## Silver

#### Learning Objective

• Recognize the propensity of silver halides to precipitate out of solution when formed, as well as silver’s electrical and thermal conductivity properties

#### Key Points

• Silver metal is used in electrical contacts and conductors, in mirrors, and in catalysis of chemical reactions.
• Silver nitrate (AgNO3) is used as the starting point for the synthesis of many other silver compounds, as an antiseptic, and as a yellow stain for glass in stained glass.
• Silver halides are highly insoluble in aqueous solutions and are used in gravimetric analytical methods.
• Silver oxide (Ag2O), produced when silver nitrate solutions are treated with a base, is used as a positive electrode (anode) in watch batteries.

#### Terms

• silverA lustrous, white, metallic element, atomic number 47, atomic weight 107.87, symbol Ag.
• emulsionA mixture of two or more liquids that are normally immiscible (nonmixable or unblendable).

## Properties of Silver

Silver is a soft, white, lustrous transition metal. It has the highest electrical conductivity of any element and the highest thermal conductivity of any metal. The metal occurs naturally in its pure, free form (native silver). It also occurs naturally as an alloy with gold and other metals and in minerals such as argentite and chlorargyrite.

Most silver is produced as a byproduct of copper, gold, lead, and zinc refining. Silver metal is used in electrical contacts and conductors, in mirrors, and in catalysis of chemical reactions. Its compounds are used in photographic film. Dilute silver nitrate solutions and other silver compounds are used as disinfectants and microbiocides.

## Compounds of Silver

Silver metal dissolves readily in nitric acid (HNO3) to produce silver nitrate (AgNO3), a transparent crystalline solid that is photosensitive and readily soluble in water. Silver nitrate is used as the starting point for the synthesis of many other silver compounds, as an antiseptic, and as a yellow stain for glass in stained glass.

Silver reacts readily with sulfur or hydrogen sulfide (H2S) to produce silver sulfide (Ag2S), a dark-colored compound familiar as the tarnish on silver coins and other objects. Silver sulfide also forms silver whiskers when silver electrical contacts are used in an atmosphere rich in hydrogen sulfide.

$4Ag+ O_{ 2 }+2{ H }_{ 2 }S\rightarrow 2{ Ag }_{ 2 }S+2{ H }_{ 2 }O$

## Silver Halides

Silver chloride (AgCl) is precipitated from solutions of silver nitrate in the presence of chloride ions. The other silver halides used in the manufacture of photographic emulsions are made in the same way, using bromide or iodide salts. Silver chloride is used in glass electrodes for pH testing and potentiometric measurement and as a transparent cement for glass. Silver iodide has been used in attempts to seed clouds to produce rain. Silver halides are highly insoluble in aqueous solutions and are used in gravimetric analytical methods.

## Other Compounds of Silver

Silver oxide (Ag2O), produced when silver nitrate solutions are treated with a base, is used as a positive electrode (anode) in watch batteries. Silver carbonate (Ag2CO3) is precipitated when silver nitrate is treated with sodium carbonate (Na2CO3):

$2Ag{ NO }_{ 3 }+2{ OH }^{ }\rightarrow { Ag }_{ 2 }O+{ H }_{ 2 }O+2{ { NO }_{ 3 } }^{ }$

$2Ag{ NO }_{ 3 }+Na2CO3\rightarrow { Ag }_{ 2 }{ CO }_{ 3 }+2{ NaNO }_{ 3 }\\$

Silver fulminate (AgONC), a powerful, touch-sensitive explosive used in percussion caps, is made by reaction of silver metal with nitric acid in the presence of ethanol (C2H5OH). Other dangerously explosive silver compounds are silver azide (AgN3), formed by reaction of silver nitrate with sodium azide (NaN3), and silver acetylide, formed when silver reacts with acetylene gas.

## Uses of Silver Compounds

Alkaline solutions of silver nitrate can be reduced to silver metal by reducing sugars such as glucose. This reaction is used to silver glass mirrors and the interior of glass Christmas ornaments. Silver halides are soluble in solutions of sodium thiosulfate (Na2S2O3), which is used as a photographic fixer.

Silver metal is attacked by strong oxidizers such as potassium permanganate (KMnO4) and potassium dichromate (K2Cr2O7) and in the presence of potassium bromide (KBr). These compounds are used in photography to bleach silver images, converting them to silver halides that can either be fixed with thiosulfate or redeveloped to intensify the original image. Silver cyanide solutions are used in electroplating of silver.