Laboratory Vessels, Chemical

utensils made of glass, quartz, porcelain, platinum, or other material that are used in chemical experimentation. Such utensils must be resistant to chemical reagents, easy to clean, resistant to heat, and display a low coefficient of thermal expansion. Chemical laboratory vessels can be subdivided according to use into volumetric, nonvolu-metric, and special-purpose.

Volumetric vessels display precise graduation and cannot be heated. Like all laboratory apparatus, they differ in volume, diameter, and shape. They include pipettes, which are used for drawing off liquids (0.1–100 ml) and gases (100 ml and higher); various burettes (1–100 ml), which are used in titration and for the measurement of precise volumes, for example, microburettes and volumetric, weight, piston, and gas burettes; volumetric flasks (10–2,000 ml), which are used for measuring off and storing specific volumes of liquids; and graduates, which have less precise graduations.

Nonvolumetric, or all-purpose, chemical laboratory vessels include vessels used for heating, such as test tubes (5–25 ml), beakers (5–1,000 ml), retorts (up to 3 liters), and flasks (10–1,000 ml); the last can be flat-bottomed, round-bottomed, or conical. They also include vessels that are not used for heating, such as test tubes made of thick glass for centrifuging; funnels for transferring and filtering liquids; separatory funnels (25 ml and higher), which can be cylindrical, pear-shaped, or globular; crys-tallizers, which are flat-bottomed vessels; and special and all-purpose condensers, which are used for cooling and condensing vapors and for collecting condensates. Other nonmeasuring laboratory vessels are siphons of various shapes and sizes, which are used for transferring liquids; water jets, which accelerate filtration and create a vacuum over a boiling liquid during distillation; vials that serve as containers from which a liquid is transferred to another vessel, for example, in burettes during titration; weighing bottles with ground-in stoppers, lids, which are used for storing substances; and various droppers for measuring out liquids.

Special-purpose chemical laboratory vessels include distillation flasks; adapters, which are in the form of bent tubes and are used to connect condensers to receptacles; dephlegmators—column packings in the form of tubes and featuring a wide upper section fitted with a tap—which are used in fractional distillation; and Kjeldahl’s flasks, pear-shaped vessels used for nitrogen determination. Other special-purpose vessels are desiccators, used for slow drying and storage of substances that readily absorb atmospheric moisture, for example, vacuum desiccators; various types of bottles for rinsing gases in order to rid them of impurities; Kipp generators, which are used to produce laboratory quantities of such gases as CO₂ and H₂S; and variously shaped tubes, for example, calcium-chloride U-tubes, for drying gases and for cleaning gases of mechanical impurities.

Glass is the most widely used material for chemical laboratory vessels, although other materials are also often used. Vessels made of quartz are necessary when working with especially pure substances, as well as for heating to temperatures of 1200°C and in a vacuum. Platinum laboratory vessels are primarily used when working with hydrofluoric acid. Such vessels are not recommended for use with PbSO₄, PbO₂, SnO₂, Bi₂O₃, Sb₂O₃, and other easily reducible compounds and with sulfur- and phosphorus-containing compounds in the presence of reducing agents. They are also not recommended when fusing iron-rich substances or substances that release halogens in the presence of oxidizing agents, for example, aqua regia. Crucibles made of silver or gold are suitable when fusing various substances with alkalis at 900°–1000°C, whereas those made of nickel or iron are used when fusing a given substance with Na₂O₂. Porcelain laboratory vessels are more durable and heat-resistant than glassware, but they are opaque and heavy. In addition to beakers, cups (for evaporation), and crucibles, porcelain is used to make mortars, Büchner funnels, spatulas and spoons used for sampling substances, and boats used when roasting various substances. Crucibles made of high-refractory materials, such as Alundum, alumina, or corundum, are used when heating at temperatures of 1200°–3000°C. Chemical laboratory vessels are often made from polymer materials, for example, polyethylene and fluoro-plastic, which combine chemical stability with valuable physicomechanical properties. Such vessels are usually recommended when working with aggressive substances, such as hydrofluoric acid.