Sorption-spectrophotometric determination of Ni(II) in natural waters using nitrazine yellow immobilised on a polyamine-modified polyacrylonitrile fibre.

A solid-phase sorption-spectrophotometric procedure was developed for the determination of Ni(II) in natural waters using nitrazine yellow (NY) immobilised on PPA-1, an anion-exchange fibre obtained by treating hydroxylamine-activated polyacrylonitrile with polyethylene polyamine. Reagent immobilisation and Ni(II) binding were characterised by absorption and diffuse-reflectance spectroscopy, Raman spectroscopy, conductometry and scanning electron microscopy with energy-dispersive X-ray analysis (SEM-EDS). In aqueous solution, the absorption maximum of NY shifts from 465 to 583 nm on complexation with Ni(II) (Δλ = 118 nm), whereas on the fibre the Kubelka-Munk maximum shifts from 470 to 620 nm (Δλ = 150 nm), so that the analytical signal is measured where the reagent-loaded blank absorbs only weakly. For the solid-phase determination (50 mL sample, pH 6.4, 0.2000 g of NY-loaded fibre, 10 min contact time, no free reagent added), ΔF(R) at 620 nm was linear in the mass of Ni(II) over 4.2-88.5 µg per 50 mL (84-1770 µg L-1; R2 = 0.9985, ten levels, four replicates each). The limits of detection and quantification (3sy/x/b and 10sy/x/b) were 72 and 240 µg L-1, and the repeatability was 0.1-0.9% (RSD, n = 4). The solution reaction, used only for spectral characterisation, has a molar absorptivity of 3.8 × 104 L mol-1 cm-1 at 583 nm and a Sandell sensitivity of 0.0016 µg cm-2. Cu(II), Fe(II), Fe(III), Zn(II), Mn(II), Co(II), Pb(II) and Hg(II) were tolerated at a tenfold mass excess over Ni(II) (recoveries 99.3-100.0%). Ni(II) added to Zarafshan River water at 400-1000 µg L-1 was recovered at 97.5-104.0% (RSD 4.2-7.3%, n = 5) without significant bias. Because the LOQ exceeds the WHO drinking-water guideline value (70 µg L-1), the procedure in its present form is suited to Ni(II)-contaminated surface waters and wastewaters; monitoring at the guideline level would require prior preconcentration. The procedure is solvent-free and uses an inexpensive, disposable fibrous sensing phase that is easily separated from the sample.