A pipette is a type of chemical dropper used in laboratory experiments to measure and transport fixed volumes of chemicals. There are several designs of pipettes but they all function in the same way. The pipette is inserted into a liquid-filled container, where it creates a vacuum. The vacuum suction draws up a designated amount of liquid and dispenses it into another. Pipettes are made with either borosilicate, aluminosilicate or quartz.
Micropipettes are made to measure and dispense small quantities of liquid. Micropipettes dispense between 1 and 1,000 microliters. They are often used in microinjection and measuring experiments, as they are extremely accurate.
A volumetric pipette, like a volumetric flask, is used to measure and dispense one specific quantity of liquid to add to another. Several different sizes are available to measure a predetermined amount of liquid, with popular sizes being 10, 25 and 50 milliliters. Volumetric pipettes are most often used in titration experiments.
A graduated pipette works similarly to a graduated cylinder. It is marked with a series of graduated lines to measure the quantity of a liquid before and after it is dispensed. It is not as accurate as a volumetric pipette, but it serves its purpose.
A pasture pipette is the basic pipette used to withdraw and dispense a random amount of liquid. It is not calibrated to a specific volume. This type of pipette resembles a large dropper.
The accuracy of the pipette depends upon several factors. The most important factor is the training and experience of the person performing the experiment. An inexperienced chemist may make mistakes that affect the measurements. Another factor is the type of liquid that is being pipetted. Regular use of corrosive liquids may damage the pistons and o-rings of the pipette, resulting in inaccurate measurements.
In order to ensure the accuracy of a pipette, it must be calibrated regularly. A pipette can be calibrated rather easily, without being sent out to a professional. Before calibrating a pipette, verify that it is draining properly.
Start the calibration by setting the dial at the highest volume. Using deionized water, pipette the liquid into a flask on an analytical balance. Measure and record the weight. Perform the procedure five times.
Repeat the process with the dial on the lowest and middle settings. Compare the measurements and look for discrepancies. If systematic discrepancies are noticed, adjust the calibration wheel clockwise or counterclockwise as necessary. If there are random discrepancies, the chemist's technique may need improvement.