Chapter 5: Mastering Laboratory Skills 91
dAR. Rm y CHERvENAk CommENTS:
Actually, I’ve never used hydrochloric acid to clean
glassware. If the glassware is extremely dirty, I soak it in a
base bath (concentrated ethanolic potassium hydroxide).
Once the soak is complete, rinse the glassware thoroughly
with water and dry with acetone.
ISTRIALNdU STRENGTH CLEANING
When I worked in chemistry labs back in the early
1970s, the standard method for cleaning glassware was
to use a solution of chromium trioxide (or potassium
dichromate) in concentrated sulfuric acid. On the plus
side, this solution works wonders. Glassware that goes
in filthy comes out pristine and sparkling. The only minor
drawbacks are that chromic-sulfuric acid solution is
very expensive, extremely toxic, a known carcinogen,
hideously corrosive, emits deadly chromyl chloride fumes,
may explode unpredictably, and is almost impossible
to dispose of safely. But, boy, does it ever clean glass
squeaky clean.
Fortunately, there are other cleaning solutions that are
almost as good and somewhat safer to use. One of the
best is a concentrated solution of sodium hydroxide or
potassium hydroxide in 91% isopropanol or 95% ethanol.
Although this solution is extremely corrosive (and
requires using full protective gear including goggles and
face shield) and very flammable, it is inexpensive, does
not contain heavy metal ions, and is easy to dispose of
safely. It’s strong enough to dissolve glass, literally, so you
must either make it up as needed or store it in a nonglass
container that is rated for storage of strong alkalis. On
the few occasions I use it—usually to clean a very dirty
piece of expensive glassware—I mix up a fresh batch just
large enough to do what needs to be done. When the job
is finished, I neutralize the alkali with dilute acid and flush
the neutralized solution down the drain with lots of water.
Note that I do not recommend using this cleaning solution,
but mention it only for completeness.
contents properly, rinse the glassware thoroughly under
running hot tap water, and then sit it aside inverted to dry.
Make sure that it’s in an area devoted to dirty glassware or is
otherwise marked as requiring thorough cleaning.
- If the glassware is very dirty—such as a test tube or flask that
contains a precipitate that won’t rinse away—use a brush and
hot sudsy water to remove as much of the contamination as
possible. If the glassware still looks dirty, submerge it in a sink
or plastic tub that contains hot sudsy water, allow it to soak
for several hours or overnight, and again use a brush and hot
sudsy water to scrub it out. If the glassware still looks dirty,
try soaking it overnight in a 1 M solution of hydrochloric acid
(hardware store muriatic acid diluted one part acid to ten
parts water is fine for this purpose) and then scrubbing it out
again. If the glassware then appears clean, rinse it with tap
water and set it aside to drain until you are ready to do a final
cleaning. Otherwise, discard the glassware.
- Glassware that has undergone preliminary cleaning may
appear clean to the eye, but it’s not clean enough for lab
use. A good preliminary cleaning removes nearly all of the
contamination, but a final cleaning is necessary to ensure the
glassware is really clean. Begin by filling a sink or tub with hot
sudsy water. Using an appropriate brush or brushes, scrub
the entire surface of the glassware, inside and out. Rinse the
glassware thoroughly under hot running tap water and then
examine it to ensure it is pristine. Invert the glassware on a
drying rack and allow it to drain completely. Finally, rinse the
glassware, particularly the inside, with distilled water and
invert it on the drying rack to allow the final rinse to drain.
Although it no doubt horrifies our friends who do quantitative
analyses, I add one drop (literally) of dishwashing liquid
(actually, I use Kodak Photo-Flo) per liter of distilled rinse
water. The dishwashing liquid or Photo-Flo breaks the surface
tension and allows the remaining water to sheet cleanly off
the glassware rather than beading, leaving the glassware dry
to the touch almost immediately.
- Once the glassware is completely dry, return it to its
storage location.
wHEN To CLEAN ANd wHEN To dISCARd
Don’t get too attached to your glassware. It’s usually better
to discard seriously dirty glassware and replace it with new,
particularly if the glassware is an inexpensive item such as
a test tube, pipette, beaker, or Erlenmeyer flask. For more
expensive glassware, such as a volumetric flask or burette,
more heroic measures may be justified. (Of course, you
shouldn’t be doing things with expensive glassware that
cause it to become seriously dirty anyway; most expensive
glassware is intended only for volumetric use, not for
use as a reaction vessel.) If scrubbing the glassware
with soap and water doesn’t work and soaking it in dilute
hydrochloric acid doesn’t work, it’s best just to discard it. If
you do decide to risk using the alcohol/hydroxide solution
and it doesn’t work, it’s time to replace the glassware.
