Gas-liquid reactions in microstructured devices have recently gained much attention in scientific research and industry. Enhanced heat and mass transfer can be employed to overcome mass transfer limitations in gas-liquid reactions. Helically coiled capillaries can further increase mass transfer due to Dean vortices, which narrow the residence time distribution, too. In this work, a colorimetric technique is implemented in order to visualize local mass transfer phenomena and concentration gradients of gas-liquid reactions in straight and helically coiled capillaries. The method enables on-line and non-invasive investigation of mass transfer and chemical selectivity in microchannels with high spatial resolution. Straight and helically coiled capillaries are fabricated from FEP tubes with inner diameter of 1.6 mm. Bubbles are generated by a hypodermic needle, which is placed in the center of the FEP tube generating a stable slug flow. Total volumetric flow rate is varied from 1.6 to 6.5 mL/min and volume ratios of gas/liquid flow from 0.17 to 6.0. Selectivity experiments are performed with the consecutive oxidation of leuco-indigo carmine.

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