An Experiment in Diffusion, Water Pollution, and Bioassay Using Polyethylene Film as A Semipermeable Membrane

Abstract

Introduction Water-filled polyethylene bags are permeable to oxygen. They thus provide excellent devices for measuring the oxygen concentration of waters which are so full of pollutants or particulate matter that they are virtually impossible to test by conventional means (2). A water-filled bag is placed in the water to be tested and it is allowed to remain there for at least 24 hours. Oxygen diffuses through the polyethylene membrane until the concentration of dissolved oxygen in the water within the bag is equal to the concentration of oxygen in the water surrounding the bag. The water in the bag is then tested by standard methods for its dissolved oxygen concentration (1, 3). A small fish or other aquatic organism can be placed in another waterfilled bag. By its living or dying in the test situation, the bioassay organism is also used as a measure of the dissolved oxygen concentration of the surrounding medium. I have found that modifications of these methods, which were developed for research work, provide a novel approach whereby students may learn, by experimentation, about water pollution, bioassay, and about the diffusion of a gas through a semipermeable membrane. The method is so flexible that its applications are limited only by the imagination of the instructor. The experiment is easily adapted to large