A volumetric pipette is a glass tube used to measure and transfer liquid accurately. It has markings calibrated to operate at 20°F and comes in different sizes. Proper use requires temperature control, cleanliness, and no trapped air bubbles. Meniscus should be viewed at eye level to avoid errors.
A volumetric pipette, also spelled pipette, is an accurately graduated glass tube designed to aspirate, measure and, frequently, transfer an amount of liquid. Often the volumetric pipette has a zero volume mark, below which the liquid must be held in the pipette and will not be transferred. The markings, sometimes just a fill line, are usually calibrated to operate accurately at 20°F (68°C). The better the quality, the more accurate the pipette calibration; ASTM specification E969 lists class A as “precision pipettes” and class B as “general purpose pipettes”.
The pipettes have two openings; liquids should be aspirated and then carefully prevented from spilling until the transfer is complete. Because many fluids pose health risks, suction is often provided by a bulb or some other suction device, rather than using the mouth. The volumetric pipette can be found in laboratories, with capacities from microliters (1 µl = 10-6 liters) to hundreds of milliliters (1 ml = 10-3 liters). Large volume pipettes often have a large cylindrical center to increase capacity.
Proper use of the volumetric pipette requires temperature control or temperature adjustment, which depends on an accurate knowledge of the operating temperature and a correction table or correction data, often provided by the manufacturer. Also, the pipette must be used clean and dry. If wet, the pipette should be filled and emptied with some of the liquid to be transferred and this liquid should be discarded. This procedure prevents dilution of the sample to be transferred. Also, if the solution is not carefully prepared and well characterized, the use of an accurate volumetric pipette will not compensate.
Once the volumetric pipette is filled, it is imperative that there are no trapped air bubbles or debris. At the top notch, the liquid will form a curve, as gravity pulls the liquid down most effectively in the center, while at the edges and touching the pipette, the liquid experiences upward capillary action. The effect is to produce a meniscus: a volumetric pipette is generally designed to have the lowest point of the meniscus aligned with the calibration mark. To ensure this is the case, the meniscus and line should be viewed at eye level, thus avoiding parallax errors. If there is a drop hanging at the bottom of the pipette, it should be tapped on the side of the vessel from which it was taken to remove it.
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