Ground glass joint

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^{[citation needed]}.

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^{[citation needed]}.

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History

Crude versions of conically-tapered ground glass joints have been made for quite a while, particularly for stoppers for glass bottles and retorts. These days, ground glass joints can be precisely ground to a reproducible taper or shape. They are made to join two glassware pieces together. One of the glassware items to be joined would have an inner (or male) joint with the ground glass surface facing outward and the other would have an outer (or female) joint of a correspondingly-fitting taper with the ground glass surface facing inward.

Joint types

Two general types of ground glass joints are fairly commonly used: joints which are slightly conically-tapered and ball and socket joints (sometimes called spherical joints)^{[citation needed]}.

Conically-tapered joints

• 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 which stands for "Standard Taper". This symbol is followed by a number, a slash, and another number. The first number represents the outer diameter in millimeters (mm) at the base of an inner joint or the inner diameter in mm at the tip of an outer joint, in both cases where the applicable diameter is at a maximum in the joint. The second number represents the ground glass length of the joint in mm. The most commonly encountered joints are 14/20 and 24/40. These joint sizes apply only to glassware in the US. There are also European ISO standard joints with common joint sizes of 10/19, 14/23 and 29/32. The US and ISO joints differ only in the length not in the slope, and can be used in combination. The stopper joints of chemical bottles, volumetric flasks, and separatory funnels often do not use the precision standard taper ground glass joints. Stopper joints are designated (if at all) only by the maximum diameter number^{[citation needed]}.

Ball and socket joints

For ball and socket joints, the inner joint is a ball and the outer joint is a socket, both having holes leading to the interior of their respective tube ends to which they are fused. The ball tip is a hemisphere with a ground glass surface on the outside which fits inside of the socket where the ground glass surface is on the inside. Ball and socket joints are labeled with a size code consisting of a number, a slash, and another number. The first number represents the outer diameter in mm of the ball at its base or the inner diameter in mm at the tip of a socket, in both cases where the diameters are their maximum in the joints. The second number represents the inner diameter of the hole in the middle of the ball or socket, which leads to the inner diameter of the tube fused to the joint.

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 union of the joints to help keep them together.

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 which needs to be closed off for an experiment, there are often conically-tapered inner ground glass stoppers available for such a purpose. In some cases, small hook-like protrusions made of glass may be fused onto the rest of the glass item near a joint to allow an end loop of a small spring to be attached so the spring helps keep joints temporarily together. The use of a special very small size of conically-tapered fitting for glass, plastic, or metal parts called a Luer fitting or adapter has become more widespread. Originally, Luer fittings were mostly used to connect the hub of a needle to a syringe. Where the use of ground glass presents a problem such as in the production or distillation of diazomethane, which may explode on contact with rougher surfaces, equipment with smooth glass joints may be used.

Keck clips

  To prevent assembled glassware from coming apart, Keck clips are used to hold the pieces together. These are usually made of polyacetal, and are colored according to joint sizes.

An older method was to have barbs on the ends of each joint; a spring was used to apply tension.

Lubrication and sealing

A rather thin layer of grease particularly made for this application can be applied to the ground glass surfaces to be connected and the inner joint is inserted into the outer joint such that the ground glass surfaces of each are next to each other to make the connection. In addition to making a leak-tight connection, the grease allows to joints to be later separated more easily. However, a potential drawback of using such grease is that if it is used on laboratory glassware for an extended period of time in high temperature applications (such as for continuous distillation), the grease may eventually cause contamination of the chemicals.

PTFE (Teflon) sleeves and PTFE sealing rings^[1] are used in between joints to fit them together instead of grease. PTFE tape is another alternative that is finding widespread use.

See also

• Laboratory glassware

References