Silver(I) Sulfide

OVERVIEW

Silver(I) sulfide (SILL-ver one SUL-fide) is a grayish-black heavy powder. Most people are familiar with the compound as tarnish, the black coating that covers silver tableware and jewelry when they are exposed to the air.

HOW IT IS MADE

Silver(I) sulfide occurs naturally as the minerals acanthite and argentitde, from which they can be extracted by grinding, crushing, and washing the mineral ore. The compound can also be prepared synthetically by passing hydrogen sulfide (H₂S) gas through an aqueous solution of silver nitrate.

H₂S + 2AgNO₃ → Ag₂S + 2HNO₃

KEY FACTS

OTHER NAMES:

Argentous sulfide

FORMULA:

Ag₂S

ELEMENTS:

Silver, sulfur

COMPOUND TYPE:

Binary salt (inorganic)

STATE:

Solid

MOLECULAR WEIGHT:

247.80 g/mol

MELTING POINT:

925°C (1520°F)

BOILING POINT:

Not applicable; decomposes

SOLUBILITY:

Insoluble in water; soluble in nitric acid, sodium cyanide (NaCN), and potassium cyanide (KCN)

The silver(I) sulfide precipitates out of solution and can be filtered and purified by washing with hot water.

COMMON USES AND POTENTIAL HAZARDS

Naturally occurring silver(I) sulfide can be used as a source of silver metal. The sulfide is roasted in air, converting silver(I) sulfide to silver(I) sulfate. The sulfate can then be treated chemically to obtain silver metal. The process finds little commercial application since other, more economically efficient, sources of silver are available.

The primary use of silver(I) sulfide is in the production of glazes for ceramics. The compound gives a glassy or metallic sheen to the glaze. It is also used in the process known as niello, first used by the early Greeks and Romans to produce a black, metallic inlay on the surface of pottery.

Silver(I) sulfide is an irritant to the skin, eyes, and respiratory system. No long-term effects of exposure to the compound have been reported. When absorbed through cuts in the skin or ingested it may produce a condition known as argyreia, which causes skin and mucous membranes to develop a bluish-black color.

FOR FURTHER INFORMATION

Krampf, Robert. "Cleaning the Silver." Edgerton Explorit Center. http://www.edgerton.org/kidscorner/silver.html (accessed on November 5, 2005).

"Material Safety Data Sheet." ESPI Metals. http://www.espimetals.com/msds's/silversulfide.pdf (accessed on November 5, 2005).

"Mineral Acanthite/Argentite, The." Amethyst Galleries. http://mineral.galleries.com/minerals/sulfides/acanthit/acanthit.htm (accessed on November 5, 2005).

Patnaik, Pradyot. Handbook of Inorganic Chemicals. New York: McGraw-Hill, 2003, 845.

Interesting Facts

• Tarnish is caused by a chemical reaction between silver in tableware and sulfur compounds that occur in eggs. Tarnish can be removed by soaking tableware in a solution of warm baking soda in water in a pan lined with aluminum foil. A chemical reaction occurs in which aluminum replaces silver, forming aluminum sulfide (Al₂S₃) and free silver: 2Al + 3Ag₂S → 6Ag + Al₂S₃. The reaction occurs only if the silver tableware is in physical contact with the aluminum foil because an electric current must flow between the two metals.
• The silver present in electric contacts exposed to hydrogen sulfide gas may begin to grow long filaments known as silver whiskers. These whiskers have been known to grow as long as 8 centimeters (3 inches) in length and can cause catastrophic failures in the electrical contacts.

Words to Know

AQUEOUS

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

PRECIPITATE

A solid material that settles out of a solution, often as the result of a chemical reaction.

SYNTHESIS

A chemical reaction in which some desired chemical product is made from simple beginning chemicals, or reactants.