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

The dynamic behaviour of the optical oxygen probe at the presence of the liquid film is well described by unsteady one-directional oxygen diffusion into the probe membrane through liquid film. It is shown that the asymmetry caused by hyperbolic form of the Stern–Volmer relation between the probe signal and oxygen concentration is strongly suppressed in the presence of liquid film for the optical probes used in this work. In such case, the response calculated with respect to the mean oxygen concentration in the membrane describes well the optical oxygen probe dynamics.Liquid film effects on the signal of the optical and the polarographic oxygen probes are compared. Special attention is paid to find properties of membranes minimizing the liquid film effect on the rate of its response. From this point of view, the most suitable material for the optical probe membrane is one with high diffusivity but low solubility. Thus, silicone (used by optical probe manufacturers in these days) is the least suitable material. When both probes are covered with the same membrane, the response of polarographic probe is faster and less influenced by the liquid film compared with optical one. From this point of view, the polarographic probe is more suitable for monitoring of fast changing oxygen concentration in bulk liquid as it occurs, e.g. measurement of kLakLa in gas–liquid dispersions, usually with the need to know the LLLL parameter characterizing the liquid film diffusion resistance. In addition, only the polarographic probe allows determination of a local value of the parameter LLLL (under the given hydrodynamic conditions where the probe is situated) which is not possible with an optical probe. The optical probe 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 membrane resistance as it occurs for diffusivity measurements in liquids. A modified method for evaluation of the oxygen diffusion coefficient in liquids from the dynamic probe response is presented.