A separator for separating a first from a second phase of a liquid in a tubular container has an elastic float with a circular sealing edge for sealing engagement with the inside of the tubular container in a sealing position. At the underside of the float, a ballast is attached. The density of the ballast is greater than the density of the float and the density of the entire separator is in a range between the density of the first phase and the density of the second phase of the liquid. The float has a local constriction and, in the area of the constriction, a membrane.