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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.
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.
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.
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).
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.
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
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Ground_glass_joint". A list of authors is available in Wikipedia.|