EFFECT OF pH POLYMERIZATION OF SiLIcIc ACID
July, 1956
curve c without overlapping it; identical spectra would give perfectly parallel curves. Kinetic Measurements.-The reaction was carried out in sealed tubes of Corning No. 728 alkali-resistant glass. It was demonstrated in the early stages of this work that Pyrex glass was attacked by the strongly alkaline solutions and non-reproducible rate constants were obtained. Several filled tubes were sealed, plunged into the constant temperature bath simultaneously, and two minutes was allowed for the bath to regain temperature equilibrium. Then the first tube was withdrawn, plunged into cold running water, and the timer started. The remaining tubes were withdrawn from the bath a t appropriate intervals and
1007
the reaction quenched. After allowing sufficient time for each sample to reach room temperature the tube was opened and a 10.00-ml. aliquot was titrated potentiometrically with standard acid. Rate constants were corrected for expansion of the solvent since the concentration o,f the reactants is different a t temperatures other than 25 . Thus, values of k obtained a t 100" were corrected by multiplying k by 1.04, a t 110" by 1.05, and at 120" by 1.05. Temperature Control.-The thermometer was calibrated against a platinum resistance thermometer which had been checked recently by the National Bureau of Standards. The temperature was controlled within f 0 . 2 " .
EFFECT OF pH ON POLYMERIZATION O F SILICIC ACID BY KATSUMI GOTO Faculty of Engineering, Hokkaido University, Sappor, J a p a n Received March 13, 1066
In view of discordant results reported by earlier investigators, the effect of pH on the polymerization of silicic acid has been further studied. The reaction rate constant a t various p H values in the range from 7 to 10 was calculated from the rate of disappearance of molecularly dispersed silica, on the assumption that the polymerization reaction is third order with respect to the concentration of molecular silica. The value of the calculated constant increased linearly with pH, indicating that the polymerization occurred more rapidly at higher p H . The time required for complete depolymerization of the particles of colloidal silica formed a t various pH's, indicated that larger particles are formed a t higher p H .
Many workers1-* have reported the effect of pH on the polymerization of silicic acid in water, but their conclusions are not identical. Iwasaki, et uL,lhave stated recently that the polymerization is most rapid a t the neutral point. Similar conclusions have been drawn from measurements of gel time.2-4 On the other hand, Brady6 has concluded from light scattering measurements that the optimum pH for polymerization is about 8, while Greenberg and Sinelair', on the basis of similar experiments, have reported a maximum rate of polymerization a t pH 8.6. It is our belief that the above disagreement arises from the tacit assumption in each case that the polymerization proceeds to the same equilibrium state regardless of pH. This assumption seems to be incorrect when it is considered that the solubility of amorphous silica changes with pH, as has been described in the previous paper.$ The present investigation was carried out to obtain a clear picture of the effect of pH on the rate of polymerization of silicic acid, and also to determine whether the equilibrium state of polymerization has been affected by pH. Experimental I. Changes in Concentration of Molecularly Dispersed
Silica.-These changes were measured a t various p H ' s in a manner similar to that reported in the previous paper.9 Adjustment of p H was carried out by rapid mixing of sodium silicate solution with the silicic acid solution prepared by the ion-exchange method. The latter also contained a large amount of molecularly dispersed silica. (1) I. Iwasaki, T. Tarutani, T. Katsura a n d H. Arino, J. Chem. Soc.
To simplify presentation of data, the following equation was assumed to be applicable to the polymerization of silicic acid
where C is the concentration of molecularly dispersed silica a t time 1; S, the solubility of amorphous silica; and k is a constant. Values of k for the polymerization a t various p H ' s are plotted on a logarithmic scale in Fig. I , which clearly shows the greater polymerization rate a t higher p H . Polymerization may be a reaction too complex to be fully dealt with by such a simple assumption. Nevertheless, the results indicated in Fig. 1 still show a t least qualitatively the effect of p H . 11. Depolymerization Time.-The measurements of depolymerization time were carried out to determine whether there were any differences in the polymers formed a t different pH's.
10-7 A I
-
-
Q '
7
T-
2E
-
V v
2
Jap., Pure Chem. Sec., 7 6 , 856 (1954).
(2) R. C. Merrill and R. W. Spencer, THISJOURNAL, 54,806 (1950). (3) I. A. Heald, K. B. Coates and J. E. Edwards, J. AppE. Chem., 5, 425 (1955). (4) R. K. Iler, THISJOURNAL, 67, 604 (1953). (5) C.B. Hurd and H. A. Letteron. ibid., 36, 605 (1932). (Ci) A. P. B r a d s , A . G . B l o w n 2nd €1. Huff, J. Coll. Scz., 8, 252 (1953). (7) S. A. Greenberg and D. Sinclair, THISJ O U R N A59, L , 435 (1456). ( 8 ) G. B. Alexander, J . A m . Chem. Soc.. 76, 2094 (1954). (9) IC. Goto, J . Chem. Soc., Pure Chem. Rec., 76,1304 (1955).
I
7
I ' 8
I
I I)
PH. Fig. 1.--Values of k at various pH's.
1
10
GRANTW. SMITHAND HOWARD W. JACOBSON
1008 4oo
Vol. 60
trapolating this straight line to the point where M1/a = 0. I By a plying these procedures to depolymerization of colloidarsilica, values of have been obtained for colloidal T
silica formed a t various pH's. They are shown in Fig. 2 and indicate that larger particles are formed a t higher pH. Depolymerization experiments were carried out by diluting one volume of colloidal silica solution with 100 volumes of the solution containing 1 g./l. NaSCOa (pH 10.8). The colloidal silica solutions used in these experiments were prepared by aging a t various pH's for 6 days and contained 2 g. of SiOzper liter.
h
'8 300
v
c
200 7
6 Fig. 2.-Values
8
9
10
PH.
for colloidal silica formed at various pH's. Suito, et aL.,'O have derived the following equations for the dissolution of fine particles, assuming the dissolution rate to be proportional to the surface area of the particles. n'/a M1/a = K(T t ) of
T
-
~ = b v o
where M is the total weight of particles at time t ; 7 , the time necessary for complete dissolution (in the present study T means the time necessary for complete depolymerization); UO, the initial radius of particle; ~t, the number of particles; and b and K are proportionality constants. Therefore, the MVa versus t plot becomes a straight line for monodisperse systems, and T can be obtained by ex(10) E. Suito, N. Hirai and K . Taki, J . Chem. Soc., 1 3 , 713 (1951).
Discussion I n no case has polymerization reached true equilibrium, since in this 'case the system, a t iiifinite time, would consist of water containing a single large particle of amorphous ' silica. After aging the colloidal solutions for only 6 days, surely only a pseudo-equilibrium is established. It might therefore be postulated that as the monomer disappeared to form colloidal particles, these colloidal particles further polymerize, or at least increase in particle weight, and this rate of increase in particle size is also a function of pH, since the largest particles are formed a t highest pH. From the foregoing evidence, it is clear that hydroxyl ions promote the polymerization of silicic acid over a pH range wider than that reported by previous workers. It is very interesting to note that the polymerization takes place more rapidly a t higher pH, where colloidal silica particles are highly charged and depolymerize very rapidly. This might be evidence for the fact that the same mechanism is involved both in polymerization and in depolymerization. Acknowledgment.-The author is indebted to Prof. Y. Uzumasa, Prof. Q. Okamoto and Assist. Prof. T. Okura for their valuable advice and encouragement ,
~
CHARACTERISTICS OF ADSORPTION OF COMPLEX METAL-AMMINES AKD OTHER COMPLEX IONS OF ZINC, COPPER, COBALT, NICKEL AND SILVER ON SILICA GEL1 B Y G R A N T IT.SMITHAND HOWARD w.JACOBSON Contribution f r o m the College of Chemistry and Physics, T h e Pennsylvania State University, University Park, Pa. Received April 18, 1066
The adsorption isotherms for complex metal ammines, ethylenediamine-metal complexes, and diethylenetriamine-metal complexes on silica gel are shown. The extremely complex nature of the adsorption of metal ammines is demonstrated for the nickel and co per ammines. Metal ammines are adsorbed ill the fol!owing decreasing order, in millimoles adsorbed per gram of silica gel? zinc, copper, cobalt, nickel, silver. For diethylenetriamine complexes, those with higher formation constants were more highly adsorbed. For ethylenediamine complexes studied, six-coordinate systems were more highly adsorbed than four-coordinate. An interpretation of the adsor tion process in terms of hydrogen bonding of ligand to silica surface is presented. Complex copper ammines, copper etEylenediamines and copper diethylenetriamines dissociate during adsorption on silica gel. This is shown by a comparison of the absorption spectra of solutions of the complex metal ions before and after adsorption. The ratio, animonia:copper ion adsorbed is higher than that of the complex species originally in solution. The more stable a given complex ion, the closer the ratio for the adsorbate agrees with that of the 8peCieS initially in solution.
Introduction This study was undertaken to correlate the relative adsorption of complex metal ammines and other nietaI complexes in which ethylenediamine (1) Tliis paper is baaed on part of a thesis submitted by Howard Wayne Jaoobson in partial fulfillment of the requirements for the degree of Doctor of Philosophy at The Pennsylvania State University, August, 1953.
(abbreviated en) and diethylenetriamine (abbreviated dien) are the ligands, with the structure and stability of the complex entities. The unusually strong adsorPtiol1 of ComPlex metal ammines on silica gel was first reported by Smith and Reyerson.2 adsorbed On the surface (2) G. W. Smith and L. H. Reyerson, J . A m . Chem. Soc., 63, 2584 (1930).
