(1) Occurrence of gold : Gold is mainly found in native state either as vein gold, placer gold or alluvial gold. It is also present to a small extent in the combined state as sulphide, telluride and arsenosulphide. It is considered to be the king of metal.

Some important ores of gold are:

(i) Calaverite, AuTe₂ (ii) Sylvanite, AuAgTe₂ and

(iii) Bismuth aurite, BiAu₂

(2) Extraction of gold : (i) Mac-Arthur-Forest Cyanide process : The powdered gold ore, after concentration by Froth-floatation process, is roasted to remove easily oxidisable impurities of tellurium, arsenic and sulphur. The roasted ore is then treated with a dilute solution of KCN in presence of atmospheric oxygen when gold dissolves due to the formation of an aurocyanide complex.

4Au + 8KCN + 2H₂O + O₂ → 4K[Au(CN)₂] + 4KOH

(ii) Plattner’s chlorine process : The roasted ore is moistened with water and placed in wooden vats with false perforated bottoms. It is saturated with current of chlorine, gold chloride thus formed is leached with water and the solution is treated with a reducing agent such as FeSO₄ or H₂S to precipitate gold.

The impure gold thus obtained contains impurities of Ag and Cu. The removal of Ag and Cu from gold is called parting. This is done by heating impure gold with conc. H₂SO₄ (or HNO₃ when Ag and Cu dissolve leaving behind Au.

Cu + 2H₂SO₄ → CuSO₄ + SO₂ + 2H₂O

Properties of Gold: Gold is a yellow, soft and heavy metal. Gold and Ag are called noble metals since they are not attacked by atmospheric oxygen. However, Ag gets tarnished when exposed to air containing traces of H₂S . Gold is malleable, ductile and a good conductor of heat and electricity.

Pure gold is soft. It is alloyed with Ag or Cu for making jewellery. Purity of gold is expressed in terms of carats. Pure gold is 24 carats. Gold ’14 carats’ means that it is an alloy of gold which contains 14 parts by weight of pure gold and 10 parts of copper per 24 parts by weight of the alloy. Thus the percentage of gold in ’14 carats” of gold is = 100/24 × 14 = 58.3% .

Most of the jewellery is made from 22 carat gold (91.66% pure gold). Gold is quite inert. It does not react with oxygen, water and acids but dissolves in aqua regia

Oxidation states of gold: The principal oxidation states of gold are + 1 and + 3 though + 1 state is more stable than + 3.

Compounds of gold

(1) Auric chloride, AuCl₃ : It is prepared by passing dry Cl₂ over finely divided gold powder at 573 K

It is a red coloured crystalline solid soluble in water and decomposes on heating to give gold (I) chloride and Cl₂

It dissolves in conc. HCl forming chloroauric acid

AuCl₃ + HCl → H[AuCl₄]

(2) Aurous sulphide, Au₂S : It is prepared when H₂S is passed through an acidified solution of potassium aurocyanide, K[Au(CN)₂]

2K[Au(CN)₂] + H₂S → Au₂S + 2KCN + 2HCN

It is a dark brown solid, not attached by dilute mineral acids and hence is probably the most stable gold compound.

Potassium Permanganate, (KMnO₄)

Potassium permanganate is a salt of an unstable acid HMnO₄ (permanganic acid). The Mn is an +7 state in this compound.

Preparation : Potassium permanganate is obtained from pyrolusite as follows.

Conversion of pyrolusite to potassium manganate : When manganese dioxide is fused with potassium hydroxide in the presence of air or an oxidising agent such as potassium nitrate or chlorate, potassium manganate is formed, possibly via potassium manganite.

By chemical method : The fused dark-green mass is extracted with a small quantity of water. The filtrate is warmed and treated with a current of ozone, chlorine or carbon dioxide. Potassium manganate gets oxidized to potassium permanganate and the hydrated manganese dioxide precipitates out. The reactions taking place are,

When CO₂ is passed

2CO₂ + 4KOH → 2K₂CO₃ + 2H₂O

When chlorine or ozone is passed

2K₂MnO₄ + Cl₂ → 2KMnO₄ + 2KCl

2K₂MnO₄ + O₃ + H₂O → 2KMnO₄ + 2KOH + O₂(g)
The purple solution so obtained is concentrated and dark purple, needle-like crystals having metallic lustre are obtained.

Electrolytic method : Presently, potassium manganate (K₂MnO₄) is oxidized electrolytically. The electrode reactions are,

At anode: 2MnO₄^2– → 2MnO₄^– + 2e^–

At cathode: 2H⁺ + 2e^– → H₂(g)

The purple solution containing KMnO₄ is evaporated under controlled condition to get crystalline sample of potassium permanganate.

Physical properties

KMnO₄ crystallizes as dark purple crystals with greenish luster (m.p. 523 K).

It is soluble in water to an extent of 6.5g per 100g at room temperature. The aqueous solution of KMnO₄ has a purple colour.