Abstract: A method for producing a liquid medium composition, including mixing a first liquid A1 containing a particular compound and a second liquid B1 containing a linking substance, and the method is carried out using a production apparatus. The production apparatus has a container 1 provided with at least two openings 2, 3 in a wall, the two openings are connected to each other by a tube 4 outside the container, and the tube has a part with a shape and flexibility that render the tube placeable as a pumping tube into a peristaltic pump 5. In the production method, the peristaltic pump is operated, and the first liquid and the second liquid are mixed by circulating them to form the object liquid medium composition in the container.