Distillation Glossary: Key Terms for Laboratory Use

Distillation is one of the most commonly used scientific processes in the world. With applications across multiple disciplines and industries, distillation takes several forms, including simple, fractional, and vacuum, with even more specific variations, depending on the desired outcomes and component parts. Understanding how to create a distillation system begins with understanding the different pieces of available equipment. The following information gives you a good starting point.

Distillation

Distillation is the separation of a liquid mixture into its component parts on the basis of differences in boiling points. The process involves separating a mixture by boiling away the more volatile liquid.

Fractional Distillation

Fractional distillation is the process by which a chemical compound is separated into components by distillation. In fractional distillation, the compound is heated, and as each constituent component comes to a boil, the vapors are separated and cooled to remove them in pure form.

Distilland

Distilland is the material in a distillation apparatus that is to be distilled.

Distillate

Distillate is the material in a distillation apparatus that is collected in the receiver.

Vigreux Column

A Vigreux Column is a modification of the air condenser. It is usually used as a fractionating column for fractional distillations. It is usually made of borosilicate glass with 24/40 ground glass joints. Typical sizes are 200 mm and 300 mm which refer to the effective separation length or indentation length.

Unlike straight-walled columns, a Vigreux Column has a series of downward-pointing indentations on the inside wall. These serve to dramatically increase the surface area without increasing the length of the condenser. Because of their added complexity, Vigreux columns also tend to be considerably more expensive than traditional straight-walled designs.

Condensers

Liebig Condensers

The Liebig Condenser is the most basic water-cooled design, with the straight inner tube, it makes it cheaper to manufacture. The Liebig Condenser is much more efficient than a simple retort due to its use of liquid for cooling. Water can absorb much more heat than the same volume of air. Its constant circulation through the Liebig's water jacket keeps the condenser's temperature constant. Therefore, a Liebig condenser can condense a much greater flow of incoming vapor than an air condenser or retort.

Additionally, stainless steel 'wool' or another heat-conductive, nonreactive material can be loosely placed in the inner cylinder of a Liebig condenser, substantially increasing the reflux effect and the overall efficiency of the distillation. The cooling water should flow from top to bottom in this instance, as that also increases the efficiency and helps prevent thermal shock to the glassware.

West Condenser

A variant of the Liebig condenser, the West Condenser has a more slender design. The narrower coolant jacket may render more efficient cooling with respect to coolant consumption.

Allihn Condenser

The Allihn Condenser consists of a long glass tube with a water jacket. A series of large and small constrictions on the inside tube increases the surface area upon which the vapor constituents may condense. This equipment is ideally suited for laboratory scale refluxing.

Again, a Liebig readily substitutes for an Allihn condenser, taking care to enter coolant at the cooler point to maintain a correct thermal gradient; i.e., the higher fitting in this case.

Graham Condenser

Graham Condenser with 24/40 ground glass joints. A Graham condenser has a coolant-jacketed spiral coil running the length of the condenser serving as the vapor/condensate path.

Friedrichs Condenser

A Friedrichs Condenserhas 24/40 ground glass joints and hose-barb connections on top. A Friedrichs condenser, also known as a spiraled finger-condenser, consists of a large, spiraled internal cold finger-type capillary tube disposed within a wide cylindrical housing. Coolant flows through the internal cold finger; accordingly, vapors rising up through the housing must pass along the spiraled path.

Round-Bottom Flasks

Round-Bottom Flasks are flasks with spherical bottoms used as laboratory glassware, mostly for chemical or biochemical work. They are typically made of glass for chemical inertness, and they are usually made of heat-resistant borosilicate glass. There is at least one tubular section known as the neck with an opening at the tip.

Two or three-necked flasks are common as well. Round bottom flasks come in many sizes, from 5 mL to 5 L, with the sizes usually inscribed on the glass. In pilot plants, even larger flasks are encountered.

The ends of the necks are usually conical (female) ground glass joints. These are standardized and can accept any similarly-sized tapered (male) fittings. Standard Taper 24/40 is common for 250 mL or larger flasks, while smaller sizes, such as 14 or 19, are used for smaller flasks.

Because of the round bottom, cork rings or support stands and clamps are needed to keep the round bottom flasks upright. When in use, round-bottom flasks are commonly held at the neck by clamps on a stand.

The round bottoms on these types of flasks allow more uniform heating and/or boiling of liquid. Thus, round-bottom flasks are used in various applications where the contents are heated or boiled. Chemists usually use round-bottom flasks in distillation as distilling flasks and receiving flasks for the distillate. One-neck round-bottom flasks are used as the distilling flasks in rotary evaporators.

Significantly, this flask shape is also more resistant to fracturing under vacuum, as a sphere more evenly distributes stress across its surface.

Ground Glass Joints

Ground glass joints are used in laboratories to quickly and easily fit leak-tight apparatus together from commonly available parts. For example, a round bottom flask, Liebig condenser, and oil bubbler with ground glass joints may be rapidly fitted together to reflux a reaction mixture. This is a large improvement compared with older methods of custom-made glassware, which was time-consuming and expensive, or the use of less chemically- and heat-resistant corks or rubber bungs and glass tubes as joints which took time to prepare as well.

To connect the hollow inner spaces of the glassware components, ground glass joints are hollow on the inside and open at the ends, except for stoppers.

Conically Tapered Joints/Standard Taper Symbol

The conically tapered ground glass joints typically have a 1:10 taper and are often labeled with a symbol consisting of a capital T overlaid on a capital S, meaning "Standard Taper." A number, a slash, and another number follow this symbol. The first number represents the outer diameter (OD) in millimeters (mm) at the base (widest part) of the inner joint. The second number represents the ground glass length of the joint in millimeters.

The most common U.S. joints are 14/20 and 24/40. These sizes apply only to glassware in the U.S. There are also European ISO standard joints with common joint sizes of 10/19, 14/23, 19/26, 24/29, and 29/32. The US and ISO joints differ only in the length, not the slope, and some can be combined.

The stopper joints of chemical bottles, volumetric flasks, and separatory funnels often do not use the precision standard taper joints. Stopper joints are designated (if at all) only by the maximum diameter number.

Frozen Joints

Standard taper ground glass joints sometimes freeze or seize. Joints may freeze for a few reasons, such as:

• The lack of lubrication (with grease, sleeves, or tape)
• Attack of the ground glass surfaces by strong bases
• Leaching of greases by organic solvents
• Dirt or other debris
• Allowing sealed vessels to cool (creating a pressure difference across the joint)

Frozen joints may be removed by working solvent into the joint while rocking the stopper, heating the joint, or cooling the stopper.

Adapters

For either standard taper joints or ball-and-socket joints, inner and outer joints with the same numbers are made to fit together. When the joint sizes are different, ground glass adapters may be available (or made) to place in between to connect them. Special clips or pinch clamps may be placed around the joints to hold them in place.

Round-bottom flasks often have one or more conically-tapered ground glass joint openings or necks. Conventionally, these joints at the flask necks are outer joints. Other adapters, such as distillation heads and vacuum adapters, are made with joints that fit in with this convention. If a flask or other container has an extra outer ground-glass joint on it that needs to be closed off for an experiment, there are often conically-tapered inner ground-glass stoppers for that purpose.