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 and Otto W. Post. A Craig tube consists of two parts. The first is a stout-walled

test tube A test tube, also known as a culture tube or sample tube, is a common piece of laboratory glassware consisting of a finger-like length of glass or clear plastic tubing, open at the top and closed at the bottom. Test tubes are usually placed in ...

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, 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 The mother liquor (or spent liquor) is the solution Solution may refer to: * Solution (chemistry), a mixture where one substance is dissolved in another * Solution (equation), in mathematics ** Numerical solution, in numerical analysis, approx ...

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 Filter paper is a semi-permeable paper barrier placed perpendicular to a liquid or air flow. It is used to separate fine solid particles from liquids or gases. The raw materials are different Pulp (paper), paper pulps. The pulp may be made from soft ...

, 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