S. A. Greenbergl Seton Hall University South Orange, New Jersey
The Chemistry of Silicic Acid
Although a great deal of information on this chemistry has been reported in the last five years ( I ) , many incorrect older concepts still appear in the current literature, in freshman textbooks, and in treatises on inorganic chemistry. Frequent references are made to the theoretical, but not to the actual existence of silicic acid H,Si04, to the composition of silicic acid as HBiOa, to the preparation of solutions of dimers or trimers of silicic acid, and to many other aspects of the chemistry of the acid for which no definite evidence has been advanced. In the present paper the currently accepted approach to the chemistry is reviewed for those who must present a clear picture to students and are not in this field of chemistry. The element silicon contains four outer electrons, 3s23p2. When the element shows a coordination number of four, these electrons form 3sp3 hybrid bonds (2). Because in SiFe-, SiP20i and in compounds with organic groups, silicon is octahedrally coordinated by six atoms (3, 4, 6),3sp3d2hybridization has been snggested (2). However, recent Raman studies (6) are interpreted to indicate that monosilicic acid in solution exists as Si(OH)4rather than HpSi(OH)~. An analysis of solubility (7) and emf measure ments (8) has demonstrated that the acid is dibasic and dissociates in two steps &(OH),
+ H20
=
SiO(0H)r-
+ H10f
SiO(0H)a-+ HzO = SiOn(0H);
pK, = 9 8(20°)
+ HaO+
(1)
pKz = 11.8(20°) (2)
Present address: Portland Cement Association, Skokie, Ill.
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lovrnol o f Chemicol Education
The pK values, however, are not sufficientlydifferent to 'ause two, breaks in the acid-base titration curve (Fig. 1). The monomeric species (H4Si04,H3Si04-, and H2Si04-) are found only in solution and in the vapor phase. When the solutions are evaporated,
I
i
I
I
1x0
1:l
1 :2
1:3
I 1:A
Mole Ratio Na90:Si09 hydroxide rolvtionr b y Figure 1. Neutralization curves of 0.1 rilico ot 20' (A1 ( 9 ) and 25' 101 110).
silica gels form. Several methods for the preparation of monosilicic acid have been reported. (I) Dilute solutions of sodium silicate above pH 11 contain monomeric silicic acid and its ions (11). ( 2 ) Aqueous solutions in contact with solid silica
are saturated with the undirsociated acid and its ions (1, 7,s). (3) Monosilicic acid solutions have been prepared by hydrolyzing silicon tetrachloride or orthosilicic acid esters in aqueous solutions a t low pH and a t low temperatures (1). (4) Solid silicates in which none of the four oxygens of the SiOl tetrahedron is shared by silicon atoms are reported t o form monomeric acid solutions in water maintained at pH 3 with HCI (1,12). ( 5 ) When noncolloidal solntions of sodium silicate a t 0' and pH 3 are passed through cation exchange resins in hydrogen form, monomeric solutions are produced (1j. (6) Brady (IS) has discussed the presence of orthosilicic acid as a volatile species in steam a t pressures above a few atmospheres. Monosilicic acid Si(OH)4 is the fundamental monomeric unit in polymeric glass, silica gels, crystalline silicas, and silicate structures (14). However, in the orthosilicates the oxygens of the SiOntetrahedra are not shared in siloxane Si-O-Si bonds. Also in hydrated calcium silicates it has been proposed that hydroxyl bonds hold the tetrahedra together (15). Cummius and Miller (12) reported that the silica in calcium silicate hydrate disperses in acidic solutions, in which the rate of condensation reactions is slow (16, 17). I t would be useful t o an understanding of the silicate structures to know what the molecular weight of the silicic acid is. Silica dissolves in water accordine to the eouation derived by O'Connor and ~reenberg~(18)
where k7 is the rate constant and Csionis the concentration of silanol groups. The polymerization of silicic acid in acid solutions has been reported to proceed by different mechanisms (21). Alexander has demonstrated that below pH 3.2 a t 1.90" the reaction is third order and above this pH value the reaction is second order. The details of the mechanisms are not too well understood. The thermodynamic functions have been calculated for the equation (22).
In this equation it may be seen that solid sllica is in equilibrium with saturated solutions of silicic acid and the equilibrium constant K is equal to the activity of monosilicic arid. Several conclusions were drawn from the solubility results. (1) The free energy states of colloidal silica and quartz are sufficiently different (1.32 kcal/mole) to result in an appreciable solubility (0.012%) a t 25' for the amorphous silica and a small solubility for quartz (0.003rr/o). (2) There 1s a negative entropy rhange ( A S = -28 en) for colloidal silica dissolving in water which demonstrates the strong interaction between silicic acid and water (22). Several authors (1) have proposed that tetrahedral silicic acid and water have the same oxide structures. I n one case the small silicon atoms fill the spaces and in the latter ease the small protons are present. (3) The equilibrium data show that only monosilicic acid exists in neutral solutions (I). Literature Cited
where Cis the concentration of monosilicic acid, S is the surface area, kl is the rate constant for solution or depolymerization, and kz is the rate constant for polymerization in neutral solutions. The surface area is proportional to the two-thirds power of the concentration. I n alkaline solutions since the polymerization rate is low, lctC may be neglected. Therefore, the rate of solution is proportional to surface area or to the available Si-0-Si bonds on the surface and the rate of polymerization to the concentration of monomeric species as well as the surface area. The formation of polysilicic acid polymers such as silica gel, quartz, and silicates in alkaline solutions is believed t o proceed by condensation reactions between silanol groups (16, 19, $0).
or between monomeric and polymeric silicate ions II
-SiO-
and silanol groups
I I I I + HOB-I = -SiOSi+ OH(5) I I I I The kinetics for polymerization in alkaline solution has been represented by (17). S i O
( 1 ) For excellent discussion see R. K. ILER,"The Colloid Chem-
istry of Silica and the Silicates," Cornell University Press, Ithsca, N. Y., 1955. ( 2 ) STONE,F. G. A., AND ROCHOW, E. G., J. Inorg. Nuclea~ Chem., 1 , 112 (1955); ALLRED, L., ROCHOW, E. G., AND STONE,F. G. A,, ibid. 2, 416 (1956). S . A. A,, Z. K ~ i s t .92, , 155 (1935). ( 3 ) KETELAAR, L., "The Nature of the Chcmied Bond," Cornell ( 4 ) PAULING, University Press, Ithrtea, N. Y., 1948, p. 382. S., ET AL., abstracts of 133rd American Chem( 5 ) KIRSCHNER, ical Socicty Meeting, Inorganic Division, April 1958; Chem. Eng. News, 36, 49 (1958). D., AND EDWARDS, J. O., J. Inorg. Nuelem Chem., ( 6 ) FORTNUM, 2 , 264 (1956). S. A,, AND PRICE,E. W., J . Phys. Chem., 6 1 , ( 7 ) GREENBERG, 1529 (I!%?\. ...~ >-.,( 8 ) GREENBERG, S. A,, J. Am. Chern. Soc., in press. (9) BACON, L. R., AND WILLS, J. H., PranklinImtitute, 258,347 (1954). R. W . , J. Phys. Chem., 30, 1100 (1926). (10) HARMAN, S. A,, J . Phys. Chem., 61, 9601 (1957). (11) GREENBERG, (12) CUMMINS, A. B., AND MILLER,L. B.,Ind. Eng. Chem., 26, 6881 (1934). (13) BRADY,E. L., J . Phy8. Chm., 57, 706 (1953). S. A., J. Phys. Chem., 60, 325 (1956). (14) GREENBERG, J . U., Brit. J . Appl. Phys., 3 , 277 (1951). (15) BERNAL, S. A,, AND SINCLAIR, D., J . Phys Chm., 59, (16) GREENBERG, 435 (1955). S. A., J . Polymer Sci., 27, 523 (1958). (17) GREENBERG, (18) T . L..A N D GREENBERG. S. A,. J . P ~ I I SChem.. . . . O'CONNOR. 62,1195 (1958j. (19) HURD,C. B., Chem. Rare., 22, 403 (1938). K . D., A N D INNES,W. B., Ind. Eng. Chem., 44,2857 (20) ASHLEY, IlQii7\~ - - - . ,. (21) ALEXANDER, G. B., J . A n . Chem. Soe., 7 6 , 2094 (1954). S. A,, J . Phys. Chem., 61, 196 (1957). (22) GREENBERG, \
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36, Number 5, May 1959
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