  We confront, quantitatively, the theoretical description of the
reaction-diffusion of a second order reaction to experiment. The reaction at
work is \ca/CaGreen, and the reactor is a T-shaped microchannel, 10 $\mu$m
deep, 200 $\mu$m wide, and 2 cm long. The experimental measurements are
compared with the two-dimensional numerical simulation of the
reaction-diffusion equations. We find good agreement between theory and
experiment. From this study, one may propose a method of measurement of various
quantities, such as the kinetic rate of the reaction, in conditions yet
inaccessible to conventional methods.
