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Ground glass joints are used in laboratories to quickly and easily fit leak-tight apparatus together from interchangeable commonly available parts. For example, a
round bottom flask Round-bottom flasks (also called round-bottomed flasks or RB flasks) are types of Laboratory flask, flasks having spherical bottoms used as laboratory glassware, mostly for chemistry, chemical or biochemistry, biochemical work. They are typicall ...
,
Liebig condenser The Liebig condenser (, ) or straight condenser is a piece of laboratory equipment, specifically a condenser consisting of a straight glass tube surrounded by a water jacket. In typical laboratory operation, such as distillation, the condenser ...
, and
oil bubbler A gas bubbler is a piece of laboratory glassware which consists of a glass bulb filled with a small amount of fluid — usually mineral or silicone oil, less commonly mercury. The inlet to the bulb is connected to a ground glass joint, whil ...
with ground glass joints may be rapidly fitted together to
reflux Reflux is a technique involving the condensation of vapors and the return of this condensate to the system from which it originated. It is used in industrial and laboratory distillations. It is also used in chemistry to supply energy to reactions ...
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 chemical resistant and heat resistant corks or
rubber bung A laboratory rubber stopper or a rubber bung or a rubber cork is mainly used in chemical laboratories in combination with flasks and test tube and also for fermentation in winery. Generally, in a laboratory, the sizes of rubber stoppers can be var ...
s and glass tubes as joints, which took time to prepare as well. 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 Ground glass is glass whose surface has been ground to produce a flat but rough (matte) finish, in which the glass is in small sharp fragments. Ground glass surfaces have many applications, ranging from ornamentation on windows and table glassw ...
surface facing inward. 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.


History

Crude versions of conically tapered ground glass joints have been made for quite a while, particularly for stoppers for glass
bottle A bottle is a narrow-necked container made of an impermeable material (such as glass, plastic or aluminium) in various shapes and sizes that stores and transports liquids. Its mouth, at the bottling line, can be sealed with an internal stopp ...
s and
retort In a chemistry laboratory, a retort is a device used for distillation or dry distillation of substances. It consists of a spherical vessel with a long downward-pointing neck. The liquid to be distilled is placed in the vessel and heated. The n ...
s. Crude glass joints could still be made to seal well by grinding the two parts of a joint against each other, but this led to variations between joints and they would not seal well if mated to a different joint. These days, ground glass joints can be precisely ground to a reproducible taper or shape, and joints of the same specification are fully interchangeable.


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).


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, meaning "Standard Taper". This symbol is followed by a number, a slash, and another number. The first number represents the outer diameter (OD) in millimeters at the widest point of the inner (male) joint. The second number represents the ground glass length of the joint in millimeters. Internationally the ISO sizes are used with 14/23, 19/26 and 24/29 very common in research laboratories, with 24/29 the most common. In the US the ASTM sizes (equal to the now obsolete Commercial Standard 21) are used with common sizes being 14/20, 19/22, 24/40 and somewhat 29/42. In the US 24/40 is most common. :


Ball-and-socket joints

For ball-and-socket joints (also known as spherical 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. This type of joint separates freely and must be held together with a clamp. 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 millimeters of the ball at its base or the inner diameter in millimeters at the tip of the 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. If the angle standard taper fittings make with glassware is not perfectly set, the glass is extremely rigid and brittle, presenting a fracture risk on some setups. A ball and socket joining method allows some flexibility in the mating angles of the pieces being joined, which can be particularly important with heavy flasks or long pieces of glassware that would otherwise be difficult to support and potentially snap under bending loads. A common example of this is the collection flask on a rotary evaporator, whose weight increases significantly as it fills. A ball and socket allows the flask to plumb itself without placing a bending load on the joint. Such a socket might also be used on a larger, but more typical, distillation setup at the head and before the condenser. This allows the long span of the condenser, the non-perfect angle of the receiving bend and the filling flask to be supported more easily as their angle with the still head has a few degrees of positioning freedom. They can also be found as the necks on pilot plant production flasks, where large volumes and masses are present, and on some
Schlenk line The Schlenk line (also vacuum gas manifold) is a commonly used chemistry apparatus developed by Wilhelm Schlenk. It consists of a dual manifold with several ports. One manifold is connected to a source of purified inert gas, while the other is ...
s, where the long spans of fine glass benefit from a little flexibility between pieces. Generally, when considering smaller glassware, ball and sockets are far outnumbered by standard tapers.


Flat joints

For flat joints the edge of a tube is ground flat perpendicular to the tube and pressed against a similar flat surface, with the mating force being applied by an external clamp. The rim of the joint is flared, sometimes into a wide flange, to give a larger surface for the joint to seal over. Aside from joints using a
gasket Some seals and gaskets A gasket is a mechanical seal which fills the space between two or more mating surfaces, generally to prevent leakage from or into the joined objects while under compression. It is a deformable material that is used to c ...
or
O-ring An O-ring, also known as a packing or a toric joint, is a mechanical gasket in the shape of a torus; it is a loop of elastomer with a round cross-section, designed to be seated in a groove and compressed during assembly between two or more par ...
, this is the only type of ground-glass joint used for very large diameters, since at those scales conical joints become impractical to manufacture and are prone to binding. Flat joints are mostly seen on large columns and reaction vessels, although they are used in some smaller applications such as flasks with removable lids. Glass vacuum desiccators use a flat ground-glass joint to seal their wide lids.


Connections


Threaded connections

Round slightly spiral threaded connections are possible on tubular ends of glass items. Such glass threading can face the inside or the outside. In use, glass threading is screwed into or onto non-glass threaded material such as plastic. Glass vials typically have outer threaded glass openings onto which caps can be screwed on. Bottles and jars in which chemicals are sold, transported, and stored usually have threaded openings facing the outside and matching non-glass caps or lids.


Hose connections

Laboratory glassware Laboratory glassware refers to a variety of equipment used in scientific work, and traditionally made of glass. Glass can be blown, bent, cut, molded, and formed into many sizes and shapes, and is therefore common in chemistry, biology, and anal ...
, such as
Buchner flask Buchner is a German surname. Notable people with this surname include the following: * Andreas Buchner (1776–1854), German historian * Annemarie Buchner (1924-2014), German Olympian * August Buchner (1591–1661), German influential Baroque poet ...
s and
Liebig condenser The Liebig condenser (, ) or straight condenser is a piece of laboratory equipment, specifically a condenser consisting of a straight glass tube surrounded by a water jacket. In typical laboratory operation, such as distillation, the condenser ...
s, may have tubular glass tips serving as hose connectors with several ridged hose barbs around the diameter near the tip. This is so that the tips can have the end of a rubber or plastic tube mounted over them to connect the glassware to another system such as a vacuum, water supply, or drain. A special clip may be placed over the end of the flexible tube surrounding the connector tip to prevent the hose from slipping off the connector. A number of brands, including Quickfit, have begun using threaded connections for hose barbs. This allows the barb to be unscrewed from the glassware, the hose pushed on and the setup screwed back together. This helps avoid accidentally breaking the glass and potentially doing serious harm to the chemist, as will sometimes occur when pushing the hoses directly onto the glass.


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 flask Round-bottom flasks (also called round-bottomed flasks or RB flasks) are types of flasks having spherical bottoms used as laboratory glassware, mostly for chemical or biochemical work. They are typically made of glass for chemical inertness; an ...
s 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 for that purpose. In some cases, small hook-like glass protrusions 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 used to connect the hub of a needle to a
syringe A syringe is a simple reciprocating pump consisting of a plunger (though in modern syringes, it is actually a piston) that fits tightly within a cylindrical tube called a barrel. The plunger can be linearly pulled and pushed along the inside ...
. Where the use of ground glass presents a problem, as in the production or distillation of
diazomethane Diazomethane is the chemical compound CH2N2, discovered by German chemist Hans von Pechmann in 1894. It is the simplest diazo compound. In the pure form at room temperature, it is an extremely sensitive explosive yellow gas; thus, it is almost u ...
(which may explode on contact with rougher surfaces), equipment with smooth glass joints may be used.


Joint clips

To prevent a joint from separating during a reaction process, various types of plastic or metal clips or springs can be used to secure the two sides together. They are available in a variety of materials for different temperature and chemical environments. Patented in 1984 by
Hermann Keck Hermann Keck (1919 – 2010) was a German scientist who invented the Keck Clip. Keck was born in Treysa Treysa, an independent town until 1970, is the biggest ''Stadtteil'' of the German town Schwalmstadt. It was incorporated into Schwalms ...
, plastic joint clips are usually made of
polyacetal Polyoxymethylene (POM), also known as acetal, polyacetal, and polyformaldehyde, is an engineering thermoplastic used in precision parts requiring high stiffness, low friction, and excellent dimensional stability. As with many other synthetic pol ...
, and are colored according to joint sizes. Polyacetal melts at a reasonably low temperature (around 175 °C) and begins to soften around 140 °C. As glassware temperatures are recommended up to 250 °C, care needs to be taken that clips made from this material are not being used to hold glass together that will get this hot. Typical problem areas include a flask over the plate (which may drop off the end of the column as it gets hot) and the connection the condenser makes to the still head (which will reach high temperatures and may allow the condenser to fall off). As such, different clips should be used at these points or the glassware should be clamped such that these elements can't slide apart or don't need the clip. Polyacetal clips suffer another problem in that the material is strongly affected by corrosive gases. This effect can be so dramatic that the clip will fall apart in minutes of exposure to minute quantities leaking through even greased, ground tapers. Importantly, this failure mode is sudden and without warning. PTFE joint clips are sometimes used, as its recommended temperature peak matches that of most practical chemistry work. Its highly inert nature also makes it immune to degradation around the corrosive gases. However, it is both expensive and will begin producing
perfluoroisobutylene Perfluoroisobutene (PFIB) is the perfluorocarbon counterpart of the hydrocarbon isobutene and has the formula (CF3)2C=CF2. An alkene, it is a colorless gas that is notable as a highly toxic perfluoroalkene. Few simple alkenes are as toxic. Safet ...
if heated to beyond its specified temperature; so care must be taken to avoid this, given the level of risk the result presents. The same is true of using Krytox and chemically resistant Molykote (PTFE thickened, fluoro based) oils and greases for
glassware upTypical drinkware The list of glassware includes drinking vessels (drinkware) and tableware used to set a table for eating a meal, general glass items such as vases, and glasses used in the catering industry. It does not include laboratory glass ...
seals. A high grade
stainless steel Stainless steel is an alloy of iron that is resistant to rusting and corrosion. It contains at least 11% chromium and may contain elements such as carbon, other nonmetals and metals to obtain other desired properties. Stainless steel's corros ...
joint clip is a final option. Naturally, this can withstand the entire temperature spectrum of
borosilicate glass Borosilicate glass is a type of glass with silica and boron trioxide as the main glass-forming constituents. Borosilicate glasses are known for having very low coefficients of thermal expansion (≈3 × 10−6 K−1 at 20 °C), ma ...
and is reasonably inert. Though, lower grades of stainless steel are still rapidly attacked in the presence of the corrosive gases and the clips themselves are often as expensive as PTFE. Some glassware features barbs (devil's horns, Viking helmet) sticking out the sides of the tapers. Small stainless steel springs are used on these to hold the joint together. The use of springs is of particular benefit when dealing with positive pressures, as they apply enough force for the glass to operate, but will open the taper if an unexpected excursion occurs. This method is considered quite old fashioned, but is still used on some of the most well known and high-end glassware available. For situations where the simple spring action of metal wires or plastic is not strong enough or are not convenient for other reasons, screws can be used to hold joints together. Plastic collars are often used on microscale equipment.


Hermetic sealing

A thin layer of PTFE material or grease is usually 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. The use of this helps provide a good seal and prevents the joint from seizing, allowing the parts to be disassembled easily. Although
silicone grease Silicone grease, sometimes called dielectric grease, is a waterproof grease made by combining a silicone oil with a thickener. Most commonly, the silicone oil is polydimethylsiloxane (PDMS) and the thickener is amorphous fumed silica. Using this ...
used as a sealant and a lubricant for interconnecting ground glass joints is normally assumed to be chemically inert, some compounds have resulted from unintended reactions with silicones.


Frozen joints

Sometimes conical ground glass joints can lock together, preventing the user from rotating them - this is known as freezing or locking. Ball and socket-type joints are much less susceptible since they have more degrees of rotation than a conical joint. This may happen for a variety of reasons: * Lack of lubrication between the two glass surfaces. If organic solvents come into contact with the joint, they can slowly dissolve the grease, leaving a dry glass-glass surface. * Exposure to a strong base (hydroxide, phosphate, etc.) may dissolve some of the SiO2 surface, generating
silicic acid Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
(H4SiO4 / Si(OH)4) * Solids from reaction mixtures * Allowing sealed vessels to cool, which creates a vacuum within the joint * Strong differential heating or cooling of the joint components, which can cause
shrink-fitting Shrink-fitting is a technique in which an interference fit is achieved by a relative size change after assembly. This is usually achieved by heating or cooling one component before assembly and allowing it to return to the ambient temperature after ...
. Frozen joints may be removed by working solvent into the joint while rocking the stopper, heating the outer joint, or cooling the inner stopper. The last two methods employ the property of
thermal expansion Thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to a change in temperature, usually not including phase transitions. Temperature is a monotonic function of the average molecular kinetic ...
to create a small space between the two surfaces. There are also specialized glassblowing tools to unfreeze the joint.


See also

*
Closure (container) A closure is a device used to close or seal a container such as a bottle, jug, jar, tube, or can. A closure may be a cap, cover, lid, plug, liner, or the like. The part of the container to which the closure is applied is called the finish. Ot ...


References

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