A bubble extraction apparatus which utilizes a "hydrodynamic trap" effect is examined. The behavior of the trapping device, combined with various columns, can be described by means of a simple two-parameters model even when a concentration gradient is found to occur in the column. Experimental results on Sulphan Blue, Rhodamine B and picric acid are reported and checked against the model. The values of the parameters thus determined appear useful to investigate substances with respect to their surface activity and/or bubble conditioning effects.