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 plastics up to a point, and relatively easy to customize. Borosilicate glasses are often used because they are less subject to thermal stress and are common for reagent bottles. For some applications quartz glass is used for its ability to withstand high temperatures or its transparency in certain parts of the electromagnetic spectrum. In other applications, especially some storage bottles, darkened brown or amber (actinic) glass is used to keep out much of the UV and IR radiation so that the effect of light on the contents is minimized. Special-purpose materials are also used; for example, hydrofluoric acid is stored and used in polyethylene containers because it reacts with glass.[1] For pressurized reaction, heavy-wall glass is used for pressure reactor.
Contents |
There are many different kinds of laboratory glassware items, the majority 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, and pressure, like pressure reactor. 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.
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.
Borosilicate glass, which makes up the majority of lab glass, may fracture if rapidly heated or cooled through a 150 °C (302 °F) temperature gradient. This is particularly true of large volume flasks, that can take hours to safely warm up. Gentle thermal cycling should be used when working with volumes more than hundreds of mLs to two liters. Whenever working with borosilicate glass, it is advisable to avoid sharp transitions between temperatures when the heating and cooling elements have a high thermal inertia. Glassware can be wrapped with tinfoil or insulated with wool to smooth out temperature gradients.
500 °C (932 °F) is the maximum service temperature for borosilicate glass as, at 510 °C (950 °F), thermal strain begins to appear in the structures. Operation at this temperature should be avoided and only intermittent. Bear in mind that glassware under vacuum will also have around one atmosphere of pressure on its surface before heating and so will be more likely to fracture as temperature transitions increase. Vacuum operation should be used if the atmospheric temperatures required are above a few hundred degrees Celsius, as this often has a dramatic effect on boiling points; significantly lowering them.
Borosilicate anneals at 560 °C (1,040 °F), this removes built in strain in the glass.
At 820 °C (1,510 °F), borosilicate glass softens and is likely to deform. And at 1,215 °C (2,219 °F) it becomes workable.
Quartz glass is far more resilient to thermal shock and can be operated continuously at 1,000 °C (1,830 °F). Thermal strain appears at 1,120 °C (2,050 °F), annealing occurs at 1,215 °C (2,219 °F) and it becomes workable at 1,685 °C (3,065 °F).
It is common for students and those new to working with glassware to set hotplates to a high value initially to rapidly warm a solution or solid. This is not only bad practice, as it can scorch the contents, it will almost universally burst large flasks, and this is one of the reasons why large flasks are often heated in water, oil, sand and steam baths or using a mantle that surrounds most, or all, of the flask.
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]
Grease can be washed out of tapers by the flow of solvents past them. Reagents may react with the grease or it may leak from the tapers at higher temperatures. The latter is prone to occurring when the system is under vacuum. Grease leaking from tapers can, of course, contaminate an operation either passively or by actively reacting with something passing by. For these reasons, it is advisable to apply a light ring of grease at the fat end of the taper and not its tip, to keep the material away from insides of the glassware. If the grease smears over the entire taper surface on mating, too much is being used. Using greases specifically designed for this purpose is also a good idea, as these are often better at sealing under vacuum, thicker and so less likely to flow out of the taper, become fluidic at higher temperatures than Vaseline (a common substitute) and are more chemically inert than other substitutes.
Grease allows chemists to easily see when a taper is leaking, as bubbles can usually be seen flowing through the taper.
When contamination is a serious concern, PTFE (Teflon) sleeves and PTFE sealing rings can be used in between joints to fit them together instead of grease.[3] PTFE tape can also be used, but requires a little care when winding onto the joint to ensure a good seal is produced.
Keck clips and other clamping methods can be used to hold glassware together.
An absolute vacuum produces a pressure difference of one atmosphere, approximately 14 psi, over the surface of the glass. The energy contained within an implosion is defined by the pressure difference and the volume evacuated. Flask volumes can change by orders of magnitude between experiments. Whenever working with liter sized or larger flasks, chemists should consider using a safety screen or the sash of a flow hood to protect them from shards of glass, should an implosion occur. Glassware can also be wrapped with spirals of tape to catch shards, or wrapped with webbed mesh more commonly seen on scuba cylinders.
Glass under vacuum becomes more sensitive to chips and scratches in its surface, as these form strain accumulation points, so older glass is best avoided if possible. Impacts to the glass and thermally induced stresses are also concerns under vacuum. Round bottom flasks more effectively spread the stress across their surfaces, and are therefore safer when working under vacuum.
When connecting glassware, it is often tempting to use Keck clips on every joint, but this can be dangerous if the system is sealed or the exhaust is in any way restricted; e.g. by wash flasks or drying media. Many reactions and forms of operation can produce sudden, unexpected surges of pressure inside the glass. If the system is sealed or restricted, this can blow the glass apart. It is safer to only clip the joints that need holding together to stop them falling apart and to purposefully leave one or more unclipped; preferably those that are connected to lightweight, small objects like stoppers, thermometers or wash heads, that are pointing vertically upwards and not connected to other items of glassware. By doing so, any significant surge of pressure will cause these specifically chosen tapers to open and vent. This may seem counterintuitive, but it is safer and easier to deal with a controlled escape as opposed to the entire volume being uncontrollably released in an explosion.
This is a prerequisite for a lot of laboratory work as it protects the work itself and decreases the possibility of thermal strain fracturing the glass; see service temperatures for more information on this.
A common method is to fill a bowl surrounding the flask with water, oil, sand or steam, or to use a wrap around heating mantle.
However, baths can be extremely dangerous if they spill, overheat or ignite, they have a high thermal inertia (and so take a long time to cool down) and mantles can be very expensive and are designed for specific flask volumes. There are two alternative methods that can be used instead, where appropriate.
When a heat source's minimum temperature is high, the glassware can be suspended slightly above the surface of the plate. This will not only reduce the ultimate temperature on the glass, it will slow down the rate of heat exchange and encourage more even heating; as there is no longer direct contact via a few points with the plate. Doing so works well for low boiling point operations.
If the glassware must be run at higher temperatures, a teepee setup can be used; so named as it looks a little like a tipi. This is when the glassware is suspended above the plate, but the flask is surrounded by a skirt of tinfoil. The skirt should start at the neck of the flask and drape down to the surface of the plate, not touching the sides of the flask. Having the base of the skirt cover the majority of the plates surface will effect better heat transfer. The flask will now be warmed indirectly by the hot air collecting under the skirt but, unlike simply suspending the glassware, it can now reach hundreds of degrees Celsius and is better protected from drafts.
Both these methods are useful as they are either cheaper or free, effective, safe and feature low thermal inertia transfer methods, meaning the chemist does not have to wait for a bath to cool down after use.
Baths are most useful when the heat source has little or no control over it. With the advent of variable temperature hotplates and wrap around mantles, their necessity has somewhat declined. The same can be said for many round bottom flask operations, which require the use of a bath.
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).
Ground glassware should be disassembled as soon as it is safe to do so after a reaction, as this will help avoid the tapers seizing. Tapers will seize either from thermal activity or something from the reaction penetrating the taper and thickening upon cooling or exposure to the atmosphere. High concentrations of certain reactants can chemically seize a taper such that is essentially impossible to open. Exposure time plays a role in this occurring, and so should be minimized. PTFE sealing methods are also of use for such applications as they produce an intermediate layer between the glass that is highly inert and solid, meaning it can not be displaced and the glass can never come in contact with its mating taper.
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.
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.
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.
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.
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.
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.
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.
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.
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.
More examples are featured in the gallery. This is a small sampling of stopcock valves; many additional variations exist in both plug boring and joint assembly.
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 piece of female glass. Screwing the plug in part-way first engages one or more O-rings, made of rubber or plastic, near the plug's base, which seals the female joint off from the outer atmosphere. Screwing the plug valve all the way in engages the plug's 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 it is convenient 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 and are attached to a threaded sleeve in such a way that the sleeve can be 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 they 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, the 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 been 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 valve's 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 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 (also known as gas diffusing stones or diffusors) as well as scrubbers, and gas-washing bottles (or Drechsel bottles [6]) 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.
As frits are made up of particles of glass that are bonded together by small contact areas, it is wise to avoid using them in strongly alkaline conditions, as these can dissolve the glass to some extent. This is not normally a problem, as the amount dissolved is usually minute, but the equally minute bonds in a frit can be rotted away, causing the frit to fall apart over time. As such, consideration should be given to using frits in such solutions and they should be rapidly and thoroughly rinsed when cleaning the glass with bases like KOH.
There are many different methods of cleaning laboratory glassware. Most of the time, these methods [7][8] are tried in this order:
If the glassware are still dirty, more drastic methods may be needed. This includes soaking the piece in a saturated solution of sodium or potassium hydroxide in an alcohol ("base bath"),[8] 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. Care should be taken using strongly alkaline solutions to clean fritted glassware, as this will degrade the frit over time.
More aggressive methods involving aqua regia (for removing metals from frits), piranha solution and chromic acid (for removing organics), and hydrofluoric acid baths are generally considered unsafe for routine use because of possible explosions and the corrosive/toxic materials involved.[8]
Chromic acid is not a preferred method if the glassware is to be used for the biological sciences, as chromate ions can implant themselves in the glass and produce anomalous results when it is subsequently used for cell cultures; to which the ions are toxic. A proprietary alternative known as NoChromix is available, which is essentially a sachet of largely ammonium persulphate and a smaller amount of surfactant. This is poured into a bottle of concentrated sulphuric. Like concentrated hydrogen peroxide, ammonium and sodium persulphate are strong oxidisers, yet they are not hydroscopic and are more stable. This allows them to be more easily stored and used. When mixed with concentrated sulphuric, they begin releasing oxygen, which can oxidise the carbonaceous dehydration products formed from organic residues by the sulphuric to carbon dioxide; 'burning' them off the glass. The rate of effervescence is slower than that of strong piranha solution, allowing more time for deposits to mechanically break up and for the mixture to be used before fully decomposing. This same method is used in some PCB etching tanks, where sodium persulphate (fine etch crystals) are combined with sulphuric acid to oxidise the copper surface and then make it water soluble as it's sulphate.
|
|||||||||||||||||||