Abstract: The present invention concerns a mixer-settler, an arrangement comprising a train of at least two mixer-settlers and a method for measuring and controlling volumetric O/A ratio and phase disengagement time of organic and aqueous phases in a dispersion. A continuous flow of dispersion is led via an inlet channel (6) from the uptake channel (4) though a measurement chamber (5) to an outlet channel (9) which leads the flow to the pump-mixer unit (1). At predetermined time intervals the continuous flow of dispersion is interrupted by closing the inlet and outlet valves (12, 13) to retain a sample of dispersion in the measurement chamber (5) for the measurement of the O/A ratio and phase disengagement time.