SILICIC ACID GEL6
57
REFERENCES (1) EQQERT, J., AND K ~ ~ S T EA.: R , Kinotech. 18, 381 (1936). T . H.: J. Am. Chem. SOC.61, 048 (1939). (2) JAMES, (3) JAMES, T . H.: J. Phys. Chem. 43, 701 (1939). T . H., AND WEISBBEROER, A.: J. Am. Chem. SOC.80, 98 (1938). (4) JAMES, ( 5 ) LUMIBRE,A,, LUMIBRE, L., AND SEYEWETZ, A.: Phot. Korr. 62, 183 (1926). (6) LUTHER, R., A N D LEUBNER, A.: Brit. J. Phot. 69, 632 e t seq. (1912). (7) c f . NIETZ,A. H.: Theory of Development. D. Van Nostrand Company, New York (1922). (8) RABINOWITSCH, A. J.: Trans. Faraday SOC.34, 920 (1938). A. J . : Trans. Faraday SOC.34, 921 (1938). (9) RABINOWITSCH, (10) RABINOWITSCH, A. J., AND PEISBACHOWITSCH, 9.: Z. wiss. Phot. 33, 94 (1934). (11) REINDERS, W.: Trans. Faraday SOC. 34, 936 (1938). G. M.: Z. Elektrochem. 36,584 (1929). (12) cf. SCHWAB, (13) c f . SHEPPARD, S. E., AND MEEB,C. E. K.: Investigations on the Theory of the
Photographic Process. Longmans, Green and Company, London (1907). AND MEYER,G.: J. Am. Chem. SOC. 4, 689 (1920). (15) SHIBERSTOV, V.: Photo-Kino Chem. Ind., 2, 141 (1933). (16) WULFF,P., AND SEIDL,K.: Z. wiss. Phot. 28, 239 (1930). (14) SHEPPARD, S. E.,
STUDIES ON SILICIC ACID GELS. I X THEEFFECT OF A CHANGE OF PHUPON THE TIME OF SETOF SOME ACID GELS CHARLES B. HURD AND HARRIS W. PATON' Department of Chemistry, Union College, Schenectady, New York Received January 80, 1059 INTRODUCTION
A recent study of the time of set of silicic acid gels, prepared from solutions of sodium silicate and acetic acid in this laboratory ( 5 ) , has shown that, for mixtures of acid reaction, the time of set is proportional to the hydrogen-ion concentration. A report by Hurd, Frederick, and Haynes (2) showed a similar result when hydrochloric, nitric, or sulfuric acid was used. A discussion of the theory has also been given (1). It was observed, incidentally, that addition of extra acid to mixtures already prepared delayed their setting. We have thought that it would be very important to discover whether there might be some particular time, during the setting of the already prepared mixture, when the introduction of the extra acid would have a particularly marked effect. If such Present address: Corning Glass Works, Corning, New York.
58
CHARLES B. HURD AND HARRIS W. PATON
a point were found, it would mark the time during the setting of the gel mixture when the process would be taking place which is so sensitive to the hydrogen-ion concentrat’ion. If no significant point were found, it would indicate that the whole process would appear to be equally sensitive to the hydrogen-ion concentration. The experiments to be described were devised to test this point. They cover, unfortunately, only a part of the pH range, namely, from 4.57 to 5.08, but appear worth reporting because of their bearing upon the theory. EXPERIMENTAL
The technique employed has been reported several times in this Journal (5, 2). A solution of sodium silicate equivalent to 1.26 N sodium hydroxide was obtained by dilution of “E” brand sodium silicate, prepared by the Philadelphia Quartz Company. This silicate has a soda:silica weight ratio of 1 to 3.19. A 2.01 N solution of acetic acid was used. The tests were carried out a t 25OC. in an electrically controlled water thermostat. The solution of sodium silicate was always poured into the acetic acid solution. Double quantities of the mixtures were prepared, and were mixed by pouring back and forth. The time of set was determined in one sample in a 100-cc. Pyrex Griffin beaker by the “tilted rod” method of Hurd and Letteron (4). The pH was measured in the other sample by the quinhydrone method, as recommended by Hurd and Griffeth (3). Since addition of a solution of acid to any silicic acid gel mixture decreases the silica concentration, parallel series were carried out where equal volumes of pure water in the one series and acetic acid solution in the other were used. The length of time from the preparation of the original mixture to the addition of the acid solution of water was varied from zero, where the mixtures were made up complete a t first, to about 85 per cent of the time required for the original mixture to set. Beyond that point the gel was set sufficiently so that liquid could not be mixed with it homogeneously. The concentrations are easily calculated. For example, in the first mixture of the second section of table 1 they are as follows: SiOz, 0.86; CHsCOONa, 0.52; CH3COOH,0.65; all concentrations are in gram-moles per liter. A word of explanation may be useful. In the first result in table 1, the extra 5 cc. of acid solution was added a t the start, and the mixture set in 88 min. a t 25OC. In other words, 25 cc. of silicate solution was poured into 30 cc. of acid solution. The second result was obtained by mixing first 25 cc. of silicate solution and 25 cc. of acid solution. This 50 cc. of mixture was placed in the 25°C. thermostat in a 100-cc. beaker, and covered with a watch glass. At exactly 15 min. after mixing, an extra 5 cc. of acid solution was introduced and the result mixed. It set 79 min. after the
59
SILICIC ACID GELS
TABLE 1 The effect of addition of extra acetic acid solution or of wafer upon the time of set and the p H of various miztures of sodium silicate solution and acetic acid ACID A D D E D
1
Original mixture ec. 5.0 5.0 5.0 5.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
Original mixture 10.0 10.0 10.0 10.0 0.0 0.0 0.0 0.0 0.0 0.0
Original mixture 15.0 15.0 15.0 15.0 0.0 0.0 0.0 0.0 0.0 0.0
WATEB ADDED
= 25.0
1
~
TIME OF ADDITION
minutes
0.0 0.0 0.0 0.0 0.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 0.0
0.0 15.0 26.0 39.0 0.0 8.0 15.0 20.5 30.0 40.0 45.0 46.0
88.0 79.0 75.0 65.5 57.0 67.5 66.5 64.5 63.0 61.0 60.0 58.0 59.0 57.0
'
4.57 4.57 4.57 4.57 4.82 4.82 4.82 4.82 4.82 4.82 4.82 4.82 4.82 4.82
cc. of silicate solution and 25.0 cc. of acetic acid solution
0.0 0.0 0.0 0.0 0.0 10.0 10.0 10.0 10.0 0.0 = 25.0
TIME OB BET
cc. of silicate solution and 30.0 cc. of acetic acid solution
cc.
= 25.0
1
0.0 8.0 15.0 22.0
0.0 10.0 15.0 20.0
88.0 74.0 60.0 49.0 32.0 46.0 42.0 39.5 37.5 32.0
4.57 4.57 4.57 4.57 4.98 5.03 5.03 5.03 5.03 4.98
cc. of silicate solution and 20.0 cc. of acetic acid solution
0.0 0.0 0.0 0.0 0.0 15.0 15.0 15.0 15.0 0.0
0.0 3.0 6.0 8.0 0.0 3.0 6.0
8.0
88.0 67.5 47.5 36.0 11.0 21 .o 20.0 17.5 15.3 11.0
4.57 4.57 4.57 4.57 5.08 5.42 5.42 5.42 5.42 5.08
time of first mixing. The last result in the first section of table 1 set 57 min. after mixing. KOextra acid had been added.
60
CHARLES B . HURD AND HARRIS W. PATON
The pH in each case is for the mixture which actually set. The data of these three tables are plotted on figure 1. The straight-line relation is very evident.
FIQ.1. Effect of addition of extra acid upon the time of set of silicic acid gels DISCUSSION
The data show, quantitatively, a straight-line relation between the time of set of the mixture and the time which had elapsed from the time of mixing the original solution to the time of addition of the extra acid or water. The limits of these straight lines are a maximum time of set, when the extra acetic acid solution, or water, was added at the start,-that is, as part of the original mixture,-and a minimum time of set when the original mixture set without addition. These linear relations are very significant. For example, if we consider the second line of the first section of table 1, we see that the extra 5 cc. of acid added 15 min. after the gel mixture was prepared gave a mixture whose setting time was 79 min. If this extra acid had been added at the start, the time of set would have been 88 min., while if it had never been added, the original mixture would have set in 57 min. We may consider that this setting process is proceeding in the original solution at a rate such as to make it set in 57 min., and that, after the addition of the extra acid, the rate became much slower, so that 88 min. would have been required for setting. Calculating that after the original
SILICIC ACID QELS
15 min., the fraction
61
57 - 15 '57x must 5 still occur at a rate such that 57
88 min. are necessary, we have for our total time of set t = 15.0
- 15 X 88 = 15.0 + 64.7 = 79.7 min. + 5757
The time measured was 79.0 min. It is obvious that such a calculation is possible for all points and that these calculated setting times will agree with the actually measured values &s well as the coincidence of the points with the straight lines. The latter agree with the straight-line relations within the experimental error, except for one value a t 30 min. on curee 1 for acid and several values for water. It would appear, then, that we may change the rate of the setting process by dilution or by addition of more acid; the latter includes both a dilution of the solution and an increase in the hydrogen-ion concentration. The effect of either of these procedures is to change the time of set by an amount capable of calculation, namely, as the sum of two time intervals. It appears to make no difference where the extra acid solution or water is added between the limits of the original mixing time and 85 per cent of the setting time. Over this wide range, the time of set of the final mixture appears to be equal to the sum of the two time intervals. It would be very interesting if an addition of either were possible even nearer to the time of set, but it proved impossible to mix the result uniformly. The general weight of opinion of workers in this field favors the polysilicic acid fibrillar theory to explain the formation of silicic acid gels (7, 6). The theory postulates that a monosilicic acid is first formed and that condensation reactions occur, building up more and more complex acids (1) until finally the whole mass sets. There is, apparently, no especially significant point in the setting process a t which addition of more acid has a particularly marked effect. SUMMARY
The time of set of a silicic acid gel mixture, produced by mixing solutions of sodium silicate and acetic acid, may be increased by addition of q o r e water or more acetic acid solution. The original mixtures were acid and the temperature was kept constant a t 25°C. There is, apparently, no time during the setting when the process shows any unusual sensitivity to change in the hydrogen-ion concentration, the process being uniformly affected. The time of set may be calculated, if the time of the addition of water or acetic acid solution is known, since the relation is linear. The total
02
E. D. FISHER AND C. H. SORUM
time is shown to be the sum of two intervals, each dependent on the concentration of silica and of hydrogen ion. The bearing of this experiment upon the fibrillar condensation theory of silicic acid gel structure has been discussed. REFERENCES (1) HURD,C. B.: Chem. Rev. 22, 403 (1938). (2) HURD,C. B., FREDERICK, K. J., AND HAYNES,C. R . : J. Phys. Chem. 42,s(1938). R. L.: J. Phys. Chem. 39, 1155 (1935). (3) HURD,C. B.,AND GRIFFETR, (4) HURD,C. B., AND LETTERON, H . A.: J. Phys. Chem. 38, 604 (1932). C. L., AND MILLER, P. S.:J. Phys. Chem. 38,663 (1934). (5) HURD,C. B.,RAYMOND, (6) JORDIS,E . : Z. anorg. Chem. 44, 200 (1905). (7) TREADWELL, W.D., AND K ~ N I CW.: , Helv. Chim. Acta 16, 54 (1933).
T H E INFLUENCE OF SOL CONCENTRATION ON FLOCCULATION VALUES E. D. FISHER
AND
c . H. SORUM
Department of Chemistry, University of Wisconsin, Madison, Wisconsin Received March 24, lgS9 INTRODUCTION
The influence of sol concentration on the flocculation values of pure ferric oxide and chromium hydroxide hydrosols has been previously reported (3, 5 ) . With these pure sols the flocculation values decreased for monovalent ions, remained constant for divalent ions, and decreased for trivalent and tetravalent ions as the sols were diluted. This behavior conformed to the flocculation rule formulated by Burton and Bishop (1). When these pure sols were made impure by adding small amounts of stabilizing electrolytes, the flocculation values for all ions decreased as the sols were diluted. In this paper a more detailed study of the behavior of chromium hydroxide sols will be presented, as well as observations on other typical systems. Changes in flocculation values when the sols were dilited with alcohol are also included. CHROMIUM HYDROXIDE
Table 1 shows the effect of the addition of chromium chloride on the flocculation values of a pure chromium hydroxide sol a t various dilutions. The monovalent coagulating ions chosen are widely separated in the Hofmeister series; two polyvalent ions are also included. The sol was prepared by precipitating the hydroxide from a chromium chloride solution
