The Grasshopper's Guide to
Author: J. M. McCormick
Last Update: June 30, 2006
A Chemistry Koan
A Chemistry Student approached the
Master and asked, “Master, how do I achieve enlightenment in Chemistry?” The
Master replied, “Wash your glassware.”
Clean glassware is essential in chemistry. The
problem is that the tolerance for shmutz varies with the work you are doing, and
sometimes a chemist does not know how important clean glassware is to an
experiment until it has failed. This document is designed to give an
undergraduate chemistry student a brief introduction to what chemists mean by
"clean" and how it can be achieved.
There are two broad degrees of clean in chemistry;
quantitative and normal. Quantitatively clean glassware is required for
the most demanding applications where a quantity is being measured at high
precision, such in analytical or physical chemistry. At this
level of cleanliness there are no residues (e. g., grease) or other impurities on
Normal clean glassware is free of most contamination but some contaminates (e.
g., grease) are tolerated. Glassware that has been cleaned normally is used where
a high degree of precision is not required, such as in a synthesis.
Health and Safety Considerations
Even a task as simple as washing glassware at the sink is
potentially hazardous. You must wear eye protection, appropriate for the
task, at all times. Gloves are recommended, even for general cleaning, if
the glassware contained an irritant, lachrymator or toxic material. Before
cleaning be sure that any excess reagent has been disposed of properly and the
vessel in which it was contained has been triple
rinsed into the waste
General Cleaning Tips
The key to cleaning is doing it a timely manner; letting
dirty glassware sit for long periods of time guarantees a harder cleaning job.
- •Disassemble your apparatus as soon as possible after
you are finished with it. Remove all stopcocks and stoppers from addition
funnels, separatory funnels and the like. Ground glass stopcocks and
stoppers will freeze in place if certain reactants (e. g., bases) were used in
them. Triple rinse all surfaces with an appropriate solvent to remove
traces of solvents and reaction mixtures, place the rinses in the appropriate waste container.
- •Separate glassware that must be quantitatively clean
from that which does not. In this way you do
not waste time trying to quantitatively clean items that do not
need to be.
- •Graduated cylinders, beakers, Erlenmeyer flasks,
burets and pipettes that were only used to dispense or store reagents generally only need to
rinsed with a compatible solvent followed by tap water and a final DI water
rinse, if desired. Air dry on a drying rack. In some cases you may need
to be more thorough, as described below.
- •Büchner funnels, etc. should be rinsed
with an appropriate solvent to remove substances that are clinging to them.
Running solvent through them backwards using gravity (never use vacuum to
speed up this process!) can help remove contamination from the inside of the
funnel and from the surface of fritted funnels. Follow this by tap water and DI water rinses and air dry.
General Cleaning Procedure
The following steps should be followed for glassware for
which a simple solvent rinse is not sufficient. If you need quantitatively
clean glassware, these should be the first steps toward this goal, and more
aggressive cleaning methods may be required (vide infra).
- •Degrease your glassware’s ground glass joints by
wiping them with a paper towel soaked in a small amount of ether, acetone or
other solvent (CAUTION! wear appropriate gloves and minimize your
exposure to the vapors).
- •Place the glassware in a warm concentrated aqueous
solution of Alconox, or other detergent, and let it sit for several minutes.
- •Scrub. Be sure that your brush is in good shape
before scrubbing (not rusty, bristles are not matted down); replace it if
- •Rinse thoroughly with tap water and give a final rinse
with DI water.
The water will sheet cleanly off the glass, if it is
quantitatively clean. If water does not sheet off the glass, and you desire the
glassware to be quantitatively clean, first repeat the above soaking and scrubbing
steps. If, after a second cleaning, bits of solid still adhere to the glass,
or if there is clearly a greasy residue on the glass, more aggressive action
must be taken.
More Aggressive Cleaning Methods
The following cleaning methods are two of the more commonly used ways to
remove contaminants from glassware. They are usually used after normal
cleaning has failed, and they are often
used together, because each is effective at removing different types of
contaminants. Care must be taken using either one because of the corrosive
nature of the solutions used.
- •If the contaminant is a metal-containing compound, soak
the piece of glassware in a 6 M HCl solution. DANGER! this solution can cause
severe burns! Wear appropriate gloves. Once the solid has dissolved, copiously rinse the item with tap
water, and then repeat the general cleaning steps above. This method will
also remove some organic residues (not grease).
- •If the contaminant is organic, submerge the item in a base bath
(a saturated NaOH or KOH solution in ethanol, methanol or isopropanol). DANGER! the base bath will dissolve skin
and alcohols are flammable! Wear butyl gloves and keep ignition sources away from
the base bath. Be sure that the piece of
glassware is completely filled with the solution and is sitting upright. After
several minutes of soaking, carefully remove the item (it will be slippery), and
rinse thoroughly. If the glassware is not quantitatively clean at this point,
the general cleaning steps may need to be repeated, or a longer soaking time
in the base bath, may be needed.
- NEVER soak the following items in a base bath for
contaminated with metal-containing compounds
- »Glass fritted
glassware (pipettes, volumetric flasks)
- »Any glassware
contaminated by an oxidizing agent
- »Anything that
has not been washed according to the above steps first
- Glass fritted funnels and volumetric glassware can be
rinsed briefly with the base bath solution to remove small amounts of
grease, but prolonged exposure to the caustic solution can damage these
Even More Aggressive Cleaning Methods
Sometimes 6 M HCl and a base bath are not sufficient, and
even more aggressive methods must be employed. CAUTION! all of
these methods will do severe damage to the eyes, skin, mucous membranes and
lungs. Extreme caution should be exercised when using these methods.
Wear butyl gloves (not latex or nitrile exam gloves), eye protection and a lab coat.
Work in the hood.
Undergraduate students must check with their faculty supervisor before using
these methods, and they must be under the direct supervision of a faculty member
at all times when using these methods (no exceptions).
- Aqua Regia This is an extremely powerful
oxidizing solution prepared from 1 part concentrated HNO3 and 3
parts concentrated HCl (it is recommended that 1 part H2O be added
if the aqua regia will be stored to minimize the generation of Cl2).
It is the only acidic solution that will dissolve gold and will oxidize just
about everything else. Extreme caution must be used when working with
aqua regia because it generates Cl2 and NOx gases in
addition to causing severe tissue damage. Clean the glassware before
soaking in aqua regia and then rinse thoroughly with water.
- Acidic Peroxide Solution This is most
conveniently prepared by dissolving the commercially-available "NoChromix"
mix in concentrated
H2SO4 per the package directions. An alternative preparation is to prepare a
solution by mixing equal proportions of concentrated H2SO4
and aqueous H2O2 solutions (remember to add the acid to
the H2O2). A 3% H2O2
solution is usually sufficient, and under no circumstances should H2O2
solutions greater than 10% be used. The H2O2/H2SO4
solution is both a strong oxidant and a strong reductant, so care must be
taken when using it. Another acidic peroxide cleaning solution can
be prepared by dissolving 36 g (NH4)2S2O8
(ammonium peroxydisulfate) in 2.2 L of 98% H2SO4 (can be
made right in the bottle of H2SO4, if the bottle is
loosely stoppered). The procedure for these solutions is the same
as for aqua regia as are the precautions for their use.
- Chromic Acid This is a solution of CrO3
in concentrated H2SO4. A premeasured mix is
available under the name "Chromerge", which should be treated in the same way
as aqua regia or acidic peroxide solutions. Because high-valent chromium
is carcinogenic, teratogenic and causes severe environmental damage, the use
of chromic acid is not recommended.
- Hydrofluoric Acid Concentrated solutions of HF
will remove just about everything from glass and will even etch the surface of
the glass itself. It should not be used on calibrated volumetrics.
HF causes severe, painful burns that do not heal well, and prolonged or
intense exposure can lead to a very slow, painful death. It
is not to be used by any students at Truman under any
- Cuvettes Generally, you only need to rinse a
cuvette in the appropriate solvent and wipe the outside with a Kimwipe
immediately after use. If something has adhered itself to a cuvette, it
is best to soak the cuvette in solvent first and gently coax the solid off the
side with a cotton swab. Never use a brush on a cuvette! If
this fails, one of the acidic cleaning solutions mentioned above can be used
(but never HF!). It is not recommended that base bath be used on
cuvettes, because it tends to etch glass surfaces.
- Fritted Funnels These can generally be cleaned
by inverting and allowing to solvent to flow by gravity through the frit in
reverse (do not use vacuum to speed this process). Solvent can also be pulled through the frit
(in the normal direction) under vacuum.
Recalcitrant gunk can usually be removed by soaking in acid, followed by
copious rinsing with water under vacuum. Because HF and the base bath
solution etch glass, they should not be used on fritted funnels (a brief
exposure to a base bath is not usually fatal to a frit, but prolonged soaking
should be avoided).
- Protein Contamination Usually proteins can be
removed scrubbing with detergent, but occasionally protein defies
removal. In that event, you can proceed to the more aggressive acidic
solutions, or you can prepare a peptidase solution (an enzyme that degrades
proteins). The enzymatic approach is a bit slower than the forcing
methods, but it is gentler and so can be used in situations that the
contaminated item is incompatible with acid.
Wet glassware can be dried by
1) placing it on the drying rack (or invert on a paper towel), 2) placing it in
the drying oven (for items that are water-wet only, no flammable solvents) or 3)
rinsing with a solvent such as acetone, methanol or ethanol and then gently
blowing compressed air into the vessel until it is dry. The first method
is preferred for drying quantitatively clean glassware (provided that the prongs
of the drying rack are not inside the item, thus contaminating it). Volumetric
glassware and cuvettes are never to be placed in drying ovens, even if they are
not quantitatively clean. The third method is acceptable only when
the compressed air supply is known to be free oil and other contaminants.
An alternative to blowing air into the item is to use an aspirator, or house
vacuum, to pull air into the item.
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