What Is a Volumetric Pipette?
A volumetric pipette, also spelled pipet, is a carefully graduated, glass tube designed to suck up, to measure, and frequently, to transfer a quantity of liquid. Often, the volumetric pipette has a zero volume marking, below which marking the liquid is to be retained in the pipette, and is not transferred. The markings — sometimes just a fill line — are usually calibrated to perform accurately at 68° F (20° C). The better the quality, the more accurately the pipette is calibrated; ASTM's E969 specification lists class A as the "precision pipet" and class B as the "general purpose pipet."
Pipettes have two openings; liquids must be drawn up by suction and then carefully prevented from leakage until transfer can be accomplished. Since many fluids present health risks, the suction is often provided by either a bulb or some other suction device, rather than using the mouth. The volumetric pipette may be found in laboratories, ranging in capacity from microliters (1 µl = 10-6 liter) to hundreds of milliliters (1 ml = 10-3 liter). Large volume pipettes will often have a wide cylindrical center to increase capacity.
Proper use of the volumetric pipette requires either temperature control or temperature adjustment, which depends on an accurate knowledge of the temperature of use and a correction chart or correction data, frequently provided by the manufacturer. In addition, the pipette should be used clean and dry. If wet, the pipette should be filled and emptied with some of the liquid to be transferred, and that liquid should be discarded. This procedure prevents dilution of the sample to be transferred. Additionally, if the solution is not carefully prepared and well-characterized, using an accurate volumetric pipette will not compensate.
Once the volumetric pipette is filled, it is imperative there are no entrapped air bubbles or detritus. At the top marking, the liquid will form a curve, since gravity pulls downward on the liquid most effectively at the center, whereas at the edges and touching the pipette, the liquid experiences an upward capillary action. The effect is to produce a meniscus — a volumetric pipette is generally designed to have the lowest point of the meniscus line up with the calibration marking. To assure this is the case, the meniscus and the line should be viewed at eye-level, thus avoiding parallax error. If there is a hanging drop on the bottom of the pipette, it should be touched to the side of the vessel from which it is drawn to remove it.
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