Silica, ionization

Hydrated amorphous silica dissolves more rapidly than does the anhydrous amorphous silica. The solubility in neutral dilute aqueous salt solutions is only slighdy less than in pure water. The presence of dissolved salts increases the rate of dissolution in neutral solution. Trace amounts of impurities, especially aluminum or iron (24,25), cause a decrease in solubility. Acid cleaning of impure silica to remove metal ions increases its solubility. The dissolution of amorphous silica is significantly accelerated by hydroxyl ion at high pH values and by hydrofluoric acid at low pH values (1). Dissolution follows first-order kinetic behavior and is dependent on the equilibria shown in equations 2 and 3. Below a pH value of 9, the solubility of amorphous silica is independent of pH. Above pH 9, the solubility of amorphous silica increases because of increased ionization of monosilicic acid. 

Figure 12.7 Cliromatograms of a polycarbonate sample (a) microcolumn SEC ti ace (b) capillary GC ti ace of inti oduced fractions. SEC conditions fused-silica (30 cm X 250 mm i.d.) packed with PL-GEL (50 A pore size, 5 mm particle diameter) eluent, THE at aElow rate of 2.0ml/min injection size, 200 NL UV detection at 254 nm x represents the polymer additive fraction ti ansfeired to EC system (ca. 6 p-L). GC conditions DB-1 column (15m X 0.25 mm i.d., 0.25 pm film thickness) deactivated fused-silica uncoated inlet (5 m X 0.32 mm i.d.) temperature program, 100 °C for 8 min, rising to 350 °C at a rate of 12°C/min flame ionization detection. Peak identification is as follows 1, 2,4-rert-butylphenol 2, nonylphenol isomers 3, di(4-tert-butylphenyl) carbonate 4, Tinuvin 329 5, solvent impurity 6, Ii gaphos 168 (oxidized). Reprinted with permission from Ref. (14).

I Most of the GC conditions given in this book are for 0.25-mm ID columns, but 0.32- or 0.53-mm ID columns also can be used. The wide bore fused silica columns are found to be more inert, probably because of the greater film thicknesses. A splitter arrangement with a jet separator is used with 0.53-mm ID columns. This arrangement shown in Figure 11.1 has the advantage of simultaneous flame ionization quantitation. 

Where-high purity MU is provided for higher pressure WT boiler plant FW, some form of continuous analyzers for measuring treated water pH and conductivity are almost always installed, as are sodium (Na) ion-selective electrodes for detection of sodium leakage. Automatic online silica analyzers also may be installed, but they measure only reactive (ionizable) silica (Si02), not colloidal or total silica, so caution is required where unforeseen silica leakage may present a problem. 

Ionic silica is not totally removable by DI. Colloidal silica is difficult to remove by both DI and reverse osmosis (RO) it may cause some resin fouling as well as leaking into the treated water. Where the cation effluent is maintained at a pH of 2.0 to 3.0, however, silica tends to both depolymerize and ionize thus enabling its effective removal in strongly basic, anion resin beds. 

The purity of 1 and 2 is assessed by analytical gas-liquid chromatography (GC) on a Hewlett-Packard 5890 gas chromatograph equipped with a flame-ionization detector and fitted with a 50 m x 0.2 mm HP-5 fused silica glass capillary column using linear temperature programming from an initial temperature of 150°C for 5 min to a final temperature of 200°C for 10 min at a rate of 5°C/min. 

Sodium dodecyl sulfate present in hydrophilic ointments has been determined by TLC on silica gel with flame ionization detection, which was considered better than the colorimetric method. TLC is preferred to HPLC in this case due to the low sensitivity of the refractive index detector that makes difficult the analysis of small amounts of sodium dodecyl sulfate [284]. 

The primary method for detecting methyl parathion and metabolites in biological tissues is gas chromatography (GC) coupled with electron capture (BCD), flame photometric (FPD), or flame ionization detection (FID). Sample preparation for methyl parathion analysis routinely involves extraction with an organic solvent (e g., acetone or benzene), centrifugation, concentration, and re suspension in a suitable solvent prior to GC analysis. For low concentrations of methyl parathion, further cleanup procedures, such as column chromatography on silica gel or Florisil are required. 

Chromatographic Characterization of TTXs. The vast majority of reports have identified TTX and anhydro-TTX in bacterial cultures using HPLC, TLC, and GC-MS. Yasumoto et al. (30) showed that TTX-like substances extracted from a Pseudomonas sp. culture could bind to activated charcoal at pH 5.5 and be eluted with 20% ethanol in 1% acetic acid. In addition, HPLC analysis demonstrated TTX and anhydro-TTX-like fluorophors following strong base treatment. These compounds migrated on silica gel comparably to TTX and anhydro-TTX. Furthermore, when analyzed by electron ionization (EI)-MS and fast atom... 

Reaction products were analyzed by on-line gas chromatography with a Shimadzu GC-14A gas chromatograph equipped with a 50 m CP Sil-5 fused silica capfllary column and a flame ionization detector. Reaction intermediates were identified by GC-MS. Samples were taken after 50 h on stream when the activity of the catalyst was stable, with n-nonane and n-dodecane as internal standards. Space time was defined as t = e Voat/vgas, where e is the void fraction of the... 

CUSO4, HgCli Fresh water Silica gel G 0.1-1% Identification of ionized 72... 

Fraczkiewicz, R. In silica prediction of ionization. In ADME/Tox Approaches,... 

Chlornitrofen in water samples was determined by GC/MS using an SPB-1 fused-silica capillary (15 mx 0.53-mm i.d., 0.5-pm film thickness), with the ionization... 

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