Silicate polysilicate

Operationally, there is a similarity between and metal ions (Lewis acids) and OH" and bases (Lewis bases). The OH group on a hydrous oxide surface has a complex-forming oxygen-donor group like OH" or OH groups attached to other elements (phosphate, silicate, polysilicate). Proton and metal ions compete with each other for the available coordinating sites on the surface ... 

Polysilicates are effective in potable HW systems and provide threshold effect protection against hardness destabilization and red water. They also function by laying down a protective calcium silicate film. Polysilicates used for this type of application typically require a dose rate of 8 to 15 ppm Si02 above that of the natural orthosilicate found in the MU water. Where polysilicates are employed for dualtemperature and LPHW systems, however, the silicate reserve required is higher and ranges from 50 to 150 ppm. 

The product of the fusion of silica with sodium carbonate, sodium silicate (strictly called sodium poly trioxosilicate but usually metasilicate), dissolves in water to give a clear, viscous solution known as waterglass . It hydrolyses slowly and silica is precipitated. Besides the metasilicate, other silicates of sodium are known, e.g. the poly-tetroxosilicate (orthosilicate), Na4Si04. Only the silicates of the alkali metals are soluble in water. Other silicates, many of which occur naturally, are insoluble, and in these substances the polysilicate anions can have highly complicated structures, all of which are constructed from a unit of one silicon and four oxygen atoms arranged tetrahedrally (cf. the structure of silica). Some of these contain aluminium (the aluminatesilicates) and some have import ant properties and uses. 

Magnesium forms an array of silicates having varying structures. Such a wide variety of silicates include metasilicate, orthosilicate, pyrosilicate, polysilicates, and a number of complex silicates, such as asbestos and talc in combination with other metal ions. Many such silicates occur in nature either as complex silicates or as discrete magnesium silicate. Some important magnesium silicates are listed below ... 

Another route for the production of materials involves the reaction of hydrolysis-condensation of metal alkoxides with water. We study here the important case of amorphous silica synthesis. In this case [38,39,44], silicic acid is first produced by the hydrolysis of a silicon alkoxide, formally a silicic acid ether. The silicic acids consequently formed can either undergo self-condensation, or condensation with the alkoxide. The global reaction continues as a condensation polymerization to form high molecular weight polysilicates. These polysilicates then connect together to form a network, whose pores are filled with solvent molecules, that is, a gel is formed [45],... 

Busey, R. H., and Mesmer, R. E. Ionization equilibriums of silicic acid and polysilicate formation in aqueous sodium chloride solutions to 300°C. J. Inor. Chem. 16, 2444-2446 (1977). 

Regardless of the t)q)e of soluble metal silicates used, they are subject to the same molecular speciation in aqueous solution resulting in a mixture of monomeric tetrahedral ions, oligomeric linear or cyclic silicate ions, and polysilicate ions. Sodium metasilicate, an example of a soluble metal silicate, can be prepared in anhydrous form or in the presence of water of crystallization as the penta- or nona-hydrate. It is readily soluble in water [8]. 

VI 0 OH, UjO, borates and polyborates, silicate and polysilicates, borosilicates and aluminosilicates, columbates, tantalates, P04 ", As04 , antimonates, antimonites, aisenites, germanates, germanites, V04 , Cr04 , Mo04 , W04, S04 , COj , NOj", selenate, selenite, tellurate, tellurite, iodate, organic hydroxyl and carboxylate... 

Many zeolites were crystallized from strongly basic solution containing silicate and aluminate. In strongly basic solution, silicate exists as polymerized-state polysilicates with a variable distribution of molecular weight. Aluminate reacted with polysilicate to form various zeolite structures via condensation reaction under hydrothermal or solvothermal conditions. Therefore, in the study of the synthetic chemistry of zeolites, one of the key issues is to understand the existing state and reactivity of the polymerized silicate. 

The state and structure of silicate ions in sodium salt solution Sodium silicate solution (e.g., water glass) is one of the most commonly used chemicals in the synthesis of zeolites, which will supply the silicon species as well as Na+ and OH. Therefore, it is important to study the polymerization state and distribution of the polysilicate ions in sodium silicate solution under hydrothermal conditions. 

In the early 1980s, 29Si-NMR was used to determine the distribution of polysilicate anions in sodium silicate solution for the first time. For example, McCormick et al. studied in detail the state and distribution of polysilicate anions in sodium silicate solution with [Si02] = 1 3 mol% and R = = 1 3.[2] A typical 29Si-NMR spec-... 

Based on analyses on the 29Si-NMR spectra of the sodium silicate solution with various concentrations of Si02 ([Si02]) and moduli R (777 ), McCormick and colleagues obtained detailed information on the state and distribution of the polysilicate anions under the following conditions, summarized in Table 5.1. 

In 1981, Harris et al. systematically studied the state and distribution of polysilicate anions in potassium silicate solution through analyses of their 29Si-NMR spectra.[3] They found that the state and distribution of polysilicate anions in potassium silicate solution are similar to those in sodium silicate solutions. Figure 5.3 shows the state and distribution abundance (the number in parentheses indicates the relative abundance of the polymerized silicate with the assumption of 100 monomers.) of 18 types of polysilicate anions existing in potassium silicate solution with a concentration of... 

Table 5.1 State and distribution of the polysilicate anions in sodium silicate solutions with different moduli (R)...

Figure 5.3 The state and distribution abundance of the polysilicate anions that exist in potassium silicate solution (0.63mot/L, K Si— 1.5 1.0)...

Their results indicated the presence of some highly polymerized silicate ions as well. According to the pre-described studies, it could be concluded that the state and distribution of polysilicate anions in both Na+ and K+ basic solutions are dependent on both [Si02] concentration and the alkalinity of the solution. Increased pH value and decreased [Si02] concentration result in an increased concentration of silicate monomer. 

In addition, Harris s studies indicated that the polymerization state of the polysilicate anions in the potassium silicate solution is similar to that in the sodium silicate solution with the same silica concentration. The difference lies in their relative distribution abundance. 

Figure 5.5 Distribution of the state of polysilicate anions in TMAS solution with various TMA/ Si02 ratios. Note Mono means the monomer of the silicate ions Di means the dimer of the polysilicate anions D4R means the double 4-membered ring octa-polysilicate. Reproduced from [5] with permission of Wiley-VCH...

Hoebbel et al. carefully studied the state of polysilicate anions in TBAS solution.181 Their results indicated that there mainly existed monomeric, dimeric, trimeric, tetra-meric, pentameric, and heptameric silicate ions and poly silicate anions in the form of double 3-, 4-, and 5-membered rings in TBAS solution. Compared with the TMAS and TEAS solutions that mainly contain double 3- and 4-membered-ring-type silicate ions under some conditions, the distribution of these species in TBAS solution is quite even. However, the distribution of these species in solid crystalline structures is quite different to that in solution. It was found that there are a large number of double 5-membered rings in the solid crystalline structure obtained from the corresponding solution. Studies indicated that there are some similarities between the properties of TPAS and TBAS solutions. Owing to space limitation, studies on TPAS solution will not addressed here. 

Many factors can affect the polymerization process of silicate anions. The most important one is the acidity of the solution. The polymerization state of polysilicate anions and the polymerization process of SiC>2 strongly depend on the pH value of the solution. Her systematically studied the polymerization process and the corresponding mechanism of silicate anions in acidic, neutral, slightly basic, and strongly basic solutions.[11]... 

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