26 DIY Science: Illustrated Guide to Home Chemistry Experiments
ACURATEC , ACCURATER, ACCURATEST?
With the exception of graduated cylinders, many people
think of volumetric flasks as the least accurate volumetric
instruments, with burettes being more accurate, and
pipettes most accurate of all. In reality, although graduated
cylinders are in fact less accurate, the Class A and Class
B standards are remarkably similar for the other three
types of volumetric glassware, if you compare identical
capacities. Of course, pipettes are made in much smaller
capacities than burettes or volumetric flasks. Those
small pipettes have extremely high accuracy, but only for
measuring correspondingly small quantities of liquids.Volumetric flasks are available in glass (Class A or Class B) and
plastic (Class B only), in sizes from 10 mL to 2000 mL or larger.
A home lab should ideally have a set that includes 25 mL, 50 mL,
100 mL, 250 mL, 500 mL, and 1000 mL flasks. But volumetric
flasks, particularly good ones, aren’t cheap. If you can afford only
one volumetric flask, choose a 100 mL flask. If you can afford
two volumetric flasks, add a 500 mL flask for making up stock
solutions that you use in larger amounts, such as dilute acids and
bases. If you can afford three, add a 25 mL flask for mixing up
small quantities of stock solutions of expensive chemicals, such
as silver nitrate, expensive indicators with short shelf lives that
you buy by the gram, and so on. If you’re on a very tight budget,
you can do without a volumetric flask and use a graduated
cylinder instead. You’ll give up some accuracy, but your results
will be accurate enough for all of the labs in this book.
mICRCALEoS EqUIpmENT
Using microscale equipment and procedures has many
advantages. Microscale equipment and procedures are less
expensive than standard equipment and procedures, which is a
major reason for the popularity of microscale chemistry.
Using microscale equipment and procedures means that
chemicals are needed in very small quantities, which are safer
to work with and easier to dispose of properly. Microscale also
makes it economically feasible to do experiments with very
expensive chemicals, such as gold, platinum, and palladium
salts. Setup and teardown is faster, allowing more time for actual
experiments, and cleanup usually requires only rinsing the
equipment and setting it aside to dry.
Against these advantages, there are several disadvantages to
microscale chemistry. First and foremost, everything is on such
a small scale that it can be difficult to see what’s going on. For
example, you may need a magnifier to examine a precipitate (or
even to determine whether there is a precipitate). Because of the
small scale, measuring or procedural errors so small that they
would have no effect on a traditional scale experiment can greatly
affect the outcome of a microscale experiment. With traditional
equipment and procedures, a milligram balance (or even a
centigram model) is sufficiently accurate to do reasonably precise
quantitative work, but equivalent precision in microscale requires a
very expensive tenth-milligram (0.0001 g) or hundredth-milligram
(0.00001 g) analytical balance. Finally, although it sounds odd,
like most people we find microscale experiments less satisfying
emotionally than larger-scale experiments. Watching two drops of
reagent combine in a reaction plate to yield a precipitate smaller
than a grain of sand just isn’t as thrilling as combining 10 mL of
each reagent in a test tube and watching the precipitate settle to
the bottom of the tube, where it can be isolated, purified, and used
for subsequent experiments and tests.Still, the ubiquity of microscale chemistry means that it’s
important for you to become familiar with basic microscale
equipment and procedures. Most of the experiments in this book
use semi-microscale, such as test tubes and small beakers and
flasks. If you want to gain experience with microscale procedures,
you can substitute microscale equipment and techniques. The
only microscale equipment you need to do the experiments in
this book is described in the following sections.The recent trend in chemistry labs, particularly school and university labs, is to substitute
microscale chemistry equipment and procedures for traditional semi-micro or macroscale
equivalents. Microscale chemistry, often called microchemistry, is just what it sounds like.
Instead of using standard test tubes, beakers, and flasks to work with a few mL to a few hundred
mL of solutions, you use miniaturized equipment to work with solution quantities ranging from
20 μL (microliters, where one μL equals 0.001 mL) to a couple mL.