Silver nitrate

}} Silver nitrate also known as lunar caustic is a soluble chemical compound with chemical formula AgNO₃. This compound is a versatile precursor to many other silver compounds, such as those used in photography. Comparatively, it is far less sensitive to light than the halides. It is called lunar caustic because silver was called luna by the ancient alchemists.

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

Preparation

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:

4 Ag (s) + 6 HNO₃ (aq) → 4 AgNO₃ (aq) + 3 H₂O (l) + NO (g) + NO₂ (g)

Applications

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

Halide abstraction

The silver cation quickly and effectively irreversibly reacts with halide anions to produce the insoluble silver halide. This reaction is commonly used in inorganic chemistry to abstract the halide as the insoluble silver salt:

Ag⁺ (aq) + X^- (aq) → AgX (s)     (X = Cl, Br, 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 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. The color of precipitate varies with the halide: white (silver chloride), pale yellow/cream (silver bromide), yellow (silver iodide). 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 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.

Medicine

Silver salts have antiseptic properties. Until the development and widespread adoption of antibiotics, AgNO₃ used to be dropped into newborn babies' eyes at birth to prevent contraction of gonorrhoea 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. 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 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.

Safety

As with all silver salts, silver nitrate is toxic and corrosive. Brief exposure to the chemical will not produce immediate or even any side effects other than the purple skin stains, but with more exposure, side effects will become more noticeable. It is also very poisonous and can cause burns. Long-term exposure can cause permanent blue-grey staining of eyes, mouth, throat and skin, (argyria) and may cause eye damage. Short contact can lead to deposition of black silver stains on the skin. Besides being very destructive of mucous membranes, it is a skin and eye irritant.

References

External links

• International Chemical Safety Card 1116
• NIOSH Pocket Guide to Chemical Hazards