This is a standard 300 mL BOD bottle with black PVC plastic coating that blocks all visible light up to 800 nm. This inhibits the production of oxygen by algae. It is recommended for use in marine photosynthesis projects in which you need to compare oxygen in a light and a dark bottle. This bottle is supplied complete with a glass robotic stopper and an opaque black cap. To replace stopper use Wheaton Number 227670.