Silver nitrate

Silver nitrate is a soluble chemical compound with chemical formula AgNO₃. This nitrate of silver is a versatile precursor to many silver compounds, such as those used in photography, although this salt is far less sensitive to light than the halides. A spill of AgNO₃ on the skin results in a white stain which turns dark after roughly an hour, resulting from a combination of silver metal and silver sulfide.

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Synthesis and structure

Silver nitrate crystals can be produced by dissolving silver metal in a solution of nitric acid and evaporating the solution. The equation is as follows:

3 Ag_(s) + 4HNO_3(aq) → 3 AgNO_3(aq) + 2 H₂O_(l) + NO_(g)

In solid silver nitrate, the silver ions are three-coordinated in a trigonal planar arrangement.^[1]

Applications

Precursor to other silver compounds

Silver nitrate is the least expensive salt of silver and offers several other advantages as well. It is non-hygroscopic, in contrast to the fluoroborate and perchlorate salts. It is relatively stable to light. Finally it dissolves in numerous solvents. The nitrate can be easily replaced by other ligands, rendering AgNO₃ versatile. Treatment with solutions of halide ions gives a precipitate of AgX (X = Cl, Br, I). 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 silver oxide:^[2]

2 AgNO₃ + 2 NaOH → Ag₂O + 2 NaNO₃ + H₂O

Organic synthesis

Silver nitrate is used as 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.^[3]

Medicine

  Silver salts have antiseptic properties. AgNO₃ is sometimes dropped into newborn babies' eyes at birth to prevent contraction of gonorrhoea or chlamydia from the mother. Eye infections and blindness of newborns is reduced by this method. This protection was first used by Credé in 1881 and is still in use.^[4][5][6] 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. 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.

The Canadian physician 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.^[7]

Biology

In histology, silver nitrate is used for silver staining, for demonstrating proteins and nucleic acids. For this reason it is also used to demonstrate proteins in PAGE gels. It is also used as a stain in scanning electron microscopy.

Analytical chemistry

The presence of chloride, bromide, or iodide ions can be tested by adding silver nitrate solution. Samples are typically acidifed 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. Addition of AgNO₃ to the resulting solutions produces a precipitate in the presence of halides. The color of precipitate varies with the halide: white (silver chloride), pale yellow (silver bromide), yellow (silver iodide). AgBr and especially AgI photo-decompose to the metal, as evidence by a grayish color on exposed samples.

Coordination chemistry

Silver nitrate is also often used in inorganic and organic chemistry to abstract halides in the form of the insoluble silver halide salt.

Toxicity

As with all silver salts, silver nitrate is toxic and corrosive.

References

1. ^ P. Meyer, A. Rimsky et R. Chevalier "Structure du nitrate d'argent à pression et température ordinaires. Example de cristal parfait" Acta Crystallographica (1978) B34, 1457-1462. doi:10.1107/S0567740878005907.
2. ^ Campaigne, E.; LeSuer, W. M. "3-Thiophenecarboxylic (Thenoic) Acid" Organic Syntheses, Collected Volume 4, p.919 (1963). (preparation of Ag₂O, used in oxidation of an aldehyde). http://www.orgsyn.org/orgsyn/pdfs/CV4P0919.pdf
3. ^ Cope, A. C.; Bach, R. D. "trans-Cyclooctene" Organic Syntheses, Collected Volume 5, p.315 (1973); Vol. 49, p.39 (1969). http://www.orgsyn.org/orgsyn/pdfs/CV5P0315.pdf
4. ^ Peter.H (2000). "Dr Carl Credé (1819-1892) and the prevention of ophthalmia neonatorum". Arch Dis Child Fetal Neonatal 83: F158-F159.
5. ^ Credé C. S. E. (1881). "Die Verhürtung der Augenentzündung der Neugeborenen". Archiv für Gynaekologie 17: 50-53.
6. ^ Bulletin of the WHO: Credé's method still valid?
7. ^ Cryderman v. Ringrose (1978), 89 D.L.R. (3d) 32 (Alta S.C.) and Zimmer et. al v. Ringrose (1981) 4 W.W.R. 75 (Alta C.A.).