STUDIES ON SILICIC ACID GELS. 111
THEEFFECT OF
THE
ADDITION OF SOMESOLUTES ON OF SET
CHARLES B. HURD
AND
THE
TIME
DANIEL H. CARVER
Department of Chemistry, U n i o n College, Schenectady, N e w York Received J u l y 22, 1932 INTRODUCTION
The setting of a gel of hydrated silica, or a silicic acid gel as it is commonly called, is affected, among other things, by the concentrations of the solutions, the type of silicate, the temperature, the pH of the mixture, and the presence of additional soluble materials. Several investigations are being conducted in this laboratory upon the effects of some of these factors upon the time of set. In connection with a study of the effect of the pH of the mixture, the idea suggested itself that measurements be made upon the pH of mixtures containing certain additional solutes such as ammonia, the amines, pyridine, and certain others. In this paper are presented some of our typical results. The effect of the change in the pH of the mixture and the specific effects of certain substances are shown. An attempt has been made t o explain certain of these results. The gel mixtures were prepared by mixing solutions of sodium silicate with dilute acetic acid. Such a process produces in the mixture sodium acetate together with excess acetic acid. A possible, though not practicable, way of avoiding the presence of these soluble materials has already been pointed out by Hurd and Letteron (l),that is, to start with a pure suspension of colloidal silicic acid in water. The results of Werner (2) and Kroger (3) show that the removal of the last traces of electrolytes is impossible. It is also obviously impossible to determine the time of set correctly. The results of various studies reported in the literature on the effects of added materials on the time of set of the gel are difficult to interpret, because in general the specific effect of the solute has not been distinguished from the indirect effect due to a change in the pH of the mixture. We may mention here only a few of the results reported. The addition of strong bases has been found to prevent the formation of a gel, in fact, solutions of sodium hydroxide will peptize an already formed 321
322
CHARLEE B. HURD AND DANIEL H. CARVER
silicic acid gel. The effect of weak bases is less pronounced than that of strong bases. The effect of ammonia was studied by Bhatnagar and Mathur (4). Various compounds, including ammonia and pyridine, were studied by Prasad and Hattiangadi (5). The latter determined the pH of their mixtures. These compounds were found to accelerate setting. The effect of excess acid has been considered by many workers. We may mention Holmes (6), Prasad and Hattiangadi (7), Maschke (S), and Pappada (9). Excess acid has been found to increase the time of set, although Holmes, and later Hurd and Letteron, have shown that at high acid concentrations the time of set decreases rapidly, until at very high acid concentrations sodium silicate solutions cannot be mixed with the acid to give a clear solution. The effect of salts on the time of set has been considered by a number of workers, including Prasad and Hattiangadi, Pappada, and Werner. The effects of various unionized solutes have been studied, among them the alcohols, studied by Prasad and Hattiangadi, and certain gums and sugars, investigated by Wulf and Praetorius (10). The general impression received from a survey of the literature is that the addition of bases to an acid gel mixture hastens the setting and that excess acids retard the setting. The effects of other solutes have not in general been divided into their effect on the pH of the mixtures and their specific effects. EXPERIMENTAL
The determination of the pH in colloidal solutions has been the subject of much study and of voluminous discussion. The system under investigation here involves a colloidal material, namely colloidal silicic acid. In similar systems Prasad and Hattiangadi (7) have measured the pH by means of an indicator method, They note in their discussion, however, that the turbidity of the gel mixture caused difficulty in determining the color of the indicator, and also that the alcoholic solution affected the setting of the gel. A study of the various methods has led us to the belief that the quinhydrone method would prove the most applicable, especially since no alkaline gels were to be studied, A study of gels formed from alkaline mixtures has led us to believe that such gels differ considerably from the gels produced from acid mixtures, We have, accordingly, limited our study to acid gels. A series of determinations of the pH of gel-forming mixtures from the time of mixing until after the time of set, using the quinhydrone method,l has shown that only a negligible change, if any, in the pH of typical acid 1 These experiments were performed by Mr. D. P. Roehm a t Union College i n 1931-1932.
S T U D I E S ON S I L I C I C ACID G E L S
323
mixtures of various sodium silicate solutions with excess acetic acid occurs from the time of mixing until after the gel has set. As a further check on the quinhydrone method, comparisons were made of the pH values obtained on portions of the same mixture by the quinhydrone method and by the use of a special glass electrode-vacuum tube potentiometer circuit, using a McInnes and Dole electrode.2 The agreement was within the limits of experimental error. No doubt there is a possibility that the quinhydrone may have a specific effect on the time of set. The time of set and the pH were accordingly determined on portions of the same mixture, no quinhydrone having been added to the portion upon which the time of set was determined. The time of set was determined as follows. The correct amounts of solutions were drawn from bottles which had been standing for at least several hours in a water thermostat. They were mixed in a 100-cc. Pyrex Griffin beaker which was standing in the thermostat. The time of set was determined by the tilted rod method described by Hurd and Letteron (1). The method consists of inserting a small stirring rod into the gel at an angle of about 20” to the vertical until the gel is stiff enough to hold the rod in its tilted position. The glass rod is about 3 mm. in diameter and about 8 cm. long, drawn out to a stubby point and fire-polished. The effects of ammonium hydroxide, of methyl-, dimethyl-, and trimethylamines, of pyridine, and of aniline were studied. The sodium silicate was “E” brand, Philadelphia Quartz Company ~ilicate.~It was diluted with distilled water. The concentration was determined by titration with standard sulfuric acid using methyl orange, giving the sodium hydroxide equivalent. From this and the soda-silica ratio supplied by the maker, the concentration in equivalents of sodium hydroxide and gram-moles of silica per liter were calculated. The acetic acid was prepared from J. T. Baker C.P. acetic acid by dilution with distilled water. Its strength was determined by titration with standard sodium hydroxide, using phenolphthalein as an indicator. The ammonium hydroxide was prepared by diluting Grasselli C.P. ammonium hydroxide (sp. gr. 90). The methylamine was prepared and purified in our laboratorx: The other amines were purchased from the Eastman Kodak Company, the dimethylamine in the form of the hydrochloride. This amine was set free by distillation from a solution of the hydrochloride with excess sodium hydroxide. The strengths of the a By the courtesy of Mr. B. W. Nordlander of the Research Laboratory of the General Electric Company. The special apparatus employed in this determination is described in an internal report of the Research Laboratory. a The writers wish to thank the Philadelphia Quartz Company for their courtesy in supplying not only the silicate used in this research but also information concerning analysis, composition of the silicate, and other valuable data.
324
CHARLES B. HURD AND DANIEL H. CARVER
diluted ammonium hydroxide and the amine solutions were determined by titration with standard sulfuric acid, using methyl orange as an indicator. The aniline and pyridine were freshly redistilled and made up accurately by weight. TABLE 1 T h e e$ecl of weak bases on the time os set and the p H of mixtures of sodium silicate and acetic acid at 96°C. Concentrations constant throughout are as follows: NaOH, 0.395; SiOz, 0.664; CH,COOH, 0.454. CONCENTRATION OF BASE
TIME OF SET
I
pH
CONCENTRATION OF BASE
TIME OF SET
I
pH
Effect of ammonium hydroxide, NH40H Effect of methylamine, CH3NH2.HOH gram-moles p e r liter
minutes
0 0.00422 0.0105 0.0169 0.0211 0.0316 0.0422
22.60 20.50 18.00 15.75 14.00 10.25 7.25
5.52
5.63 5.97
gram-moles per liter
minutes
0 0.00422 0.0105 0.0169 0.0211 0.0316 0,0422
22.75 20.25 17.75 15.00 13.50 9.75 6.60
5.52
5.67 5.97
Effect of trimethylamine, (CHs)3N*HOH 0 0.00422 0,0105 0.0169 0.0211 0.0316 0.0422
22.50 19.80 17.00 14.50 12.50 9.00 5.60
0 0.00422 0.0105 0.0170 0.0211 0.0316 0.0422
22.75 20.75 18.75 16.75 15.75 12.50 10.50
5.52
5.67 6.00
5.52
5.63 5.87
0 0.00422 0.0105 0.0170 0.0211 0.0316 0.0422
22.15 19.30 16.00 12.75 11.oo 7.50 4.50
0 0.00422 0.0105 0,0170 0.0211 0.0316 0.0422
23.00 22.75 22.50 22.00 21.50 21 .oo 20.50
5.52
5.67 5.99
The results obtained on the time of set of the mixtures of sodium silicate and acetic acid with varying amounts of ammonium hydroxide or the various amines are given in table 1. The values for the pH of the mixtures as determined by the quinhydrone method are also tabulated.
325
S T U D I E S ON SILICIC ACID G E L S
The results shown in table 1 are plotted in figure 1. The effect of the six weak bases is shown graphically, the decrease in the time of set being greatest for trimethylamine and least for aniline. From the four curves for the trimethylamine, dimethylamine, methylamine, and ammonia, together with the pH data, there is a small, though easily evident, specific effect here. The effect of the trimethylamine is evidently the greatest. We have not found aqueous solutions of pyridine to show an effect as noticeable as that reported by Prasad and Hattiangadi. We were unable 24.
I
FIU. 1. ERFECT OF VARIOUS WEAK BASESUPON
THE
TIMEOF SET
to make pH determinations by means of the quinhydrone electrode in the case of the addition of aniline, because the aniline and the quinhydrone apparently react chemically. We have also attempted to analyze the effect of a typical ester, ethyl acetate, which was found to retard the setting of a silicic acid gel. Four series of determinations of the time of set were made, all at 22"C., and all using the same amounts of sodium silicate and acetic acid. The first series, table 2 (l),contained increasing amounts of ethyl acetate; the second contained extra amounts of acetic acid equivalent in concentration to the
326
CHARLES B. HURD AND DANIEL H. CARVER
ethyl acetate of table 2 (1);the third series contained amounts of alcohol equivalent to the ethyl acetate of table 2 (1); and the fourth contained amounts of alcohol and extra acetic acid each equivalent to the ethyl acetate of table 2 (l),in other words, the products of the complete hydrolsis of the ethyl acetate of table 2 (1). The results in table 2 show the effect of the addition of ethyl acetate and the products of its hydrolysis separately and together. The effect of the ethyl alcohol is obviously practically negligible at these concentrations. TABLE 2 The eflect of ethyl acetate and the products of the hydrolysis of ethyl acetate at 8b''C. Concentrations constant throughout are as follows: NaOH, 0.395; SiOz, 0.664; CH,COOH, 0.454. CONCENTRATION OF ADDED SUBSTANCE
TIME OF SET
(I) Effect of ethyl acetate
CONCENTRATION OF ADDBD SUBSTANCE
TIME OF SET
(2) Effect of extra acetic acid
grain-moles per liter
minutes
gram-moles per Mer
minutes
0 0.0166 0.0416 0.0830 0.125 0.166
28.50 30.00 36.50 44.00 52.25 60.00
0 0.0166 0.0416 0.0830 0.125 0.168
29.50 38.00 51 .OO 69.00 89.00 110.00
(3) Effect of ethyl alcohol 0 0.0166 0.0416 0.0830 0,1250 0.1665
30.50 30.50 31.50 31 .OO 31 .OO 31.00
(4) Effect of the products of hydrolysis of ethyl acetate
O* 0.0166 0.0416 0.0830 0.1250 0.1665
29.50 37.50 51.75 69.00 89.00 109.00
* Both ethyl alcohol and extra acetic acid are added and the concentration of each is as given in this column.
The extra acetic acid has the effect of greatly increasing the time whether alone or in the presence of its equivalent of alcohol. The ethyl acetate, therefore, apparently supplies extra acid, being hydrolyzed in the acid environment, the extra acid increasing the time of set. We have an example here, therefore, of a type of compound which causes a change in the time of set becauqe it supplies more acid to the gel mixture. Whether the ethyl acetate as such gives a specific effect is impossible to say. In the next portion of the work, a study was made of the effect of a
327
S T U D I E S ON SILICIC ACID G E L S
series of typical organic compounds upon the time of set. An aliphatic alcohol, an aldehyde, a ketone, a hydrocarbon, a sugar, and glycerine were investigated, These substances were added to the acetic acid before mixing with the sodium silicate. Table 3 shows the effects of these substances. It will be noted that the hydrocarbon, benzene, is present in very small amount, because of its slight solubility in water. It will also TABLE 3 E$ect of various organic compounds o n the time of set and on the p H at 26°C. Concentrations constant throughout are a8 follows: NaOH, 0.395; Si02, 0.664; CHaCOOH, 0.454. CONCENTRATION OF ADDED SUBSTANCE
I
T I N E OF SET
1
CONCENTRA-
pH SUBSTANCE
Effect of acetaldehyde
Effect of ethyl alcohol gram-moles per lite7
minutes
0 0.1910 0.955 1.910 2.865 3.810
20.50 21.25 25.00 28.75 33.25 37.00
gram-moles p e t
5.52 5.52
21.25 24.25 27.75 31 -50 36.50 42.50
20.5 24.0 28.5 32.0 36.0
0 1.02 2.04 3.06 4.08
5.55 5.55 5.55
5.52
Effect of acetone 0 0.1515 0.7575 1.515 2.278 3.030
minutes
liter
Effect of benzene 5.52 5.52 5.52
0 0.00001 0.00005 0.00010 0.00015 0.00020
22.00 22.25 22.00 22.00 22.25 22.00
Effect of cane sugar 0 0,065 0.130 0.195 0.260
21.00 21.25 21.25 21.50 21.25
5.55 5.55 5.55
0 0.152 0.760 1.520 2.280 3,040
'
21.00 21.25 21.75 22.00 22.25 22.50
5.55 5.55 5.55
be noted that the alcohol concentrations here are much higher than those involved in the study of the effectof ethyl acetate. The results of these six sets of data in table 3 are shown graphically in figure 2. An analysis of the effects of ethyl alcohol, acetaldehyde, acetone, benzene, cane sugar, and glycerine may be attempted as follows. The ben-
328
CHARLES B. HURD AND DANIEL H. CARVER
zene may only be added in such minute quantities that it is not surprising that we have found no apparent effect. It may be concluded, however, that benzene, if it possesses any specific effect on the time of set, does not possess a large one. The cane sugar shows an effect which is practically negligible. Glycerine shows a slight effect. It should be noted that the maximum concentration of cane sugar obtained was 0.26 molar, while the glycerine reached 3.04 molar. The effect on the pH by these compounds was also shown to be very slight. The effects of ethyl alcohol, acetaldehyde,
3s.
20.'
I i
I 1.0
CONCUVTRA~ N OF SOLUTE
FIQ.2. EFFECT OF
SOLUTES UPON THE
TIMEO F
SET
and acetone upon the pH of these sodium silicate-acetic acid mixtures when added up to concentrations of 3 to 4 molar are seen to be negligible when measured by the quinhydrone method. The effects on the time of set, however, are perfectly evident. Each one lengthens the time of set, and the effect of the acetone is greater than the effects of the other two. The effects of these compounds are apparently specific, since no change of pH could be detected. It will perhaps be possible in the future to correlate these results with the results of other investigators on other properties of these gel mixtures.
STUDIES ON SILICIC ACID GELS
329
SUMMARY
The effects of added solutes on the time of set of gel mixtures produced by mixing solutions of acetic acid and sodium silicate have been studied. An attempt has been made to separate the specific effects of the compounds themselves from the effects which they produce by changing the pH of the mixtures. Ammonium hydroxide, methyl-, dimethyl-, and trimethyl-amines, pyridine, and aniline up to 0.04 M decrease the time of set mainly by increasing the pH of the mixture. There is, however, a specific effect apparent, greatest in the case of trimethylamine. A typical ester, ethyl acetate, apparently increases the time of set chiefly because of acetic acid set free during acid hydrolysis of the ester. Cane sugar and glycerine have little effect either on the time of set or on the pH. Benzene shows no effect, although its very low solubility practically precludes any save a very unusual effect. Ethyl alcohol, acetaldehyde, and acetone show practically no effect upon the pH, but show considerable specific effects in increasing the time of set. REFERENCES (1) (2) (3) (4) (5) (6)
(7) (8) (9)
(IO)
HURDAND LETTERON: J. Phys. Chem. 36,604 (1932). WERNER:J. Am. Pharm. Assoc. 9,501 (1920). KROGER:Kolloid-Z. 30, 16 (1922). BHATNAGAR AND MATHUR: Kolloid-Z. 30,368 (1922). PRASAD AND HATTIANGADI: J. Indian Chem. SOC.6,653,893,991 (1929). HOLMES: J. Phys. Chem. 22, 510 (1918). PRASAD AND HATTIANGADI: J. Indian Chem. SOC.6,893 (1929). MASCHKE: Ann. Physik. 222,90 (1872). PAPPADA: Gaaz. chim. ital. 33,272 (1903); 36, i, 78 (1905). WULFAND PRAETORIUS: Rec. trav. chim. 49,582 (1930).
