Insoluble substances, definition

Langmuir from his study of the films of insoluble substances upon water (see Ch. ill) concluded that the film was in these cases but one molecule thick and that the molecules in the film were definitely orientated. 

To confuse matters, the solubility of a substance is often defined on its ability to go into solution at 25 °C (i.e. around room temperature). On the basis of this definition, whereas soluble substances can form a 0.10 to 1M solution at 25 °C, insoluble substances cannot form a 0.10 to 1M solution at 25 °C. 

Reactions between solids and solutions may be treated in a similar manner. Every solid has a definite solubility in every solvent, just as it has a definite vapour pressure at a given temperature. The solubility depends on the nature of the solvent and on the temperature, but is independent (in dilute solutions at least) of the presence of other dissolved or undissolved substances. So-called insoluble substances differ from soluble substances only in that their solubility is exceedingly small. 

McBain and Hutchinson (1) were the first to give a definition of solubilization, i.e. the increase in the solubility of an insoluble or slightly soluble substance in a given medium. This phenomenon involves the presence of surfactant molecules or colloidal particles in a single phase which will totally incorporate the insoluble substance. 

Humic substances in sediments and soils have commonly been, defined as heteropolycondensates of decomposing plant and animal detritus 46. For lack of a better structural definition, these macromolecular substances have been divided into three categories fulvic acids and humic acid and humin. Fulvic acids and humic acids are soluble in dilute alkaline solutions, whereas humin is insoluble. 

Organic matter extracted from earth materials usually is fractionated on the basis of solubility characteristics. The fractions commonly obtained include humic acid (soluble in alkaline solution, insoluble in acidic solution), fulvic acid (soluble in aqueous media at any pH), hymatomelamic acid (alcohol-soluble part of humic acid), and humin (insoluble in alkaline solutions). This operational fractionation is based in part on the classical definition by Aiken et al. (1985). It should be noticed, however, that this fractionation of soil organic matter does not lead to a pure compound each named fraction consists of a very complicated, heterogeneous mixture of organic substances. Hayes and Malcom (2001) emphasize that biomolecules, which are not part of humic substances, also may precipitate at a pH of 1 or 2 with the humic acids. Furthermore, the more polar compounds may precipitate with fulvic acids. 

NOM is common in sediments, soils, and near ambient (<50 °C) water. The materials result from the partial decomposition of organisms. They contain a wide variety of organic compounds, including carboxylic acids, carbohydrates, phenols, amino acids, and humic substances (Drever, 1997, 107-119 Wang and Mulligan, 2006, 202). Humic substances are especially important in interacting with arsenic. They result from the partial microbial decomposition of aquatic and terrestrial plants. The major components of humic substances are humin, humic acids, and fulvic acids. By definition, humin is insoluble in water. While fulvic acids are water-soluble under all pH conditions, humic acids are only soluble in water at pH >2 (Drever, 1997, 113-114). 

Humin Naturally occurring, organic-rich substances of biological origin that are common in the A horizons of soils. By definition, humin is insoluble in water under all pH conditions (Drever, 1997), 113 (compare with humic acid and fulvic acid). 

The product is hydrated rhodium trichloride, which, according to Claus 6 contains eight molecules of water. Leidie,1 on the other hand, concluded that the amount of water varies and does not correspond to any definite hydrate. It is an amorphous, briek-red, deliquescent substance which, on heating to 90-95° C., still retains four to five molecules of water and two of hydrogen chloride. At 100° C. it loses water and hydrogen chloride simultaneously, and at 175-180° C. it is completely dehydrated. At 360° C. it becomes insoluble in water, hut it is most... 

Soluble substances, when brought into solution, may show slight physical impurities, such as fragments of filter paper, fibers, and dust particles. Unless excluded by definite tests or other requirements however, significant amounts of black specks, metallic chips, glass fragments, or other insoluble matter are not permitted. 

Researchers have devised numerous extraction and fractionation schemes to deal with the heterogeneous nature of humic substances. Traditionally, the operational definition of humic substances as used by the International Humic Substances Society (Hayes et al., 1989) is based on the solubility in a series of acids and bases. In this scheme, humic substances are classified into three chemical groupings (1) fulvic acid, soluble in both alkali and acid solutions, has the lowest molecular weight and is generally considered the most susceptible to microbial degradation (2) humic acid, soluble in alkali but not in acid, is intermediate in molecular weight and decomposability and (3) humin, insoluble in both alkali and acid solutions, is the most... 

Diphenylyl-o-arsinic acid.— This is isolated by coupling sodium arsenite with diazotised o-aminodiphenyl in alkaline solution at 50° to 00° C. in the presence of a cupric salt. Yield, about 60 per cent. It crystallises from boiling water in bristle-like needles, M.pt. 206° C. When reduced in warm concentrated hydrochloric acid by sulphur dioxide in the presence of an iodide, it yields diphenylyl-o-dichloroarsine, a heavy oil, soluble in chloroform, insoluble in water. The chloride, with alcoholic potash, yields the oxide, an amorphous substance with no definite melting-point. 

Solubilization can be defined as the preparation of a thermodynamically stable isotropic solution of a substance normally insoluble or very slightly soluble in a given solvent by the introduction of an additional amphiphilic component or components. The amphiphilic components (surfactants) must be introduced at a concentration at or above their critical micelle concentrations. Simple micellar systems (and reverse micellar) as well as liquid crystalline phases and vesicles referred to above are all capable of solubilization. In liquid crystalline phases and vesicles, a ternary system is formed on incorporation of the solubilizate and thus these anisotropic systems are not strictly in accordance with the definition given above. 

Biochemists have found it convenient to define one set of biomolecules, the lipids, as substances, insoluble in water, that can be extracted from cells by organic solvents of low polarity like ether or chloroform. This is a catch-all sort of definition, and lipids include compounds of many dilferent kinds steroids (Sec. 15.16), for example, and terpenes (Sec. 8.26). Of the lipids, we shall take up only the fats and certain closely related compounds. These are not the only important lipids— indeed, every compound in an organism seems to play an important role, if only as an unavoidable waste product of metabolism—but they are the most abundant. 

Humic substances are those organic compounds found in the environment that cannot be classified as any other chemical class of compounds (e.g., polysaccharides, proteins, etc.). They are traditionally defined according to their solubilities. Fulvic acids are those organic materials that are soluble in water at all pH values. Humic acids are those materials that are insoluble at acidic pH values (pH < 2) but are soluble at higher pH values. Humin is the fraction of natural organic materials that is insoluble in water at all pH values. These definitions reflect the traditional methods for separating the different fractions from the original mixture. 

The factors which lead to the formation of emulsions are not definitely known. The most permanent emulsions are formed when an insoluble oil is shaken with a solution which contains a substance that interacts with one of the constituents of the oil to produce a colloid. This occurs when an oil containing free fatty acids is shaken with an aqueous solution of an alkali. A layer of soap is formed around the particles of the oil, and it is probable that a layer of oil may surround the colloidal partides of soap. When a solution of egg albumin is shaken with olive oil, a layer of the coagulated protein is formed around the drops of the oil and emulsification takes place. 

Data in Table 7 show that acetonitrile, dioxane, ethanol, and water are very poor solvents for humic acids. Humic acids are, by definition, insoluble in water, although traces of the H -exchanged substances are invariably dissolved in it. Similar trace amounts were dissolved in ethanol. Pyridine and formic acid (90%) were also poor solvents for humic substances. The extent of dissolution in pyridine was less than would be predicted from the data quoted for the H -exchanged soil in Table 4. However, the Glade humic... 

The classic definitions of soil humic and fulvic acids are based on solubility (Schnitzer and Khan, 1972). Thus, humic acid is the alkali-soluble material in soil, which is precipitated at pH 1. The material which remains soluble in the extract at pH 1 is fulvic acid. A more recent definition for aquatic humic substances is given by Thurman and Malcolm (1981). Here the material which adsorbs on an XAD column from an acid aqueous solution is defined as aquatic humus. That part of the adsorbed material which is soluble in acid and base is fulvic acid the portion insoluble in acid is humic acid. Another definition of an aquatic humic substance is based on adsorption by DEAE-cellulose columns (Miles etal., 1983). 

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