Distillation is a common operation in many laboratories for the purpose of separating and/or purifying components of a liquid mixture. The apparatus used consists of three major parts: distillation flask (or ‘pot’) to heat the mixture and volatilize the components, a condenser to cool the vapors back to liquid state, and a collection vessel. Click on the blue link distillation apparatus to see an animation of one way to set up a distillation apparatus. . Below is another picture of a distillation apparatus which uses rubber stoppers rather than the more modern standard taper glass connections.
Many of the hazards associated with the distillation process have been discussed in previous sections (glassware, flowing water, heating devices). However, the importance of the procedure is such that it is advisable to collect the warnings in this one place to make sure nothing goes unnoticed.
The apparatus is usually made of glass and therefore subject to breakage. All components of the distillation apparatus should be secured to a stable stand or rack to prevent it from falling over. All the glassware, particularly the part to be heated, should be checked for cracks prior to use. Connections between the glass parts may involve rubber or cork stoppers but in more modern apparatus standard taper connections are used. If stoppers are used, it must be known that the hot vapors will not react with the rubber or cork and thus contaminate the products. If standard taper connections are used, any lubricant used to make tight seals must also not react, melt or evaporate and contaminate the product.
The condenser must generally be connected to a source of running water to provide cooling for the vapors. The proper method is to connect the input hose to the condenser at the end furthest from the heated flask and the outflow hose nearest the heated flask. This prevents the hottest vapors from contacting the coldest water and creating a large thermal shock to the glassware. As mentioned in the section on flowing water, the hoses must be connected tightly enough to the condenser that they will not come loose if the water pressure should increase during the experiment. Usually this means that something like copper wire is twisted around the tubing at the joint to prevent it from coming away. The flow of water must be sufficient to accomplish condensation without being so fast as to cause undue hose pressure or splashing of outflow water, remembering that flow rates can change during the day after they have initially been set.
The distillation flask should preferably be a round-bottomed one rather than a flat-bottomed one for smoothness of boiling. The flask should never be more than half-filled with the liquid mixture to be distilled. Greater filling leads to bubbles and sometimes foaming that is constricted in the narrowing part of the flask and gets out of control.
To make boiling smoothest, boiling chips or tubes should be added to the liquid in the distillation flask before heating has begun. It is very important not to add chips or tubes to heated liquid as it may suddenly begin to boil and eject hot liquid out onto the operator. The chips are generally made of sharp pieces of broken ceramic or hard plastic. Tubes are usually of the capillary type with both ends open.
Heating the distillation flask requires care. The liquids being distilled are often flammable so that flame is not the preferred heat source. Heating mantles or sand baths are good sources of heat to conform to the round-bottomed flasks. Care must be taken not to let any vapors near the control switches that may spark when opening and closing. Doing the distillation inside a hood is a good practice.
One last word of caution about the apparatus is in order. There have been cases where the operators decided to make the connection between the condenser and the receiving vessel a tight one using a stopper or standard taper connection. This must NOT be done as it creates a completely sealed system. When the distillation flask is heated and vapors begin to rise they will expand and create a pressure in such a sealed system that will inevitably blow the joints apart. This generally causes vapors to escape into the surrounding room (or hopefully hood) if not the glassware to be broken. Always allow for a pressure relief opening in the distillation system between the condenser and the receiving vessel. Note in the diagram above the glass connector where the drops are falling is NOT sealed but is only loosely in the receiving flask.