Anal, Chem. 1988, 58,983-985
983
Flow- Injection Analysis of Phenols via Bromination and Detection of Unreacted Bromine at a Platinized Gas Diffusion Membrane Electrode Sir: Solid electrodes used in voltammetric flow-through detectors are subject to passivation and gradual loss of sensitivity due to electrode surface contamination by dissolved species or electrode reaction products. For example, phenols, an important class of compounds in industrial and environmental analysis (I),are known for thek tendency to polymerize upon anodic oxidation and thus form insulating films on the electrode. Well-defined voltammetric responses are usually obtainable only at very low phenol concentrations (2). The situation is advantageously exploited by numerous liquid chromatography methods that employ direct amperometric detection of phenols (3-7). However, electrode fouling has been reported in several chromatographic studies (3, 7) when the injected samples contained more than 200 ng of a phenol (3). A successful approach that avoids problems associated with electrode fouling even under very unfavorable matrix conditions is to use a metalized gas porous membrane electrode (GMF'E). Such a metalized GMPE functions as a gas diffusion electrode. It permits selective detection of soluble, volatile species that reach the unmetalized, hydrophobic side of the membrane. These volatile species then partition into the gas-filled pores of the membrane and diffuse to the porous metal electrode on the other side of the membrane where they are electrolyzed. In the pneumatoamperometric technique, a solution containing the volatile, electroactive species is purged with a nonelectroactive gas, and the gas stream is impinged on the unmetalized side of the membrane (8). We recently described a novel variation of this pneumatoamperometric method that makes gas purging unnecessary (9). Instead, the solution itself was impinged on the unmetalized face of the GPME. Volatile species in the solution partition directly into the membrane's pores and diffuse to the metalized face where they are electrolyzed. This geometry produced a sensitive flow-through detector that we used for flow injection analysis of nitrite. It detected the chemically produced volatile species, nitric oxide and iodine, in a flowing aqueous solutions. The same type of detector was employed in the present work for the flow-injection analysis of phenols by the bromination method. A platinum GPME detected excess bromine that did not react with the dissolved phenol. EXPERIMENTAL SYSTEM Flow System. The pump and injection valve have been described previously (9). The liquid manifold included a mixingreaction coil (60 cm long Teflon tubing 0.20 in i.d.) placed in a thermostated bath held at 50 "C between the injection valve and the detector. The detector, Figure 2 of ref 9, was equipped with a platinum GPME based on a porous Teflon membrane. The polyethylene spacer used in the cell had a narrow 1 mm wide, straight channel. This prevented vapor bubbles created by warming the solution from collecting on the membrane. A higher flow rate of carrier solution, as compared to the nitrite work, of 0.7 mL/min was used throughout this work to minimize bubble collection. The detector's electrode compartment was filled with 0.1 M H2S04and the platinized GPME was used without any pretreatment. Other. All chemicals were of analytical grade and were used without further purification. The aqueous carrier stream, containing 0.05 M KBrO, and various concentrations of KBr or a pure 0.05 M acetate buffer, was degassed under vacuum in an ultrasonic bath prior use. All other techniques, chemicals, and electronic equipment have been described earlier (9).
RESULTS AND DISCUSSION General Considerations. The flow-injection method described for the determination of phenols is based on their bromination in acid solution. Excess, unreacted bromine is determined by reduction at a platinum GPME electrode at the potential +0.70 V vs. SCE. At this potential, the electrode yields a low background current, typically
