16.7 What is the best method for cleaning glassware? (Chemistry)
Description
This article is from the Chemistry FAQ, by Bruce Hamilton B.Hamilton@irl.cri.nz with numerous contributions by others.
16.7 What is the best method for cleaning glassware? (Chemistry)
As scientific glassware can be used for a variety of purposes, from the
ultra-trace determination of sub-ppq levels of dioxin, to measuring %
concentrations of inorganic elements, there is no single cleaning method that
is "best" for all circumstances. Difficult and intractable deposits often
involve the use of hazardous and corrosive chemicals, and details of the
necessary safety precautions for each cleaning solution should obtained
before attempting to clean glassware. The use of heat and/or ultrasonic
agitation can greatly improve the removal rate of many deposits, especially
inorganic and crystalline deposits.
Whilst the semiconductor industry use piranha solution ( refer Section
12.9 ), and several other reactive and toxic chemicals for cleaning, those
reagents can react dangerously with the residues found in laboratories, and
their use is prohibited in some institutions. Such chemicals should only be
used after extensive prior consultation with laboratory management and safety
staff - to either identify safer alternatives, or to ensure that appropriate
protective and safety systems are in place.
If the probable composition of material deposited on the glassware is known,
then the most appropriate cleaning agent can be readily selected. There are
several safe aqueous cleaning solutions that are routinely used. If possible,
glassware should be washed or soaked immediately with an appropriate solvent
for the residue. This will make subsequent cleaning easier, but all traces
organic solvents must be removed before using any cleaning solution.
The most common aqueous-based soaking solutions are commercial formulations
that usually contain alkalis, chelating agents, and/or surfactants, and can
be used either at ambient temperature, or temperatures up to boiling ( with
ventilation - caustic fumes are noxious ). These are very effective for
general grime, most labels, pyrogens, and many common chemical residues, and
well known examples include RBS-35, Decon, Alconox, and Pyroneg. Their main
advantages are low toxicity and ease of disposal.
The next common strategy involves physical abrasion to remove deposits
inside flasks, usually with a bottle brush and an aqueous cleaning solvent
( like those above ) or a suitable organic solvent. A refinement is to add
sand, pumice, glass spheres, or walnut shell chips, along with some water
or solvent, and shake vigorously. It's important that the sand should not
have sharp edges - as it can scratch the glass. It has been suggested that
table salt in solvent ( eg petroleum spirit, methylene chloride, acetone )
is superior, as it doesn't scratch the glass, can be easily removed by
washing with water, and has minimal disposal problems [14].
The traditional glassware cleaning solution is "chromic acid", and many
analytical chemistry texts detail the preparation [15,16]. Chromium (VI) is
highly toxic ( mutagenic, carcinogenic ), and disposal is expensive, as all
solutions containing more than 5 mg/l of chromium are considered hazardous
waste in the USA. Disposal of chromic acid requires a two-stage process,
involving bisulfite addition to reduce Cr(VI) to Cr(III), followed by
neutralisation of the acid. There have also been several reports of
spontaneous explosions of chromic acid cleaning solutions [17,18,19],
consequently the use of chromic acid for cleaning glassware is declining,
and several alternative glassware cleaners have recently been evaluated [20].
Sodium dichromate dihydrate is usually used to prepare chromic acid, as
potassium dichromate is less soluble in sulfuric acid. One technique is to
dissolve 140g of technical grade sodium dichromate dihydrate in approximately
100 ml of water. Add two litres of technical grade 98% sulfuric acid to a 4-5
litre glass beaker that is sitting in a cold water bath in a fume cupboard.
Carefully stir the acid gently and pour a few mls of the dichromate solution
slowly into the acid. Keep repeating the addition every few seconds - after
the previous dose has been dispersed. As long as the stirring is gentle and
continuous, little or no splattering should occur, but the solution will
become quite warm. Allow to cool before storing in a glass-stoppered reagent
bottle. Always ensure that the stopper is sufficiently loose to release any
gas pressure. Never use a screw-capped or similar types of sealed containers.
If made correctly, the chromic acid solution should have no precipitate, will
be a deep red colour, and will last for years in a glass-stoppered bottle.
Ensure the glassware to be cleaned does not have any residual organic
solvents. Chromic acid is very effective at around 80C, but an overnight soak
at ambient temperature is commonly used. If the solution develops a green
hue, it is exhausted and should be disposed of, or regenerated, using
appropriate procedures. Slowly pouring used acid down a drain with the cold
water tap fully open is no longer considered appropriate. There is a recent
report of a technique to regenerate chromic acid cleaning solution ( by
distillation of water and oleum ) that reduces disposal quantities [21].
The major problems with chromic acid are the multiple rinses, and perhaps
even alkaline EDTA treatment [16], that are necessary to remove all the
chromium from glassware - especially if it is required for cell culture or
trace analysis, and the increasing problems of safe and legal disposal of
spent solutions.
An alternative to chromic acid is "Nochromix", which is commercial solid
formulation that contains 90-95% of ammonium persulfate ( ((NH4)2)S208 )
along with surfactants and other additives. The powder is dissolved in
water and mixed with 98% sulfuric acid. The solution is clear, but turns
orange as the oxidizer is consumed, and further additions of solid are
routinely required. It is available from Godax Laboratories, New York.
A similar bath that is reported to be very effective can be made by the
addition of 19 grams of reagent grade ammonium persulfate to two litres of
reagent grade 98% sulfuric acid [22]. Add more ammonium persulfate and acid
every few weeks, as necessary.
One popular replacement for chromic acid in organic laboratories has been
alcoholic sodium hydroxide or potassium hydroxide solutions. These remove
most deposits, with metals and hydrocarbons greases ( Apiezon ), as notable
exceptions. One advantage they have is that they will remove silicone grease
deposits from joints and stopcocks, especially if warmed to 65C, and the
glassware immersed for up to 10 minutes [23]. Prolonged immersion, even at
ambient temperature, will damage ground-glass joints, dissolve glass sinters,
and will leave glass surfaces translucent or opaque. The solution can be
prepared by either adding two litres of 95% ethanol to 120 mls of water
containing 120 grams of sodium hydroxide [16], or by dissolving 100 grams of
potassium hydroxide in 50 ml of water and, after cooling, make up to one
litre [15].
Solutions based on hydrofluoric acid, usually containing 1-5% of HF, also
rapidly attack glass, and destroy sinters, but are very effective for removal
of carbonaceous and fine silica deposits. They also remove silicone greases,
but alcoholic caustic solutions are preferred [23,24,25]. Hydrofluoric acid
is corrosive and extremely nasty if it comes in contact with humans. It
requires extensive safety precautions before use. For most deposits, only a
few minutes are required, and ultrasonic agitation often assists the removal
of deposits. Cleaned glassware usually remains transparent. Cleaners
containing HF should not be used on volumetric glassware.
Another acidic solution, comprising of a 3:1 mixture of concentrated sulfuric
acid and fuming nitric acid, is also extremely effective for removing grease
and dirt, but also requires extensive safety precautions. The grease and dirt
can often be removed more safely using hot aqueous-based cleaners.
If you have intractable organic-based deposits in flasks without standard
ground glass ( or clear glass ) joints, then some deposits can be carefully
burned off in a glass annealing furnace. The glass needs to carefully
follow a slow heating and cooling schedule to minimise thermal stresses and
distortion. My experience has been that standard joints do tend to freeze
more often after such treatment. Also note that glassblowers may not want
to coat their annealing furnace with your rubbish, so they may prohibit
the use of their furnace for such activity.
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