1
Silver
Introduction
Symbol : Ag
Scientific name: Argentum
At. No. : 47
At. Mass : 107.9 amu
Oxidation state: +1
Occurrence
Silver is found in free as well as in combined state. In our country it is found in Khanigaon, near
Bandipur in Tanahun. Some important ores of silver are:
a) Argentite or silver glance (Ag2S) b) Ruby silver (Ag2SbS3)
c) Horn silver (AgCl) d) Argentiferrous galena (PbS) which contains 0.01 to 0.1% Ag.
Extraction of silver from Argentite (Ag2S) by cyanide process
1. Concentration:
Argentite being sulphide ore is concentrated by froth flotation process. The pulverized ore is
kept in large tank containing water and pine oil. The mixture is agitated by passing
compressed air where ore forms froth with pine oil & comes to the surface where as the
impurities are settled down at the bottom of the tank.
2
2. Treatment with sodium cyanide:
The concentrated ore is treated with 0.4% to 0.7% aqueous solution of sodium cyanide and the
current of air is passed through it. The argentite ore dissolves in sodium cyanide forming
sodium argento cyanide (soluble complex).
Ag2S + 2NaCN ⇌ 2AgCN + Na2S
AgCN + NaCN(Excess) ⇌ Na[Ag(CN)2]( sodium argento cyanide)
Since, the reaction is reversible, air drives the reversible reaction to forward direction by
oxidizing sodium sulphide to sodium thiosulphate and sulphur.
2Na2S + 2H2O + O2 → 4NaOH + 2S↓
2Na2S + H2O + 2O2 → 2NaOH + Na2S2O3
If ore is horn silver, complex sodium argentocyanide and NaCl are formed.
AgCl + 2NaCN → Na[Ag(CN)2] + NaCl
3. Precipitation of Silver:
The solution obtained from above process is filtered to remove insoluble impurities. The
filtrate is then treated with zinc dust where Zn displaces silver from its complex because Zn is
more electropositive than silver.
Zn + 2Na [Ag (CN)2] → Na2[Zn (CN)4] + 2Ag
Sodium tetracyano zincate
Thus obtained silver is called spongy silver. The mass obtained above is filtered and then
strongly heated with borax or potassium nitrate to get compact silver.
4. Refining: Crude silver contains impurities like Zn, Cu, Au, Pb etc and purified by the
following methods.
Electrolytic Method:
The impure silver is purified by electrolytic method. A block of impure metal is made anode
where as a thin strip of pure silver is made cathode. A mixture of AgNO3 solution acidified
with dil. HNO3 acts as an electrolyte. On passing current, impure silver dissolves and equivalent
amount of pure silver is deposited at the cathode.
Fig: Electrolytic refining of silver
3
AgNO3(aq) → Ag+ + NO3−
At anode: Ag - e− → Ag+
At cathode: Ag+ + e− → Ag (pure)
Physical Properties
• Silver is lustrous white metal having density 10.5 g/cc.
• It is good conductor of heat and electricity.
• It is highly malleable and ductile.
Chemical Properties
a) Action of air
It is not attacked by air but is tarnished when exposed to air containing traces of hydrogen
sulphide.
2Ag + H2S → Ag2S↓ + H2↑
b) Action of acids: With acids the following reaction takes place.
i) With hot and concentrated H2SO4, sulphur dioxide gas is obtained.
∆
2Ag + 2H2SO4 Ag2SO4 + 2 H2O + SO2↑
ii) With conc. HNO3, AgNO3 and NO2 gas are obtained.
Ag + 2HNO3 → AgNO3 + NO2↑ + H2O
iii) With dil. HNO3, silver is oxidized to AgNO3 but acid get reduced to NO gas.
3Ag + 4HNO3 → 3AgNO3 + NO↑ + 2H2
c) Action of Sulphur dioxide: With SO2 at high temperature, Ag2SO4 and Ag2S is obtained.
4Ag + 2SO2 → Ag2SO4 + Ag2S
Uses of silver
1. Silver is used in making ornaments, coins, decorative articles etc.
2. It is used in silver plating, making silver mirrors etc.
3. It is used for making different alloys.
Compounds of silver
1. Silver nitrate (AgNO3)
4
It is commonly known as lunar caustic because silver was called luna by the
ancient chemists who associated silver with the moon. Caustic means "able to
corrode organic tissue by chemical action". This is just what lunar caustic does.
Improper use of lunar caustic can cause chemical burns.
Preparation
It is prepared by the action of dilute nitric acid on silver and then evaporating the
solution upto crystallization point.
3Ag + 4HNO3 → 3AgNO3 + NO + 2H2O
dil.
AgNO3 (aq.) Heated upto cryst. pt. AgNO3 (s)
Properties
1. It is colourless crystal with rhombic structure.
2. It is soluble in water and alcohol.
3. It melts at 212oC.
4. It is decomposed by light so it is stored in dark coloured bottles. It produces black stain
on skin and clothes in presence of sunlight. This is due to formation of metallic silver.
This property is used to mark the voters who have casted their votes during election.
Sunlight,organic comp.
2AgNO3 2Ag + 2NO2 + O2
5. Action of heat:
It decomposes on heating to give silver nitrite (AgNO2) and oxygen.
2AgNO3 212-450 2AgNO2 + O2
2AgNO3 Above 450 2Ag↓ + 2NO2 + O2
6. Precipitation reactions
It gives precipitates with some salt solutions which help in the detection of
acid radicals. Some of the precipitation reactions are:
AgNO3 + NaCl → AgCl↓ (White ppt) + NaNO3
AgNO3 + NaBr → AgBr↓( Pale yellow ppt) + NaNO3
.
AgNO3 + NaI → AgI↓ (yellow ppt.) + NaNO3
2 AgNO3 + Na2S → Ag2S (black ppt.) + 2NaNO3
7. Action with caustic alkali: It forms precipitate of silver oxide with NaOH.
AgNO3 + 2NaOH → Ag2O↓(Brown ppt.) + 2NaNO3 + H2O
8. Action with sodium thiosulphate: It gives white precipitate with hypo solution which
dissolves in excess of hypo forming a complex salt.
5
2AgNO3 + Na2S2O3 → Ag2S2O3↓(white ppt.) + 2NaNO3
Ag2S2O3 + 3Na2S2O3 → 2Na3[Ag(S2O3)2](Sodium argento thiosulphate)
9. Action with ammonium hydroxide: With ammonium hydroxide, a brown ppt of Ag2O
is obtained which get dissolved in excess of ammonium hydroxide forming a coplex.
2AgNO3 + 2NH4OH → Ag2O↓(brown ppt.) + 2NH4NO3 + H2O
Ag2O↓ + 2NH4NO3 + 2NH4OH → [Ag(NH3)2]NO3↓ + 3H2O
Uses of AgNO3
1. It is used for the preparation of silver halides which are used in photography.
2. It is used for making unremovable inks and hair dyes.
3. It is used as a lab reagent.
4. It is used as a dental antiseptic (dilute solution).
5. It is used for the preparation of Tollen’s reagent to identify aldehyde as the
functional group.
2) Silver Chloride (AgCl)
It occurs in nature as horn silver (AgCl) as ore.
Preparation
a. It is also prepared by adding solution of sodium chloride to AgNO3 solution.
The ppt is filtered, washed and then dried at about 70oC.
AgNO3(aq.) + NaCl (aq.) → AgCl↓ + NaNO3
white ppt
Properties
1. It is white amorphous solid insoluble in water.
2. AgCl is light sensitive compound and turns violet and finally to black on exposure to
light.
3. Action of NH3 solution
AgCl dissolves in NH3 solution by forming a soluble diamine complex.
AgCl + NH4OH → [Ag(NH3)2]Cl + 2H2O
diammine silver chloride.
(soluble complex compound)
4. Action of KCN solution:
AgCl dissolves in KCN solution forming a soluble potassium argento cyanide complex.
AgCl + KCN(excess) → K [Ag (CN)2] + KCl
Potassium argento cyanide
6
5. Action of light:
It is sensitive to light. It is converted to metallic silver when exposed to light.
Sunlight
2AgCl 2Ag + Cl2
Uses
AgCl is used in making photographic film and paper.
Frosted Silver
The ornaments and articles of silver usually contain copper from 6 to 7.5 %. To give pure white
appearance to the article, these are heated in air so that the copper near the surface is oxidized
while silver remain as such. The oxide formed on the surface is dissolved in dilute sulphuric acid
and layer of pure silver is left on the surface. This process is called frosting of silver and silver
obtained is called frosted silver.
Some Important Questions:
1. Why silver ores are leached with metal cyanides for the extraction of silver?
2. Why does silver nitrate produce black stain on the skin in the presence of sunlight?
3. What happen when AgCl is kept in excess ammonia solution?
4. What is mean by spongy silver?
Ans: The silver extracted from argentite which is soft, porous and about 95% pure is called
spongy silver.
5. Why does silver nitrate produces permanent black stain on the skin? Write an
important use of silver nitrate.
6. What happens when a precipitate obtained by adding silver nitrate solution in a
chloride salt solution is treated with ammonia solution. (Write with balanced
chemical equation)
7. Why silver ores are leached with metal cyanides for the extraction of silver?
8. How is the presence of halide ions are tested with silver nitrate solution?
The End!
