STUDIES ON SILICIC ACID GELS. VI1
THEEFFECT OF
THE
SODASILICA RATIO UPON
CHARLES B. HURD, WILLIAM FIEDLER, JR.,
AND
THE
TIMEOF SET
C. LAWRENCE RAYMOND
Department of C h e m i s t r y , U n i o n College, Schenectady,
New York
Received October 6 , 1936 INTRODUCTION
One of several theories, proposed to explain the mechanism involved in the setting of gels of hydrated silica, or silicic acid gels as they are often called, assumes that the molecular silicic acid reacts to form long chains by a process of condensation. This branching or fibrillar structure is believed to provide sufficient rigidity, so that the gel may set (8). With this theory in view it might be assumed that a gel would be produced more rapidly if one started with di- or tri-silicic acid molecules than if the starting point were a monosilicic acid such as ortho- or meta-silicic acid. I n order to check this theory, two investigations have been completed in this laboratory. The result of the first investigation, as reported by Hurd and Miller (6), has shown that the effect of temperature upon the time of set of gel mixtures produced by mixing solutions of acetic acid and solutions of sodium silicate is the same, over a variation of the soda-silica ratio from 1:1,58 up to 1:3.86. In the present investigation we have been able to show that the original soda-silica ratio has no effect upon the time of set where the compositions of the final mixtures are identical in every particular. It can thus be seen that the original soda-silica ratio does not have the result which was logically expected. GENERAL CONSIDERATIONS
In general, it is well known that the time of set of gels of hydrated silica is affected by a number of factors. The influence of the temperature has been reported (6). The concentrations of silica, of excess acid, of the hydrogen ion, and of certain sodium salts hare been shown to exercise a powerful influence upon the time of set. It was not known whether the soda-silica ratio of the sodium silicate possessed any influence, although Bogue (1) has voiced the opinion that it should be studied. In the investigation which we report here concerning the influence of the soda-silica ratio of the original silicate upon the time of set, it was 553
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C. B. HURD, W. FIEDLER, JR., AND C. L. RAYMOND
clear, of course, that no factor should be allowed to vary except the sodasilica ratio. This meant that the composition of two mixtures which were to be compared for time of set should be identical in every respect, except that the sodium silicates used possessed different soda-silica ratios. This involved building up the soda deficiency in a silicate with a higher ratio such as 1:3.25 to compare, for example, with one whose ratio would be 1:2.00. This might be done in any one of four ways. First, the extra sodium hydroxide could be added to the sodium silicate which was lower in soda, using the same amount of acetic acid for both mixtures. Secondly, the extra sodium hydroxide could be added to the acetic acid before pouring in the silicate. Third, less acetic acid could be used and the deficiency of both sodium and acetate in the one solution could be built up by adding the necessary amount of sodium acetate, placing it in the acetic acid before mixing. In a fourth procedure, the sodium acetate could be added to the one sodium silicate solution before mixing. Of these possible procedures, the first three have been tried in our investigations. We have not added the sodium acetate to the silicate, inasmuch as the addition of a strong salt solution causes the silicate to curdle. In all of the determinations, the temperature was kept constant by means of a good thermostat such as has been used here in previous investigations. The silicates used in this study have been described previously by Hurd, Raymond, and Miller ( 7 ) . The time of set was determined always in 80-cc. mixtures in 100-cc. Pyrex Griffin beakers, by the tilted-rod method (1). Thermostats employed gave a variation in temperature of less than 0.1OC. Hydrogen-ion concentration was determined by the quinhydrone method, which was found satisfactory by Hurd and Griffeth (4). T H E EFFECT OF T H E ADDITION O F SODIUM HYDROXIDE
The silicates used are classified by letters by their manufacturer.’ Their composition is shown in table 1. These were fresh silicates and were used fairly quickly. Reagent acetic acid and reagent sodium hydroxide were used. Standard quantitative methods were used for the analysis of solutions of the latter. Solutions of the sodium silicates were analyzed by determining their normality equivalent in sodium hydrdxide by titration with standard sulfuric acid solution, using methyl orange as the indicator. From this and the soda-silica ratio supplied by the manufacturer, the silica content of the solution was determined. The “natural” gel mixtures were produced by pouring a measured volume, 25 cc., of the solution of sodium silicate of the K, U, or C brands into 55 cc. of a mixture of standard acetic acid solution and water. The ’ T h e writers wish to express their appreciation to the Philadelphia Quartz Company, which has kindly supplied the silicates used in this and in other studies.
555
SILICIC ACID GELS
solutions before mixing had been kept in a thermostat at such a temperature that the heat of the immediate ionic reaction raised the temperature exactly to the temperature of the thermostat in which the time of set TABLE 1 C o m p o s i t i o n of silicates used B0DA:SILICA WEIQHT RATIO
BRAND
E. . . , . . . . . . . . . . . . . . . . . . . . . . . . . . K . . . . . . . . . . . . . . . . . .. . . . . . .. . .. . . .. . .
~
1:3.25 1:2.84 1:2.44 1:2.00
u . . . . .. . . . . . . . . .. . . .. . .. . . . . . . . . . . . . c. .. . . . . , . . .. . .. . . .. . .. .. . . . . . . . . . . .
0ODA:BILICA MOLE RATIO
1:3.36 1:2.93 1:2.52 1:2.06
TABLE 2 T i m e of set of synthetic m i x t u r e s produced b y a d d i n g s o d i u m hydroxide TIME OF S E T IN MINUTE0 TEMPERATURE
‘C.
,
i
27.4
(
i
25.9
I
25’9
1
MIXTURE
CONCENTRATION OF ACETIC ACID
K-1 K-2 K-3 K-4 K-5 K-6
0.680 0.805 0.930 1,055 1,180 1,300
75. 113, 146. 181. 207. 236.
76. 111, 146. 178. 206. 236.
U-1
0.748 0.873 0,998 1.123 1.248 1.373
82. 119. 155. 187. 218. 248,
82. 118. 155. 188. 217. 249.
0,865 0.990 1.115 1,240 1.365 1,490
70. 103. 132. 165. 193. 215.
70. 102. 131, 163. 194. 215.
u-2 u-3 u-4 u-5 U-6
c-1 c-2 c-3 c-4 c-5 C-6
Sodium hydroxide added to acid
Sodium hydroxide added t o silicate
Concentrations are all in gram moles per liter. Constant are SiOl = 0.645; NaOH from E brand = 0.385; NaOH total in K runs 0.440, in U runs 0.508, and in C runs 0.625.
was determined. In each case the silicate solution was of such a strength as to give exactly the same concentration of silica in the gel mixture. The “synthetic” gel mixtures were all made from the solution of E
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C. B. HURD, W. FIEDLER, JR., AND C. L. RAYMOND
brand silicate, which has the least soda per mole of silica of any considered in this work. As a result of the study of these synthetic mixtures, it was found that it made no difference whether the sodium hydroxide was added to the E brand silicate or to the acetic acid. This will be shown in table 2. It was also found, however, that while the curves for time of set against concentration of excess acid checked very well for natural and synthetic mixtures in the case of the K brand silicate, the curves were parallel but did not check in the case of the I:and C brands. We suspected this to be due to a small error in the analysis, which was later shown to be true. Accordingly, that part of the investigation was reserved until more satisfactory methods of analysis were worked out. The data of table 2 show that no significant difference could be found in the time of set of these mixtures whether the sodium hydroxide was added to the sodium silicate solution or to the acetic acid solution. In addition to these data shown, several series of determinations were made where the sodium hydroxide was added to the E brand silicate solution and the result allowed to stand or age for from a few seconds up to seventytwo hours. Then this mixture was poured into the acetic acid solution. No change in the time of set could be found. It is evident, therefore, either that the sodium hydroxide has no effect upon the structure of the E brand silicate in the three different concentrations needed for the synthetic K, U, or C brands, or that the changes in the structure resulting have no effect upon the time of set. DEVELOPMENT O F METHODS O F ANALYSIS
From the fact that the curves for the natural and synthetic mixtures from the first investigation either checked satisfactorily, as in the case of the E< mixtures, or were parallel but failed to check, as in the case of the U and C mixtures, there was doubt as to whether we were dealing with actual differences or with the results of inaccuracies in the analysis. Since the materials were supplied with a trade analysis and since, more particularly, a small difference in analysis and hence in the composition of the mixtures would produce a considerable difference in the time of set, it was decided that all mixtures must be checked by some satisfactory method of analysis. After receiving considerable help from the manufacturer and studying the various methods here in our laboratory, we have decided to adopt the following methods. It may be possible that they do not give results of absolute accuracy, satisfactory to all, but we have found them entirely reliable and always capable of giving results perfectly reproducible. -4fter all, we must make absolutely certain that the compositions of the natural and synthetic solutions are identical.
'
557
SILICIC ACID GELS
Sodium silicate About 2 g. of the sodium silicate was weighed accurately into a clean platinum crucible, placed in a 250-cc. casserole, and dissolved in about 100 cc. of boiling water. The sodium hydroxide equivalent was determined in the cooled solution by titration with standard hydrochloric acid, using one or two drops of methyl orange indicator. The hydrochloric acid solution was of such strength that about 40 cc. was required. The silica in the solution was dehydrated with hydrochloric acid instead of sulfuric acid, ignited and weighed in the regular manner. The silica was volatilized with hydrofluoric and sulfuric acids. We have had very much better success in dehydrating the silica with hydrochloric acid than with sulfuric acid. When a solution of sodium silicate was analyzed, it was measured by volume, otherwise the same procedure was used. The silicates used have given the results shown in table 3, three check determinations having been made in each case. TABLE 3 Composition of silicates from analysis A V E R A Q E DEVIATION
F R O M M E A N I N PARTS PER TBOUSAND
E . . . ....................
I
