Cold finger

{{Short description|Laboratory equipment}}

{{For|the medical condition causing cold fingers|Raynaud syndrome}}

Image:Sublimation_apparatus.svg. 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.]]

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).{{cite book|title=Laboratory Technique in Organic Chemistry|url=https://archive.org/details/laboratorytechni00wibe_0|url-access=registration|author=Kenneth B. Wiberg|isbn=0070700958|year=1960|publisher=McGraw-Hill}}

Typically a cold finger is used in a sublimation apparatus,{{Cite book|title=The Organic Chem Lab Survival Manual: A Student's Guide to Techniques|last=Zubrick|first=James W.|publisher=John Wiley & Sons|year=2016|isbn=978-1118875780|location=United States of America|pages=192–194|chapter=Sublimation|edition=10th }} 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.