# Why pipetting with a 30° angle?

After seeing this youtube video, I was left to wonder why to use a $30^\circ$ angle between the flask and the pipette when emptying it. I can imagine it is important to touch the flask with your pipette when emptying, because the liquid prefers being attached to a solid surface due to capillary forces. When just emptying it from above the flask droplets would remain at the tip of the pipette. But why the $30^\circ$ angle? Is this an important standard or just arbitrary?

• I believe what you want is "capillary". This occurs when the adhesion between the water and the glass is stronger than gravity, which is surprisingly often. – Williham Totland Jul 1 '14 at 10:38

The angle of 30 degrees is not so important.

You have to make sure that the pipette is straight and the liquid runs along inside of the glass. You should not hold the pipette directly into the liquid. The drop in the tip has been included in the calibration. At the end you just have to wipe the pipette at the glass edge.

For volumetric instruments, such as bulb pipettes and graduated measuring pipettes, used for delivery of a liquid, the volume delivered is always less than the volume contained, due to the film of liquid left on the inner walls of the volumetric instrument. Most pipettes are calibrated ‘to deliver’ (TD, Ex); i.e. the delivered (not the contained) quantity of liquid corresponds to the value printed on the pipette.

Volumetric instruments – like any other measuring instrument – have a limited accuracy. Various sources of error are inherent in calibration and use. When the greatest possible accuracy is desired, the pipette should be used as closely as possible to the manner in which it has been calibrated. Methods for the testing, calibration and use of volumetric instruments made from glass are specified in the international standard ISO 4787:2010. The shown handling of a pipette including the angle of about 30° is part of the standardized calibration methods:

The pipette shall be held in a vertical position. Delivery shall be made into another glass vessel (the receiving vessel), with the receiving vessel held inclined at an angle of about 30°, so that the tip of the jet is in contact with the inner surface of the receiving vessel, above the level of any collected liquid, but without movement of one against the other throughout the delivery period, and finally drawing the tip over a distance of about 10 mm.

The resulting capillary effects influence the time required for the pipette to deliver its nominal volume and the volume of the film of liquid that is left on the inner walls of the pipette. Therefore, the volume delivered depends on the manner in which the pipette is used. Since capillary effects depend considerably on the material on which the liquid runs down, it is important that a receiving vessel made of glass is used.

The main advantage is that you don't risk sprinkling your solution outside the beaker, so choose the angle you are more comfortable with to obtain this goal. Sometimes you don't see little drops spreading around!

It is a good practice but if you make multiple pipetting using the same pipette you could contaminate your reagent touching the beaker, so maybe sometimes emptying the pipette slowly, quite near to the solution, is the best thing to do.

Some pipettes must be completely empty when you use them, while others take into account that some amount of solution will remain inside. Check what type yours is but generally touching the beaker's side does not change the result.

I think it is more important to use gloves and cut your nails (the men in the photo seems to ignore it!).

• What’s wrong with long nails? D= – Jan Dec 14 '15 at 2:22
• @Jan eh if you don't use the gloves some residues might stay in your nails, if you have the gloves you might cut them ehehe – G M Aug 20 '18 at 6:50
• From my personal experience of now around five years full-time in chemistry labs with long nails the answer is: the gloves survive. (Longer than those in the picture btw.) – Jan Oct 16 '18 at 13:55