A trap is placed between the vacuum apparatus and the side-arm receiving flask to prevent contamination of the filtrate and/or prevent the filtrate from reaching the vacuum apparatus. For low-vacuum applications (when a water aspirator or a vacuum pump with an intentional leak in the system is used), the trap can be assembled with an Erlenmeyer flask, a three-hole rubber stopper, glass tubing, rubber vacuum tubing, and a hose clamp (or a valve).
Glass tubing is inserted into each hole on the stopper. To two of the glass tubes, 1-3 foot lengths of rubber vacuum tubing are connected. One length is connected to the receiving flask, and the other length is connected to the vacuum source. To the third glass tube, a 3-4 inch length of rubber vacuum tubing is connected and the tubing clamp is loosely attached. The assembled stopper is then placed in the opening of the flask.
Clamp the side-arm receiving flask to a ring stand. Then, connect the side-arm receiving flask to the trap and place the Buchner funnel or glass crucible in the mouth of the receiving flask.
IMAGE 2. Vacuum filtration apparatus with a trap.