The Grasshopper's Guide to Cleaning Glassware
Introduction
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. Glassware at this level
of cleanliness has no residues (e. g., grease) or other impurities
on it. Normal clean glassware is free of large amounts of impurities,
but some grease may be tolerated. Glassware that has been cleaned
normally is used where high degrees of precision are not required,
such as in a synthesis.
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.
Also take a minute to separate your glassware into a group which
requires a higher degree of cleaning and one that does not.
•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 these in
the appropriate waste container.
•Graduated cylinders, beakers, Erlenmeyer flasks, burets and pipettes
that were only used to dispense or store reagents generally only need
to be triple-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. Follow
this by tap water
and DI water rinses and air dry.
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 container.
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 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 necessary.
•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 or methanol). 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. Do Not soak any
glassware in base bath longer than necessary, as it can slowly dissolve
a layer of glass or even permenently etch/ frost surfaces.
NEVER soak the following items in a base bath for prolonged periods:
»Glassware
contaminated with metal-containing compounds
»Glass
fritted funnels
»Cuvettes
»Volumetric 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 soaked briefly
with the base bath solution to remove small amounts of grease, but prolonged
exposure to the caustic solution can damage these items.
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
solution for cleaning 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 (avoid using if at all possible): Concentrated
solutions of HF will remove just about
everything from glass (because it actually
dissolves a layer off the glass) and
will even etch the surface. 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
without proper safety training, permission
of faculty adviser and supervision. Remember
if you get even one small drop in your
eye...you will be blind for life.
HF
is one of the nastiest acids
and considered Extremely Dangerous!
Special
Cases
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. Solvent can also be
pulled through the frit 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.
Drying
Glassware
Wet
glassware that is not quantitatively clean 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 (1) 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.
As
I was writing cleaning procedures for the UD glass shop web site
I googled around for other useful items to include. However after
finding the Grasshopper's Cleaning Guide I knew I was wasting my
time. Many
thanks to JM McCormick- TSU for compiling all procedures into
one useful source.
If you
have questions about the safety of any glassware, please
bring it to the glass shop for evaluation.
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