Measurement of Volume
A) Erlenmeyer flasks –Erlenmeyer flasks are used primarily toprepare solutions prior to an accurate volume adjustment. Although there are volumetric markings on these flasks, they are not calibrated and should not be relied upon for exact volume measurements.
B) Beakers –Beakers are also used for preparing solutions,especially when a pH adjustment requires access to the solution by a pH probe. The volumetric markings on beakers are also not reliable.
C) Graduated cylinders –Graduated cylinders are calibrated withsufficient accuracy for most volume measurements when preparing solutions. For example, the calibration of most 100 mL graduated cylinder can be relied upon to accurately measure to within +/- 0.6 mL.
D) Volumetric flasks –Volumetric flasks are used to measure aspecific volume with the highest degree of accuracy, and areused to make standard solutions for analytical assays. For example, the calibration of a 100 mL volumetric flask can have an accuracy of +/- 0.1 mL.
E) Pipets –Pipets are glass or plastic devices that are routinely usedto measure and transfer liquids by drawing the liquid into the tube with a bulb or mechanical pump.
a. Pasteur pipets are small glass tubes used with a bulb to transfer volumes as small as a single drop and as large as a few mL. They are not graduated and are not used to measure volumes.
b. Beral pipets (transfer pipets) are plastic pipettes with a bulb at one end used for transfer of liquids. Sometimes they have calibration marks, which have a low level of accuracy. They are often disposable, sterile and individually wrapped.
c. Serological, or “blowout,” pipets are graduated glass tubes used to measure anywhere from 0.1 to 50 mL. When the liquid has drained from this pipette, the final drop in the tip is transferred with a puff of air.
d. Mohr, or “to deliver,” pipets are similar to blowout pipettes, but do not require a puff of air to accurately deliver the desired volume. They can be identified by the label ― “TD” on the top.
e. Volumetric pipets are not graduated, but are carefully calibrated to deliver a single, highly accurate volume, and are used for the transfer of exact volumes.
f. Automatic micropipettes are mechanical pumps calibrated to deliver highly accurate volumes generally less than 1.0 mL, and as little as 0.1 microliters. They are often adjustable for measuring different volumes and they always use dispensable plastic tips to actually transfer the liquids
g. Multichannel micropipettes can deliver the same volume from as many as 12 tips simultaneously. All automatic micropipettes need regular maintenance, calibration, and validation.
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