Optimization of an analytical method for the spectrophotometric determination of copper in tea and water samples after ultrasonic assisted cloud point extraction using a benzothiazole fluorescein derivative complexing agent

Abstract

In this work, 2′,7′-di(2-benzothiazolyl)-fluorescein (BTF) was synthesized by using 2-(2′,4′-dihydroxyphenyl) benzothiazole (BT) and phthalic anhydride. BTF is colorful and its absorption maximum occurs at 559 nm. Interestingly, the introduction of Cu²⁺ affords a dramatic decrease of the absorbance at 559 nm, which is due to the spirolactonization of BTF induced by Cu²⁺. BTF shows high selectivity toward Cu²⁺ over other common metal ions in real samples, such as K⁺, Mg²⁺, Ca²⁺, Na⁺, Fe³⁺, Cd²⁺, Al³⁺, Co²⁺, Cr³⁺, Mn²⁺, Ni²⁺, La³⁺ and Zn²⁺. On this basis, a cloud point extraction (CPE) was developed for copper determination by a UV-vis method. The BTF–Cu²⁺ complex was explored for pre-concentration, and dissolved in a micellar phase composed by octylphenoxypolyethoxyethanol (Triton X-114). Under optimal experimental conditions, the decrease of the absorbance value (ΔA) at 559 nm is linear with Cu²⁺ concentration in the range from 0.03 to 2 μmol L⁻¹ (1.90–127 μg L⁻¹) with a limit of detection of 7.48 nmol L⁻¹ (3σ). The method has been successfully applied for the determination of copper in tea and water samples.