Silicate ions, reactions

Scale formation Controlled scale deposition by the Langelier approach or by the proper use of polyphosphates or silicates is a useful method of corrosion control, but uncontrolled scale deposition is a disadvantage as it will screen the metal surfaces from contact with the inhibitor, lead to loss of inhibitor by its incorporation into the scale and also reduce heat transfer in cooling systems. Apart from scale formation arising from constituents naturally present in waters, scaling can also occur by reaction of inhibitors with these constituents. Notable examples are the deposition of excess amounts of phosphates and silicates by reaction with calcium ions. The problem can be largely overcome by suitable pH control and also by the additional use of scale-controlling chemicals. 

Modem production of elemental phosphoras uses a technique similar to the metallurgical processes described in Chapter 20. Apatite is mixed with silica and coke and then heated strongly in the absence of oxygen. Under these conditions, coke reduces phosphate to elemental phosphoms, the silica forms liquid calcium silicate, and the fluoride ions in apatite dissolve In the liquid calcium silicate. The reactions are not fully understood, but the stoichiometry for the calcium phosphate part of apatite is as follows ... 

Ba2+wlll be very fast while the silicate ion will diffuse very slow (if at all). Because of the vast differences in the types of diffusing species, there is no reason to expect all of them to diffuse at the same rate, particularly when we compare electrons and vacancies. Actually, this aspect of solid state reaction has been studied in great detcdl and the Kirchendall Effect deals with this aspect. 

Some of the many silicates that are readily available have silicate ions with frameworks that are similar to or the same as those present in common alkyl silicates and common organosiloxanes (5,7). In light of this, a synthetic approach to alkyl silicates and organosiloxanes based on substitution reactions becomes conceivable. 

In hard water, however, the presence of small amounts of carbonate, sulfate, or silicate ions form a protective film on the metal surface, and prevent the occurrence of the above reaction and thus, corrosion of the metal. 

Calcium Silicates Hydrates (CSH) is the major hydration product of Portland Cement. It has been intensively studied for several decades. CSH can be obtained by hydration of C3S and P-C2S or by precipitation fi om aqueous solutions containing a Ca salt and silicate ions. CSH has a wide range of chemical composition. Many studies [1-5] indicate that its molar ratio Ca/Si can vary fixrm 0.7 to 2 (or more) [6] and is in most cases near to 1.7 [7]. There has been indirect evidence that CSH (obtained by reaction between CaO and Silica) has a layered structure similar to that of natural Tobermorite and/or Jennite [8]. Recent solid state NMR [9-10], intfared spectroscopy [11], EXAFS [12] and thermogravimetry works [14] have... 

In deeper systems dominated by calcium-rich saline fluids, it has been shown that both solubility constraints and silicate reactions act to further remove bicarbonate ions as precipitated calcium and magnesium carbonates, often adjusting pH to levels greater than 9 (Barnes and O Neil, 1971 Fritz et al., 1987a Clauer et al., 1989). For example, during closed-system dissolution of magnesium olivine (forsterite), a major component of many ultramafic rocks, as the silicate water reaction proceeds water breaks down, H" " ions are consumed, carbonates precipitate, and hydroxyl ions force the pH to rise (Barnes and O Neil, 1971 Drever, 1988). 

Chelants at concentrations of 0.1% to 0.2% improve the oxidative stability through the complexation of the trace metal ions, e.g., iron, which catalyze the oxidative processes. Examples of the chelants commonly used are pentasodium diethyl-enetriaminepentaacetic acid (DTPA), tetrasodium ethylenediaminetetraacetic acid (EDTA), sodium etidronate (EHDP), and citric acid. Magnesium silicate, formed in wet soap through the reaction of magnesium and silicate ions, is another chelant commonly used in simple soap bars. 

Many productive methods have been developed for the preparation of silica sol including acidification/121 electrolysation-electrodialysis,[13] ion-exchange,[14] peptization/111 and hydrolysis of silicon compounds/101 which can be grouped into two main types. One is called the aggregation method that contains two steps the polymerization of silicate ions and the aggregation of these polysilicate anions via condensation reaction between the hydroxy groups of the particles. The other one is called the peptization method, i.e., dispersal of a precipitate of Si02 to form colloid. The acidification method will be discussed in detail below. 

Silica Precipitation [5.1,5.31,5.32]. Silicate ions may be precipitated by addition of aluminum sulfate or magnesiiun sulfate solution to the sodium tungstate solution. Also, mixtures of both reagents are used. The addition is made to the slightly alkaline or neutral solution. The respective chemical reactions are complex but can be described in a simplified matmer by the following equations ... 

For the silicate ions of reduced concentration Csid = Csi/Cinj, the stoichiometry of the reaction 10.27 implies that (Somerton and Ranke, 1983)... 

This dependence can be explained in terms of the polycondensation mechanism of formation of aluminosilica gel skeleton from Al(OH)4 hydroxoaluminate ions and (0H)4 mSi(0 )m silicate ions with different degrees of hydroxylation. At equal correlations and concentrations of the silicate and aluminate ions in solutions, the amount of silicate ions participating in the reaction of condensation should increase with the growth of their hydroxylation degree. The degree of hydroxylation of the silicate ions in alkaline solutions is determined by Equilibrium 1 and therefore it must increase with decrease in NaOH/Si02 ratio (= m) and with dilution. 

The reaction is thought to proceed via hypercoordinated silicate ions [112] ... 

The application of the formation of trimethylsilyl derivatives to the study of silicate structures has been discussed with particular reference to the sodium silicate hydrates Na2H2Si04, H20 ( = 4, 5, and 8) and the mineral hemi-morphite, Zn4(0H)2Si207,H20. The technique is based on the extraction of silicate ions from crystalline and amorphous solids as their stable trimethylsilyl derivatives followed by separation by g.l.c. Although it was found necessary to control the conditions of the trimethylsilylation reaction very carefully, it was concluded that the method is universally applicable to all silicate structures, including those soluble in water. 

The mechanism of molecular deposition of SiO from Si(OH) is apparently the reverse of dissolution of solid silica. It involves a condensation reaction catalyzed by hydroxyl ions and accelerated by the presence of salts. The process therefore occurs principally above pH 7, since it is catalyzed by hydroxyl ion, but obviously not above pH 11 where silica dissolves as silicate ion. Deposition is more rapid and condensation and dehydration of the silica are more complete in hot solution. 

The history and use of this reaction in analyzing for silica is discussed in detail in Chapter 1 and its application in characterizing silicate ions in Chapter 2. 

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