Kinetic-catalytic method for sequential determination of iron and copper using a chip coupled to a multipumping flow system

Abstract

A kinetic-catalytic spectrophotometric method for the sequential determination of iron and copper is presented in this work. For this, a multipumping flow system (MPFS) has been coupled to a monolithic flow conductor, called a chip. The reaction is based on the catalytic effect of Cu(II) and Fe(III) on hydroxylamine oxidation in a basic medium. The addition of EDTA inhibits the Cu(II) catalytic effect and the reaction becomes selective for Fe(III). The nitrite produced is spectrophotometrically measured using a modified Griess reagent. Due to the importance of the mixing order of the reagents, a special design of the chip is proposed. The sample, hydroxylamine and buffer (with EDTA for Fe and without EDTA for Cu determination) have been simultaneously propelled into the thermostated first section of the chip for mixing and reaction at 40 °C. Then, this mixture and the modified Griess reagent (sulphanilamide and N-(1-naphthyl)ethylenediamine) have been injected by confluence into the second section of the chip. Under optimal conditions, the determinable range was 3–75 μg L⁻¹ and 70–350 μg L⁻¹ for Cu(II) and Fe(III) respectively, with a sample throughput of 32 h⁻¹ for Fe(III) and 39 h⁻¹ for Cu(II). The limits of detection were 0.95 μg L⁻¹ for Cu(II) and 21 μg L⁻¹ for Fe(III). The relative standard deviations (n = 6) were 1.6% for 50 μg L⁻¹ Cu(II) and 1.3% for 100 μg L⁻¹ Fe(III). The proposed method has been successfully applied to the sequential determination of Cu(II) and Fe(III) in wastewater and pharmaceutical samples. ICP-OES has been used as a reference method for validation.