laboratory safety in metallography 2.pdf - Laboratory...

Laboratory Safety in Metallography, Metallography and Microstructures, Vol 9, ASM
Handbook , ASM International, 2004, p. 1081–1090
Laboratory Safety in Metallography
George Vander Voort, Buehler Ltd.
Chemicals, Storage, and Handling
Many of the chemicals used in metallography are toxic, corrosive, flammable, or potentially
explosive. Therefore, purchase only small quantities that are likely to be used within a
reasonably short time. Flammable solvents should be stored in fireproof steel cabinets. Acids and
bases should be stored separately, again in fireproof steel cabinets. Strong oxidants must not be
stored along with acids, bases, or flammable solvents.
Reagent-grade chemicals or solvents of highest purity are recommended. Although more
expensive, the amounts used are small, and the gain in safety and reliability compensates for the
cost difference. Chemicals may deteriorate during storage; only reasonably small quantities
should be purchased. Exposure to light can accelerate deterioration of some chemicals. Hence,
they should be stored in a closed metal cabinet.
Etchants. Most laboratories mix commonly used reagents in quantities of 250 to 1000 mL and
then store them as stock reagents. Many reagents can be safely handled in this manner. It is best
to store only those reagents that are used regularly. Glass-stoppered bottles are commonly used
as stock reagent bottles. If these bottles are opened regularly, the stopper will not become
“frozen.” However, if they are used infrequently, a frozen stopper often results. Holding the neck
of the bottle under a stream of hot water will usually loosen the stopper. If thermal expansion
does not free the stopper, it can be gently tapped with a piece of wood. Glass bottles with plastic
screw-on tops can be used so long as the solution does not attack the plastic. These bottles are
useful for holding solutions, such as nital, that can build up gas pressure within a tightly
stoppered bottle. A small hole can be drilled through the cap top to serve as a pressure relief
vent. Tightly stoppered bottles of nital and some other solutions have exploded as the result of
pressure buildup. Be certain that the reagent is safe to store, and store only small quantities. All
bottles should be clearly labeled. Polyethylene bottles are required for etchants containing
hydrofluoric acid, which attacks glass.
Most recipes for etchants or electrolytes list the ingredients by weight if they are solids and by
volume if they are liquids. In a few cases, all amounts are given in weight percentages. In most
cases, reagent compositions are not extremely critical. An ordinary laboratory balance provides
adequate weighing accuracy, while graduated cylinders provide acceptable accuracy for
volumetric measurements. These devices should be cleaned after use to prevent accidents to the
next user. For weight measurements, a clean piece of filter paper or a cup should be placed on
the balance pan to hold the chemical, to protect the pan surface, and to facilitate transfer to the
mixing beaker. A large graduated beaker is usually employed for mixing solutions. When using
hydrofluoric acid, all containers should be made of polyethylene.