A cannula is a long thin metal tube that is used to transfer air-sensitive liquids or solutions. Cannulas come in different bore sizes and lengths. A typical cannula is two to three feet long and is made up of 16-22 gauge (.047"-.016") tubing. The ends are pointed so they can be inserted through septa. In many respects, a cannula is like a very long double-ended syringe needle.
In the drawing below, a cannula is being used to transfer liquid from one Schlenk flask to another:
Transferring liquid using a cannula
Using the above drawing as an example, this is how we would transfer liquid from one Schlenk flask to another.
Turn up the nitrogen pressure on bubblerA and turn it off on bubbler B. Insert one end of the cannula through the rubber septum on the left Schlenk flask, but DO NOT submerge it below the level of the liquid.
Check to see that nitrogen is flowing out of the cannula. An easy way to do this is to stick the free end in a beaker of solvent -- you should see bubbles. If not, the cannula may be bent or clogged (usually with a bit of septum).
Once the cannula is purged, insert the other end through the rubber septum on the right Schlenk flask. The second flask should be open to bubbler B. You will notice that bubbler B will start to bubble once the cannula has been inserted into the right flask.
Now submerge the cannula below the level of the liquid in the left Schlenk flask. If necessary, turn up the nitrogen pressure on bubbler A until liquid flows through the cannula into the right flask.
Once you have dispensed the desired volume of liquid, pull the cannula up so that it is no longer submerged in the solution in the left Schlenk flask.
Important: Clean your cannula right away! See below.
There are other variations on this technique. For example, you can transfer liquid from a bottle if you use a needle to connect the bottle to a nitrogen bubbler. You can transfer into other vessles such as graduated addition funnels or Schlenk tubes. In addition, you can perform a cannula filtration by putting a small filter on the back end of your cannula.
For proper care and long life for your cannula be sure to:
Avoid bending it too sharply. It can kink shut or even crack, resulting in little or no flow or a potentially dangerous leak.
Clean your cannula as soon as you are done with it!
Flush the cannula with an appropriate solvent.
Flush the cannula with dry nitrogen.
If the cannula is clogged, you can try cleaning it with a fine cleaning wire.
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.
This page was last updated Friday, October 22, 2010.
This document and associated figures* are copyright 1996-2015 by Rob Toreki. Send comments, kudos and suggestions to us via email. *Some of these figures are adapted from the Chemglass, Inc. catalog and have been reproduced with permission.