Chemistry of Silver

Silver (name from the Anglo-Saxon sioful, symbol from the Latin argentium) is considered a precious metal. It is found only to the extent of 0.05 parts per million in the earth (i.e., you have to dig up 20 million shovels full of dirt to get one shovel full of silver!). Knowledge of the metal is of ancient origins. Refining methods are mentioned in the Hebrew scriptures and ancient Egyptian writing. Silver was once considered more valuable than gold, probably because gold was easier to locate and refine at the time. Silver is ten times more abundant than gold.

Physical Properties

Silver is also a very popular metal. Like gold, silver is a rare metal and holds great value and is also traded in similar markets. Silver is also a popular metal to make jewelry with because of both its rarity and malleability. For some time, silver was also used in dentistry, although it had to be mixed with mercury because it is not quite as malleable as gold. Silver is also a good conductor and is used in electroplating. It is also used in the manufacturing of batteries as well as a catalyst.

Chemical Properties

Silver also played a vital role in photography and consequently, medicine. The first camera-like apparatus was developed during the Middle Ages, but they had no way to make the captured images permanent. Then during the 19th century, the first permanent photos were developed using silver nitrate coated glass plates, which were used as negatives, and modern photography soon developed. However, this process is quickly becoming obsolete due to the emergence of digital photography.

The development of permanent photography catalyzed the development of modern medicine. As a result of permanent photography and the silver halides used in its development, William Conraad Roentgen was able to use radiation to create a picture of a person's skeleton, making it easier for doctors to diagnose their patients.

Even though coinage metals are known to be resistant to oxygen in air, silver will tarnish due to $$H_2S_{(g)}$$

$2Ag_{(s)} + H_2S_{(g)} \rightarrow Ag_2S_{(s)} + H_{2(g)}$

Pure silver metal is the best conductor of heat and electricity. Pure silver would be too soft for jewelry and utensils so it is usually alloyed with at least one other metal. Sterling silver, for example, is usually about 93% silver and 7% other metals, mostly copper.

Silver can be found in high concentrations as argentite, $$Ag_2S$$, but most is recovered in the refining processes of other metals such as copper. Most silver compounds are light sensitive and a lot of silver bromide and silver chloride are used by the photographic industry.

Extraction of Silver by cyanide process

Silver also occurs both in combined state as well as in free state. The important ores of silver are: Argentite (Ag2S), Copper silver glance, Horn silver, Ruby silver. The silver ores are found along with gold ores in some parts of India.

Silver is extracted from the ore-argentite (Ag2S). The process of extraction of silver is called as cyanide process as sodium cyanide solution is used. The ore is crushed, concentrated and then treated with sodium cyanide solution. This reaction forms sodium argento cyanide.

$Ag_2S + 4NaCN \rightleftharpoons \underset{\text{Sodium argento cyanide}}{2Na[Ag(CN)_2]} + Na_2S$

The solution of sodium argento cyanide combines with zinc dust and forms sodium tetra cyanozicate and precipitated silver. This precipitated silver is called spongy silver.

$Zn + 2Na[Ag(CN)_2] \rightarrow Na_2[Zn(CN)_4] + 2Ag$

The spongy silver is fused with potassium nitrate to obtain pure silver. Then the silver obtained is purified by electrolytic process.

Preparation and uses of Silver chloride and Silver nitrate

Silver Chloride

AgNO3 combines with chloride ions and forms AgCl precipitate.

$AgNO_3 + HCl \rightarrow AgCl + HNO_3$

Properties

1. AgCl is obtained as a white precipitate
2. AgCl is insoluble in water.
3. It is soluble in aqueous ammonia

$AgCl + 2NH_3 \rightarrow [Ag(NH_3)_2]Cl$

Silver Nitrate

In the preparation of silver nitrate dilute nitric acid is used. Silver combines with HNO3 and forms silver nitrate.

$3Ag + 4 HNO_3 \rightarrow 3AgNO_3 + 2H_2O + NO$

Properties

• It is colorless and is soluble in water.
On heating AgNO3 decomposes and forms silver, nitrogen dioxide and oxygen.

$2AgNO_3 \rightarrow 2Ag + 2NO_2 + O_2$

$$AgNO_3$$ forms precipitates with chlorides, bromides, phosphates and sulfides. It is extensively used in laboratories.

Contributors

Stephen R. Marsden (ChemTopics)

Binod Shrestha (University of Lorraine)