An ionic-liquid based probe for the sequential preconcentration from headspace and direct voltammetric detection of phenols in wastewaters

A conveniently assembled probe is here proposed for the electrochemical detection of the phenol index (total volatile phenols) in wastewaters, which exploits some advantageous properties of ionic liquids (ILs), such as their negligible vapor pressure, good electrical conductivity and ability to dissolve a wide range of organic and inorganic compounds. It consists of three Pt wires piercing through a Teflon rod, whose exposed end is coated with an adhesive film of ionic liquid which assured the necessary ionic conductivity between the electrodes while displaying concomitantly good solvent properties. The electrode assembly was exposed to the headspace in equilibrium with wastewater samples for controlled times, until a convenient preconcentration of volatile phenols was achieved. At the end of this step, voltammetric detection was directly performed in situ. The performance of this device was assayed in both phenol synthetic samples and in real wastewaters, after that the preconcentration step was optimized with regard to the effect of pH of the sample, preconcentration time sample and probe temperature and nature of IL. Voltammetric responses characterized by good repeatability (+/- 4%) were recorded, whose height and area depended linearly on the phenol concentration over a wide range (ca. 3 orders of magnitude). The detection limit, estimated for a signal-to-noise ratio of 3 (2 mu M at 20 degrees C and 0.2 mu M at 80 degrees C), was lower than the limit set by the European Community for phenols in wastewaters (ca. 5 mu M). The results found in real samples were in acceptable agreement with those obtained by the 4-aminoantipyrine colorimetric approach which is recommended as the standard method.