Chapter 3: Equipping a Home Chemistry Laboratory 25TCSEEH A of THE mISSING SToppER
Some volumetric flasks are sold with ground-glass
stoppers or plastic snap caps included, but for others
you must order the stopper or cap separately. Using your
thumb as a stopper is considered poor practice. It can also
be hard on your thumb. I confess, though, that at times I’ve
found myself stopperless and have used a clean-gloved
thumb as a makeshift stopper.BRETTESU
A burette (also spelled buret) is used to dispense controlled
small amounts of a liquid with great precision. Burettes are used
to perform titrations for quantitative analyses, determining
accurate concentrations of stock solutions, and so on.
Burettes are available in glass and plastic models. Although
glass burettes are breakable, we prefer them, because they
are easier to clean and are unaffected by organic solvents. The
least expensive burettes use a rubber tube and pinchcock to
control the flow of the reagent. More expensive models use a
ground glass stopcock, which must be kept scrupulously clean
and stored disassembled to prevent the ground glass joint from
binding. The most expensive models use a Teflon stopcock to
control the flow, and are much easier to clean and maintain. You’ll
want a burette if you plan to do quantitative analyses. A 50 mL
model is the best compromise of capacity, flexibility, accuracy,
and general usefulness.
vOLUMETRIC FLASKS
A volumetric flask, shown in Figure 3-16, is used to make up
a precise volume of solution. It has one graduation line that
indicates the nominal volume. To use a volumetric flask to
make up a solution, you fill it partially with distilled water, add
the chemical or chemicals to be dissolved or diluted, swirl the
flask until the solute is dissolved, and then fill the flask to the
graduation line.
Unless a volumetric flask is otherwise labeled, it is designed
to be accurate at 20°C. Small variations in room temperature
are no cause for concern, but dissolving or diluting some
chemicals, such as strong mineral acids, is highly exothermic,
and raises the temperature of the solution enough to affect
accuracy. (Although it is less common, dissolving or diluting
some other chemicals can be endothermic, and can lower the
temperature of the solution enough to affect accuracy.) If you
use a volumetric flask to make up solutions of these exothermic
or endothermic solutes, allow the solution to come to room
temperature before you top up the flask to the graduation line.
Although some glass volumetric flasks are made of Pyrex or
similar borosilicate glass, many are of flint glass. For those flasks
in particular (and even for Pyrex flasks), we prefer to do the
initial dissolving of exothermic solutes in a beaker or Erlenmeyer
flask. For example, if we’re making up 500 mL of a stock solution
of sodium hydroxide, which releases a great deal of heat as it
dissolves, we start with about 400 mL of distilled water in a
600 mL beaker, add the sodium hydroxide gradually, and stir
until it dissolves. Only after that solution has cooled to room
temperature do we transfer it to the volumetric flask (rinsing
the beaker several times with distilled water to make sure
we’ve done a quantitative transfer) and then top off the
volumetric flask.
FIGURE 3-16: A 100 mL volumetric flaskExTHERMICO DAnGERS
When you make up a solution using a concentrated or
solid strong acid or base, always gradually add the acid or
base to the water, not vice versa. If you add water to the
strong acid or base, the solution may boil almost instantly,
ejecting the chemical forcefully from the container.z