Distribution countercurrent

If a mixture, consisting of two components S, and S2, is treated with two insoluble solvents Li and L2 so that S) dissolves Lj and S2 dissolves L2, countercurrent distribution has to be applied (Fig. 6-26). The feed is charged in the middle of the extraction column and the solvents are fed at the top and the bottom of the column. During the phase contact, L, and L2 are dissolved in 5 and 1S2 according to the solubility. In an ideal case of high selectivity, the feed may be totally separated. TNvo columns are often 

Just one extraction performed on a solution of a complicated sample will likely not result in total or at least sufficient separation of the analyte from other interfering solutes. Not only will these other species also be extracted to a certain degree with the analyte, but some of the analyte species will likely be left behind in the original solvent as well. Thus, the analyst will need to perform additional extractions on both the extracting solvent, to remove the other solutes that were extracted with the analyte, and the original solution, to remove additional analyte that was not extracted the first time. One can see that dozens of such extractions may be required to achieve the desired separation. Eventually, however, there would be a separation. The process is called countercurrent distribution. 

FIGURE 11.4 A drawing depicting countercurrent distribution as discussed in the text. 

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Oxytocin [50-56-6] M 1007.2, m dec on heating, [a] -26.2"(c 0.53, N AcOH). A cyclic nonapeptide which was purified by countercurrent distribution between solvent and buffer. It is soluble in H2O, rt-BuOH and isoBuOH. [Bodanszky and du Vigneaud J Am Chem Soc 81 2504 1959 Cash et al. J Med Pharm Chem 5 413 1962 Sakakibara et al. Bull Chem Soc Jpn 38 120 1965 solid phase synthesis Bayer and... 

Figure 1. Chromatogram of fractions from countercurrent distribution of basic portion of barley extract...

Chromatography on thin layers of Avicel C (microcrystalline cellulose from American Viscose Division, F.M.C. Corp., Newark, Del.), in 1-butanol-water-acetic acid (4 5 1) with bromocresol green as indicator. Countercurrent distribution in same solvent by single withdrawal procedure with 299 transfers and 100 elements... 

Gau, W. et ah. Mass spectrometric identification of xanthophyll fatty acid esters from marigold flowers (Tagetes erecta) obtained by high performance liquid chromatography and Craig countercurrent distribution, J. Chromatogr., 262, 277, 1983. 

Hhen the distribution constant is very - f small continuous liquid-liquid extraction or f countercurrent distribution apparatus is required. 

Large sample volumes may be extracted using continuous and countercurrent distribution methods. 

Countercurrent Distribution The relationship of the distribution constant K, of the solute in a CCD process to the concentration in the various separatory funnels or stages is given... 

An additional method of increasing the efficiency of liquid-liquid extraction is based on the countercurrent distribution principle [80-861 Early countercurrent distribution apparatus... 

Define recrystallization, distillation, fractional distillation, extraction, liquid-liquid extraction, solvent extraction, countercurrent distribution, liquid-solid extraction, and chromatography. 

Find, in a reference book, a description of the Craig countercurrent distribution apparatus and discuss its design as it relates to the description of countercurrent extraction presented in this chapter. 

Countercurrent distribution is a liquid-liquid extraction process in which actually dozens of extractions are performed when extracting solvent and sample solvent are contacted in the manner depicted in Figure 11.4. It is a process involving a series of separatory funnels in a way that approximates partition chromatography. 

The partition of different lipids between two immiscible solvents (countercurrent distribution) is useful for crude fractionation of lipid classes with greatly differing polarities. Repeated extractions in a carefully chosen solvent pair increase the effectiveness of the separation but in practice mixtures of lipids are still found in each fraction. A petroleum ether-ethanol-water system can be used to remove polar contaminants (into the alcoholic phase) when interest lies in the subsequent analysis of neutral glycerides, which may be recovered from the ether phase. Carbon... 

AvV -Bis(Boc)cystine (13.2 g, 30 mmol) was reduced by Na in liq NH3 (300 mL). The soln was decolorized with NH4C1, and L-2-amino-4-bromobutanoic acid methyl ester hydrochloride (20.8 g, 90 mmol) was added and the soln lyophilized. The lyophilizate was dissolved in 0.5 M NaHC03 (250 mL) and extracted with Et20. Subsequently, the soln was acidified to pH 3.0 by 1M HC1, evacuated, and filtered through a column of Dowex 50W-X4 (H+ form, 1000 mL). The column was washed with H20 and the product was displaced by 1% pyridine (cooled to 0°C). The effluents were acidified with AcOH to pH 6.0 and taken to dryness. The residue was reprecipitated from MeOH with Et20 and then separated from the contaminating jV,V -bis(Boc)cystine by countercurrent distribution yield 11.8g (52%) mp 185-186°C [a]D22 +37.2 (c 0.5, MeOH). 

Ac-Tyr- Met-Gly-Trp- Met- Asp-Phe-NH, (5 0.55 g, 0.56 mmol) was dissolved in a mixture of anhyd DMF (10 mL) and anhyd pyridine (5mL) and was then treated with pyridine/S03 (2g, 13 mmol) at rt for 24 h. The solvents were removed and the residue was dissolved in ice-cold H20 (20 mL) while keeping the pH at 7.0 with 10% aq Na2C03. The aqueous soln was then concentrated and after addition of DMF the inorganic salts were removed by filtration. The filtrate was taken to dryness and the resulting crude product purified by countercurrent distribution in the solvent system BuOH/EtOH/H2Q (5 1 8). Frac-... 

An interesting approach149 to the synthesis of partially protected nucleosides involved fusion for 12 hours at 130° of 5 -0-acetyladenosine and 2, 3, 5 -tri-0-acetyladenosine, followed by separation, by countercurrent distribution, of the so-formed mixture of 3, 5 -di-0-acetyl-, 5 -0-acetyl-, and 2, 3, 5 -tri-0-acetyladenosine. It is noteworthy that no 2, 5 -diester was isolated. A variation154 of this procedure was the fusion, at —200°, of a mixture of 2, 3, 5 -tri-0-acetyladenosine and adenosine, which yielded the 2, 3, 5 -triacetate, the 2, 3 -diacetate, and the 3, 5 -diacetate, with the last preponderating. 

Amino acids have high melting or decomposition points and are best examined for purity by paper or thin layer chromatography. The spots are developed with ninhydrin (see Lederer and Lederer, p.44). Customary methods for the purification of small quantities of amino acids obtained from natural sources (i.e. l-5g) are ion-exchange chromatography (see p. 20) or countercurrent distribution (see p. 28). For general treatment of amino acids see Greenstein and Winitz [The Amino Acids, Vols 1-3, J.Wiley Sons, New York 1961]. 

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