Diagnostics of Organic Compounds by Optical Emission Spectroscopy

This work is focused on the optical emission spectroscopy observations of the DC non-pulsed diaphragm discharge in various water solutions of selected organic dyes (Direct Blue 106 and Solvent Yellow 94), a mixture of humic acids and alkaline metals. The emission intensity dependencies of hydrogen and oxygen spectral lines on initial conductivity are studied and compared also with respect to the initial solution conductivity. The hydrogen and oxygen intensities increase more or less linearly with the increasing solution conductivity. The oxygen emission is more or less independent on the used electrolyte in the salt. Additionally, the rotational temperature is calculated from the OH radical spectrum for the investigated electrolytes and it is in the range of 500–1500 K in the depending on the solution initial conductivity. The dependence of emission intensity on the electrolyte kind is not clear. Introduction Discharges generated in liquids have a special position among the plasma sources. The common discharge configurations are point-to plane [Joshi et al., 1995] and coaxial [Šunka et al., 1999], several experiments were done in a diaphragm (electrode-less) configuration [Kuzhekin et al., 1995]. This work focuses on the diaphragm discharge created using the non-pulsed DC high voltage with the voltage of about 2 kV. Besides the study of these discharges from the electrical point of view [Lukeš et al., 2005], chemical processes are the second main research field [Lukeš et al., 2003]. In water solutions, various chemical active species are produced by the discharge, for example radicals, hydrogen peroxide ozone etc. [Joshi et al., 1995]. These active species initiate many further chemical reactions. Therefore the processes taking place in such discharges can be applied in the fields of water treatment, surface treatment etc. [Šunka et al., 1999]. The reaction mechanisms in the electrical discharges in liquids are not fully understood yet because many different active species are created [Lukeš et al., 2001]. Some of these active species can emit the radiation, and thus they can be determined by the optical emission spectroscopy. Experimental A simplified scheme of the used batch discharge reactor is shown in Figure~1. It consists of two parts a „negative part” with an anode (negative streamers are created) and a „positive part” with the cathode (positive streamers are created). Two planar stainless steel electrodes are installed in the distance of 4 cm symmetrically with respect to the PET dielectric diaphragm (thickness of 0.25 mm) with a small pin-hole in its centre (the initial diameter of the pin-hole is 0.25 mm). To generate high voltage and to create the discharge, a non-pulsed DC high voltage source is used. To ensure homogeneous conditions in the whole volume of the liquid, a mechanical mixing system is used. Details of the reactor construction can be found in [Stará et al., 2006]. The work focuses on the degradation processes in water solutions of selected organic dyes (Direct Blue 106 and Solvent Yellow 94) and a mixture of humic acids. As the organic dyes are not electrolytes we have to add a specific amount of the NaCl electrolyte (the concentration of 4 mM) to obtain the defined conductivity of the solution, optimal for the stable discharge operating. Organic dyes, humic acids and NaCl were solved in the demineralized water to obtain the initial conductivity in the range of 150–850 μS⋅cm. The following alkaline metal salts were used as the electrolytes: NaCl, KCl, NaNO3 and KNO3. They were solved in the demineralized water to obtain the initial conductivity in the range of 150–850 μS⋅cm. WDS'06 Proceedings of Contributed Papers, Part II, 145–149, 2006. ISBN 80-86732-85-1 © MATFYZPRESS