Precipitation with polar organic substances

Selective Precipitation with Polar Organic Substances... 

Starch granules are composed of two different polysaccharides, amylopectin and amylose the former constitutes about 80 % of the most common starches. Separation of the two components can be achieved by selective precipitation involving the formation of an insoluble complex of amylose with polar organic substances. 

The instability of starch sols can be traced primarily to the presence of the linear A-fraction. The adsorptive affinity of this component is manifest in all the reactions of starch pastes and sols. It is responsible for retrogradation, the blue coloration with iodine, and the adsorption of various polar materials. The latter reaction has been utilized to effect a quantitative precipitation of the A-fraction. Thus, when a starch sol is treated with polar organic substances containing a hydrophilic group (such as hydroxyl or carboxyl) attached to a hydrophobic residue, the A-fraction adsorbs this material by polar attraction for the hydrophilic group. The resulting adsorption complex is insoluble by reason of its hydrophobic loading and consequently separates from solution. 

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. 

Crystals of an inorganic substance can often be grown by making a hot, saturated solution of the substance and then slowly cooling it. Polar organic compounds can sometimes be crystallized by similar procedures or by slow precipitation from aqueous solutions by addition of organic solvents. If you work with proteins, just the mention of these conditions probably makes you... 

In toxicology, body fluids such as blood and urine can be directly extracted using liquid-liquid extraction. This is because these samples can be easily partitioned with an organic solvent without protein precipitation (however, in some laboratories, protein precipitation will be carried out on blood samples prior to LLE) but after pH adjustment. The solvent that is used in the extraction will be chosen based on its polarity and the solubifity of the substances extracted within that solvent. 

Organic solvents have long been used for extraction and sequential extraction, which is fractionation of a sort (Flaig et al., 1975 Schnitzer, 1978). While the direct use of organic solvents in fractionation has not been widespread, nonetheless, the technique has received some attention. For instance, the separation of hymatomelanic acid from precipitated humic acid is obtained by extraction with ethanol (Oden, 1919). Ethanol has been used to bring about fractional precipitation by addition to alkaline solutions of humic acid (Kyuma, 1964 Kumada and Kawamura, 1968). There is no reason why other water-miscible solvents such as acetone and methanol should not be used in this way. Solvents that are highly immiscible with water (e.g., hexane and benzene) do not appear to remove any substantial fraction of humic substances. These are perhaps best used to remove nonhumic substances (such as fats and waxes) prior to extraction. However, recent work by Allen and MacCarthy (personal communication) has shown that more polar water-immiscible solvents, such as methyl isobutyl ketone and diethyl ether, can be used successfully to purify and fractionate humic substances. 

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