Colorimetric filtrations of metal chelate precipitations for the quantitative determination of nickel(ii) and lead(ii)

A colorimetric filtration method has been developed for the highly selective and sensitive determination of Ni(2+) and Pb(2+) ions. Determinations of Ni(2+) and Pb(2+) follow the filtration using nioxime (1,2-cyclohexanedione dioxime) and rhodizonic acid disodium salt, respectively, as colorimetric reagents. Different from regular instrumentation techniques, the metal chelate precipitations are continuously pumped into a home-made flow cell at a constant flow rate, and filtered by a cellulose acetate/nitrate membrane. The color changes of the membrane are imaged using a conventional flatbed scanner, and digitized. The special selection of individual channels in the red, green, and blue channels of the images filters the influences of coexisting ions and provides a highly selective detection of Ni(2+) and Pb(2+) cations. The linear relationship between the colorimetric response of the chosen channel and Ni(2+) or Pb(2+) concentrations indicates a quantitative detection. The detection limit for Pb(2+) is 3 μM (almost half of the Chinese wastewater discharge standard concentration), and is well below the nM level (94 nM) for Ni(2+) (a quarter of the WHO drinking water safe-exposure standard for Ni(2+)). The determinations take five to ten minutes. No shelf life issue exists because the chelating indicators react with metal directly without any pre-immobilizations.