STUDIES ON SILICIC ACID GELS. IV
THE EFFECTOF
THE
HYDROGEN-ION CONCENTRATION UPON TIMEOF SET'
THE
CHARLES B. HURD, C. LAWRENCE RAYMOND, AND P. SCHUYLER MILLER
Department of Chemistry, U n i o n College, Schenectady, N e w York Received A u g u s t 30, 1938 INTRODUCTION
In a recent study in this laboratory, Hurd and Miller ( 5 ) hsve obtained data upon the effect of temperature on the time of set of gels of hydrated silica or silicic acid gels, as they are commonly called. The data were later examined and were found to indicate the possibility of a relation between the hydrogen-ion concentration and time of set. A further study of this possible relation has been made. The results are incorporated in this article.2 HISTORICAL
Numerous investigators have studied the effects of various factors upon the time of set of silicic acid gels, among such factors being the concentration of silica, the concentration of acid, salts, or foreign materials, and the temperature. The effect of the hydrogen-ion concentration has not been thoroughly studied. The effect of the acid was noted by Holmes (2) and later by Hurd and Letteron (4). The hydrogen ion specifically was noted by Glixelli and Wiertelak (1) and by Prasad and Hattiangadi (8). Ray and Ganguli (10) determined the optimum limits of pH within which setting occurred in their experiments. To cite merely three opinions3 concerning the setting of these gels of silicic acid, Holmes has stated that the acid molecules have a dehydrating effect upon the silicic acid, while Laskin (7) believes that certain ions in solution possess a dehydrating effect. Kroger (6) believes that the sodium ions in solution have a peptizing effect upon the silicic acid. However, from the results to be cited in this article, it seems almost certain that the peptizing action in alkaline mixtures containing silicic acid is due to the hydroxyl ion. This research was supported by a Grant-in-Aidof the National Research Council. A report was made on the first half of these results before the Collojd Division of the American Chemical Society a t the meeting held in Washington, March, 1933. a Tn the interests of economy, the bibliography has been practically omitted from this article and the article itself greatly condensed. 663 2
664
HURD, RAYMOND AND MILLER EXPERIMENTAL
As previously stated, Hurd and Miller have made a study of the effect of temperature upon the time of set of various mixtures of solutions of sodium silicate and acetic acid. Five commercial brands of silicate were used, known by their maker, the Philadelphia Quartz Company, as brands “S,” ThBLE 1
Time of set o,f sodium silicate-acetic acid mixtures at 25°C. in minutes
11IXTURE
L3.99
1
1.3.33
i
Time of
114.0 158.5 219.0 266.0 298.6 342.8
100.0 153.2 206.6 267.0 298.6 355.0
I
k2.94
L2.52
1
1:2.06
set
81.3 120.2 164.0 209,O 245.4 282.0
42.2 79.4 117.5 149.7 182.0 221.4
117.5 158.5 205.6 247.2 283.8 320.0
TABLE 2
Excess acetic acid in the mixtures whose time of set i s tabulated in table 1
Soda-silica ratio
MIXTURE
1 3.98
1
1:3.33
1
1:2.94
I
L2.52
1
1:2.06
Excess acetic acid
0.177 0.302 0.427 0.522 0.677 0.802
0.237 0.362 0.487 0.612 0.737 0.862
0.185 0.310 0.435 0,560 0.685 0.810
0.242 0.367 0.492 0.617 0.742 0,869
0.127 0.252 0.377 0.502 0.627 0.752
“N,” ((I liu,,? and ((C.914 Six different mixtures of each brand with acetic acid were used. Every mixture used contained the same concentration of silica, namely,0.645 gram-mole silica per liter. From these data the time 4 The writers wish to thank the Philadelphia Quartz Company for their kindness in supplying the silicates used in this and other research. ‘
STUDIES ON SILICIC ACID GELS.
IV
665
of set of each mixture at 25°C. was determined, the figures being given below in table 1. Also from the concentrations may be obtained what we call “excess acetic acid,” namely, the difference in equivalents of the total acetic acid and the sodium of the sodium silicate, expressed as sodium hydroxide. It is thus the excess of acetic acid over that equivalent to the sodium in the sodium silicate. These data are given in table 2 . The time of set for each mixture of table 1 has been plotted as ordinate against the concentration of excess acetic acid of table 2 as abscissa. The
FIG. 1. TIMEOF SET IN RELATION TO CONCEXTRATION OF EXCESS ACETIC ACID
curve is shown in figure 1. These isothermal curves show, apparently, a linear relation between time of set and concentration of excess acid. Extrapolation to the left indicates a minimum time of set with slightly less than enough acetic acid to be equivalent to the sodium of the sodium silicate. It is well known that minimum time of set occurs in mixtures which are slightly alkaline. This linear relation between time of set and concentration of excess acetic acid might well be taken to point toward a linear relation between the time of set and the hydrogen-ion concentration since, in solutions containing an excess of weak acid, acetic acid, in the presence of a fixed concentration of
666
HURD, RAYMOND AND MILLER
sodium acetate, the hydrogen-ion concentration will be very nearly proportional to the acetic acid concentration. To test the hypothesis that a linear relation exists between the time of set and the hydrogen-ion concentration, the following series of experiments were made. A series of mixtures of solutions of "E" brand sodium silicate and acetic acid were made. This silicate is particularly clear and suitable. It was used in the form of a solution, 1.23 normal with respect to sodium hydroxide. All water used in any of these solutions was distilled water which had been freshly boiled. The mixtures were made by placing the solution of sodium silicate and the acetic acid solution in small beakers in a 23.5"C. thermostat. The amount of sodium silicate was fixed, 50 cc. of sodium silicate 1.23 normal with respect t o sodium hydroxide. The relative amounts of acetic acid and water were varied, although the total volume was fixed. The silicate was poured quickly into the acid, the result mixed thoroughly by pouring back and forth several times and then divided into two 80-cc. portions. One-portion was poured into a 100-cc. beaker in a 25°C. thermostat, where the time of set was determined by the tilted rod method. The other half was placed in an electrically insulated 25°C. thermostat where the pH was determined by the quinhydrone method, using a platinum wire electrode and solid quinhydrone. The whole technique for this method using quinhydrone had been carefully tested in this laboratory by an elaborate series of tests and found to be reliable. Particular attention should be called here to the care used to keep the mixture steadily a t 25°C. We have not found this mentioned elsewhere. In all cases, the rise in temperature upon mixing, due to the neutralization of sodium hydroxide by acetic acid, was measured previously. In the case just mentioned it was 1.5"C. The thermostat in which the two solutions were kept before mixing was then adjusted to that interval below 25"C., in this case, to 23.5%. The results of this series of experiments are given in table 3. All concentrations are in gram-moles per liter. The relation between time of set and concentration of hydrogen ions and the logarithmic relation, namely, between logarithm of time of set and pH are shown in figure 2. Between concentrations of hydrogen ion of 1.0 the relation between time of set and concentration and 7.24 X X of hydrogen ion is seen to be very nearly linear. As the concentration of hydrogen ions increases, however, the time of set is seen to be slightly less than would be indicated for a linear relation. Table 3 shows, however, a large excess of acetic acid required to produce increase in the hydrogenion concentration. In order to try a mixture where the hydrogen-ion concentration could be greatly increased without any appreciable increase in the amount of excess
S T U D I E S ON SILICIC ACID GELS.
667
IV
acid, the same type of experiment was carried out using hydrochloric instead of acetic acid. Owing to the rapid change in hydrogen-ion concentration with excess hydrochloric acid, mixtures were made containing TABLE 3 T i m e of set and p H data for mixtures of sodium silicate and acetic acid at R5"C. TOTAL
CHEOOH
EXCESS
CONCENTRATION
o a ~ +
CHGOOH
PH
0.975 0.862 0.665 0.554 0.467 0.356 0.294 0.232 0.195 0.170 0.108 0:071 0.046 0.034 0.021 0.009
4.14 4.21 4.32 4.42 4.48 4.61 4.69 4.78 4.87 4.92 5.09 5.29 5.46 5.60 5.75 6.00
1
TIME O F S E T
iOG TIME O F S E T
minutes
1,363 1.240 1,053 0.942 0.855 0.744 0.683 0.620 0.583 0.558 0.496 0.459 0.434 0.422 0.409 0.397
7.24 x 6.17 X 4.79 x 3.80 X 3.31 x 2.45 x 2.04 x 1.66 X 1.35 X 1.20 x 0.813 X 0.513 X 0.347 X 0.251 X 0.178 X 0.100 x
10-5 10-5 10-5 10-5 10-5 10-5
10-5
331.0 299.0 247.0 213.0 185.0 143.5 114.0 97.0 86.0 74.0 48.5 33.5 21.5 16.0 11.2 6.2
2.520 2.476 2.393 2.328 2.267 2.157 2.057 1,987 1.935 1.869 1.686 1.525 1.332 1.204 1.049 0.792
TABLE 4 Time of set and pH data for mixtures of sodium silicate and hydrochloric acid at 95°C. pH
1
COII'CENTRA'IION OF
H-
TIME O F S E T
LOO TIME O F SET
minutes
4.21 4.29 4.32 4.35 4.45 4.49 4.63 4.82 4.90 5.03 5.06 5.44
6.17 5.13 4.79 4.47 3.55 3.24 2.34 1.51 1.26 0.933 0.871 0.363
x
10-5
X
x x x x x x x X X X
10-5 10-5 10-5
10-5 10-5 10-5
75.0 55.0 51.0 49.0 40.0 34.5 29.5 21.0 16.5 14.0 10.9 5.5
1.875 1.740 1.708 1.690 1.602 1.538 1.470 1.322 1.217 1.146 1.037 0.740
as nearly as possible the same amount of acid with a fixed amount of silicate solutions. Checks, of course, could not be made. The excess of hydrochloric acid could not be measured, being less than 0,001. The
668
HURD, RAYMOND AND MILLER
data are given in table 4 and plotted on figure 2. The linear relation between the time of set and the hydrogen-ion concentration is evident. In fairness, it should be pointed out here that the variation in hydrogen-ion concentration is only about twentyfold, from pH = 4.21 t o 5.44. A study over a wider range is contemplated in the future. The range with acetic acid mixtures from p H = 4.14 to p H = 6.00 represented the limits for the experiment. Higher hydrogen-ion concentrations could not be reached because of curdling of the mixture. Lower hydrogenion concentrations could not be reached because the mixture set too rapidly to allow accurate measurements to be made. To investigate further the effect of the hydrogen-ion concentration upon time of set, the following series of experiments was devised. A series of
Pti
FIG.2. RELATION BETWEENTIME OF SET AND HYDROGEN-ION CONCENTRATION mixtures was made in which the amount of sodium silicate (“E” brand) was fixed. The amount of acetic acid was varied, but with it was added sufficient sodium acetate so that the pH of the final mixture remained fixed. The proper quantities were determined by preliminary experiments. The proper quantities of solutions were thermostated at 23.5”C. and mixed as before. The time of set was determined with half of the mixture and the pH with the other half, each at 25°C. The solutions used were sodium silicate 1.23 normal, acetic acid 2.007 normal, and sodium acetate 2.225 molar. In all, six different series of runs were made, each a t a fixed pH. The data are tabulated in table 5 and in figure 3 the time of set for each sample is plotted against the concentration of excess acetic acid. The curves of figure 3 show that while in Series I with pH = 4.22, the time of set remains practically constant as the excess acetic acid and sodium acetate are increased, the effect of these substances is greater and greater
TABLE 5 Time of set o j six series of mixtures each with a constant p H CONCENTRATION O F EXCESS
CHaCOONa
TOTAL
Na
CONCENTRATION OF TOTAL
CONCENTRATION OF EXCESS
CHaCOOH
CHjCOOH
TIME O F SET
Series I. pH = 4.22; H+ = 6.03 X 10-6 minutes
0.000 0.047 0.074 0,100 0.122
0.385 0.432 0.459 0.485 0.507
1 253 1.319 1.380 1.442 1.506
0.868 0.934 0.995 1 057 1,121
296. 295. 296. 296.5 297.
0.000 0.061 0.097 0.136 0.175
0.385 0.446 0.482 0.521 0.560
0.880 0.943 1.007 1.068 1.131
0.495 0.558 0.@2 0.683 0,746
189. 190. 190. 193. 199.5
0.000 0.101 0.213 0.309 0.394
0.385 0.486 0.598 0.694 0.779
0.628 0.691 0.754 0.816 0,879
0.243 0.306 0.369 0.431 0.494
101. 103. 106. 109.5 116.
Series IV. pH = 4.92; H+ = 1.20 X 0.000 0.153 0.288 0.404 0.515
0.385 0.538 0.673 0.789 0,900
0,565 0.628 0.691 0.754 0.816
Series V. pH = 5.09;
H+ = 8.13
0.180 0.243 0.306 0.369 0.431
78. 82. 87. 91. 97.
X 10-6
0.000 0.188 0.369 0.543 0.713
0.385 0.573 0.754 0.928 1,098
0.502 0.565 0.628 0.691 0.754
0.117 0.180 0.243 0.306 0.369
53. 58. 64. 71. 77.5
0.000 0.390 0.700 1,030
0.385 0.775 1.085 1.415
0.439 0.502 0.565 0.628
0.054 0.117 0.180 0.243
24.5 31.2 39.2 45.0
All concentrations are in gram-moles per liter. Constant are NaOH, 0.385, and SiO,, 0.645. 669
670
HURD, RAYMOND AND MILLER
the higher the pH. It should also be noticed that in series I the first sample, which contains no excess sodium acetate, already contains excess acetic acid, 0.868 gram-mole per liter. In series VI, on the other hand, the first sample, which contains no excess sodium acetate, contaiFs only 0.054 gram-mole per liter excess acetic acid. The shape of the family of curves is suggestive. The curves and data show clearly, however, that the time of set is increased by the presence of excess acetic acid and excess sodium acetate,
I
...
*..a
L
LO %R L l T L R
FIQ.3. TIMEOF SET OF MIXTURESWITH CONSTANT pH ACETICACID
AS A
FUNCTION OF EXCESS
and is not alone a function of the hydrogen-ion concentration. We have not been able t o distinguish between the effect of the sodium acetate and that of the acetic acid. To test further the effects of sodium salts in considerable amounts upon the time of set, three series of determinations of the time of set and pH of the mixture were made, using increasing amounts of three,different salts, sodium chloride, sulfate, and nitrate. The concentrations of sodium silicate and acetic acid were kept constant. The effects of electrolytes have
STUDIES ON SILICIC ACID GELS.
67 1
IV
been studied by other investigators, especially Prasad and Hattiangadi (9). Too little attention has been paid to the simultaneous effect of variation of the pH, however, as was pointed out by Hurd and Carver (3). The results of these three runs are given in table 6 and are plotted in figure 4. It is apparent that increasing amounts of sodium chloride show no effect upon the pH. The effect upon the time of set is marked, namely, TABLE 6 T h e affect of certain sodium salts u p o n the time of set of silicic acid gels and u p o n the pH of the mixtures CONCENTRATION
PH
0.000 0.063 0.188 0.312 0.468 0.625 0.845 0.938
5.09 5.09 5.09 5.09 5.09 5.09 5.09 5.09
NalSOl . . . . . . . . . . . . . . . . .
0.000 0.062 0.124 0.188 0.250 0.376 0.500 0.625
5.09 5.09 5.09 5.08 5.08 5.04 5.03 5.03
53.0 51.0 48.5 48.0 47.5 45.3 44.5 43.7
NaNOs. . . . . . . . . . . . . . . .
0.000 0.062 0.124 0.188 0.250 0.376 0.500 0.625
5.09 5.09 5.08 5.08 5.08 5.07 5.07 5.06
52.5 46.7 43.0 40.8 38.8 34.0 31.6 29.5
RALT
TIME OF S E T
minutes
NaCl . . . . . . . . . . . . . . . . .
I
52.5 48.0 40.0 36.5 31.3 28.0 25.6 23.9
a decrease of over 50 per cent in time of set in a mixture containing approximately 1 gram-mole per liter. The sodium sulfate caused an appreciable decrease in pH, while the sodium nitrate caused a smaller decrease. Each of the pH changes should cause an increase in the time of set, estimated a t about nine minutes in the case of the highest concentration of sodium sulfate. The effect of each of these three salts is to cause a marked decrease in the time of set. The sodium ion, therefore, in company with its anion,
t
672
HURD, RAYMOND AND MILLER
shows no net peptizing effect, such as might be expected from the interpretations of Kroger. The assumption may be made, therefore, in the interpretation of the data of table 5 that. the sodium acetate, in addition to its function of keeping the pH constant in the presence of increasing amounts of acetic acid, would contribute materially toward decreasing the time of set. The net increase might, therefore, be due to the increasing amounts of acetic acid.
FIG. 4. EFFECTOF CERTAINSODIUMSALTSON TIME OF SET DISCCSSTON OF RESULTS
In this paper an attempt has been made to study so far as possible the effect, on the time of set, of the hydrogen-ion concentration, the concentration of excess acid, and of certain salts. The precautions taken to isolate these variables so far as possible have been explained. The description of details has been necessarily limited. The concentration of silica and the temperature have been kept constant throughout. In the case of gels made from sodium silicate and acetic acid, the time of set appears t o be a nearly linear function of the concentration of hydrogen ion from pH = 4.14 t o 6.00. The presence of considerable excess acetic acid confuses the interpretation, however. In the case of gels made with
STUDIES ON SILICIC ACID GELS.
IV
673
hydrochloric acid, the excess acid required to produce the same change in hydrogen-ion concentration is so small that it may be neglected. Here the time of set appears as a linear function of the hydrogen-ion concentration, as shown by figure 2 . R7e may admit the assumptions first proposed by Hurd and Letteron, namely, (1) that in dealing with the mechanism involved in the setting of a silicic acid gel we are dealing with a process which follows the laws of an ordinary chemical reaction, so far as its velocity is concerned, and ( 2 ) that for a given silica content, the time of set measures the time when a certain fixed proportion of the silica, in whatever form, in solution has rea ct ed . Then it may be shown easily that the specific reaction rate, k , is inversely proportional to the time of set, t'. It is not necessary that we know the order of the reaction, n. From the curve of figure 2 for the hydrochloric acid gel, the time of set appears to be not only a linear function of the hydrogen-ion concentration, but to be actually proportional to the hydrogenion concentration. Since the specific reaction rate, lc, is inversely proportional to the time of set, t', and since the hydroxyl-ion concentration is inversely proportional to the hydrogen-ion concentration, it would appear that the specific reaction rate, k, is proportional to the hydroxyl-ion concentration. This may suggest the idea that in silicic acid gel mixtures in this pH range, the hydroxyl ion pIays the part of a catalyst. From other work it appears, however, that in mixtures containing silicic acid of pH greater than 8, the hydroxyl ion peptizes the silica. SUMMARY
Several series of experiments are described in which an attempt has been made to determine the effect on the time of set of silicic acid gels of the hydrogen-ion concentration and the concentrations of excess acid and of certain salts. With gels from sodium silicate and hydrochloric acid from pH = 4.2 to 5.5, the time of set appears to be a linear function of the hydrogen-ion concentration. \\'ith gels from sodium silicate and acetic acid from pH = 4.14 to 6.0, a nearly linear relation is evident between time of set and concentration of hydrogen ions. With gel mixtures of constant pH, the time of set is increased by increasing amounts of acetic acid and sodium acetate. Three salts, sodium chloride, sodium sulfate, and sodium nitrate were found to decrease the time of set considerably without causing any considerable change in the pH.
674
HURD, RAYMOND AND MILLER
REFERENCES (1) GLIXELLIAND WIERTELAK: Kolloid-Z. 46,197 (1928). (2) HOLMES:J. Phys. Chem. 22, 510 (1918). (3) HURDAND CARVER:J. Phys. Chem. 37, 321 (1933). (4) HURDAND LETTERON: J. Phye. Chem. 36, 604 (1932). (5) HURDAND MILLER:J. Phys. Chem. 36, 2194 (1932). (6) K R ~ G E RKolloid-2. : 30, 16 (1922). (7) LASKIN:Kolloid-Z. 46, 129 (1928). (8) PRASAD AND HATTIANGADI: J. Indian Chem. SOC.6,893 (1929). (9) P R A s A D AND HATTIANGADI: J. Indian Chem. Soc. 7,341 (1930). (10) RAYA N D GANGULI:J. Phys. Chem. 34, 352 (1930).
