SILICIC ACID GELS BY HARRY N. HOLMES
'
Workers in various sciences have occasion to prepare special silicic acid gels but specific directions are lacking. Requirements for such gels made from the desired acid, setting in a convenient time, and of a definite silica content, should be met. This paper offers information for such needs as well as new work on the effect of high concentrations of the acid mixed with the sodium silicate and the comparative effect of weak and strong acids on silicic acid. A study of the influence of higher temperatures on silicic acid gels is also included. Flemming,l under the direction of W. Ostwald, has made the most important contribution to this phase of the subject although Kohlrausch2 studied the effect of age on water glass solutions as measured by change in conductivity and van Bemmelen3 did invaluable work in connection with the dehydration and hydration of silicic acid gels. Maschke4 added some work on the time of formation of these gels.
Methods of Work A water glass solution sold for general purposes by a prominent chemical company was used for most of the experiments. Since carbon dioxide has a marked influence on the time of set of silicic acid gels it was necessary to have a water glass free from this substance. We diluted this sodium silicate, acidified and boiled. KO carbon dioxide could be detected in the issuing gases. Some samples of commercial water glass contain iron salts in appreciable quantities but only a trace of iron was to be found in the product we selected. It was practically colorless. Analysis showed the composiZeit. phys. Chem., 41, 427 (1902). Ibid., 12, 573 (1893). Zeit. anorg. Chem., 13, 233, 258 (1897); 18,114 (1898); 59, 62, I (1909).
Pogg. Ann., 146, go (1872).
225
(1908);
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tion of this water glass to be I NanO, 3.66 S O z . Diluted to a density of I .06 it required an equal volume of 0 . 5 2 N acid to neutralize it, using phenolphthalein as an indicator. In making the gels it was our custom to pour the water glass into an equal volume of the acid desired, mixing quickly. The mixture was considered to have set to a gel when the container, a tube of 15 mm to 2 0 mm diameter, could be tilted and sharply tapped without a flow of the gel mixture. F’lemming invented two rather complicated methods of observing the time of set, but, with a set of a few hours, their accuracy was probably no greater than by the simple method here given. In the temperature study, water baths were improvised from granite kettles covered with asbestos paper and heated by micro burners. The flames were protected from drafts by curtains of asbestos paper. Test tubes of the gel mixture hung in the baths through holes in the wooden covers. Of course the solutions were heated to the desired temperature before mixing. With these baths, by close watching, we could hold the temperature to within 0 . 5 O C of the desired point for temperatures below 5oGC. Above soG C the possible error was about I O C.
Effect of varying acidity on the same concentration of SiOz. Acid mixed with an equal volume of I . 16 density sodium silicate a t zoo
jI2
Harry N . Holmes
I n Fig. I we plot the time of set for mixtures of the same water glass with five different acids, using concentrations of the acids differing by 0 . 5 N and below the I 2\; decreasing in 0 , I N steps. It will be observed that with increasing acidity the set of the tartrate or formate gels is delayed more than with acetic, citric or phosphoric acids. Of course the normality reading on this chart does not represent the excess of free acid above that required to react with the sodium silicate but only the concentration of acid actually mixed with an equal volume of I .16 density water glass. To read this excess of free acid one should subtract I . 4 N , the normality of acid required to neutralize an equal volume of the particular water glass used, and divide the result by two since the volume of the acid was doubled. Flemming, working with hydrochloric and sulphuric acids, charted only the excess acid but as a source of convenient working directions this paper is confined to the actual concentrations mixed. It will be observed there that gels of almost immediate set are more than slightly basic, as it requires I . 4 N acid to neutralize an equal volume of the water glass used in Pig. I . Plemming iound that with gels of a much lower concentration of S O z a slight excess of hydroxyl ion produced the quickest set. It is evident from Fig. I that the quickest set with more concentrated gels occurs in the presence of a much greater excess of hydroxyl ions. With increase in the excess of hydroxyl ions the time of set is delayed more rapidly than by the catalytic action of excess of hydrogen ions, that is, the curve quickly flattens. The curves for all these acids below the normal practically coincide as shown in Fig. I . This chart is similar to Fig. I , but refers to different densities of the same water glass instead of I .16 density alone. Due partly to the lower concentration of silicic acid, the time of set was much longer than with the I . 16 density water glass, in fact, the chart had to be drawn on the scale of one hundred hours instead of four hours as in Fig. I . If it is desired to compare the effect of excess of different acids on
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silicic acid in this chart the comparison must be confined to one density, or concentration, of sodium silicate I .08 for example. Naturally a lower concentration of silicic acid sets to a gel more slowly. For the I .06 density subtract 0 . 5 2 N from the chart reading; for I .08 density subtract 0 . 7 N ; and for I . I O density subtract 0 . 8 6 N . When these results are divided by two (because the volume of acid was doubled in mixing) the chart readings are changed from the concentrations actually used to the excess of acid present.
0 Fig. 2 . Effect of varying acidity on water glass of I .06, I .08 and
I .IO
density a t
20'
Using a water glass of approximately the same composition as that described here, any experimenter can select from the range of five different acids and four different densities of sodium silicate the particular mixture that meets his requirements in time of set. I n some work a definite acid is desired. Flemming considered the time of set to depend wholly upon the concentration of silicic acid liberated, and the catalytic effect of hydrogen or hydroxyl ions with, of course, a temperature influence. In this paper we raise the question whether the concentration of free hydrogen ions is not influenced greatly, in the weaker acids, by the presence of the
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sodium salt formed in the reaction. For example, the excess of free acetic acid must be greatly repressed in ionization by the sodium acetate formed. With sulphuric and hydrochloric acids, the only acids used by Flemming, this influence is slight. The dehydrating influence of the molecules of the acid may have a great effect on the relative distribution of water between the two phases of the gel thus influencing the time of set. This influence must vary with the acid used and the concentration of its excess over the amount required to react with the water glass. 46
24
26
6
2 I
2
3
Fig. 3 Effect of extreme acidity on silicic acid gels Sodium silicate of I .16 density mixed with an equal volume of acid a t
20'
Fig. 3 presents rather unexpected results. I n Fig. I and Fig. 2 it is shown that gel mixtures containing only a slight
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excess of hydrogen ion set quickly but with increasing excess of hydrogen ion the set is greatly delayed. The curves soon flatten and run off the chart to an indefinite distance. Fig. 3 shows the return of the curves to the limits of the chart a t still higher concentrations of acid. With hydrochloric or nitric acids this return, or rise from the horizontal, begins a t about 5 N . For acetic acid the curve begins to rise a t about 1 1 N but for phosphoric not until about 2 2 N is reached. It is remarkable that on mixing 45 N phosphoric acid (ordinary 85 percent syrup) with an equal volume of 1.16water glass a gel of excellent texture results, setting almost immediately. Concentrated sulphuric acid coagulates the mixture into a rough lumpy mass but 24 N sulphuric yields a good firm gel, setting immediately. Glacial acetic acid is about 1 7 LV hence the upper limit of that curve. Flemming’s belief that the catalytic effect of hydrogen or hydroxyl ions, the concentration of the silicic acid, and temperature, are the only factors influencing the time of set scarcely seems to explain the curves here given. He did not work a t high concentration of the acids and so failed to plot the return of the curves to the vertical axis. It is quite possible that the catalytic influence of hydrogen ion is a very important factor but an increase in concentration of excess acid reaches a point where the number of molecules of the acid increase faster than the number of hydrogen ions. It is well known that sulphuric acid has its maximum specific conductivity in a 30 percent solution (approximate) and above this concentration the specific conductivity rapidly decreases. In other words, there is a concentration of the excess sulphuric acid, for example, below which hydrogen ions exert the most powerful influence on the set of silicic acid gel but above which the dehydrating influence of the nonionized molecules of sulphuric acid rapidly increases and finally preponderates. With the I , 16 density sodium silicate this point is reached apparently with about 7 N sulphuric acid. With phosphoric acid this point is reached a t about 2 0 N . Of course the ionization of the free acid is influenced by
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Harry N . Holmes
the salts present. With weak acids this repression of ionization is greatest. It is not inconceivable that there may be a catalytic effect exerted by other negative ions than hydroxyl to complicate the influences.
Fig. 4. Temperature effect on silicic acid gels Sodium silicate of I , I O density mixed with an equal volume of acid
In Pig. 4 the curves all refer to the same concentration of silicic acid., It will be noted that a slight increase in temperature accelerates the time of set much more rapidly from 20' C to 60' C than above 60" C. In other words, the curve grows steeper above this,middle range of temperature. The curves for formic and sulphuric acids are evidently quite unlike. Water glass of I . I O density mixed with an equal volume of 3 N acetic acid sets more slowly than the less acid mixture with 2 N acetic although each gel contains more than enough acetic to liberate all the silicic acid. The curves for 3 N nitric and 3 N acetic are far apart, possibly because of the greater ionization of the strong mineral
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acid as well as the different dehydration influence of the two kinds of acid molecules. At higher temperatures it is probable that dehydrating effects of nonionized molecules decrease and therefore the curves should approach each other as, in fact, they do. At any given temperature the gel from 3 N nitric sets more slowly than that from the much more acid 6 N sulphuric. This can hardly be explained on the theory of hydrogen ion catalysis. When a water glass of a higher Na20 content is acidified with sulfuric acid, for example, more sodium sulphate is formed as a by-product than when an equal volume of a water glass of the same SiOz content b u t a lower Na,O value is acidified. The question arose whether two silicic acid gels formed as described and of equal SiOz concentration would set in the same time. They did this when a strong acid was used, but with a weak acid such as acetic the sodium acetate formed repressed the ionization of the excess acetic, lessening the catalytic influence of the hydrogen ion. Unequal amounts of sodium acetate were formed in the two cases and, as a result, one gel set much sooner than the other.
TABLEI Sodium silicate mixed with equal volumes of acid 1.06 density sodium silicate
Acid
Gel set in z to 3 hours
Gel set in IO hours
8 to
I:
I. I Z
2
density sodium silicate
Gel set in to 3 hours
Gel set in IO hours
3 to
-
Hydrochloric Nitric Sulphuric Phosphoric Formic Acetic Chloracetic Trichloracetic Citric Tartaric Lactic
0.5jN 0.5jN
0.55 N 0.7jN 0.70N
0.80!l 0.80N
0.6 N 0.6 N 0.6 I .5
LV
3 .g
L V
I.2
A1
I
.o A T
0.5jN
0.6
I
1.2
.ooN 0.80 N I .oo l !
.v
N 0.9 N 1.2
N
1.2 N 1 . 2 A' 1.2N 2 . j r\r 1 . jN
!V
4.0
s
4 . 0 -16 . 0! i 3.0 N 2.0
A'
2.2 2.2N I .2 2 .j 2 . 2 A'
N
3.5 N 3.5N 1 . 4N
N
3.0
N
2 . j !I
3.0
111
......
Harry AT. Holmes Using any sodium silicate one can determine the silica content and make gels setting in approximately the same time as those of equal silica content described in this paperif strong acids are ased. The particular water glass we used a t a density of I .06, yielded, when completely neutralized, gels containing 2 9 . 4 7 grams SiOz per liter. This table is offered for further convenience in making gels. It adds to the data given by the curves. It is noteworthy that trichloracetic acid delays the gel set more than does the less ionized acetic, hence the lower concentration of trichloracetic given in Table I. Dialyzed gels may be made in which there is only a small excess of any acid (other than silicic) or soluble salts, but the removal of these substances changes the time of set from the figures given in this paper. Hatschek dialyzed some mixtures of sodium silicate and acid and then secured a set at his convenience by adding a small amount of ammonia. Por special purposes such a method may be of value.' Summary I . Directions are given for the preparation of many types of silicic acid gels setting in any required time. 2 . The temperature influence on these gels is plotted from oc c to IOOO c. 3. The effect of a great excess of acid on the time of set of the gel is plotted. As shown by other workers, gels containing a very slight excess of hydroxyl ions set in the shortest time, almost immediately if not too dilute, but with increasing excess of hydrogen ion, the time of set rapidly increases. With each acid mixed with sodium silicate there is a concentration of hydrogen ion which delays the set to an indefinite time. In this paper we show that a t still higher concentrations of the same acid the time of set of the gel again becomes measurable, rapidly decreasing to an almost immediate set.
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4. Contrary to Flemming and Ostwald’s theory that the time of set of a silicic acid gel depends only on the concentration of Si02,temperature and the catalytic effect of hydrogen or hydroxyl ions, i t is here shown that the dehydrating influence of nonionized molecules of the acid mixed with the sodium silicate is an important factor. Oberlin College
