Using a Pasteur pipette

• Pasteur pipettes are used with a bulb (also known as a teat) which slides onto the top of the pipette. Bulbs come in a number of designs and varying volumes. It is the bulb (not the size of the pipette) which controls the amount of volume which can be drawn into a Pasteur pipette (it is important to ensure the bulb volume does not exceed that of the pipette). Care should be taken when fixing bulbs to pipettes as the glass is fragile and easily broken and cuts from breaking Pasteur pipettes are a common injury within chemistry laboratories.

• The best way to generally hold a Pasteur pipette is to place the pipette across the palm of the dominant hand holding the bulb between thumb and forefinger and the pipette itself with the remaining three fingers. This gives you greatest control with the pipette and prevents the pipette slipping out of the bulb. However, in some situations other ways of holding a pipette may be appropriate.

• Squeeze the pipette bulb between thumb and forefinger to expel the air from the bulb. The amount of air displaced from the bulb will be equal to the volume of solution which will be drawn up into the pipette.

• Keep the bulb squeezed as the Pasteur pipette is inserted into the solution to be drawn up. Ensure the tip of the pipette is below the level of the liquid.

• Slowly release the pressure on the bulb by relaxing the grip between thumb and forefinger. The solution should be gradually drawn up into the pipette. The viscocity of the liquid and speed of pressure release will affect the rate of the liquid being drawn up.

• Transfer the pipette to the vessel for the liquid to be transferrred into. Ideally this should be as close as possible to the vessel the liquid is being drawn up from.

• Carefully squeeze the bulb to release the liquid slowly into the target vessel. The rate of addition is very easy to control with a Pasteur pipette. For instance dropwise additions are commonplace. The viscocity of the liquid will affect how fast a liquid can be dispensed from a pipette.

• Once addition is complete, carefully remove the pipette bulb. Take care not to drip any solution which remains in the Pasteur pipette. The Pasteur pipette can be disposed of into glass bins (note, for some compounds it may be nescessary to treat the pipette first to remove very noxious substances). The pipette bulb is reusable. If the pipette bulb has be contaminated it may need to be cleaned or discarded.

Common issues

Most issues with Pasteur pipettes arise from contamination of bulbs, which is easy to avoid. The most common factors leading to contamination are:

• Turning the pipette on the side or upside down. This will cause the liquid to flow into the bulb. This may cause contamination to your reaction (for example the plasticisers used in pipette bulbs are soluble in many solvents), to future users (as drops may run down from the pipette bulb when used with other Pasteur pipettes), or cause injury (eg corrosive liquids in the bulbs may drip onto skin during handling).

• Removing the tip of the pipette above the level of the liquid whilst drawing up a solution. When lifted from the solution, air may rush into the pipette which may splash liquid into the pipette bulb which can cause contamination as described above. This is especially common with very viscous liquids (for example concentrated sulfuric acid).

Handling volatile solvents can be tricky with a Pasteur pipette, but is a fairly common occurance. This can be particularly tricky when handling warm solvents (eg during recrystallisation). The volatility of the solvents can cause the liquid to exit the pipette due to the increase in pressure caused by the vapour rather than squeezing the bulb. This can be potentially be hazardous, for example if working with volatile corrosives or very flammable solutions near heat sources (which should generally be avoided). Drawing the solution and expelling it from the pipette several times can minimise the tendancy of the liquid to be accidentally dispensed from the pipette.

Pasteur pipettes are also very fragile and the thin glass at the tip is very prone to snapping (especially on longer pipettes), for example if knocked or caught on the edge of a bottle or vessel. It is imortant to take care when using pipettes to avoid breaking the tips of the pipette. The glass may fly off which has the potential to cause injury, both from the glass and any liquids contained in the pipette.