Speciation of iron(II), iron(III) and full-range pH monitoring using paptode: A simple colorimetric method as an appropriate alternative for optodes

A new methodology for construction of a sensor array, to acquire signal patterns, is discussed in this article. This sensor is constructed by immobilizing an ionophore on a piece of paper (or porous material such as thin layer chromatography (TLC) plate) followed by charge coupled device (CCD) camera-based detection. This allows us to use non-transparent porous materials as a support for immobilization of ionophore. Changes in RGB values of color spots on pH paper or TLC strips create a pattern. The obtained pattern was analyzed using Microsoft Excel Solver. Full-range pH monitoring and speciation of ferric and ferrous ions in solution are discussed. A commercial multi-color pH strip was used as a typical prototype for full-range pH monitoring, and 4-methyl-2,6-bis (hydroxymethyl)phenol (or its halo-derivatives) and 1,10-phenanthroline were used as ionophores for determination of Fe(III) and Fe(II), respectively. Results of analyzed patterns showed a good correlation between predicted and true values of pH (r = 0.9986), Fe(II) (r = 0.9989) and Fe(III) (r = 0.9978) with acceptable relative standard error.