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

The values of the diffusion coefficient of PAN (1-(2'-pyridylazo)-2-naphtho l) in various ethanol-water solutions at 298 K were determined using both f low injection (FI) and nuclear magnetic resonance (NMR) techniques. The tra nsient Fl: 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 statis tical moments method varied in the range from 2.43 x 10<SUP>-10</SUP> to 8. 31 x 10<SUP>-10</SUP> m<SUP>2</SUP> s<SUP>-1</SUP>. As expected, the diffus ion coefficient values obtained by curve-fitting were found to be more reli able. 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 Fl results though they w ere from 5% to 16% lower in value. This deviation was attributed to the ass ociation effects facilitated by the experimental conditions under which the NMR measurements were taking place, i.e. in quiet solutions and at conside rably higher concentrations than those used by the Fl technique. The diffus ion coefficient of PAN in pure water was calculated as 2.21 x 10<SUP>-10</S UP> m<SUP>2</SUP> s<SUP>-</SUP>1<SUP></SUP> by extrapolating the Fl results to zero ethanol concentration. The results reported in the present investi gation can be used for studying the electrochemical properties of PAN in et hanol-water solutions as well as for elucidating the sensing mechanism of a n optode based on immobilisation of PAN into Nafion membranes. (C) 1999 Els evier Science B.V. All rights reserved.