Silicic acid, defined

It is not a little remarkable that glass, which is applied so extensively and to so many purposes, and the varying properties of which exercise so marked an influence in its different applications, has, notwithstanding, never been, tho subject of a series of decisive experiments, calculated to establish the theory of its fabrication. Not that this theory has not been nearly if not wholly elucidated hut that it is scarcely to be looked for in the works published on glass. The greater part of these wero written at a period when the part which silicic acid performed was not defined or understood and among the recent works on the subject, Knapp s is the only one which disousses it in a thoroughly scientific manner. 

The model involves four basic assumptions. First, it is postulated that an adsorbed phase of silicic acid exists on the particle surface, the equilibrium of which can be expressed by a Langmuir isotherm (Equation 1, symbols are defined in table of nomenclature). 

The texture of precipitated silica-alumina depends on the texture of the silica when silica results from a sol-gel transition. The condensation of silicic acids leads to the formation of primary spherical particles (sol) which aggregate in defined conditions, forming the tridimensional network of the gel [1]. In the gel framework each primary particle of silica is connected to two or three particles [1] and the gel pores are the cavities existing between these particles [2], The size of the particles, conjugated to their connectivity, defines the surface area, the volume and diameter of the gel pores. Thus, the silica texture could be controlled by mastering the size and the packing of the silica particles [3], characteristics which depend on the conditions of preparation of silica sol and gel. 

The kinetics of the silicic acid condensation was investigated using the molybdate method introduced by Alexander [14]. Upon addition of a molybdate solution to a silicate solution, monomeric and dimeric silicic acid react to form the yellow molybdosilicate [SiMoi204o] [5]. The change in the absorption of this solution reflects the decrease in monomeric and dimeric silicic acid due to the condensation reaction. Because of the reaction equilibrium between silicic acid and the molybdate as well as between monomeric, dimeric, and oligomeric silicic acids, it is important to adhere to a well-defined, constant, and reproducible time protocol for the formation of the molybdosilicate. 

Figure 2 Depth profiles for major nufrienfs (nifrafe (Pacific only), phosphate, and silicic acid) and filterable concentrations (that passing a 0.4-nm filter) of frace nufrienf elemenfs (zinc, cadmium, nickel, copper, and manganese) in the central North Pacific (diamonds, 32.7° N, 145.0° W, Sep. 1977) and North Atlantic (squares, 34.1° N, 66.1 °W, Jul. 1979). Manganese concentrations in the Pacific were analyzed in acidified, unfiltered seawater samples. The units molkg are defined as moles per kilogram of seawater. Data from Bruland KW and Franks RP (1983) Mn, Ni, Cu, Zn and Cd in the western North Atlantic. In Wong CS, Boyle E, Bruland KW, Burton JD, and Goldberg ED (eds.) Trace Metals in Sea Water, pp. 395-414. New York Plenum.

The term "colloid is defined as matter in a state of subdivision from 1 to 100 nm in diameter (17). Silicic acid oligomers are linear and cyclic structures generally smaller then 1 nm. The smallest three-dimensional polymers having internal siloxane bonds and exterior silanol groups are about 1 nm in diameter (18) and are thus the smallest possible colloidal particles. 

The second characterizes the process demand for H at hydrolysis and little depends on the composition of alumimun silicates. It defines direct correlation of the mineral solubihty vs. solution acidity, i.e., defines the slope of curves on the left at pH < 6 in Figure. 2.54. 

Although esters of type Si(OR)4 (equation 14.66) are known, no well-defined silicic acid (H4Si04) has been established. 

A method that is said to distinguish alpha and beta from gamma silicic acid was developed by Nemodruk and Bezrogova (73a), who defined thz gamma silicic acid as that which did not react with molybdic acid reagent at 100°C in 20 min, whereas beta reacted completely. 

In order to explain the mechanism of the polymerization of silicic acid, we would have to start with the fact that the tetravalent silicon is still unsaturated coordinaiively. Both of the strongly defined secondary valences which are active in the fluoride complexes of silicon must also play a role in the hydrated oxfde. 

The conversion of a sol of spherical particles to a uniform gel coctaiuing all the liquid in the sol is not easily understood. When particles collide it is assumed that adhesion can occur but in the case of silica particles there is reason to believe that the attachment is through the formation of Si-O-Si bonds. One reason for thinking so is that the same factors that promote polymerization of moncrr.er and low molecular weight silicic acids also promote the conversion of a sol of coUoidal silica particles to a gel. Thus sols consisting of well-defined spherical particles form gel least rapidly at about pH 2 and the process is accelerated by fluoride ior.s at low pH. 

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