Determination of the diffusion coefficient of some highly toxic organic substances by using a diffusion tube CA Hemijska industrija
Authors: Bastic Jelena , Skala Dejan , Ivanovic Slavica 
Published: July 2008

The determination of specific characteristics of materials commonly used in the preparation of cloths for the protection of the human body, lungs and other human organs, based on the principles of the effective adsorption of highly toxic organic vapors initially requires the development of specific and reliable techniques for the preparation of a gas stream with the relevant highly toxic organic substances. The dynamic method, one of the techniques commonly used for material characterization, is based on the procedure of determining the concentration of different substances in the gas stream at the inlet and after passing through a specific thin layer of the adsorption material (outlet). Therefore, the generation of a gas phase containing highly toxic organic substances in a defined quantity during a specific time interval, is the requirement that must previously be fulfilled before using the dynamic method for testing specific materials for protection of the human body. Usually, a dynamic method of determining the quality of cloths prepared with specific adsorbents requires a relatively low volumetric flow rate of the gas phase containing the test substance which may be achieved by using a diffusion tube (DT) as the standard equipment for the generation of test substances. A very wide range of concentrations of the test substances in a specific gas phase (inert, air) can be obtained by varying the working conditions (temperature, pressure, gas flow rate) and the geometrical size of the diffusion tube, taking into account that the concentration of toxic substances, is determined as the ratio of their diffusion rate and from the DT and the carrier gas flow rate. The diffusion rate of the test substances from the DT is caused by the concentration gradient between the partial pressure of the saturated vapor at one end of the DT capillary and the partial pressure at the outlet of the diffusion tube according to Fick's law. The mass rate of the generated test substances from the DT is constant for constant conditions (temperature and pressure) in the reservoir of the DT On the basis of a detailed theoretical calculation such a rate (R) may be defined by the equation: R = 16.624 DM(P/T)(A/L)log [P/(P- pv)l, mg/min where: D - the diffusion coefficient, cm/s; M - the molar mass of the test substance, g/mol; P - the pressure in the DT kPa; pv - the vapor pressure of the test substance, kPa; A - the cross sectional area of the capillary of the DT cm2: L - the length of the capillary of the DT cm; T - the temperature, K. A procedure for the constant isothermal vapor generation of hydrogen cyanide (HCN) and 2-bromo-2-chloro-1,1,1-trifluoroethane (halotane) into air by a using diffusion tube is described. The DT was placed into a thermostat 0.5?C) and the diffusion rates of the substance were measured during some time interval and the above equation was used for the determination of the diffusion coefficient. The obtained data were correlated with the calculated ones on the basis of an empirical correlation from the literature. The relative differences in the experimentally determined and theoretically calculated values of the diffusion coefficient for hydrogen cyanide at different temperature (19-23.2?C) were 26-31 % and for halotane (26-45?C) 8-14%.