76 DIY Science: Illustrated Guide to Home Chemistry Experiments
mRIoH ppETTES vERSUS voLUmETRIC pIpETTES
A Mohr pipette is finely graduated from zero to its maximum capacity, and
so can be used to measure and dispense any arbitrary amount of liquid up to
its maximum capacity. A volumetric pipette, which is instantly recognizable
by the bulge in its center, is designed to contain a specific amount of liquid
when it is filled to the index line, like a volumetric flask but more accurate.
The mechanics of using either of these types of pipettes are the same, but
volumetric pipettes are seldom needed in home chemistry labs.
Remember that most Mohr pipettes are calibrated To Deliver (TD or EX)
and most volumetric pipettes are calibrated To Contain (TC or IN). The
designation determines how you treat the last drop in the pipette.
With a TD pipette, the last drop is not part of the measured volume, and so
remains in the pipette. With a TC pipette, also called a blow-out pipette, for
obvious reasons, that last drop is part of the measured volume and so must
be transferred to the receiving vessel.CLIBRATINGA A voLUmETRIC fLASk wITH A BALANCE ANd A
THERmomETER
Although small volumes of liquids can be measured accurately with a pipette
or similar instrument, it’s much more difficult to measure larger volumes
volumetrically. For example, a 100 mL volumetric flask may be accurate
only to 0.5 mL, plus or minus. But if the mass of that volumetric flask when
filled with water is less than the maximum capacity of your balance, you can
calibrate that flask by mass, using the known density of distilled water at
various temperatures, listed in Table 5-1. To do so, place the empty flask (less
stopper) on the balance pan and tare the balance. Fill the flask with distilled
water until the water level is a few millimeters under the index line on the
flask and replace the flask on the balance pan. Add water dropwise to the
flask until the bottom of the meniscus just touches the index line, and then
note the mass of the water.
For example, if you’re calibrating a 100 mL volumetric flask at 20°C, the mass
of the water should be 99.82 g. The actual mass is likely to be somewhat
different, particularly if you’re using a student-grade volumetric flask. You
can calculate the actual capacity of the flask by dividing the actual mass of
the water by the density of water at the ambient temperature. For example,
if the actual mass of the water is 100.08 g, dividing that mass by 0.9982071
tells you that the actual capacity of the flask is about 100.26 mL. Record that
actual capacity and use it rather than 100.00 mL for calculations when you
use the flask to make up solutions of known concentrations.FIGURE 5-8:
Use a pipette pump or pipette bulb
to transfer liquids safely and accurately.