,RAYMOND, C:. L., ANDMILLER, P. 5.: J. Phys. Chem. 88,663 (1934). (10) MUNRO,L..4., AND ALVES,C. A. : Can. J. Research B16, 353 (1937). (11) MUNRO,L.A,, AND PEARCE, J. A . : Can. J . Research B16,390 (1938). (12) MUNRO,L.A., AND PEARCE, J. 8.:Can. J. Research B17,266 (1939). (13) PRABAD, M.,AND DESAI,D. N.: J . Indian Chem. SOC. 16, 117 (1939). M.,AND HATTIANGADI, R. R . : J. Indian Chem. SOC.6, 991 (1929). (14). PRASAD, (15) TOURKY, A. R.: Z. anorg. allgem. Chem. 240, 198 (1939).
AN ANOMALOUS CONCENTRATION CELL. THE CONSTITUT I O S OF SOLUTIONS OF STANXOUS CHLORIDE I N WATER AND I N HYDROCHLORIC ACID L. It. ALLISON, E. J . HARTUNG,
. ~ N DE.
HEYMANS
Department of Chemzstry, Universzty of Melbournr, Melbourne, Australia Received June $0, 1939
Some time ago, a concentration cell of stannous chloride with the more dilute solution superimposed on the more concentrated was prepared for a lecture demonstration. The more concentrated solution (25 per cent) was prepared in 2.33 N hydrochloric acid, and the other by dilution in the ratio 1: 10 with water. On superimposing one solution on the other, and introduoing a small tin rod, it was found that the tin crystals, instead of being deposited in the more concentrated region, were formed in the dilute solution, contrary to what would be expected. It was later found that, when the two solutions were prepared in acid of the same normality, the cell behaved in a normal manner and the tin crystals were deposited in the concentrated region. Subsequent investigation has shown that the behavior of the anomalous cell may be readily explained on the assumption that the activity of the stannous ions is smaller in the strongly acid solution of stannous chloride in high concentration than in the weakly acid solution of stannous chloride in low concentration. This is most probably due to the complexity of these solutions. I. THE IAIQUID-JTJNCITONPOTENTIAL
The anomalous cell
1
25% SnClt ! 2.5% SnCla Sn 2.33N HCl 0.233 N HCl Sn+
-
