Laboratory Glass

Laboratory Glass. Special high requirements are imposed on glass for laboratory apparatus. 1) Chemical resistance. The main requirement concerns the lowest possible alkalinity of the glass; in this respect, quartz apparatus and apparatus made of quartz glass are best, as they do not release alkali to water at all. Resistance to acids is also very important; low-grade glass is corroded, for example, by hydrochloric acid fumes (it becomes dull). No glass is completely resistant to the action of strong alkali solutions; glasses with a high content of alkali-insoluble oxides (lime, barium oxide, oxides of rare earths) have increased resistance. Laboratory glass should not release other substances besides alkali; cases are known of leaching by water and reagents, especially when heated, of arsenic, fluorine, antimony, tin, selenium, and boron compounds used in glass production.- 2) Thermal resistance. Laboratory glass must be resistant to sudden temperature changes; this depends on a low coefficient of expansion. The linear coefficient of expansion for "non-devitrifying" glass is about 6.10^-6; for low grades, 20-30.10^-6. Generally, glass better withstands sudden heating than sudden cooling. Thermal resistance is also related to the requirement for the highest possible thermal conductivity of laboratory glass. Thermal resistance also depends on proper annealing and the absence of stresses in the glass mass. 3) Fusibility. For glassblowing work, easily fusible glass is more convenient; conversely, for retorts, combustion tubes, or new apparatus; tubes, rods, and flasks are also produced by the Klin¹ plants. The trust Goslaborsnabzhenie; (now Goslabreaktivsbyt) published in 1926 a catalog of the "Druzhnaya Gorka" plant for laboratory glass. Analysis of better-known foreign grades of laboratory glass shows the following average composition: Glass Name Bohemian for tubes Thuringian . . . . Jena 16 III . . . . » 69 III ... . . » 1920 ..... Apparatus...... Pyrex (USA)........ Resist ........ Easily fusible for tubes ....... Soviet glass plant "Druzhnaya Gorka" No. 23 ........ Composition in percent SiO₂ B₂O₃ TiO₂ Al₂O₃ Fe₂O₃ MnO ZnO BaO CaO MgO K₂O Na₂O 74.4 69.9 67.3 74.5 65.3 80.6 78.84 69.64 69.5 2 12 4.6 15 10.5 13.6 2.75 0.07 2.5 5 8.5 3.5 3.5 0.93 3.19 4.9 0.1 3.0 0.47 0.06 0.4 0.18 4.2 3.9 7.2 3.8 0.8 0.7 0.28 6.40 8.2 0.6 0.97 0.37 18.3 6.6 1.93 5.6 16.5 14 11 7.7 4.1 2.78 For retorts, combustion tubes, or new apparatus; refractory glass is required. Alkalis lower the melting point, as does boric acid and borax; magnesium and zinc oxides increase refractoriness. Lime and alumina cause rapid solidification of softened glass.-4) Mechanical strength. The addition of titanium, molybdenum, etc. oxides increases the elasticity of glass ("shatter-proof glass").-5) Optical properties. Optically noticeable defects are more likely to serve in laboratory glass as indicators of shortcomings: glass with bubbles, streaks, devitrified usually easily cracks at such marked places. For work with short-wave radiation (mitogenetic rays, quartz lamp, X-rays), apparatus made of glass that transmits the corresponding rays is required; conversely, in other cases, glass ("euphos") that does not transmit ultraviolet rays is required. Many types of glass are colored by the rays of a quartz lamp (including Jena glass 16 III). Laboratory glass should be colorless. Slight discoloration is not significant; however, for vessels in which color, turbidity, etc. optical phenomena are determined (cuvettes, test tubes, vessels for colorimeters), completely colorless glass is necessary. For vessels in which optical determinations are made, a certain refractive index may be required (cuvettes, cover glasses). 6) Electrical conductivity. Generally, glass is considered an insulator, so its minimum electrical conductivity is expected. However, for special needs (glass electrodes), glass with increased electrical conductivity is required.--7) Hygroscopicity. Low-grade glass attracts moisture; such is unsuitable for vessels in which weighings are performed (flasks, crucibles, beakers, watch glasses, etc.). As can be seen, there is no need to pursue an "ideal" universal glass for laboratory apparatus, but it is necessary to select glass depending on the purpose of the vessels, guided by expediency. In all respects, Pyrex glass has the highest merits; quartz and quartz glass apparatus is brittle and hygroscopic. In the USSR, laboratory apparatus is produced by the "Druzhnaya Gorka" plant (glasses No. 16, No. 23 and molybdenum- Pyrex glass is produced in the USSR by plants in Leningrad and Merefe; its characteristics: sp. gr. 2.25; refractive index 1.4784; linear coeff. of expansion 3.3×10^-6; softening temperature 642°C. Lit. -production. see literature for the article Glass production. I. Obergard.