STUDIES ON SILICIC ACID GELS. VI
INFLUENCE OF TEMPERATURE AND ACID UPON
THE
TIMEOF SEITI
CHARLES B. HURD Department of Chemistry, Union College, Xcheneetady, New York Received June $0, 1096 INTRODUCTION
I n a study of the setting of silicic acid gels in this laboratory, Hurd and Letteron (1) have noted an interesting relation between the “time of set” of the gel mixture and the temperature. They found that the logarithm of the time of set could be represented as a linear function of the reciprocal of the absolute temperature. By making certain rather simple assumptions and treating the process as a chemical reaction, they were able to calculate the value of an energy term analogous to the “heat of activation.” The values for several series of different mixtures of solutions of sodium silicate and acetic acid gave the average value 16,940 calories. This was, of course, within the range for chemical reactions. This result was so interesting that it was decided to run a careful check upon the constancy of this value. In the study reported by Hurd and Miller (2) using several different brands of sodium silicate, with sodasilica ratios ranging from 1:3.86 to 1:2.00, a close agreement in the values for this heat of activation was discovered. With six different series, each of five different silicates, variations of not over 2 per cent in this heat of activation were found. The average value was found to be 16,640 calories. We considered at that time that a sufficiently exhaustive study had been made of the effect of temperature upon time of set of these mixtures involving acetic acid. The constancy of this quantity for these gels of sodium silicate-acetic acid mixtures has suggested an investigation involving other acids. It would be very interesting to check the constancy of the same quantity for other acids and to compare the values for the different acids. I n this way an idea could be obtained as to whether this quantity is the same for any acid-sodium silicate mixture, or varies according to the acid employed. In this paper will be found the results of a study involving comparisons of four possible acids. 1
Presented a t the Twelfth Colloid Symposium, held a t Ithaca, New York, June
20-22, 1935. 21
22
CHARLES B. HURD EXPERIMENTAL
I n any study of the setting of silicic acid gels, the factors of temperature and hydrogen-ion concentration play a very important part. The latter has been discussed, among others, by Prasad and Hattiangadi (4) and Hurd, Raymond, and Miller (3). As the latter have pointed out, the time of set is proportional to the concentration of hydrogen ions for concentraTABLE 1 Time o j set o j gel mixtures using different acids Concentration of NaOH = 0.386 gram-mole per liter. Concentration of Si02 = 0.636 gram-mole per liter NO.
CONCENTRATION OF ACID I N
TIYB1 OF SET I N MINUTE0 AT
25.1"C.
1
38.4'C.
I
53.0'C.
Series I. Acetic acid
1 2 3 4 5
1 2 3 4
1 2 3 4
1
3 4 2
0.496 0.621
54.5 109.5 194.5 266.5 322.0
15.0 31.0 57.0 84.0 97.5
4.4 9.0 15.8 23.3 28.8
0.466 0.518
4.62 4.45 4.25 4.01
75.5 146.0 228.0 415.0
19.0 42.5 70.5 118.0
5.5 10.8 19.5 34.5
0.500 0.625 0.750 0.875
5.16 4.57 4.21 3.92
27.0 90.5 195.5 341.0
7.4 25.5 56.0 96.5
2.2 8.0 17.3 31.0
5.31 5.04 4.87 4.74
25.0 43.0 65.5 87.5
7.5 13.0 18.0 24.4
2.1 3.5 5.6 7.5
0.844
1.041 1.240
0.414 0.440
I i 0.502 0.564 0.627 0.690
I t is possible to duplicate the hydrotions of the latter from lo-' to gen-ion concentration in an acid mixture of solutions of sodium silicate and acetic acid, owing to buffering effect of the sodium acetate formed, and with sufficiently close agreement merely by mixing measured volumes of the solutions of sodium silicate and acetic acid. Such a thing is, of course, completely impossible with any strong acid. I n that case a more complicated technique must be employed.
23
STUDIES ON BILICIC ACID GELS. V I
The work to be described includes a study of all the weak acids found suitable. The acid must be s@ciently weak so that the hydrogen-ion control is satisfactory. It must be sufficientlysoluble at 25'C. The acids found suitable were acetic, tartaric, citric, and succinic. The ionization constants are 1.8 X l0-q 1.1 X and 6.6 X 10-5 for the 8 X first hydrogen at 25°C. Monobasic and polybasic acids are represented.
E.%$
.o
3.10
3.20
fx
103
3.30
FIQ.1 . Effect of temperature on time of set of silicic acid gels with different acids
It is to be regretted that boric acid was not available because of its low solubility. Mixtures were made as previously described 'in this series of papers. The silicate used was E brand2 with a soda-silica weight ratio of 1:3.19. a The authors wish to thank the Philadelphia Quartz Company for its kindness in supplying all the silicate used in this and other research.
24
CHARLES B. IlURD
All distilled water was freshly boiled to eliminate carbon dioxide. Standard chemically pure reagents were used. Mixtures were run at three different temperatures in carefully controlled water thermostats. The mixtures of 80 cc. were thermostated in covered 100-cc. Griffin form Pyrex beakers. The time of set was determined by the “tilted rod” method. Check determinations were always run, agreement in time to 2 per cent or less being obtained. It should be noted that in these containers a t the highest temperature, the temperature of the gel mixtures remained a fraction of a degree below the temperature of the TABLE 2 Values for the slope of the curves f o r log time temperature
set against reciprocal absolute
OJ”
ACID hllXTURE
1 2 3 4 5
1 2 3
4 5
~
1
1
I I
Average ... . . . , /
Acetic
Tartaric
3820 3780 3790 3680 3650
3920 3890 3750 3760
1
Acetic
Tartaric
17500 17300 17350 16850 16720
17950 17800 17170 17220
17140
I
Citric
~
Citric ~
17020 16950 16850 16950
1 17630
I
Succinic
3720 3700 3680 3700
~
1
3700 3780 3780 3740
1
1
-__ Succinic
16950 17300 17300 17230
I
16940
I
17190
bath. The gel mixture temperature was recorded. The data are given in table 1,the times of set being the average of several determinations. These data were plotted, using the logarithm of the time of set as ordinate with the reciprocal of the absolute temperature as abscissa. The curves obtained are apparently linear. Figure 1 shows the type of graph obtained. It should be noted that the ordinates have been adjusted to permit the four curves to be plotted on the same drawing. In obtaining the slopes, of course, each set of curves was plotted separately, obtaining steeper lines and better accuracy. The slope was determined from the curves obtained. The values of the
STUDIES ON SILICIC ACID GELS. VI
25
slopes are given in table 2. By multiplying the slope of the curves by 2.303R1 the heat of activation should be obtained. Those values are included in table 3. DISCUSSION
The data presented here and in the previous work of Hurd and Miller show certain characteristics which are worthy of discussion. The calculated heats of activation are essentially constant for a given acid. It must be remembered that the best results which we have been able to secure for the time of set, after six years of work, show differences up to 2 per cent. At higher temperatures evaporation causes an error which we are unable to avoid with our method of determining the time of set. The heat of activation is determined from the slope of a curve. To secure better results would require a very long series of determinations, such as were reported by Hurd and Miller for acetic acid mixtures. A complete series of runs should be performed a t OaC. We have carried out enough of these for the acids discussed in this paper to convince us that our reported results are correct. A comparison of the values for a given acid will show an appreciable decrease in the heat of activation as one goes down the series for each acid. This was apparent in the more accurate data on acetic acid reported by Hurd and Miller, and is apparent in the data reported here for acetic and tartaric acids. A comparison of the average results shows an appreciable difference in the value for the different acids. In the study of the reaction of iodine and acetone, catalyzed by various acids, Rice and his coworkers (5, 6) and Rice and Urey (7) have reported that the heat of activation is decreased somewhat by the addition of a salt of the acid catalyst, when a weak acid is used. Our results do not show this effect, but do show a somewhat lower heat of activation in the case of acetic and tartaric acids as more acid is used. They have also reported that their reaction showed a lower temperature coefficient for weak acids than for strong acids. The same general effect is shown here, tartaric acid having the highest heat of activation and also the highest dissociation constant, although the differences are not particularly marked. A striking difference between the effect of the acid should be noted between the reaction studied by Rice and his coworkers and the setting of the silicic acid gels. In the iodine-acetone reaction, the acid serves as a catalyst. In the setting of gels of silicic acid in the acid range, the presence of more acid retards the setting of the gel. We can give no explanation a t present for the similarity and differences of these results.
26
CHARLES B. HURD SUMMARY
The effect of temperature upon the time of set of gels of silicic acid, made by mixing solutions of sodium silicate with solutions of acetic, tartaric, citric, or succinic acids, has been studied. The heats of activation are essentially constant. The heat of activation is slightly higher for the strongest acid, tartaric acid. (1) (2) (3) (4) (5) (6) (7)
REFERENCES HURDAND LETTEROS:J. Phys. Chem. 36, 604 (1932). HURDAND MILLER:J. Phys. Chem. 36,2194 (1932). KURD,RAYUOND, A N D MILLER: J. Phys. Chem. 38, 663 (1934). PRASAD AND HATTIANQADI: J. Indian Chem. SOC. 6, 893 (1929). RICEAND KILPATRICK: J. Am. Chem. Soo. 46, 1401 (1923). RICEAND LEMKIK:J. Am. Chem. SOC.46, 1896 (1923). RICEAND UREY:J. Am. Chem. SOC.62, 95 (1930).
