Use of a Pipette

Pipettes are for the transfer of known volumes of liquid from one container to another. Pipettes that deliver a fixed volume are called volumetric or transfer pipettes (Figure 1a). Other pipettes are known as measuring pipettes. They are calibrated with graduated markings along the side so that any volume up to the maximum capacity can be delivered (Figure 1b).

Liquids are drawn into the pipette through a slight vaacuum by using a pipette bulb (Figure 2). Never use your mouth to pipette.

The volume of a volumetric pipette is recorded to two decimal places (i.e. – 10.00 mL). To read the volume of a measuring pipette, estimate the last digit to within the finest division on the pipette.

Steps to prepare and use a volumetric pipette:

1. Inspect the pipette to make sure that the tip is not chipped and the top end of the pipette is smooth and flat. If the top end of the pipette is chipped, or not smooth, it is very difficult to pipette.

2. To fill a pipette, a rubber bulb (Figure 3) is used to provide suction to pull up the solution. Keep the tip of the pipette below the surface of the liquid. It is easier to draw up solution from a smaller, narrower container (i.e. 50 mL beaker) to avoid sucking up air. Avoid sucking liquid into the pipette bulb. (Note:Never dip a pipette into the reagent bottle. Obtain the solution with a small clean and dry beaker. Pipette the solution out of the beaker.)

3. Acclimatize the pipette with the solution tobe used in the analysis by drawing up a small amount of the solution. Remove the bulb and tilt the pipette horizontally to rinse the inner walls of the pipette. Drain completely. Repeat this step three times.

4. Fill the pipette with solution past the calibration mark. Remove the bulb and quickly place the index finger of the hand holding the pipette over the exposed end of the pipette.

5. Tilt the pipette slightly and wipe away any liquid on the outside surface. Slowly release pressure on the index finger so that the bottom of meniscus approaches the calibration mark at eye level. At the mark, apply pressure on the index finger to stop the level of the liquid.

6. Touch the tip of the pipette on the wall of the container. This will drain any drop that remains on the pipette tip.

7. Transfer the pipette to the receiving container and release the pressure on the index finger. Drain the solution into the receiving container with the pipette tip touching the wall of the container. After draining, wait 10 seconds before removing the pipette. At this point, the calibrated amount of liquid has been transferred. Look closely at the tip of the pipette. A small portion of solution remains in the tip (Figure 4). Do not blow out the pipette.

8. After use, clean the pipette by rinsing with distilled water.