Cold finger

A cold finger is a piece of laboratory equipment that is used to generate a localized cold surface. It is named for its resemblance to a finger and is a type of cold trap. The device usually consists of a chamber that a coolant fluid (cold tap water, or perhaps something colder) can enter and leave. Another version involves filling the device with a cold material (examples: ice, dry ice or a mixture such as dry ice/acetone or ice/water).[1]

For the medical condition causing cold fingers, see Raynaud syndrome.
Cold finger used in sublimation. The raw product (6) is in the bottom of the outer tube (4) which is heated (7) while under vacuum (through side-arm 3). The sublimated material collects (5) on the cold finger proper, cooled by a coolant (blue) circulated through ports 1 and 2.

Typically a cold finger is used in a sublimation apparatus,[2] or can be used as a compact version of a condenser in either reflux reaction or distillation apparatus. Many commercially available rotary evaporators can be purchased with a cold finger in place of a Dimroth condenser, for example. When used as a condenser in a rotary evaporator, cold fingers can be cooled to a lower temperature of −78 °C (dry ice), compared with water condensers that can be cooled to −40 °C (ethylene glycol/water mixture). The lower temperature achieved reduces the quantity of volatile material exhausted into the air.

Media
  • A cold finger which includes a vacuum outlet.
    A cold finger which includes a vacuum outlet.
  • Camphor being sublimed. The crude product in the bottom is dark brown; the white purified product on the bottom of the cold finger above is hard to see against the light background.
    Camphor being sublimed. The crude product in the bottom is dark brown; the white purified product on the bottom of the cold finger above is hard to see against the light background.
  • Dark green crystals of nickelocene, freshly sublimed on a cold finger.
    Dark green crystals of nickelocene, freshly sublimed on a cold finger.

References
  1. Kenneth B. Wiberg (1960). Laboratory Technique in Organic Chemistry. McGraw-Hill. ISBN 0070700958.
  2. Zubrick, James W. (2016). "Sublimation". The Organic Chem Lab Survival Manual: A Student's Guide to Techniques (10th ed.). United States of America: John Wiley & Sons. pp. 192–194. ISBN 978-1118875780.
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