Refining of metals

The metals prepared by different methods contain impurities. The methods used for the purification of metals are called refining. The refining method depends on the nature of metal and the nature of impurities. Some common methods are as follows
1. Distillation
Volatile metals like zinc and mercury are purified by boiling the impure metal to get vapors of the pure metal which is condensed and collected.
2. Liquation
Low melting metals like tin and lead are purified by this method. The impure metal is melted on the sloping floor of a furnace. The metal melts and flows down leaving behind the high melting impurities.
3. Poling
Impure metal is melted and stirred with green logs of wood. The impurities rise to the surfaces, get oxidised and removed as gases (CO2) or slag. The metal may get oxidised (eg:- Cu to Cu2O). The hydrocarbons in green wood reduces the metal oxide to the metal. Example:- Refining of impure Cu and Sn.
4. Cupellation
Impure silver and gold contain base metals like lead and bismuth as impurities. These are removed by heating the metal placed in a cupel (boat shaped crucible made of cement or bone ash) in a reverberatory furnace in a current of hot air. The impurities are oxidised and carried away by the current of air. The process is stopped when a shining bright surface appears.
5. Electrolytic refining
The method is based on the process of electrolysis. The crude metal is made the anode a thin sheet of pure metal the cathode. The electrolyte is the solution of a salt of the metal. On passing electricity the metal dissolves from the anode and an equal number of metal ions of the solution gets deposited at the cathode. The impurities settle down below the anode mud (eg:- refining of Cu using CuSO4 solution as electrolyte).
At anode Cu ------> Cu 2+ + 2electron
At cathode Cu 2+ + 2electron -------> Cu
6. Zone refining
Metals of very high purity can be obtained by this method. The impure metal rod is mounted horizontally and heated by a circular electric heater at one end in an atmosphere of inert gas to form a thin molten zone. By slowly moving the heater, the molten zone is moved from one end of the rod to the other end. Pure metal crystallises while impurities pass into the molten zone. By repeated the passes of the molten zone very high purity can be attained at one end. The other end is discarded. Ge, Si and Ga used in semi conductors are refined in this manner.
7. Vapour phase refining
By this chemical reaction the metal is converted to a compound, which forms a vapour, which is decomposed to get pure metal.
8. Monds process
Crude nickel is heated with carbon monoxide to form volatile nickel carbonyl. The impurities remain as solids. The vapours on further heating decomposes to give pure nickel.
Ni + 4Co -------> Ni(CO)4
Ni(CO)4 --------> Ni + 4CO
9. Van Arkel de Boer method
Crude titanium or zirconium is heated with iodine to get vapours of the tetraiodide. The vapours are then decomposed on a tungsten filament kept at high temperature on which the pure metal gets deposited. The pure metal is then peeled off from the filament after cooling.
Ti + 2I2 ---525k----> TiI4 ----1675k----> Ti + 2I2
Zr + 2I2 ----870k---> ZrI4 ----2075k----> Zr + 2I2

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Mercury halides

Mercury forms halides in the two oxidation states, +1 and +2.
1. Mercury(1)Chloride(Hg2Cl2)
Mercury(1) chloride or mercurous chloride is known as calomel.
Mercury(1)chloride is prepared by heating a mixture of mecury(2)chloride and mercury in iron vessel.
HgCl2 + Hg -------> Hg2Cl2
It can also be obtained by reduction of mercury(2)chloride by reducing agents like tin(2)chloride in limited quantity.
2HgCl2 + SnCl2 --------> HgCl2 + SnCl4

When heated, mercury(1)chloride decomposes into mercury(2)chloride and mercury.
Hg2Cl2 -------> HgCl2 + Hg
The action of aqueous ammonia on the solid mercury(1)chloride gives a mixture of black finely divided mercury and white mercury amino chloride. This reaction is an example of disproportion reaction.
Hg2Cl2 + 2Nh3 -------> Hg(NH2)Cl + Hg + NH4Cl

Uses of mercury(1)chloride
Calomel is used in making standard calomel electrodes used as secondary reference electrode. It is also used as a purgative in medicines.

2. Mercury(2)chloride (HgCl2)
Mercury(2)chloride or mercury chloride is known as corrosive sublimate.
Mercury(2)chloride may be prepared by heating the metal in chlorine gas.
Hg + Cl -------> HgCl2
It is also prepared by heating a mixture of mercury(2)sulphate and sodium chloride in the presence of traces of MnO2
HgSO4 + 2NaCl -------> HgCl2 + Na2SO4
Manganese dioxide prevents the formation of mercury(1)chloride.

Mercury(2)chloride is a white crystalline solid, but from aqueous solution it crystallizes into colourless needles. It is a covalent compound sparingly soluble in water. Mercury(2)chloride gives a white precipitate on reduction with stannous chloride, SO2, formaldehyde etc, which changes to grey on standing owing to the formation of metallic mercury.
2HgCl2 + SnCl2 -------> Hg2Cl2 + SnCl4
Hg2Cl2 + SnCl2 -------> 2Hg + SnCl4
Mercury(2)chloride reacts with aqueous ammonia to form infusible white precipitate of mercury amino chloride.
HgCl2 + 2NH3 --------> Hg(NH2)Cl + NH2Cl
Gaseous ammonia or ammonium chloride on reaction with Mercury(2)chloride forms fusible white precipitate of diammine Mercury(2)chloride.
HgCl + 2NH3 -------> Hg(NH3)2Cl2

Uses of mercury(2)chloride
Mercury(2)chloride is used in the preparation of mercuric iodide.

3. Mercury(2)Iodide (HgI2)
Mercury(2)iodide is obtained as a scarlet precipitate on addition of potassium iodide to a solution of mercury(2)chloride.
2KI + HgCl2 --------> HgI2 + 2KCl

Mercury(2)iodide readily dissolves in excess of potassium iodide solution due to the formation of potassium tetra iodo mercurate(2)Complex K2[HgI4].
HgI2 + 2KI -------> K2[HgI4]
This potassium tetraiodo mercurate(2)complex forms light yellow crystals of K2[HgI4].2H2O. The complex dissolves in potassium hydroxide solution to give Nessler's reagent which forms a brown precipitate or colouration with ammonia due to the formation of the iodide of Million's base, Hg2NI.H2O

Uses of mercury iodide
Mercury(2)iodide is used for preparing Nessler's reagent and in the treatment of skin infection.

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Silver nitrate (Lunar caustic) AgNO3

Silver nitrate is prepared by dissolving silver in dilute nitric acid.

3Ag + 4HNO3 ------> 3AgNO3 + 2H2O + NO

Silver nitrate on heating decomposes to form silver, nitrogen dioxide and oxygen.

2AgNO3 ---------> 2AgNO2 + O2

AgNO2 ---------> Ag + NO2

Silver nitrate is also decomposed by organic matter, such as glucose, paper, skin and cork. It has also a caustic and destructive effect on organic tissues.

Uses of silver nitrate
Large quantities of silver nitrate are used in the production of light sensitive plates, film and papers. In the laboratory it is used as a reagent for the detection of halide ions. It is used in making inks and hair dyes. In small doses, silver nitrate is used as a medicine for nervous diseases.

Silver halides
Silver fluorides may be prepared by the action of hydrofluoric acid on silver(1) oxide.

Ag2O + 2HF -------> 2AgF + H2O

Silver chloride, silver bromide and silver iodide are prepared by the action of silver nitrate on sodium halide.

Ag+ + x+ --------> AgX
(Where X = Cl, Br or I)

Ie Ag+ + Cl- --------> AgCl


Silver fluoride is soluble in water whereas the silver chloride, silver bromide and iodide are insoluble in water.
Silver chloride is highly soluble in ammonia solution due to the formation of Diammine silver(1) chloride complex [Ag(NH3)2]Cl.
Silver bromide is slightly soluble in ammonia solution and silver iodide insluble in ammonia solution.
All the silver halides dissolve in thiosulphate to form thiosulphate complex of silver and in cyanide solution to form dicyano complexes of silver(1).

Uses of silver halides
Silver chloride is used in photography for making printing paper. Silver bromide is used for the production of films and plates in photography and silver iodide used for the production of colloidal emulsion plates in photography.

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Copper sulphate penta hydrate (CuSO4 5H2O)

Copper sulphate penta hydrate is known as blue vitirol and is the most common oxosalt of copper(2)
Copper sulphate is prepared industrially by blowing a current of air through copper scrap and dilute sulphuric acid.

2Cu + 2H2O + O2 --------> 2CuSO4 + 2H2O

The crude copper(2) sulphate solution obtained contain iron(2) sulphates as impurity Dilute nitric acid is added to oxidize iron(2) to iron(3) sulphate which remains in solution after crystallization and CuSO4 5H2O crystallizes out.

The crystalline copper(2) sulphate, CuSO4 5H2O has the structure in which four water molecules are coordinated to the central copper cation in square planar structure. The fifth water molecule is held by hydrogen bonds between a sulphate anion and a coordinated water molecule. The fifth hydrogen bonded water molecule is deeply embedded in the crystal lattice and hence not easily removed.


1. Copper sulphate penta hydrate is a blue coloured crystalline solid, soluble in water.

2. Action of heat
On heating, copper sulphate penta hydrate gives trihydrate at 305 kelvin, monohydrate at 373 kelvin. On further heating it forms white anhydrous copper sulphate at 573 kelvin. The white anhydrous copper sulphate decomposes to give copper oxide and sulphur trioxide on heating to 673 kelvin.

At 3o5 kelvin CuSO4 5H2O ---------> CuO + SO2

At 373 kelvin CuSO4 3H2O ---------> CuO + SO2

At 573 kelvin CuSO4 H2O ---------> CuO + SO2

At 673 kelvin CuSO4 ---------> CuO + SO2

3. Copper(2) sulphate forms well defined crystalline double salts with sulphates of strongly electropositive metals of the type M2SO4.CuSO4 6H2O like (NH4)2SO4. CUSO4 6H2O. These double salts are isomorphous with the double salts of bivalent metals, Fe, Co, Ni.

4. If an aqueous solution of copper(2)sulphate is saturated with ammonia, the blue compound [Cu (NH3)4]SO4 H2O crystallizes on evaporation.

Uses of CuSO4 5H20
Copper(2)sulphate is used in copper plating and electro refining of metals. It is used as a mordant in dyeing and in electroplating. It is also used as germicide and fungicide under the name Bordeaux mixture which is a mixture of CuSO4 and Ca(OH)2.

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Indicators in acid - base titration

The reaction between an acid and a base is called neutralization. It is very fast and the equilibrium constant for a neutralization reaction is so large that it nearly proceeds to completion. An acid-base titration is a simple and convenient volumetric method for quantitatively estimating the concentration of one, if that of the other is known. A known volume of the solution of an acid or base is transferred to a titration flask with the help of a pipette. we add indicator and start adding known volumes of the other solution in steps with the help of a burette. The point at which the reaction is observed to be complete is called the end point of the titration and is noted by the change in the colour of the indicator. For accurate estimation it is necessary for it to coincide with the equivalence point corresponding to the stoichiometric amounts of the acid and base in the neutralization reaction. A number of weak organic acids and bases which can change its colour with in its limits with variations in the pH value of the solution to which it is added act as indicators. The choice of indicator depends on the abrupt change of the pH during neutralization process near the equivalence point.

Different theories have been put forward to explain the role of indicators in the acid-base titrations's like Ostwald's ionic theory, Quinonoid theory etc. Ostwald's theory considers indicator to be a weak acid or base whose unionised forms differently coloured. In presence of acid or base, ie pH change, there is ionization of indicator and hence the colour change appears.
For example
phenolphthalein is a weak acid (PhH)

PhH <_-_-_-_-_-_-> Ph- + H+ ...........(1)
(colourless (Pink in base)
in acid)

H+ + OH- <-_-_-_-_-_-_> H2O

In presence of an acid (H+) equilibrium (1) is displaced towards the left hand side (a case of LeChatelier's principle); when strong base like NaOH is added, this equilibrium is displaced towards right hand side and there is colour change from colourless to pink when pH changes. This indicator is not suitable for titrating weak base since weak base can't furnish enough OH- that can react with H+ of the phenolphthalein and can impart pink colour only after excess of weak base is added.

Methyl orange
Methyl orange behaves like a weak base (MeOH)

MeOH <-_-_-_-_-_> Me+ + OH- .........(2)
(yellow in base) (red in acid)

OH- + H+ <-_-_-_-_-_> H2O

In presence of a base, equilibrium (2) is displaced towards left hand side and appears yellow in base solution. On the addition of strong acid, OH- of MeOH is removed and hence equilibrium (2) is displaced towards right hand side when solution appears red. Thus there is colour changes from golden yellow to red when medium changes from basic to acidic. This indicator is not used in the titration of weak acid since it will not remove OH- of the indicator and can make colour change only after excess of weak acid has been addded.

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