7.5 Silver
🎯 Learning Outcomes
After completion of the chapter, you should be able to:
- Describe the occurrence of silver
- Describe the extraction of silver by the cyanide process
- Explain the preparation and uses of silver chloride and silver nitrate
1 Silver — Occurrence & Ores
Occurrence
Silver occurs in both the native and combined states. The main producers of Ag metal are Mexico, the USA, Peru, Canada, Australia, etc.
Ores of Silver
| Ore | Formula |
|---|---|
| Argentite or silver glance | Ag₂S |
| Horn silver | AgCl |
| Ruby silver | Ag₃SbS₃ |
| Argentiferous galena | PbS (0.01–0.1% Ag) |
2 Extraction — The Cyanide Process
Large lumps of ore are crushed with the help of a jaw crusher. The crushed ore is pulverised by ball mills into a fine powder.
The crushed ore is then mixed with water and a little pine oil in a tank. It is then agitated by blowing compressed air. Sulfide ore particles are wetted by pine oil and attach to air bubbles — they rise to the surface as froth, while gangue particles are wetted by water and sink. The froth is skimmed off.
The concentrated ore is then agitated with NaCN solution using air; soluble sodium argento-cyanide is formed.
As the reaction is reversible, air is blown to remove Na₂S as Na₂S₂O₃ and S. This helps the reaction proceed in the forward direction.
The solution is then filtered and treated with zinc metal. Zinc, being more electropositive, displaces silver from the solution.
The precipitated silver — called spongy silver — is removed by filtration and dried.
The dried silver is heated with KNO₃ in a crucible. The impurities in the silver get oxidised on the surface, and the liquid silver forms a compact mass on cooling. After that, impure silver is purified by the electrolytic method.
In this process, the anode is made up of impure Ag and the cathode is made up of pure silver. Here, 6% AgNO₃ solution with 1% HNO₃ is used as an electrolytic solution. When an electric current is passed, impure silver from the anode dissolves, and pure silver is deposited on the cathode. Other impurities settle down as anode mud.
3 Silver Chloride (AgCl)
Preparation
By adding any soluble chloride to silver nitrate:
The white precipitate is separated by filtration, washed, and dried.
Uses of Silver Chloride
- AgCl is photosensitive and is used for making photographic paper.
4 Silver Nitrate (AgNO₃)
Preparation
By dissolving silver metal in dilute HNO₃ (e.g. HNO₃ : H₂O = 1 : 1) and evaporating the resulting solution to the crystallisation point.
Silver nitrate is a colourless/white crystalline solid. It is also called lunar caustic because it is a corrosive silver compound, and it can stain skin black when exposed to light — silver compounds are reduced to metallic silver, giving a dark, moon-like appearance (hence “lunar”).
Uses of Silver Nitrate
- Laboratory reagent in volumetric analysis for the estimation of chlorides, bromides, cyanides, and thiocyanates, as well as in qualitative analysis to detect Cl⁻, Br⁻, I⁻, CN⁻, S²⁻, C₂O₄²⁻, CrO₄²⁻, etc.
- In medicine, photography, and electro-refining.
- Making indelible inks (voter’s ink).
- Preparation of various silver compounds, like Tollens’ reagent.
- As a dental antiseptic.
📘 Chapter Summary
- Silver occurs both native and combined — argentite (Ag₂S) and horn silver (AgCl) are its key ores.
- Extracted by the cyanide process: ore is leached with NaCN to form a soluble complex, then precipitated with zinc as spongy silver.
- Electrolytic refining with an impure anode and pure cathode gives high-purity silver; impurities settle as anode mud.
- Silver chloride is photosensitive, used in photographic paper.
- Silver nitrate (“lunar caustic”) is a key laboratory reagent and the starting material for many silver compounds.