A carbonator reactor includes a cylindrical body, a nozzle for supplying a gas stream, inside the carbonator reactor and above the surface of a liquid phase and where the nozzle is located at the top of the reactor body, an inlet, an outlet, means for regulating the temperature and the pressure, a stirring system and at least one baffle regulating the stirring of the liquid phase and the mass transfer of the gas into the liquid surface, at least one impeller having inclined blades that make an angle from 5° to 60° with respect to the vertical axis. The reactor prepares sodium carbonate and has a configuration for the mass transfer of a gas phase in a liquid phase. A method for the preparation of sodium carbonate by means of the carbonator reactor by capturing CO2 in an NaOH aqueous solution, directly on the free surface of the liquid phase.