Separation and Purification Technology 54 (2007) 349–354 Cloud point extraction for Co and Ni determination in water samples by flame atomic absorption spectrometry Valfredo Azevedo Lemos ∗ , Robson Silva da Franc ¸a, Bruno Oliveira Moreira Laborat´ orio de Qu´ ımica Anal´ ıtica (LQA), Universidade Estadual do Sudoeste da Bahia, Campus de Jequi´ e, Jequi´ e-BA 45200-000, Brazil Received 25 July 2006; received in revised form 10 October 2006; accepted 11 October 2006 Abstract Cloud point methodology was successfully employed for preconcentration of trace cobalt and nickel prior to their determination by flame atomic absorption spectrometry (FAAS). The metals react with 2-[2 ′ -(6-methyl-benzothiazolylazo)]-4-bromophenol (Me-BTABr) in a surfactant Triton X-114 medium. Dilution of the surfactant-rich phase with acidified methanol was performed after phase separation, and the cobalt and nickel content was measured by FAAS. The proposed procedure allowed the determination of cobalt and nickel with detection limits of 0.9 and 1.1 gL -1 , respectively. The method was applied to metal determination in water samples. The validation of the procedure was carried out by analysis of a certified reference biological material, NIST 1570a Spinach Leaves. © 2006 Elsevier B.V. All rights reserved. Keywords: Me-BTABr; Cloud point extraction; Cobalt; Nickel; FAAS 1. Introduction Cobalt is a naturally occurring element found in rocks, soil, water, plants, and animals. It is an essential micronutri- ent required for the growth of both plants and animals. Cobalt can be beneficial for humans because it is part of Vitamin B 12 . However, the metal can also be harmful, because exposure to high levels of cobalt can result in lung and heart effects and der- matitis. Nickel is a very abundant natural element and it can be combined with other metals, such as iron, copper, chromium, and zinc, to form alloys. Many people are highly sensitive to nickel. The most common harmful health effect of nickel in humans is an allergic reaction. People can become sensitive to nickel when jewellery or other things containing it are in direct contact with the skin for a long time. Studies for cobalt and nickel determina- tion in water and biological matrices are very important because it is a good tool for environmental and toxicological monitoring [1–3]. The determination of extremely low concentration of cobalt and nickel is generally associated to extraction steps due to insufficient sensitivity or matrix interference [4–6]. Cloud point ∗ Corresponding author. Fax: +55 73 35289630. E-mail address: vlemos@uesb.br (V.A. Lemos). extraction (CPE) can be an alternative to conventional sol- vent extraction due to a number of possible advantages, such as reducing of the consumption of a solvent, disposal costs, and extraction time [7,8]. The solubility of several non-ionic and zwitterionic surfactants in water decreases as temperature increases, with materials that are fully soluble at room temper- ature becoming partially insoluble forming separate phases at higher temperatures. The temperature at which this substances and water separate is known as the cloud point temperature [9]. The concentration of the surfactant-rich phase is the critical micellar concentration. CPE methodologies are based on this property. The use of CPE in procedures for separation and pre- concentration of metal ions has been centered on the extraction of these metallic substances as sparingly water-soluble chelate complexes. Several CPE methods for cobalt and nickel determination have been described in the literature. Ligands such as dithizone [10], 1-(2-pyridylazo)-2-naphthol (PAN) [11], ammonium pyrrolidinedithiocarbamate (APDC) [12], 1-nitroso-2-naphthol [13], and 2-amino-cyclopentene-1-dithiocarboxylic acid (ACDA) [7] have been employed for cloud point extraction in several procedures for Co and Ni. These ligands are widely employed due several advantages obtained, such as capacity to form complexes with a large variety of metals and low solubility in water. The hydrophobicity of ligands and complexes are the 1383-5866/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2006.10.004