SIMULTANEOUS EXTRACTION ACID METHYLATION OF CHLOROPHENOXYACETIC ACIDSFROM AQUEOUS-SOLUTION USING SUPERCRITICAL CARBON-DIOXIDE AS A PHASE-TRANSFER SOLVENT

The rapid and efficient methylation and extraction of organic acids fr om aqueous solution using supercritical carbon dioxide containing meth yl iodide and tetrahexylammonium hydrogen sulfate was shown to be anal ogous to similar reactions in conventional liquid-liquid extractive al kylation systems. The test analytes 2,4-dichlorophenoxyacetic acid (2, 4-D) and 2,4,5-trichlorophenoxy-acetic acid (2,4,5-T) were only extrac ted as their methyl esters by unmodified carbon dioxide at moderate te mperature and pressure when methyl iodide was present in the extractio n solvent. Conversion to these derivatives from 1 ppm solutions was co mplete in 30 min with yields of 89% and 103% for 2,4-D and 2,4;5-T, re spectively, and relative standard deviations better than 10%. The pres ence of methanol modifier in the supercritical carbon dioxide was dele terious to recovery of the methyl esters. A method in which the deriva tizing reagent was continually supplied to the extraction vessel was f ound to be more demanding of reagents and equipment and gave less repe atable results than a simpler static reaction system.