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Lead Acetate - Lab Notes
WARNING! Soluble lead compounds are toxic.
If you choose to attempt this or any of the procedures described on this site, you do so entirely at your own risk. In order to use this web site and/or any information contained herein, you must read and agree to the Terms of Use.
Lead acetate [Pb(CH3COO)2], or "salt of Saturn"
as alchemists and early chemists called it, finds use in qualitative chemical
analysis and also in the manufacture of other lead compounds, such as litharge
(PbO; used in assaying and refining silver and gold). Lead acetate
forms colored complexes with some of the anthocyanin pigments (for example,
see http://www.crscientific.com/newsletter-10.html),
as well as producing colorful chromates, antimonates, iodides, and other
compounds. It is very useful in the laboratory.
|
| Figure 1. Colorless crystals of
lead acetate. A
concentrated Pb(OAc)2 solution became syrupy when heat-evaporated
down to a fraction of its volume. To avoid the possibility of complete
solidification upon cooling, some dilute, aqueous Pb(OAc)2
was added. The resulting solution was allowed to cool to room temperature,
yielding the crystals shown above. |
While irrational fear is counter-productive, it's important to avoid being
cavalier with soluble lead compounds. Though lead salts are not
as toxic as their mercury counterparts, ingestion, inhalation, or skin
absorption can still cause death or neurological and physiological damage.
Lead acetate is absorbed through skin. Vinyl, latex, or nitrile
gloves should be worn when handling the compound or its solutions.
If lead acetate contacts bare skin, it should be washed off immediately
with water.
|
| Figure 2. Pb metal often contains appreciable
amounts of Sn and Sb. These form insoluble compounds (e.g., SnO2
• xH2O) when treated with certain acids, especially under
oxidizing conditions. A traditional means of separating Pb, Sn, and
Sb involves concentrated HNO3; the Pb stays in solution
as lead nitrate. |
|
| Figure 3. Crystals of lead acetate on a microscope slide. Magnification ca. 40x. Macroscopically, crystals of this compound are almost colorless, acquiring a slight beige tint if basic lead carbonate (etc) is present. |
Lead acetate (or sometimes, lead nitrate) is used to make "sulfide
test paper". The presence of H2S causes the moistened paper
to turn black; the aqueous Pb2+ ions react with S2-
to form the black, insoluble PbS. This is a very sensitive test. Decades
ago, when lead paint was in common use, white-painted houses sometimes displayed
the same effect; white siding slowly turned black due to atmospheric
H2S.
Pb(OAc)2 , upon exposure to atmospheric CO2, becomes
partially insoluble (Merck Index,
10th Edition, entries for "lead acetate" and "lead subacetate"). It
tends to acquire an ivory or off-white tinge in the process.
Efflorescent compounds (including lead acetate) can be messy to crystallize
by simple evaporation, since concentrated solutions tend to creep up the
sides of the container. This can leave crystal growths on the edges,
on the rim, and even on the surrounding bench top. It seems that the
growing crystals have a wick-like effect on the mother liquor, drawing it
up the walls of the crystallizing dish.
Figure 4. In experiments with other compounds, we have noted
that certain ones "climb" up the walls of the crystallizing dish as
the mother liquor evaporates. This property was also observed
when trying to crystallize lead acetate by complete evaporation at
room temperature. The results are shown in the photograph.
It probably didn't help that seed crystals were not used. There
were no nucleation points in the center of the dish, so the big
crystals accumulated near the edges.
Nearly all the carbonates, acetates, oxides, and hydroxides of lead
are converted to PbO upon heating above 500°C, though Pb3O4
("red lead") is typically an intermediate stage, especially if there is
much air access to the sample and the heating is applied for a long time.
Lead chloride, however, remains chemically unchanged. Oxychlorides,
where present, tend to decompose back into PbCl2 and PbO on strong
heating. PbCl2 boils around 950°C (presenting a toxic
inhalation hazard), though some will of course vaporize at lower temperatures
(ventilation required!).
Many lead compounds are reducible to Pb metal by roasting in the presence
of charcoal in a reducing flame. The laboratory must be ventilated
well when performing such operations. Even PbO can volatilize appreciably
if heated strongly enough.
Containers that have been used to hold lead compounds or solutions
should be presumed to have some residue in them, even when they look empty.
The treatment and recycling of laboratory-scale Pb wastes may be the subject
of a future article.
Again, a word of caution-- soluble lead salts are toxic. Lead compounds must be stored
in a securely locked cabinet where children, animals, and untrained
personnel cannot get to them. They must be labeled conspicuously
as to their toxicity.
If planning to handle or work with lead compounds, please find and
download the relevant material safety data sheets (MSDS).Copyright Information
Legal Notice: In order to use this website or any information contained herein, you must read and agree to the Terms of Use. Malicious or negligent use, handling, or disposal of toxic materials can invite criminal prosecution.
Works Cited:
Merck Index, 10th Edition. Rahway, New Jersey: Merck and Company, Inc., 1983.
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