Watch glass

Caesium fluoride sample on a watch glass

A watch glass is a circular concave piece of glass used in chemistry as a surface to evaporate a liquid, to hold solids while being weighed, for heating a small amount of substance and as a cover for a beaker. The latter use is generally applied to prevent dust or other particles entering the beaker; the watch glass does not completely seal the beaker, so gas exchanges still occur. When used as an evaporation surface, a watch glass allows closer observation of precipitates or crystallization, and can be placed on a surface of contrasting color to improve the visibility overall. Watch glasses are also sometimes used to cover a glass of whisky, to concentrate the aromas in the glass, and to prevent spills when the whisky is swirled.[1] Watch glasses are named so because they are similar to the glass used for the front of old-fashioned pocket watches. In reference to this, large watch glasses are occasionally known as clock glasses.

Uses

Crystal solids on a watch glass with folded paper above
Drying crystal solids using a watch glass and passing a stream of dry air from an inverted funnel

One of the generic uses of a watch glass as mentioned previously includes as a lid for beakers. In this case a watch glass is placed above the container, which makes it easier to control and alter vapour saturation conditions.[2] Moreover, a watch glass is often used to house solids being weighed on the scale. Prior to weighing desired amount of solid, a watch glass is placed on the scale, followed by taring or zeroing the scale so that only the weight of the sample substance is obtained.[3] Not only can a watch glass be used for observing precipitation patterns and crystals, it can also be used for drying solids. When further drying is required, a watch glass is often used in cases where a particular type of solid needs to be separated from its comparatively volatile solvent. The solid is spread on a watch glass and, often time, a folded filter paper is placed above to keep out airborne particles from contaminating the product. To maximize the drying rate, a watch glass can be placed inside a fume hood to provide good air circulation for an adequate amount of time. Another technique used in chemistry laboratories to increase the drying rate is passing a gentle stream of dry air or nitrogen gas over the watch glass from an inverted funnel clamped above it.[4]

Types

Front and back image of different sizes for watch-glass used in laboratory.

There are two types, Glass watch glasses and Plastic watch glasses used in the laboratory. These watch-glasses are available in various sizes and widths. Watch-glass are usually thicker than any other glass or plastic lab-ware.[5]

  • Glass watch glasses – This can be reused after it is sterilized in an autoclave or a laboratory oven. The glasses made provide high resistance to thermal shock, chemical resistance and withstand mechanical durability.
  • Plastic watch glasses – These are disposable watch-glass used in laboratory work to avoid any cross-contamination while preparing the samples. They are very good at implementing in low-temperatures and has an operating span of −70 degree Fahrenheit to 275 degree Fahrenheit (−56.6667 to 135 degree Celsius), it can also resist from UV light degradation. These plastic watch glasses are less expensive and light in weight.[6]

See also

References

  1. ↑ Graham, Lawrence. "E-pistle 2007/030 – Whisky Glasses; a Study". Malt Maniacs. Canada. Retrieved 28 June 2015.
  2. ↑ Helmenstine, Anne Marie. "Watch Glass – Photo". About. Retrieved 28 June 2015.
  3. ↑ Jones, Chad. "Chemistry Lab Equipment: Watch Glass". Answers. Retrieved 28 June 2015.
  4. ↑ Lehman, John W. (2008). The Student's Lab Companion: Laboratory Techniques for Organic Chemistry (2nd ed.). Prentice Hall. pp. 156–157. ISBN 9780131593817.
  5. ↑ "Watch Glasses Information | IHS Engineering360". www.globalspec.com. Retrieved 2016-02-17.
  6. ↑ "Types | globalspec".
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