Determination of the diffusion coefficient of 1-(2′-pyridylazo)-2-naphthol in ethanol–water solutions using flow injection and nuclear magnetic resonance techniques

Abstract

The values of the diffusion coefficient of PAN (1-(2′-pyridylazo)-2-naphthol) in various ethanol–water solutions at 298 K were determined using both flow injection (FI) and nuclear magnetic resonance (NMR) techniques. The transient FI concentration curves monitored by the detector were described by the axially dispersed plug flow model with an axial dispersion coefficient obeying Taylor's theory. The diffusion coefficient of PAN determined in 40–100% (v/v) ethanol solutions in water by both curve-fitting and the statistical moments method varied in the range from 2.43×10 −10 to 8.31×10 −10 m 2 s −1. As expected, the diffusion coefficient values obtained by curve-fitting were found to be more reliable. A pulsed field gradient (PFG) spin echo NMR experiment was also used to measure the diffusion coefficient of PAN in the same solutions. The NMR results were found to follow the same trend as the FI results though they were from 5% to 16% lower in value. This deviation was attributed to the association effects facilitated by the experimental conditions under which the NMR measurements were taking place, i.e. in quiet solutions and at considerably higher concentrations than those used by the FI technique. The diffusion coefficient of PAN in pure water was calculated as 2.21×10 −10 m 2 s −1 by extrapolating the FI results to zero ethanol concentration. The results reported in the present investigation can be used for studying the electrochemical properties of PAN in ethanol–water solutions as well as for elucidating the sensing mechanism of an optode based on immobilisation of PAN into Nafion membranes.