Silica, generally

The thickening mechanism of precipitated silicas is similar to that of the fumed silicas. Precipitated silicas have more silanol groups than fumed silicas, but they have a lower concentration of silica (precipitated silicas generally contain some sodium sulphate). 

Silica—Generally, limit silica to 150 ppm as SiOg to prevent silica scale. 

Ground silica, generally available below 300 mesh in size (5 m2/g), is used as a cheap heat resistant filler for a variety of compounds. There is no effect on the rate or state of cure. 

As an example of this complexity, the fluorescence decays of Py on dry silica generally can only approximately be fitted with... 

Talc brightness usually ranges in the mid 80s and is not typically controlled through specific processing. Finally, titanium dioxide and precipitated silica generally exhibit ISO brightness values greater than 97. 

Cabot Corp., Untreated firmed silica general application guide. Technical Bulletin at www.cabot-corp.com. 

Free silica n. Si02- Silica generally present in small amounts in natural deposits of claylike minerals and diatomaceous earth and usually considered to be a contaminant. 

Colloidal solutions in which the particles are sufficiently small to be more soluble than massive amorphous silica generally contain 110-150 ppm SiO, as monomer, and if stabilized at about pH 9, also HSiO, ions. 

Thus it is possible to state that from a practical standpoint, amorphous silicas generally do not cause silicosis. Also, it appears that even w.hen allowance is made for differences in particle size, the quartz surface (and especially tridymite) is more fibrogenic than amorphous silica. 

It has been obvious for many years that clay dusts and other aluminosilicates do not cause silicosis. Therefore, when alumina is administered along with silica there must be some combination of the oxides on the surfaces to create harmless surfaces of aluminosilicates. This type of exchange has been dealt with in earlier chapters. As shown from tier s work in Chapter 4, the creation of aluminosilicate sites on the surface of silica generally reduces the tendency of proteins to be absorbed from neutral solution. 

The most common stationary phase used in HPLC is chemically modified silica. Generally, octadecyl (Cig) phases are used in the determination of DFAA. Spherical 5 /um resins are commonly employed with standard column dimensions of 250x4 nun. Columns of smaU-er dimensions may be used as well, but the resin particle diameter should then be 3 /an. This configuration minimizes solvent consumption. A guard column should be used to protect the analytical column from small particles as present in unfiltered samples and polymeric material and thus extends the lifetime of the analytical column. 

The speciation of silica generally depends on pH. Figure 7.5 shows the distribution of silicic acid and silicate as a function of pH. As the figure shows, the pK of silicic acid is 9.84. This means that at pH 9.84, the concentration of silicic acid and silicate compounds is the same, so a 50/50 blend of the compounds coexist. Hence, at the pH of most RO systems, generally 7-8.5, the form of silica present is primarily sihcic add. 

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