A Craig tube is an item of apparatus used in small-scale (up to about 100 mg) preparative and analytical chemistry, particularly for recrystallisation. It was invented by

Lyman C. Craig Lyman C. Craig (born 1906 in Palmyra Township, Warren County, Iowa; died 1974) was a chemical researcher who worked at The Rockefeller Institute for Medical Research from 1933 onward. In 1944 he published a seminal work on countercurrent distributi ...

and Otto W. Post. A Craig tube consists of two parts. The first is a stout-walled test tube with a working volume of about 1-5 ml (say, 7-8 cm in length and 1-1.5 cm in diameter). There is a constriction towards the open end of the tube. The second is a loosely-fitting generally-cylindrical stopper, possibly with a teardrop-shaped head, of glass or of another inert material such as

PTFE Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene that has numerous applications. It is one of the best-known and widely applied PFAS. The commonly known brand name of PTFE-based composition is Teflon by Chem ...

, which seats on the constriction. Recrystallisation is carried out in the usual manner, by dissolving the solid to be purified in a solvent and causing crystals of the solid to form. The stopper can be used to protect the solution from atmospheric contamination. The crystals are separated from the mother liquor by placing the tube and stopperinvertedin a centrifuge tube, followed by

centrifugation Centrifugation is a mechanical process which involves the use of the centrifugal force to separate particles from a solution according to their size, shape, density, medium viscosity and rotor speed. The denser components of the mixture migrate ...

. The stopper allows the mother liquor to pass into the centrifuge tube but retains the crystals, which can subsequently be recrystallised again or collected. The apparatus has the advantages that the crystallised product is relatively dry, is free from contamination by fibres from filter paper, and can be recovered more efficiently than from a sinter funnel. Craig tubes can be made by competent glassblowers, and are also available commercially.

References

Laboratory glassware {{Chem-stub