In-line flow injection extraction preconcentration through a passive hydrophilic membrane. Determination of total phenols in oil by flow injection analysis

An in-line flow injection extraction-preconcentration procedure for the determination of total phenols in oil is described. The reaction between phenolic compounds and 4-aminoantipyrine in the presence of K2S2O8 as oxidizing reagent was used. The phenols were extracted and preconcentrated from a xylene solution by using a more selective passive hydrophilic Spectrapor membrane which also removed interferences. The phenols deprotonated after diffusion to the basic acceptor stream and the preconcentrated phenolate was injected into a carrier stream containing 4-aminoantipyrine as colour reagent. The carrier stream then merged with the oxidant stream. followed by detection at 500 nm. The system was suitable for the determination of total phenols in oil at a sampling rate of 12 samples per hour with an RSD of batter than 1.3%. The detection limit was 0.09 mg/l for phenol, 0.18 mg/l for o-cresol and 0.02 mg/l for m-cresol. The results of the proposed system compared favourably with a standard manual 4-AAP method and a standard GC procedure.