Solvent selection extraction, solutes from water

Values of the predicted j8 are plotted against Xca in Fig. 3.17 as a curve, together with calculated from the tie-line data of Beech and Glasstone (loc. cit.) by Eq. (3.101). Agreement is seen to be very satisfactory, and since 0 is greater than unity, ethyl acetate is a selective solvent for extracting ethanol from its water solutions. 

On the basis of the considerations discussed in this chapter, determine the relative desirability of the following solvents for selectively extracting acetone from dilute water solutions at ordinary temperatures. a. Chloroform... 

Extraction is an essential step when analyzing solid samples. In some cases homogenization with a solvent suffices, but in others the sample must first be coimninuted. Water, solutions of acetic acid or sodium chloride, or more complex saline solutions are used as solvents. Mixtures of water and methanol or water and ethanol are also employed. The choice of solvent depends on the degree of selectivity desired in the extraction and whether the extraction yield is intended for quantitative analysis. Optimization of the extraction procedure is required in all cases, to fit the nature of the sample to be analyzed and the range of molecular weights of the peptides to be separated. For example, water has been used as the extraction solvent for cheese (33) and legumes (34). Saline solutions have been utilized to extract peptides from meat (35-38) and flour (39,40). Benedito de Barber et al. (41) examined differences in the extractability of amino acids and short peptides in various solvents (1M acetic acid, 70% ethanol, and distilled water) they concluded that extraction with 1M acetic acid yielded the maximum amino acid and peptide contents. 

Caro et al. made a noncovalent and a semicovalent MIP, respectively, for 4-nitrophenol (4-NP). Their purpose was to extract selectively 4-NP from river water in the presence of ten other phenolic pollutants. The MIPs were used in an online MISPE procedure employing dichloromethane as wash solvent. Figure 4 (Fig. 1 of the original) shows the experimental setup. The pump on the left is used to deliver the conditioning solutions, the sample, and the wash solvent. The two pumps on the right deliver the organic and the aqueous components of the HPLC... 

Electrospray Ionization/Mass Spectrometry (ES/MS), a soft-ionization desorption technique using polar solvents such as water, methanol, or acetonitrile, was used for direct measurement of cations in solution. The first measurements carried out with mono or bis(crown-6) calix[4]arenes from an equimolar cation-extractant solution confirm that the calixarenes mono(crown-6) extract only one cesium cation. On the contrary, in the same conditions, bis(crown-6) calix[4]arenes can extract two cesium cations for a ratio Cs/BC6 equal to 2.5. The binuclear complex (composed of two cesium cations) is the major species. Cesium/sodium selectivity measurements implementing various mono or bis(crown-6) calix[4]arenes were in agreement with liquid/liquid results.42... 

Solid-phase extractions (SPE), in which DOM is selectively concentrated on a solid-phase extractant such as XAD-2 resin or Ci8 adsorbent. In these methods, both inorganic solutes and water are removed concurrently, and a suitable solvent is used to desorb the concentrated, desalted DOM from the solid-phase extractant. In principle, all inorganic solutes may be removed by SPE. 

In a cryptate complex, the cation is enclosed wholly or partially in a hydrophobic sheath, so that not only are salts of this complexed cation soluble in nonpolar organic solvents but also extractable from aqueous solutions into organic solvents immiscible with water (144). Specific cryptands may be used to selectively complex metals from crude materials or wastes, particularly if they are immobilized on a polymer support (101, 114, 145). 

Liquid-liquid extraction (LLE) is the traditional method to extract organic compounds from water. The low molecular weight compounds are transferred from one liquid phase to another immiscible or partially immiscible liquid by shaking them in a separation funnel. LLE is still a common method, but has several drawbacks such as low selectivity, labor intensivity, and the use of large amount, of organic solvent. LLE has been used to extract hydrolysis products of degradable polyesters such as PLA and its copolymers from the buffer solution [115]. 

In certain cases it is desirable to selectively remove a volatile solute from a solution that contains other, less volatile, solutes as well as the solvent. Some examples are the reduction of ethanol content from alcoholic beverages or from dilute alcoholic extracts of aromatic flavors and fragrances from plant sources such as fruits or flowers. Conventional pervaporation would facilitate removal of water from such mixtures while retaining ethanol and the higher molecular weight organics that comprise the characteristic aroma and flavor profile of the products of interest. On the other hand, membrane distillation or osmotic distillation cannot retain the volatile components at all. 

Supercritical fluids such as carbon dioxide can be used as solvents to extract organic compounds from aqueous solutions. In order to achieve recoveries of these products often in low concentration, cosolvents as methanol or other alcohols have been added to improve the solubility and the selectivity of the primary fluid. To optimize the extract recovery, the knowledge of phase equilibria of the ternary system carbon dioxide-methanol-water is required at different temperatures and pressures. 

The standard CCC separation is solely based on the difference in the partition coefficients of solutes between the two phases of a solvent system. Most alkaloids have basic properties with pK values ranging from 6 to 12, but usually 7 to 9. Although the free base is soluble only in organic solvents, protonation of the nitrogen in the free base usually results in a water-soluble compound. This behavior serves as the basis for the selective extraction or... 

The molecular formula of the solute may suggest the type of solvent which maybe selective for its extraction, based on probable affinities between related functional groups. Thus, to extract organic acids or alcohols from water, an ester, ether, or ketone (of sufficient molecular weight to have very limited solubility in the aqueous phase) might be chosen as the solvent. The pH of aqueous phase feeds may also be very important. The sodium or potassium salts of an organic salt may well prefer the aqueous media at pH >10, but in the acidulated form may readily extract into the organic phase if the pH is low. 

In the recoveiy of ethanol and many other otganic solutes from aqueous solution, coextraction of water has a large effect on the process economics. Solvents may be compared hy plotting the selectivity (k = separation factor between ethanol and water) versus the solvent capacity for ethanol, expressed as Kff. Figure 15.2-6 la such a plot for extraction of ethanol from relatively dilute aqueous solution by many different solvents.3 This figure includes data fiom Roddy 35 Souissi and Thyrion,37 and Munson and King. 

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