Illustrated Guide to Home Chemistry Experiments

Chapter 5: Mastering Laboratory Skills 75

FIGURE 5-6:

Make up highly exothermic solutions in a separate container

flasks are made of Pyrex or similar glass, there’s no reason
to risk your volumetric flask. For an accurate measurement,
you need to allow the solution to cool to room temperature
anyway, so you might as well make up the solution in a separate
container, as shown in Figure 5-6, and do a quantitative transfer
to the volumetric flask once the solution has cooled.

To do so, fill the volumetric flask with distilled water to
about 75% of its capacity (depending on the solubility of the
chemical) and transfer that water to a beaker. Dissolve the
chemical in the beaker, allow the solution to cool to room
temperature, and use a funnel to pour the solution into the
volumetric flask. Rinse the beaker several times with a few mL
of distilled water, and transfer the rinse water to the flask,
rinsing the funnel as you do so. Add distilled water to the
volumetric flask until the bottom of the meniscus matches the
index line on the flask.

Inexpensive volumetric flasks (and some professional-
grade models) come with a plastic snap-on cap. Most
professional-grade volumetric flasks come with a ground-glass
stopper. If your volumetric flask has a ground-glass stopper, be
very careful using it. If you don’t lubricate the stopper properly,
it may weld itself to the flask, rendering the flask useless.
This is particularly likely to happen when you are making
up concentrated solutions of sodium hydroxide, potassium
hydroxide, or other strong bases that can etch glass, but it
can happen with any solution. To avoid problems, always
apply a small amount of silicone-based lubricant to the ground-
glass part of the stopper and spread it evenly, as shown in
Figure 5-7. Apply just enough lubricant to cause the ground
glass to clear, and no more. When you finish using the
volumetric flask, wash thoroughly and set it aside to dry. Store
the stopper separately.

FIGURE 5-7:

Lubricate the ground glass stopper

UPISnG A IPETTE

A Mohr pipette, which is also called a serological pipette or a

graduated pipette, has graduations that you can use to measure

and dispense small quantities of liquids accurately. For example,

a 1 mL Mohr pipette has 0.01 mL graduations and can be inter-

polated to 0.001 mL, and a 10 mL Mohr pipette has 0.1 mL

graduations and can be interpolated to 0.01 mL. Pipettes are

indispensable for dealing accurately with small quantities of liquids.

To transfer an accurately measured quantity of fluid with a Mohr

pipette, take the following steps:

1. Obtain a suitable amount of the liquid to be measured in
   a beaker. Pipetting directly from the reagent bottle risks
   contaminating the entire contents of the bottle.


2. Draw a small amount of the liquid into the pipette and use
   it to rinse the inside of the pipette thoroughly. Discard the
   rinse liquid in the appropriate container.


3. Use a pipette pump or pipette bulb, as shown in Figure
   5-8, to draw liquid from the container until the meniscus
   is well above the zero index graduation on the pipette.


4. Remove the pipette pump or pipette bulb, and quickly
   place your index finger over the top end of the pipette.


5. Wipe the outside of the pipette with a paper towel or rinse
   it with distilled water into the sink or a waste container.
   Make sure that the partial drop suspended from the tip of
   the pipette is removed.


6. Reduce the pressure of your index finger enough to
   allow the liquid to drain slowly into the beaker or a waste
   container, watching the level closely until it falls to the 0.00
   index mark. Make sure to read the meniscus at eye level.


7. By altering the pressure of your fingertip, transfer the
   desired volume of the liquid to the reaction vessel and
   discard the excess.