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A cannula (plural cannulae or cannulas) is a tubing that can be inserted into a reservoir or body (for medical uses), often for the delivery or removal of fluid. Lab cannula are a special type of tubing used in air-free synthesis chemistry to transfer air-sensitive liquids and solutions, though can also be used for transferring other liquids.
This article will talk only about the one type used in chemistry.
A cannula consists of a thin tube, with a small tube diameter, made of a chemically or biologically inert tubing, usually silicone rubber or PTFE. Stainless steel cannula are also encountered in chemistry labs, though they're unsuitable to carry halogens or haloacids. Often, a typical cannula has an outer diameter of 1.5-2 mm and an inner diameter or 0.5-1 mm, and the length varies, but most often they're never shorter than 30 cm. When used in Schlenk lines, cannulae are always used in combination with rubber septa. Needles are rarely used for PTFE tubing, more often they're used for silicone rubber cannulae.
Cannulae can be purchased from medical suppliers. Thin tubing/hoses can sometimes be purchased from hardware or electronics stores.
PTFE cannula can be purchased from lab suppliers.
- Transfer air sensitive liquids and solutions
- Safe handling of foul-smelling, corrosive or extremely toxic liquid reagents (good for handling volatile lachrymatory agents)
Avoid bending the cannula too sharply, especially if it's PTFE, as such bent will irreversible damage the tubing. It can kink shut or even crack, resulting in little or no flow or a even perforating the cannula, causing it to leak.
Since cannulae are usually used for air-sensitive work, they are commonly kept in a hot oven before immediate use, to reduce the adsorption of water molecules. Like the glassware setup used to carry out the reaction, cannulae are usually subjected to three vacuum-refill cycles to remove any traces of air.
Cutting a cannula is a bit tricky. If you use a wire cutter or scissors, you usually collapse the tube and pinch it shut. A good technique is to score the cannula with a file and then snap it just as you would do when cutting glass tubing. Then use a file to sharpen a point on the end of the cannula.
After use, clean your cannula as soon as you are done with it. To clean it, do the following:
- If the cannula is clogged, you can try cleaning it with a fine cleaning wire.
- Flush the cannula with an appropriate solvent.
- Flush the cannula with dry nitrogen or argon.
Handling pyrophoric liquids
While handling pyrophoric material (e.g. tert-butyllithium), traces of the compound at the tip of the needle or cannula may catch fire, and cause a clog. Some workers prefer to contain the tip of the needle or cannula in a short glass tube flushed with an inert gas, and sealed via two septa.
Instead of exposing the needle tip to the air, it is withdrawn into the inerted tube. Where desired, it may be inserted into a flask via two septa (one on the tube, one on the flask). Used this way, needle tip fires are eliminated, reducing the obvious hazards. Also, there is a reduced tendency for the needle tip to clog due to the reaction of traces of the reagent with air to give salts.