Separation of water-soluble mixtures

The essential basis of the scheme for the separation of water-soluble compounds is, therefore, distillation of (a) an aqueous solution of the mixture, (b) an alkaline (with sodium hydroxide) solution of the mixture, and (c) an acidic (with sulphuric oj phosphoric acid) solution of the mixture. The residue will contain the non-volatile components, which must be separated from inorganic salts and from each other by any suitable process. 

Fig. 124. Separation of water-soluble vitamins on silica gel GF254. Solvent water (CS). 30 fxg of i, 4, 5, 6 and 7 applied 10 fig of 2 5 (ig of 3. 81 mixture of 1—7 in these amounts. TL-chromatogram photographed in UV (254 nm) [8]...

Gennaro (51) proposed a method for the separation of water-soluble vitamins by means of the ion interaction reagent using octylamine < -phosphate or octylamine salicylate buffer (at pH 6.4) as the interaction reagent and the mobile phase at a flow rate of 1 mL/min, and a 2.5-p.m Spherisorb ODS C18 column (250 X 4.6 mm) as the stationary phase. The column effluent was monitored at 210 nm. Retention times of pantothenic acid obtained with octylamine t)-phos-phate and octylamine salicylate were 64.0 and 9.8 min. The method was used for the determination of pantothenic acid in a model mixture of water-soluble vitamins and also in a commercial multivitamin isotonic salt dietetic drink (Fig. 9). 

The above procedure for the separation of a water-soluble mixture is summarised in Table XI,9,2. 

TABLE XI,9,2. GENERAL SCHEME FOR SEPARATION OF A WATER-SOLUBLE MIXTURE... 

The separation of the physical isomerides is effected in the following manner the mixed nitrites are dissolved in acetone and precipitated by the addition of water, the mixture being cooled to 0°. This process is repeated several times, when eventually the more soluble or /3-nitrite is separated from the less soluble or a-variety. 

The theory and development of a solvent-extraction scheme for polynuclear aromatic hydrocarbons (PAHs) is described. The use of y-cyclodextrin (CDx) as an aqueous phase modifier makes this scheme unique since it allows for the extraction of PAHs from ether to the aqueous phase. Generally, the extraction of PAHS into water is not feasible due to the low solubility of these compounds in aqueous media. Water-soluble cyclodextrins, which act as hosts in the formation of inclusion complexes, promote this type of extraction by partitioning PAHs into the aqueous phase through the formation of complexes. The stereoselective nature of CDx inclusion-complex formation enhances the separation of different sized PAH molecules present in a mixture. For example, perylene is extracted into the aqueous phase from an organic phase anthracene-perylene mixture in the presence of CDx modifier. Extraction results for a variety of PAHs are presented, and the potential of this method for separation of more complex mixtures is discussed. 

Figure 9 A synthetic mixture of water-soluble carboxylic acids separated by anion-exchange chromatography. Column 0.3 cm x 300 cm Diaoion CA 08, 16-20 p (Mitsubishi Kasei Kogyo). Eluant 200 mM HC1. Detection reaction with Fe3-benzohy-droxamic acid-dicyclohexy carbodiimide-hydroxylamine perchlorate-triethyl amine with absorbance at 536 nm. Analytes (1) aspartate, (2) gluconate, (3) glucuronate, (4) pyroglutamate, (5) lactate, (6) acetate, (7) tartrate, (8) malate, (9) citrate, (10) succinate, (11) isocitrate, (12) w-butyrate, (13) a-ketoglutarate. (Reprinted with permission from Kasai, Y., Tanimura, T., and Tamura, Z., Anal. Chem., 49, 655, 1977. 1977 Analytical Chemistry).

There is a very wide choice of pairs of liquids to act as stationary and mobile phases. It is not necessary for them to be totally immiscible, but a low mutual solubility is desirable. A hydrophilic liquid may be used as the stationary phase with a hydrophobic mobile phase or vice versa. The latter situation is sometimes referred to as a reversed phase system as it was developed later. Water, aqueous buffers and alcohols are suitable mobile phases for the separation of very polar mixtures, whilst hydrocarbons in combination with ethers, esters and chlorinated solvents would be chosen for less polar materials. 

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