Silica, generally water samples

Mineral Oil Hydraulic Fluids. Methods are available for analysis of the hydrocarbon components of mineral oil hydraulic fluids (predominantly straight and branched chain alkanes) in environmental samples. Some of these methods are summarized in Table 6-3. In general, water and sediment samples are extracted with a suitable solvent in a Soxhlet extractor (for solid samples) or in separatory funnel or shake flask (for liquid samples) (Bates et al. 1984 Peterman et al. 1980). The extract is cleaned up on silica gel or Florisil columns using a nonpolar solvent to elute the nonpolar alkanes. Analysis is usually performed by GC/MS (Bates et al. 1984 Kawamura and Kaplan 1983 Peterman et al. 1980). Method performance has not been reported, although 82% recovery of aliphatic hydrocarbons was reported for rainwater (Kawamura and Kaplan 1983). 

Basic tests, such as hardness, alkalinity, chloride, silica, and phosphate, can generally be undertaken with low-priced simple equipment. The individual methods normally incorporate suitable protocols to minimize interferences. Thus, almost any equipment and any test method can be selected that meets the rule of fitness-for-purpose. (Typically this is a mix of portability, ease of testing, and speed of handling multiple water samples, combined with accuracy, suitable detection limits, price of equipment and replacement reagents, and service company testing protocol uniformity.)... 

Alkalinity. The alkalinity of a water sample is its acid-neutralizing capacity. Bicarbonate and carbonate ions are the predominant contributors to alkalinity in most waters, and their chemical equilibria generally maintain the pH of 5—9. The presence of enough hydroxide ion to affect the alkalinity determination in natural waters is rare. Silica, borate, or phosphate do contribute to the overall alkalinity if present in large enough quantities. 

The problem of analyzing water samples is essentially no different from that of analyzing aqueous solutions in general. You are referred to Ref. 4 for a compilation of some of the many commonly analyzed substances in water and the procedures employed for their analysis. Measurements made include acidity or alkalinity, biochemical oxygen demand, carbon dioxide, chlorine, dissolved oxygen, electrical conductivity, fluoride, particulate and dissolved matter, ammonia, phosphate, nitrate, silica, sulfate, sulfite, sulfides, turbidity, various metal ions, bacteria, microorganisms, and so forth. 

Para-substituted, X = H, CH3, OCH3, Cl, and NO2, dithioacetal derivatives were synthesized and chemically immobilized onto the surface of silica gel for the formation of five newly modified silica gel phases. The metal sorption properties of these silica gel phases were studied and the results revealed a general rule of excellent affinity for the selective extraction of Hg(II) in the presence of other interfering metal ions, giving rise to a range of 94-100% extraction of the spiked Hg(II) in the metal ion mixture. The potential applications of these modified silica gel phases for selective extraction of Hg(II) from two different natural water samples by a column technique, followed by cold vapor atomic absorption analysis, were also studied. The substituent effect on the process of selective extraction of Hg(II) by the modified silica gel phases was also presented. The structures of silica gel-immobilized-dithioacetals are given in Scheme 19. 

Therefore, the online approach is generally preferred to the off-line mode. Online SPE has been applied to the determination of phenols in water samples using small precol-mnns with different adsorbents such as octadecyl-bonded silica [50,133-135], styrene-divinylbenzene copolymers PLRP-S or PRP-1 [133-145], and graphite carbon [134,146]. Recently, Cis and PSDVB extraction disks have been applied to the off-line extraction of phenols from water samples [147-149]. 

Allophane and halloysite were the subject of detailed consideration by Ross and Kerr [1934]. They employed optical. X-ray, thermal dehydration, and chemical analysis to investigate a number of selected samples of allophane from different localities. They found wide ranges of chemical compositions (Si02, 25.19 to 33.96% AI2O3, 30.41 to 36.53% H20 , 12.84 to 21.20% H20, 14.43 to 20.28%) and of indices of refraction (1.472 to 1.496). From these results and from those of other publications they concluded that allophane is an amorphous material commonly associated with halloysite. It has no crystal structure and no definite chemical composition. The name allophane should be restricted to mutual solid solutions of silica, alumina, water, and minor amounts of bases, but the specimen should include all such materials, even though the proportions of these constituents may differ. This broad definition by Ross and Kerr of the range of properties of allophane is generally accepted. 

Both ascending and descending paper chromatographic techniques have been used and, when thin-layer supports are employed, the use of either silica gel or cellulose is applicable. As the number of carbohydrates present in the sample is often small, the careful choice of solvent will generally make it unnecessary to perform the two-dimensional separations that are often needed when large numbers of substances, such as amino acids, are present. Reference solutions of each carbohydrate can be made up in concentrations of approximately 2 g 1 1 dissolved in an isopropanol solvent (10% v/v in water) and samples of about 10 fx should give discernible spots after separation. 

It is generally accepted that the only important polar adsoiption sites on the silica surface are the silanol functions, i.e., hydroxyl groups, that are attached to silicon atoms (2). They can interact with the sample molecules by hydrogen bonding and various physical observations ctAi be used to prove this statement. Complete dehydration of silica by beating, i.e., removal of all surface hydroxyl groups, yields a hydrophobia silica which no longer shows adsorption for unsaturated and polar molecules and is no more wetted by water (15). Chemical modification of the surface hydroxyls such as used in the preparation of chemically bonded phases also eliminates the selective adsorption properties of the silica. ... 

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