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threaded-glass

Laboratory glassware

Laboratory glassware refers to a variety of equipment, traditionally made of glass, used for scientific experiments and other work in science, especially in chemistry and biology laboratories. Some of the equipment is now made of plastic for cost, ruggedness, and convenience reasons, but glass is still used for some applications because it is relatively inert, transparent, more heat-resistant than some plastic up to a point, and relatively easy to customize. Borosilicate glasses—formerly called Pyrex—are often used because they are less subject to thermal stress. For some applications quartz is used for its ability to withstand high temperatures or its transparency in certain parts of the electromagnetic spectrum. In some applications, especially some storage bottles, darkened brown glass is used to keep out much of the outside light so that the effect of light on the contents inside is minimized. Special-purpose materials are also used; for example, hydrofluoric acid is stored and used in polyethylene containers because it reacts with glass.

Applications

There are many different kinds of laboratory glassware items, the majority of which are covered in separate articles of their own; see the list further below. Such glassware is used for a wide variety of functions which include volumetric measuring, holding or storing chemicals or samples, mixing or preparing solutions or other mixtures, containing lab processes like chemical reactions, heating, cooling, distillation, separations including chromatography, synthesis, growing biological organisms, spectrophotometry, and containing a full or partial vacuum. When in use, laboratory glassware is often held in place with clamps made for that purpose, which are likewise attached and held in place by stands or racks. This article covers aspects of laboratory glassware which may be common to several kinds of glassware and may briefly describe a few glassware items not covered in other articles.

Production

Most laboratory glassware is now mass-produced, but many large laboratories employ a glass blower to construct specialized pieces. This construction forms a specialized field of glassblowing requiring precise control of shape and dimension. In addition to repairing expensive or difficult-to-replace glassware, scientific glassblowing commonly involves fusing together various glass parts—such as glass joints and tubing, stopcocks, transition pieces, and/or other glassware or parts of them to form items of glassware, such as vacuum manifolds, special reaction flasks, etc.

Various types of joints and stopcocks are available separately and come fused with a length of glass tubing, which a glassblower may use to fuse to another piece of glassware.

Lubrication and sealing

A thin layer of 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 grease helps to provide a good seal and prevents the joint from seizing, allowing the parts to be disassembled easily.

PTFE (Teflon) sleeves and PTFE sealing rings have been used in between joints to fit them together instead of grease.

Glassware Joints

Ground glass joints

In a lab experiment or process—such as a distillation or a refluxground glass joints make it possible to rapidly assemble the set-up from component glassware items in a leak-tight but non-permanent way. Using old technology, this was often done with rubber (or possibly cork) stoppers inserted between the component glassware items. Holes could be made in such stoppers to insert glass tubes or the ends of some glass items. However, rubber (and of course cork) are not as chemically inert or heat-resistant as glass and degrade with age. In order to connect the hollow inner spaces of the glassware components, these types of joints are hollow on the inside and open at the ends, except for stoppers.

Two general types of ground glass joints are fairly commonly used: joints that are slightly conically-tapered and ball and socket joints (sometimes called spherical joints).

Conically tapered joints

Conically tapered ground glass joints consist of a male and a female half which are manufactured to a standard 1:10 taper. Apart from stoppers, most conically tapered joints are hollow to allow liquids or gases to flow through. An example of the use of conically-tapered joints is to join a round bottom flask, Liebig condenser, and oil bubbler together to allow a reaction mixture to be refluxed.

Ball-and-socket joints

Here, 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. Ball and socket joints are used where some degree of free-play is necessary, such as when joining a cold trap to a gas manifold for a Schlenk line.

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, known as Keck clips, may be placed around the union of the joints to help keep them together.

O-ring joints

There are also glass joints available sometimes which use an O-ring between them to form a leak-tight seal. Such joints are more symmetrical in theory with a tubular joint on each side having a widened tip with a concentric circular groove into which an elastomer O-ring can be inserted between the two joints. O-ring joints are sized based on the inner diameter in mm of the joint. Since they can come apart rather easily, a clip or pinch clamp is needed to hold them together. The elastomer of the O-ring is more limited in high temperature resistance than other types of glass joints using high temperature grease.

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.

Glass-to-metal transition joints

Occasionally, it may be desired to fuse a glassware item to a metal item with a tubular pathway between them. This requires the use of a glass-to-metal transition joint. Most glass used in laboratory glassware does not have the same coefficient of thermal expansion as metal, so fusing the usual type of glass with metal is likely to result in cracking of the glass. These special transition joints have several short sections of special types of glass fused together between the metal and the usual type of glass, each having more gradual changes in thermal expansion coefficients.

Hose connections

Laboratory glassware, such as Buchner flasks and Liebig condensers, 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 con

Glassware Valves

Describing glassware can be complicated since manufactures provide conflicting names for glassware. For example ChemGlass calls a glass stopcock what Kontes calls a glass plug. Despite this it is clear there are two main types of valves used in laboratory glassware, the stopcock valve and the threaded plug valve. These and other terms used below are defined in detail since they are bound to conflict with different sources.

Stopcock valve

Stopcocks are often parts of laboratory glassware such as burettes, separatory funnels, Schlenk flasks, and columns used for column chromatography. The stopcock is a smooth tampered plug or rotor with a handle, which fits into a corresponding ground glass female joint. The stationary female joint is designed such that it joins two or more pieces of glass tubing. The stopcock has holes bored through it which allow the tubes attached to the female joint to be connected or separated with partial turns of the stopcock. Most stopcocks are solid pieces with linear bores although some are hollow with holes to simple holes that can line up the joints tubing. The stopcock is held together with the female joint with a metal spring, plastic plug retainer, a washer and nut system, or in some cases vacuum. Stopcocks plugs are generally made out of ground glass or an inert plastic like PTFE. The ground glass stopcocks are greased to create an airtight seal and prevent the glass from fusing. The plastic stopcocks are at most lightly oiled.

Stopcocks are generally available individually with some length of glass tubing at the ports so that they can be joined by a glass blower into custom apparatus at the point of use. This is especially common for the large glass manifolds used in high vacuum lines.

The more examples are featured in the gallery. This is a small sampling of stopcock valves many additional variation exist in both plug boring and joint assembly.

Threaded plug valve

Threaded plug valves are used significantly in air-sensitive chemistry as well as when a vessel must be closed completely as in the case of Schlenk bombs. The construction of a threaded plug valve involves a plug with a threaded cap which are made so that they fit with the threading on a corresponding pieces of female glass. Screwing the plug in part way first engages one or more o-rings, made of rubber or plastic, near the plugs base which seals the female joint off from the outer atmosphere. Screwing the plug valve all the way in engages the plugs tip with a beveled constriction in the glass which provides a second seal. This seal separates the region beyond the bevel and the o-rings already mentioned.

With solid plugs a tube or area exists above and below the bevel and turning the plug controls access. In a number of cases its convent to fully remove a plug which can give access to the region beyond the bevel. Plugs are generally made of an inert plastic such as PTFE with and are attached to a threaded sleeve in such a way that the sleeve can been turned without spinning the plug. The contact with the bevel is made by an o-ring fitted to the tip of the plug or by the plug itself. There are a few examples where the plug in made of glass. In the case of glass plugs the joint contact is always a rubber o-ring but are still prone to shattering.

Not all plugs are solid. Some plugs are bored with a T-junction. In these systems the plug extends beyond the threaded sleeve and is designed to form an airtight fitting with glass tubing or hosing. The shaft of the plug is bored from beyond the threaded sleeve to a T-junction just before the bevel plug contact. When the plug is fully sealed region beyond the bevel is separated from the plug shaft as well as the bore which leads out of its shaft. When the plug bevel contact is released the two regions are exposed to each other. These valves have also be used as a grease free alternative to straight bored stopcocks common to Schlenk flasks. The high symmetry and concise design of these valves has also made them popular for capping NMR tubes. Such NMR tubes can be heated without the loss of solvent thanks to the valves gas tight seal. NMR tubes with T-bore plugs are widely known as J. Young NMR tubes named after the brand name of valves most commonly used for this purpose. Images of J. Young NMR tubes and a J. Young NMR tube adapter are in the gallery.

Fritted glass

Fritted glass is finely porous glass through which gas or liquid may pass. It is made by sintering together glass particles into a solid but porous body. This porous glass body can be called a frit. Applications in laboratory glassware include use in fritted glass filter items, scrubbers, or spargers. Other laboratory applications of fritted glass include packing in chromatography columns and resin beds for special chemical synthesis.

In a fritted glass filter, a disc or pane of fritted glass is used to filter out solid particles, precipitate, or residue from a fluid, similar to a piece of filter paper. The fluid can go through the pores in the fritted glass, but the frit will often stop a solid from going through. A fritted filter is often part of a glassware item, so fritted glass funnels and fritted glass crucibles are available.

Laboratory scale spargers, scrubbers, and gas-washing bottles are similar glassware items which may use a fritted glass piece fused to the tip of a gas-inlet tube. This fritted glass tip is placed inside the vessel with liquid inside during use such that the fritted tip is submerged in the liquid. To maximize surface area contact of the gas to the liquid, a gas stream is slowly blown into the vessel through the fritted glass tip so that it breaks up the gas into many tiny bubbles. The purpose of sparging is to saturate the enclosed liquid with the gas, often to displace another gaseous component. The purpose of a scrubber or gas-washing bottle is to scrub the gas such that the liquid absorbs one (or more) of the gaseous components to remove it from the gas stream, effectively purifying the gas stream.

Cleaning laboratory glassware

There are many different methods of cleaning laboratory glassware. Most of the time, these methods are tried in this order:

  • The glassware is soaked in a detergent solution to remove grease and loosen most contamination
  • Gross contamination and large particles are removed mechanically, by scrubbing with a brush or scouring pad.
    • Alternatively, the first two steps may be combined by sonicating the glassware in a hot detergent solution
  • Solvents known to dissolve the contamination are used to rinse the glassware and remove the last traces

If the glassware are still dirty, more caustic methods may be needed. This includes soaking the piece in a saturated solution of sodium or potassium hydroxide in an alcohol ("base bath"), followed by a dilute solution of hydrochloric acid ("acid bath") to neutralize the excess base. Sodium hydroxide cleans glass by dissolving a tiny layer of silica, to give soluble silicates.

Older methods involving aqua regia (for removing metals from frits), piranha solution and chromic acid (for removing organics) are generally considered unsafe because of possible explosions and the corrosive/toxic materials involved.

References

Gallery

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