Definitions and solvent classification

In modern terms, asphaltene is conceptually defined as the n-pentane-insoluble and benzene-soluble fraction whether it is derived from coal or from petroleum. There are a number of procedures used to isolate asphaltene (2-7), all of which appear to be reproducible (8) but do not necessarily provide equivalent end-products. The similarity between coal- and petroleum-derived asphaltenes begins and ends at the definition of the separation procedure. Puzinauskas and Corbett s (9) comments on asphalt may be paraphrased and applied to asphaltene. They state that the broad solvent classification is unfortunate it leads to misconceptions that petroleum and coal materials are alike, or at least similar. However, these two classes of materials differ not only in their origin, mode of manufacture and uses, but also in their chemical composition and physical behavior. 

This function is used to study solvent effects and for classification of solvents. Moreover, the dielectric constant enters the definition of the molar refractivity. 

A review of the selected definitions suggests that there are many important determinants of solvent quality for specific apphcation. Some solvent parameters are conflicting, some not well quantified, and each solvent application requires a unique set of solvent performance criteria. It can be thus anticipated, prior to any analysis, that the chemical structure can be used as the best means of solvent classification for any application. Such a classification is used in this book because of its broad application. Chemical names used are the common names beeause they are generally understood by all solvents users. 

ICH Q3A(R) has classified impurities as organic impurities, inorganic impurities, and residual solvents. This classification system provides a useful framework for the discussion of impurities and serves to create fixed and consistent definitions for impurities in pharmaceutical substances. 

Section 2.3 describes various methods for the characterization and classification of mobile and stationary phases for chromatography. In section 2.3.1 the solubility parameter is introduced as a quantitative definition of the word polarity . Section 2.3.2 describes the characterization of GC stationary phases according to Rohrschneider and section 2.3.3 the classification of LC solvents according to Snyder. The applicability of the different methods is summarized in section 2.3.4. 

While most of the solvents of interest here are aprotic, in the sense defined in the introduction, they may be further classified with regard to their solvent type. Kolthoff [4] has categorized these solvents in a useful way. Only the classifications useful for nonaqueous studies have been included below. That is, the protic solvents (amphiprotic solvents in Kohltoff s terminology) are not included in this chapter. The classes and definitions are as follows ... 

If the classification of lipides were to be examined, some points would have to be made clear. In the first place, a definition of lipides should be made that excludes esters of mineral acids such as orthophosphoric acid. The phosphoric esters of alcohols and phenols are alkalino-stable—excluding those possessing a free carbonyl group moreover, they are not very soluble in organic solvents. The definition of lipides should therefore take into account the hydrolysis of ester linkage by heating with alkalies and the solubility of lipides in some organic solvents. 

If the sponge is left to dry in the sun, this adsorbed water will evaporate, leaving only a small proportion of water bound chemically to the salts and to the cellulose of the sponge fibers. Like water in sponge, water is held in food by various physical and chemical mechanisms (Table 3.1). It is a convenient oversimplification to distinguish between free and bound water. The definition of bound water in such a classification poses problems. Fennema (1985) reports seven different definitions of bound water. Some of these definitions are based on the freezability of the bound component, and others rely on its availability as a solvent. He prefers a definition in which bound water is that which exists in the vicinity of solutes and other non-aqueous constituents, exhibits reduced molecular activity and other significantly altered properties as compared with bulk water in the same system, and does not freeze at -40"C."... 

Although the definitions can be quite unambiguous, experimental classification into contact or solvent separated, or inner and outer sphere ion pairs most certainly is not unambiguous, and may even, at best, be only a guess. This is exactly the same problem as is encountered when discussing the formal and experimental distinctions between complexes and ion pairs. 

Solvents can be classified as polar, non-polar and apolar, with the definitions being as above. However, a more general classification uses the relative permittivity. There are high relative permittivity solvents such as (at 25°C) ... 

Lipids are defined as water-insoluble compounds extracted from living organisms by weakly polar or nonpolar solvents. The definition is based on a physical property in contrast to the definitions of the other basic building blocks in this chapter (proteins, carbohydrates, and nucleic acids), which are based on chemical stracture. Consequently the term lipid covers a stmcturally diverse group of compounds including/afiy acids, glycerolipids, sphingolipids, terpenes, steroids, and carotenoids. There is no universally accepted scheme for their classification. 

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