Equilibrium sampling through membranes of freely dissolved chlorophenols in water samples with hollow fiber supported liquid membrane

The freely dissolved concentration ([C.sub.free]) of pollutants is generally believed to be bioavailable and thus responsible for toxic effects. The [C.sub.free] of organic weak acids and bases consists of a dissociated and a nondissociated fraction. By using chlorophenols as model compounds, a negligible-depletion extraction technique, equilibrium sampling through membranes (ESTM), was developed for the measurement of the nondissociated part of the [C.sub.free]. Polypropylene hollow fiber membranes (280-[micro]m i.d., 50-[micro]m wall thickness, 0.1-[micro]m pore size, 15-cm length) were impregnated with undecane in the pores in the fiber wall as liquid membrane and filled with buffer solution in the lumen as acceptor. Then, the hollow fiber membranes were placed into the sample (donor) for an equilibrium extraction after sealing the two ends. The chlorophenol concentrations in the acceptor were then determined by direct injection into a HPLC system. Finally, the [C.sub.free] of the nondissociated and the dissociated species of a chlorophenol were calculated based on its measured concentration in the acceptor, its p[K.sub.a] value, and the measured pH in sample and acceptor. Theoretically calculated distribution coefficients (D = 8-970) agree well with the experimental enrichment factors ([E.sub.e(max)] = 6-1124), and the equilibration time was observed to increase with increasing distribution coefficients (hours to days). The freely dissolved concentration of five chlorophenols, with a wide range of p[K.sub.a] (4.9-9.2) and log [K.sub.ow] (2.35-5.24), were successfully determined in model solutions of humic acids and at low-ppb levels in fiver and leachate water.