Silver nitrate is an inorganic compound with chemical formula AgNO
3. This compound is a versatile precursor to many other silver compounds, such as those
used in photography. It is far less sensitive to light than the halides. It was once called
lunar caustic because silver was called luna by the ancient alchemists, who believed that
silver was associated with the moon. In solid silver nitrate, the silver ions are
three-coordinated in a trigonal planar arrangement.
Discovery Albertus Magnus, in the 13th century, documented
the ability of nitric acid to separate gold and silver by dissolving the silver. Magnus
noted that the resulting solution of silver nitrate could blacken skin. Its common name
at the time was nitric acid silver. Synthesis
Silver nitrate can be prepared by reacting silver, such as a silver bullion or silver
foil, with nitric acid, resulting in silver nitrate, water, and oxides of nitrogen. Reaction
byproducts depend upon the concentration of nitric acid used.
3 Ag + 4 HNO3 → 3 AgNO3 + 2 H2O + NO Ag + 2 HNO3 → AgNO3 + H2O + NO2
This is performed under a fume hood because of toxic nitrogen oxide(s) evolved during
the reaction. Reactions
A typical reaction with silver nitrate is to suspend a rod of copper in a solution of
silver nitrate and leave it for a few hours. The silver nitrate reacts with copper to form
hairlike crystals of silver metal and a blue solution of copper nitrate:
2 AgNO3 + Cu → Cu(NO3)2 + 2 Ag Silver nitrate decomposes when heated:
2 AgNO3(l) → 2 Ag(s) + O2(g) + 2 NO2(g) Qualitatively, decomposition is negligible
below the melting point, but becomes appreciable around 250° C and totally decompose at 440°
C. Most metal nitrates thermally decompose to
the respective oxides, but silver oxide decomposes at a lower temperature than silver nitrate,
so the decomposition of silver nitrate yields elemental silver instead.
Uses Precursor to other silver compounds
Silver nitrate is the least expensive salt of silver; it offers several other advantages
as well. It is non-hygroscopic, in contrast to silver fluoroborate and silver perchlorate.
It is relatively stable to light. Finally, it dissolves in numerous solvents, including
water. The nitrate can be easily replaced by other ligands, rendering AgNO3 versatile.
Treatment with solutions of halide ions gives a precipitate of AgX. When making photographic
film, silver nitrate is treated with halide salts of sodium or potassium to form insoluble
silver halide in situ in photographic gelatin, which is then applied to strips of tri-acetate
or polyester. Similarly, silver nitrate is used to prepare some silver-based explosives,
such as the fulminate, azide, or acetylide, through a precipitation reaction.
Treatment of silver nitrate with base gives dark grey silver oxide:
2 AgNO3 + 2 NaOH → Ag2O + 2 NaNO3 + H2O Halide abstraction
The silver cation, Ag+, reacts quickly with halide sources to produce the insoluble silver
halide, which is a cream precipitate if Br- is used, a white precipitate if Cl− is used
and a yellow precipitate if I− is used. This reaction is commonly used in inorganic
chemistry to abstract halides: Ag+ + X−(aq) → AgX
where X− = Cl−, Br−, or I−. Other silver salts with non-coordinating anions,
namely silver tetrafluoroborate and silver hexafluorophosphate are used for more demanding
applications. Similarly, this reaction is used in analytical
chemistry to confirm the presence of chloride, bromide, or iodide ions can be tested by adding
silver nitrate solution. Samples are typically acidified with dilute nitric acid to remove
interfering ions, e.g. carbonate ions and sulfide ions. This step avoids confusion of
silver sulfide or silver carbonate precipitates with that of silver halides. The color of
precipitate varies with the halide: white, pale yellow/cream, yellow. AgBr and especially
AgI photo-decompose to the metal, as evidence by a grayish color on exposed samples.
Organic synthesis Silver nitrate is used in many ways in organic
synthesis, e.g. for deprotection and oxidations. Ag+ binds alkenes reversibly, and silver nitrate
has been used to separate mixtures of alkenes by selective absorption. The resulting adduct
can be decomposed with ammonia to release the free alkene.
Biology In histology, silver nitrate is used for silver
staining, for demonstrating reticular fibers, proteins and nucleic acids. For this reason
it is also used to demonstrate proteins in PAGE gels. It can be used as a stain in scanning
electron microscopy. Medicine
Silver salts have antiseptic properties. Until the development and widespread adoption of
antibiotics, dilute solutions of AgNO3 used to be dropped into newborn babies' eyes at
birth to prevent contraction of gonorrhea from the mother. Eye infections and blindness
of newborns was reduced by this method; incorrect dosage, however, could cause blindness in
extreme cases. This protection was first used by Credé in 1881. Fused silver nitrate, shaped
into sticks, was traditionally called "lunar caustic". It is used as a cauterizing agent,
for example to remove granulation tissue around a stoma. General Sir James Abbott noted in
his journals that in India in 1827 it was infused by a British surgeon into wounds in
his arm resulting from the bite of a mad dog to cauterize the wounds and prevent the onset
of rabies. Dentists sometimes use silver nitrate infused swabs to heal oral ulcers. Silver
nitrate is also used by some podiatrists to kill cells located in the nail bed. Silver
nitrate is also used to cauterize superficial blood vessels in the nose to help prevent
nose bleeds. The Canadian physician C. A. Douglas Ringrose
researched the use of silver nitrate for sterilization procedures on women. A specialist in obstetrics
and gynaecology, Ringrose believed that the corrosive properties of silver nitrate could
be used to block and corrode the fallopian tubes, in a process that he called "office
tubal sterilization". The technique was ineffective; in fact at least two women underwent abortions.
Ringrose was sued for malpractice, although these suits were unsuccessful.
Disinfection Much research has been done in evaluating
the ability of the silver ion at inactivating Escherichia coli, a microorganism commonly
used as an indicator for fecal contamination and as a surrogate for pathogens in drinking
water treatment. Concentrations of silver nitrate evaluated in inactivation experiments
range from 10–200 micrograms per liter as Ag+. Silver's antimicrobial activity saw many
applications prior to the discovery of modern antibiotics, when it fell into near disuse.
Its association with argyria made consumers wary and led them to turn away from it when
given an alternative. Against warts
Repeated daily application of silver nitrate can induce adequate destruction of cutaneous
warts, but occasionally pigmented scars may develop. In a placebo-controlled study of
70 patients, silver nitrate given over nine days resulted in clearance of all warts in
43% and improvement in warts in 26% one month after treatment compared to 11% and 14%, respectively,
in the placebo group. Safety
As an oxidant, silver nitrate should be properly stored away from organic compounds. Despite
its common usage in extremely low concentrations to prevent gonorrhea and control nose bleeds,
silver nitrate is still very much toxic and corrosive. Brief exposure will not produce
any immediate side effects other than the purple, brown or black stains on the skin,
but upon constant exposure to high concentrations, side effects will be noticeable, which include
burns. Long-term exposure may cause eye damage. Silver nitrate is known to be a skin and eye
irritant. Silver nitrate is currently unregulated in
water sources by the Environmental Protection Agency. However, if more than 1 gram of silver
is accumulated in the body, a condition called argyria may develop. Argyria is a permanent
cosmetic condition in which the skin and internal organs turn a blue-gray color. The United
States Environmental Protection Agency used to have a maximum contaminant limit for silver
in water until 1990, when it was determined that argyria did not impact the function of
any affected organs. Argyria is more often associated with the consumption of colloidal
silver solutions rather than with silver nitrate, since it is only used at extremely low concentrations
to disinfect the water. However, it is still important to be wary before ingesting any
sort of silver-ion solution. References
External links International Chemical Safety Card 1116
NIOSH Pocket Guide to Chemical Hazards