2.4- Dichlorophenol techniques

In addition to the Hquid—Hquid reaction processes, there are many cases in both analytical and industrial chemistry where the main objective of separation is achieved by extraction using a chemical extractant. The technique of dissociation extraction is very valuable for separating mixtures of weakly acidic or basic organic compounds such as 2,4-dichlorophenol [120-83-2] and 2,5-dichlorophenol [583-78-8] which are difficult to separate by... 

In the chlorination of 2,4-dichlorophenol it has been found that traces of amine (23), onium salts (24), or triphenylphosphine oxide (25) are excellent catalysts to further chlorination by chlorine ia the ortho position with respect to the hydroxyl function. During chlorination (80°C, without solvent) these catalysts cause traces of 2,4,5-trichlorophenol ( 500 1000 ppm) to be transformed iato tetrachlorophenol. Thus these techniques leave no 2,4,5-trichlorophenol ia the final product, yielding a 2,4,6-trichlorophenol of outstanding quaUty. The possibiUty of chlorination usiag SO2CI2 ia the presence of Lewis catalysts has been discussed (26), but no mention is made of 2,4,5-trichlorophenol formation or content. 

The technique has been used in combination with HPLC using ECD for the determination of phenolic compounds with a large variety in polarity, viz. phenol, 4-CP, 2,5-dichlorophenol, 2,4,5-trichlorophenol, 2,3,5,6-tetrachlorophenol, and PCP, in natural water samples [203]. The membrane provides very efficient cleanup, and detection limits below 0.1 xg/L for the phenols were obtained (for chromatogram, see Figure 16.7). The technique has also been used for the extraction of five nitrophenols (2-nitrophenol, 3-nitrophenol, 4-nitrophenol, 2,3-dinitrophenol, and 2,4-dinitrophenol) [204]. 

A chromatogram obtained for a standard sample mixture with each CP at the 1 ng ml i (ppb) level (not shown) was well-defined, with a well-behaved baseline. This chromatogram was obtained by means of the column-switching technique, which allows pre-concentration of the samples. By coupling with the columnswitching technique, which enabled on-line preconcentration (by a factor of 50), the detection limit for 2,4-dichlorophenol was lowered to 0.4 nM (S/N = 3) from 20 nM. The detection limits were estimated to be in the range of 0.038 ng mL (0.23 nM) for 2,6-DCP to 0.361 ng mL i (2.23 nM) for 3,5-DCP (S/N = 3). 

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