Optical Flow Analysis Methods for pH Measurements with Water Dispersions of Polyaniline Nanoparticles

Optical flow analysis methods for pH measurements with two commercially avai-lable water dispersions of polyaniline (PANI) nanoparticles are presented [1-3]. The nanoparticles were obtained from Ormecon GmbH (D1012W) and Panipol Ltd (Panipol W). The pH measurements are based on the acid-base equilibrium of PANI and were carried out either by combining the automated sequential injection analysis system (SIA) and UV-vis (800 nm) or Raman spectroscopy (excitation wave-length: 633 nm). The detection was done by using the SIA technique for transporting the sample to the light path of the UV-vis and Raman spectrophoto-meters. In both methods, fresh pH reagent (PANI) was used in each measurement, thus overcoming the problem with hysteresis (memory effect), which is usually observed with PANI films. Fast pH measurements can be done between pH 7-10.5 depending on the measuring technique.

The D1012W nanoparticles were characterized with UV-vis spectroscopy in pH buffer solutions between pH 2-12 and a protonation constant of log K =4.4 (equilibrium constant of the protonation reaction) was calculated from these data. The protonation constant provide important information about the pH sensitivity of different types of PANI and can therefore be used for choosing right PANI materials for specific applications where either pH sensitivity or insensitivity is required. The acid-base equilibrium of the nanoparticles is established entirely in the solution phase. The protonation equilibrium is therefore established faster in small nanoparticles than in films, which usually have a thickness of at least several hundreads of nanometers. The protonation constant of the D1012W-1 nanoparticles will be compared with the protonation constant of PANI films prepared from HCl, H2SO4, HNO3 and HClO4 resulting in films with high pH sensitivity.

It is also shown that the PANI nanoparticles dispersed in water solutions undergo the same redox chemistry as solid PANI films. The chemical structure of the repeating unit of PANI is very similar to conventional acid-base and redox indicators, like indophenols, azo compounds and variamine blues. It is therefore not surprising that the PANI nanoparticles can be used both as acid-base and redox indicators in water solu-tions. Potential application areas of the presented optical pH measurement methods are e.g. automated flow analysis systems where the application of the glass pH elec-trode is difficult and determination of pH under elevated pressures. It should be stressed, that this is a fundamental study showing the applicability of PANI nanopar-ticles in optical pH measurements.

[1] www.zipperling.de
[2] www.panipol.com
[3] T. Lindfors, L. Harju, A. Ivaska, Anal. Chem., accepted 6.3.2005