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Glassware Cleaning
WARNING: If you choose to attempt any of the experiments or procedures described on this site, you do so entirely at your own risk. These procedures can be very dangerous and should be attempted only by competent persons.
Most of the time it is sufficient simply to use detergent and water to clean
glassware; however, there are certain types of residue that are completely
resistant to scrubbing in detergent. Obviously, choice of cleaning
solution depends on what this residue is. For example, a mortar and
pestle with stubborn remnants of manganese dioxide is best cleaned by soaking
with HCl under a fume hood (chlorine gas will be formed). There is,
as every lab-experienced person knows, no universal cleaning agent.
However, there are a few which serve a great variety of situations.
Before the era of "green chemistry", every chemist was familiar with the
concoction of concentrated sulfuric acid and chromium trioxide (or sometimes,
potassium dichromate) for cleaning glassware. This vicious
formulation can cause skin burns, spontaneous fires, chromium poisoning, and
possibly cancer; neophytes should thus stay well clear of it. 1
For those experienced at handling "nasty stuff", the CrO3-H2SO4
system is still on the market today under the familiar name of Chromerge®.
Some workers, however, prefer less-dangerous alternatives, and a number
of labs are getting away from the chromic-sulfuric acid treatment.
Many larger institutions have stopped using it altogether, although for a
very few applications it is still superior.
As all-around cleaning solutions go, a very useful and far less toxic formula
is caustic alkali (NaOH or KOH, preferably the latter) dissolved in alcohol.
Of course, don't get it on your skin or in your eyes (!); be sure to
wear gloves, an apron, and safety goggles when working with it.
Because of the ease with which they destroy and dissolve living tissues, NaOH
and KOH are still dangerous; however, they at least don't leave toxic
heavy metal wastes sitting around the lab, and they don't present the [potential]
fire/explosion hazard of chromic-sulfuric acid.
Instead of the usual ethanol, 91% isopropyl alcohol (U.S.P.) also works
well to dissolve caustic alkali. The alcohol may need to be swirled
gently and let stand for a while to dissolve the caustic pellets. The resulting
solution of NaOH or KOH, when left in contact with the glassware surface,
will clean most common residues from it. This writer has tried intervals of
15 to 30 minutes with success; be careful, however, as your results
may vary. Repeat: There is no universal glass cleaning recipe. Furthermore, we do not know of a recipe for "alcoholic KOH" that is ideal for every condition. Glass erosion rate varies not only with concentration and temperature, but also with glass composition and other factors. You will have to experiment. Some glass types erode rapidly, others very slowly.If the alkali solution stands in contact with the glass for too long, it will lead to a hazy or frosted appearance. Aesthetics aside, this makes the glass more likely to adsorb stubborn contaminants to its surface during subsequent use.
Do not use caustic alkali to clean optical cuvettes (for e.g., a spectrophotometer); you will ruin them.
Though microwaves do have their uses in the lab, do not heat caustic alkali in a microwave oven. Whether the alkali is solid or in solution, it will destroy glass in a most disastrous way.
Alcoholic KOH / NaOH is also useful for cleaning porcelain mortar &
pestles of even the most stubborn grime. Anyone who has
ever pulverized mineral samples with charcoal in the mortar 2 will
know how filthy the porcelain can become. One porcelain mortar which really
appeared to have been ruined in this manner was rescued from the junk pile
by treatment with alcoholic NaOH solution. The only side effect was
that the surface, when ground, now made a sound even more grating than fingernails-on-chalkboard.
When finished treating glassware (etc.) with the alcoholic-caustic, pour
the solution into a suitable container where it can be neutralized with dilute
acid (or, if not too contaminated from the first cleaning, re-used to clean
more glassware). Just remember that neutralizing this material can generate
dangerous amounts of heat, perhaps even leading to unwanted boiling and spattering.
Finally, the cleaned glassware is treated in some variation of the following, basic procedure:
1.) thorough rinsing of the glass under tap water;
2.) a wash-down of the glass with dilute acetic acid (vinegar);
3.) another rinse under the faucet, and
4.) a final rinse in distilled water.
Notes:
1 Hexavalent Cr in conc. H2SO4 is not quite the "worst of the worst" of lab formulations, but it's up there. It must be treated with a great deal of respect. (Extra caution: the presence of chlorides in the mix will produce the volatile and all-around horrid chromyl chloride)
Never (!) dump Cr(VI) wastes into the environment. Years ago, some industries ignored this simple warning and caused environmental disasters which may never be fully recovered. Cr(VI) can be reclaimed / recycled in the lab, by e.g., conversion to the much less-hazardous Cr2O3 (an artist pigment), so there really isn't any excuse to dump hexavalent Cr anyway.
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2 Normally the sample is crushed by itself and heated on a block with flux, but certain tests work much better when powdered charcoal is actually intermixed with the mineral particles.
A porcelain mortar and pestle can be used for pulverizing minerals if (1.) the mineral sample is fairly soft and breaks apart easily, and (2.) you're not planning to test for small amounts of Ca, Mg, Al, or Na - any of which can be introduced into a sample from the porcelain.
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