Liquid film effect on dynamics of optical oxygen probe. Comparison with polarographicoxygen probes. Diffusion coefficients measuring technique

摘要

The dynamic behaviour of the optical oxygen probe at the presence of the liquid film is well described byunsteady one-directional oxygen diffusion into the probe membrane through liquid film. It is shown thatthe asymmetry caused by hyperbolic form of the Stern-Volmer relation between the probe signal andoxygen concentration is strongly suppressed in the presence of liquid film for the optical probes used inthis work. In such case, the response calculated with respect to the mean oxygen concentration in themembrane describes well the optical oxygen probe dynamics.Liquid film effects on the signal of the optical and the polarographic oxygen probes are compared. Specialattention is paid to find properties of membranes minimizing the liquid film effect on the rate of itsresponse. From this point of view, the most suitable material for the optical probe membrane is one withhigh diffusivity but low solubility. Thus, silicone (used by optical probe manufacturers in these days) isthe least suitable material. When both probes are covered with the same membrane, the response ofpolarographic probe is faster and less influenced by the liquid film compared with optical one. From thispoint of view, the polarographic probe is more suitable for monitoring of fast changing oxygen concen-tration in bulk liquid as it occurs, e.g. measurement of k_La in gas-liquid dispersions, usually with theneed to know the L_Lparameter characterizing the liquid film diffusion resistance. In addition, only thepolarographic probe allows determination of a local value of the parameter L_L(under the given hydrody-namic conditions where the probe is situated) which is not possible with an optical probe. The opticalprobe is more suitable for measurements under steady-state conditions and for slower oxygen changes,even under the conditions, when the liquid film resistance is high in comparison with the membraneresistance as it occurs for diffusivity measurements in liquids. A modified method for evaluation of theoxygen diffusion coefficient in liquids from the dynamic probe response is presented.