Silver Nitrate

OVERVIEW

Silver nitrate (SILL-ver NYE-trate) is a colorless to transparent to white crystalline solid with no odor and a bitter metallic taste. In pure form, the compound is not affected by light, but trace amounts of organic impurities may catalyze the conversion of silver ions (Ag⁺; silver atoms with a positive charge) to grayish neutral silver atoms (Ag⁰) that give the salt a grayish tint. Silver nitrate is the most widely used of all silver compounds, finding application in the synthesis of other silver compounds, as a catalyst in certain industrial chemical reactions, as an antiseptic and germicide, and in photographic processes.

KEY FACTS

OTHER NAMES:

Silver(I) nitrate; lunar caustic

FORMULA:

AgNO₃

ELEMENTS:

Silver, nitrogen, oxygen

COMPOUND TYPE:

Salt (inorganic)

STATE:

Solid

MOLECULAR WEIGHT:

169.87 g/mol

MELTING POINT:

212°C (414°F)

BOILING POINT:

440°C (824°F); decomposes

SOLUBILITY:

Soluble in water, glycerol, and hot ethyl alcohol; moderately soluble in acetone

The therapeutic effects of silver compounds, including silver nitrate, have been known for many centuries. Both the Greeks and Romans, for example, used aqueous solutions of silver nitrate to treat wounds and cuts. In 1881, the German physician Carl Crede (1819–1892) developed the practice of applying a 2 percent solution of silver nitrate to the eyes of newborn babies to prevent gonorrheal ophthalmia, a bacterial infection of the eyes that may result in blindness in a child.

The use of silver nitrate in printing and photography dates to discoveries made in the 1720s by the German chemist Johann Schulze (1687–1744). Schulze found that a mixture of silver, nitric acid, and chalk turns purple or black when exposed to light. That simple discovery formed the basis of the modern science of photography. In 1802, Thomas Wedgwood (1771–1805), son of the founder of the famous Wedgwood pottery company, used silver nitrate to make temporary negative prints on paper, producing shades of gray as well as pure black and white. In 1835, the British mathematician William Henry Fox Talbot (1800–1877) made the first permanent paper negative from paper coated with silver nitrate and common table salt (sodium chloride).

Two years later, the French inventor Louis Jacques Mandé Daguerre (1787–1851) coated a copper plate with silver and washed it with nitric acid to create a plate on which he made the first Daguerreotype, an early form of photography. By the 1850s, Daguerre's technique was in wide use, with the substitution of glass plates coated with silver nitrate to obtain images produced by exposing the plates to light. By the end of the nineteenth century, emulsions of silver nitrate in celluloid were being used for making photographs, a process that was modified in the 1930s by the substitution of cellulose acetate for celluloid. Silver nitrate remains an important component of the photographic process today.

HOW IT IS MADE

Silver nitrate is made by dissolving metallic silver in weak nitric acid.

2Ag + 2HNO₃ → 2AgNO₃ + H₂

The solution is then evaporated to recover the crystalline silver nitrate, which is heated to remove impurities, dissolved in water, and re-purified.

COMMON USES AND POTENTIAL HAZARDS

The primary use of silver nitrate is in the production of other silvers salts used in the production of photographic film. Compounds such as silver bromide (AgBr) and silver iodide (AgI) decompose when exposed to light, forming free silver:

AgBr → Ag⁰ + Br⁻ and AgI → Ag⁰ + I⁻

While silver bromide and silver iodide are nearly colorless, the free silver atoms formed in this reaction are black. Thus, portions of a photographic film that have been exposed to light turn black where free silver has formed.

Interesting Facts

• Film speed numbers, such as ISO 100 or ISO 4000, correspond to the size of silver nitrate particles on the film. The finer the grain, the more detailed the image quality.
• Silver nitrate is not toxic to humans or other mammals. But it is toxic to fish and other aquatic organisms. For this reason, it should not be discarded in lakes and rivers.

In addition to its use in treating the eyes of newborn babies, silver nitrate has a number of other medical applications. It is used as a germicidal wall spray in medical facilities, as a topical (on the skin) anti-infective agent, for the cauterization of wounds, and as a general antiseptic. Cauterization is the process by which the skin surrounding a wound is burned in order to seal the wound. Other uses of silver nitrate include:

• The silvering of mirrors, a process by which a thin coating of silver metal is attached to the back of a piece of glass to form a mirror;
• As a catalyst in the manufacture of ethylene oxide, an important raw material in the production of plastics;
• In the silver plating of metals and plastics;
• In the manufacture of indelible printing inks;
• For hair dye; and
• As a flower preservative, a process that involves adding a small amount of silver nitrate solution to the water in which flowers are stored.

Silver nitrate is an irritant that can cause inflammation and burning of the skin, eyes, and respiratory system. It is also toxic by ingestion. In sufficient amounts, silver nitrate can cause severe damage to the respiratory tract and lungs, blindness, and even death. Solutions of silver nitrate can stain the skin dark purple or black, although such stains do not necessarily indicate that serious damage has occurred.

Words to Know

ANTISEPTIC

A chemical that prevents the growth of bacteria and viruses that cause disease.

AQUEOUS

Referring to solution that consists of some material dissolved in water.

CATALYST

A material that increases the rate of a chemical reaction without undergoing any change in its own chemical structure.

EMULSION

A temporary mixture of two liquids that normally do not dissolve in each other.

THERAPEUTIC DRUG

A compound that has healing properties.

FOR FURTHER INFORMATION

Dunn, Peter M. "Dr Carl Credé (1819–1892) and the Prevention of Ophthalmia Neonatorum." Archives of Diseases in Childhood (September 2000): F158-F159. Also available online at http://fn.bmjjournals.com/cgi/content/full/83/2/F158 (accessed on November 5, 2005).

Grimm, Tom, and Michele Grimm. The Basic Book of Photography. New York: Plume Books, 2003.

Patnaik, Pradyot. "Silver Nitrate." Handbook of Inorganic Chemicals. New York: McGraw-Hill, 2003, 841-842.

"Silver Nitrate." New Jersey Department of Health and Senior Services. http://nj.gov/health/eoh/rtkweb/1672.pdf (accessed on November 5, 2005).

See AlsoCellulose Nitrate; Silver Iodide

silver nitrate (AgNo₃) Colourless, solid compound. It is the most important salt of silver because it is very soluble in water. Silver nitrate is used in photography, chemical analysis, silverplating, mirrors, inks, and dyes.

silver nitrate (sil-ver ny-trayt) n. a salt of silver with caustic, astringent, and disinfectant properties. It is used in the form of a stick to destroy warts. Trade name: AVOCA.