Phase during extraction

Fig. 124. Distribution of complex acids between organic and aqueous phases during extraction and stripping of tantalum and niobium.

Two experimental problems have been recognised in the application of sequential extractions for PTMs fractionation in soil and sediments. One is the PTMs readsorption and redistribution among phases during extraction, meaning that PTMs... 

If your solvent has reasonable solubility in water then it is advisable to i—ave most or all of it at this stage on a rotary evaporator. This is not. vays necessary but omitting it can lead to losses of product to the iu ceous phase during extraction. The most commonly encountered. . iccples of this type of solvent are alcohols and tetrahydrofuran, and the riem gets worse as the polarity of the product increases. 

It was considered previously that the most effective phase transfer catalysts are quaternary ammonimn bases. However, preliminary experiments with crown-ethers had already shown that these compounds are more powerful phase transfer catalysts than quaternary ammonium bases and are more selective [106, 177]. This is explained by differences in the mechanism of catalytic action. The mechanisms of reaction acceleration in two-phase systems with crown-ethers are as yet little studied, but simple examination of salt extraction with crown-ethers shows that the salt in the aqueous phase G>oth anion and cation) passes into the organic layer, whereas only anions paired with the onium cation pass from the aqueous into the organic phase during extraction with onium salts. This considerable difference in the mechanism of action of the two groups of ion-carrying catalysts is the basis for the prospective use of crown-ethers and their analogs instead of quaternary ammonium bases in many fields. 

A large contact area between the polsrmer-rich and polsrmer-lean phases is a very important requirement in extraction fractionation to reduce mass-transfer limitations between the phases during extraction. In the simplest extraction technique, direct extraction, this is achieved hy using finely divided polsrmer particles or powder. Direct extraction is commonly performed in Sohxlet extractors. Diffusion limitations, pol3oner swelling, and equipment plugging make direct extraction a technique of limited use. 

A solution of 3.4 grams of N-bromosuccinimide in 60 cc of absolute dioxane is added drop wise in the dark, during the course of 5 minutes, to a stirred solution, heated to 60°C, of 9.2 grams of ergocryptine in 180 cc of absolute dioxane. The reaction mixture is stirred at this temperature for 70 minutes and is concentrated to a syrup-like consistency in a rotary evaporator at a bath temperature of 50°C. The reaction mixture is subsequently diluted with 300 cc of methylene chloride, is covered with a layer of about 200 cc of a 2N sodium carbonate solution in a separating funnel and is shaken thoroughly. The aqueous phase is extracted thrice with 100 cc amounts of methylene chloride. The combined... 

Centrifuging the screw-cap vial can easily break emulsions, which often form during extraction. The vial will survive up to 6000 g if rubber stoppers are inserted into the centrifuge cup to provide a flat base for the vials. The required phase (usually the top layer) can be easily removed with a pipet or, if it is to be discarded, it can be removed using a disposable pipet connected by tubing to a suction flask and vacuum line. 

Note Empirical volume of organic extract (Vex) acetone (200 mL) - - ethyl acetate-cyclohexane [1 1 (v/v), 100 mL] — empirical volume shrinkage (5mL) — empirical transfer of acetone into the aqueous phase during the liquid-liquid partition (10 mL) = Vex = 285 mL. 

The specificity of the NaOAc-HOAc solutions for the carbonate phase may be measured by other major elements released during extraction. For example, only limited attack of the alumosilicate components in the soil... 

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