trap has been used to trap solvent. Oxygen can be condensed into a cold trap if a pump has sucked air through the trap when the trap is very cold, e.g. when cooled with liquid nitrogen. Besides oxygen, many hazardous gases emitted in reactions, e.g. sulfur dioxide, chloromethane, condense into cold traps. Construction Cold traps (C in the figure) usually consist of two parts: The bottom is a large, thick round tube with ground-glass joints (B in the figure), and the second is a cap (A in the figure), also with ground-glass connections. The length of the tube is usually selected so that, when assembled, the total reached is about half the length of the tube. Arrangement Cold traps should be assembled such that the down tube is connected to the source of gas whilst the cap is connected to the source of vacuum. Reversing this, connecting the down tube to the source of vacuum, places the inlet of the vacuum directly above the condensate, increasing the chances of vapour phase condensate moving up the (uncooled) down tube (towards the pump) or, should the trap begin to fill to an appreciable volume, liquid phase condensate being pulled into the pump. See also Sublimation Cold finger References Laboratory glassware Vacuum systems Gases Gas technologies