Analysis of the spectrophotometric response of a nitrosyl complex generated within an electronic conducting polymer film

The chemical reaction between diffusing nitrite and an iron-substituted Keggin-type heteropolytungstate [(H2O)FeIIPW11O39] 5−, immobilized in a poly(N-methylpyrrole) film, generates a nitrosyl complex spectrophotometrically revealed around 465 nm. The analysis of transient absorption data from a theoretical model developed by Bartleu and Gardner (Philos. Trans. R. Soc. London, Sci: A, 354 (1996) 35) demonstrates that the rate-determining step of the chemical process is the reaction kinetics rather than the nitrite diffusion throughout the film when a relatively thin film (≈0.10 μm, Γ[(H2O)FeIII] = 1.0×10−8 mol cm−2) is investigated in the presence of 30 mM nitrite. Moreover, the conversion into nitrosyl complex is expected to be total. The representation of the maximum absorbance in the ln(A(∞) -A(t)) t form gives a linear plot, and the rate constant extracted from the slope is 0.4 M−1 s−1. The spectrophotometric response of the nitrosyl complex is also predicted when Γ0[(H2O)FeIII]0 and CNO2−0 are varied. For Γ[(H2O)FeIII]0contained in the range of 0.45× 10−8 to 3.35× 10−8 mol cm−2, a similar limiting case with a linear In(A(∞) -A(t)) -t plot would be expected. For much larger Γ0[(H2O)FeIII] 0 values, the kinetics of the chemical reaction would be controlled by the diffusion of nitrite throughout the film and the expected transient A (t) /A (∞) profiles would be dependent on the nitrite concentration.