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 attacks glass.^[1]

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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. 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.

Ground glass joints

Main article: Ground glass joint

In a lab experiment or process—such as a distillation or a reflux—ground 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^[2] 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.^[2]

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.

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.^[2]

PTFE (Teflon) sleeves and PTFE sealing rings have been used in between joints to fit them together instead of grease.^[3]

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 connector.

Stopcocks

See also: stopcock

Stopcocks are basically valves. They are often parts of laboratory glassware such as burettes, separatory funnels, and columns used for column chromatography. The stationary outer body of the stopcock is typically made of glass, since it is fused with the rest of the glass item. The inner plug or rotor, which can be rotated inside the body to control flow through the stopcock, has one (or more) holes going through it which serve as a fluid pathway(s).

The inner plug or rotor can be made of plastic (usually PTFE) or glass. When it is plastic, the stopcock body's inner glass surface contacting it is typically smooth glass. When it is made of glass, the contacting glass surfaces are typically both ground-glass surfaces with stopcock grease used between them for lubrication. High vacuum glass manifolds typically use specially fitted, non-interchangeable all-glass stopcocks. Such stopcocks are often available separately with some lengths of glass tubing at the ports so that glass blowers can use them to make custom glass manifolds for vacuum lines.

Rotaflo is a brand of specialty stopcocks. They generally consist of a threaded PTFE plug which fits a correspondingly threaded glass barrel. By screwing down the barrel, the flow can be stopped completely.

PTFE stopcocks usually wear out fairly quickly. On the other hand, they are interchangeable, making replacements easy, compared with fitted glass joints.

O-ring joints

There are also glass joints available sometimes which use an O-ring between them to form a leak-tight seal.^[2] 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.

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.^[4] 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.^[5]

  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 ^[6][7] 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"),^[7] 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