CLINICAL LAB SCIENEC

CHAPTER 8: PIPETTING AND USE OF GLASSWARE 195
Most pipettes are also calibrated at a 20ºC temperature for accuracy and
standardization. The TD pipette is designed to deliver the volume indicated
on the pipette when gravity is used to empty the pipette, leaving a
small quantity of solution in the tip. Never should the liquid be aspirated
by use of the mouth, nor should it be “blown out” by the mouth from the
tip of the pipette. A frosted circle around the large open end of the pipette
indicates that it is a “blowout” type, but this is done by use of an instrument
such as a bulb or similar device rather than by using the mouth.
• To contain pipette (TC)
A to contain pipette is indicated with a “TC” inscription near the large
open end of the pipette that indicates that the pipette is designed to
contain the volume stated near the end of the pipette. Liquid may adhere
to the lumen (open space) within the pipette, and it must be rinsed out
to complete an accurate transfer of the solution to be delivered.
Buret Pipettes
Buret pipettes are similar to serological pipettes (Figure 8-12) except for the
addition of a valve used to start and stop the flow of the solution. The use of a
buret pipette requires slightly more care than other types of pipettes, but when
properly used, this type of pipette provides for amazing accuracy to within a
small percentage of 1 mL. A buret pipettte delivers a solution in precisely measured
volumes with the flexibility to use variable amounts easily. Burets
are used primarily for titration, by delivering one reactant to another
until the precise end point of the reaction is reached. They are also
valuable for delivering accurate amounts of diluents such as reagents
to tubes to perform chemical end-point reactions on patient samples.
Buret pipettes are not used as often as in the past, when they were
commonly used in manual procedures to facilitate the setting up of a
number of tests of the same type, allowing a given volume of reagent
to be pipetted quickly and easily into test tubes or cuvettes. Samples
were then introduced into the measured amounts of reagents by use
of a different type of pipette, called a micropipette, to complete a test
reaction.
Using a Buret
Be sure the transfer pipette is dry or conditioned with the titrant, so the
concentration of solution will not be changed. Before titrating, condition
the buret with titrant solution and check that the buret is flowing
freely. To condition a piece of glassware, rinse it so that all surfaces
are coated with solution, then drain. Conditioning two or three times
will ensure that the concentration of the material being titrated is not
changed by a stray drop of water. Lubricants that are inert and that
will not react with most solvents may be used to lubricate the stopcock,
preserving the rubberized sealing washer, as well as allowing for
smooth turning of the stopcock.
FIGURE 8-12 Buret pipette with
support stand.
Source: Delmar/Cengage Learning.
Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.