Constituents of An Alloy

 

Introduction

An Alloy is a homogenous mixture of two or more metals or a non-metal.

An alloy of mercury with another metal is called amalgam.

Alloys are usually harder than their components but very often less ductile and less malleable. Thus the hardness of gold is increased by addition of copper to it. The melting point of an alloy is always lower than the melting points of the constituent metals. Other properties such as reactivity towards atmospheric oxygen and moisture, mechanical strength, ductility, colour etc. also under goes a change when an alloy is made from its constituents (metals). This change of properties is very useful and makes an alloy beneficial.

Some of the alloys along with their composition is given below.

1.     Brass :-

        It consists copper-50-90%.

        Zinc. : 20-40%

        and small amounts of tin, lead and iron.

2.     Bronze : It consists copper 60-90%.

3.     Tin : 5-35%

        and also contains small amounts of lead, iron and zinc.

        Duralumin : It consists A1 : 95%, Cu = 4%

        Mn = 0.5%, Mg = 0.5%

4.     Gun Metal : It consists copper 85 - 90%

5.     Zinc. : 1-3%

6.     Tin : 8-12%

        The composition of alloys may differ slightly depending upon the quality of the alloy though the main components remain the same.

 

Preparation of Alloys

Alloys are prepared from the techniques of fusion, compression or simultaneous electro - deposition. Generally the components are mixed together in proper properties in a fuse clay crucible, melted and stirred with a piece of charcoal to avoid oxidation. The molten mixture is now allowed to cool. When an alloy is obtained e.g. brass is prepared by above melted.

 

Analysis of an alloy

The complete analysis of an alloy involves two steps.

1. Qualitative Analysis :

This involves identification of the components of the alloys.

2. Quantitative Analysis :

This involves determination of the components of the alloy. It involves the separation of the components from the alloy quantitatively followed by determination of percentage of each component volumetrically or gravimetrically.

In this project we will carry out qualitative analysis only.

 

Objectives of Project :

In this project, our aim is to know the various metals present in the given sample of alloy.

 

Experiment - 1 [Constituents of Alloy]

Aim : To analyze a sample of brass qualitatively.

Requirements : China dish, test-tube funnel, filter paper and common laboratory reagents.

Theory :

Brass is an alloy of copper and zinc. with the following.

Composition :

Cu = 60-90% and Zn. = 10-40%.

Thus Cu and Zn. form the main constituents of brass. Both these metals dissolved in 50% of nitric acid due to formation of nitrates which are soluble.

3 Cu + 8HNO₃ (Dil) 3 Cu (NO₃)2 + 2NO + 4H₂O

                                    or

Cu + 8H+ + 2NO₃– 3 Cu+2 + 2NO + 4H₂O

4Zn + 10HNO₃ (Dil) 4 Zn (NO₂)2 + N₂O + 5H₂O

4Zn + 2NO₃– + 10H+ 4 Zn+2 + N₂O + 5H₂O

The solution is boiled to expel the oxides of nitrogen and the resulting solution is tested for Cu2+ and Zn+2 ions.

Procedure :

1. Place a small piece of brass in a china dish and heat this with minimum quantity of 50% HNO3 so as to dissolve the piece completely.

2. Continue heating the solution till a dry solid residue is obtained.

3. Dissolve the solid residue in dil. HCl and filter. Add distilled water to the filtrate.

4. Pass H2S gas through the filtrate. A black precipitate of copper sulphide is obtained. Separate the black ppt. and keep the filtrate for the test of Zn+2 ions Dissolve black ppt. by heating them with 50% HNO3. To this solution add ammonium hydroxide solution. Appearance of deep blue colouration in the solution shows the presence of copper ions in the solution.

5. To test Zn+2 ions, boil the filtrate to remove H2S gas, then add solid NH4Cl to this and heat to dissolve NH4Cl. Add excess of NH4OH so that a solution is ammoniacal. Now pass H2S gas through this ammoniacal solution. Dirty white or grey precipitation indicate zinc. Separate the precipitates and dissolve it in minimum amount of dil. HCl. Boil to expel H₂S gas and add potassium Ferro cyanide solution, white or bluish white ppt. confirm Zn+2 ions in the solution.

Result :

The given sample of brass contains copper and zinc. metals as the main constituents.

 

Experiment - 2 [Constituents of Alloy]

 

Aim : To analyze a sample of bronze qualitatively.

Requirements : China dish, test-tube funnel, filter paper and common laboratory reagents.

Theory :

        Bronze is an alloy of copper and tin with the following.

Composition :

Cu = 88-96% and Sn. = 4-12%.

Thus copper and zinc. form the main constituents of bronze. Both these metals dissolved in nitric acid.

3 Cu + 8H+ + 2NO₃– 3 Cu2+ + 2NO + 4H₂O

4Sn + NO₃– + 10 H+ 4 Sn+2 + NH₄+ + 3H₂O

(Cold and Dil. Acid)

Sn + 4NO₃– + 4H+ H₂Sn O₃ + 2NO₂ + H₂O

(Conc. acid) (Metastannic Acid)

Excess of nitric acid is removed by heating the solution. The resulting solution now would contain Cu+2 ions and metastannic acid. This solution is acidified with dil. HCl and H₂S gas is passed when the sulphides of copper and tin are formed.

Cu+2 + S₂- CuS (Black ppt.)

H₂SnO₃ + 2H₂S SnS2 (Black ppt.) + 3H₂O

The sulphides are separated by boiling the ppt. with yellow ammonium sulphide when SnS2 goes into solution as thiostannate where as CuS is not affected.

SnS₂ + (NH₄)2S (NH₄)2 SnS₂ (Soluble)

Ammonium thiostannate.

CuS + (NH₄)2S CuS (Unaffected)

Black ppt.

The soluble black ppt. is tested for Cu+2 ions and the solution is tested for Sn2+ ions as in elementary qualitative analysis.

Procedure :

1. Take about 1g. of small pieces of bronze in a china dish and add to it 5-10 ml. of dil. HNO₃.

2. Heat the contents slowly to dissolve copper and tin completely and then boil the contents to a paste to remove excess of HNO₃. All this is carried out in cup board.

3. Dissolve this dry mass in distilled water containing HCl (1:1) to get a clear solution.

4. Transfer the solution in a test tube and pass H2S in excess i.e. till the precipitation is complete. Filter and reject the filtrate.

5. Take the black ppt. in a test tube and add to it 2-3 ml. of yellow ammonium sulphide and heat. Filter the contents. Black residue is tested for Cu+2 ions and filtrate is tested for Sn+2 ions.

6. Analysis of black residue :

Transfer a little of the black ppt. into a test tube. Add to it 2-3 ml. of 50%. HNO3 and boil the contents of the tube. A light blue or green sol. indicates the presence of Cu+2. Divide this sol. into two parts.

(a) To one part add excess of NH4OH a deep blue colouration confirms the presence of Cu+2 ions.

(b) Acidify the second part with acetic acid and add K4 [Fe (CN)6] i.e. potassium ferrocyanide solution. A reddish brown ppt. confirms the presence of Cu+2 ions.

7. Analysis of filtrate :

Boil the filtrate with 1 ml. of dil. HCl. A yellow ppt. is obtained. Dissolve in 1 ml. conc. HCl. To this solution add 0.5 g. of zinc. dust and boil it for 2-3 minutes. Filter and to filtrate add 1-2 ml. of mercuric chloride solution. A white ppt. turning grey on standing confirms the presence of Sn+4 ions.

Result :

The given sample of bronze contains - Cu and Sn as the main constituents.

 

Experiment - 3 [Constituents of Alloy]

Aim : To analyze a sample of duralumin qualitatively.

Requirements : China dish, test-tube funnel, filter paper and common laboratory reagents.

Theory :

Duralumin is an alloy of copper and tin with the following.

Composition :

Al = 95%, Cu = 4%, Mn = 0.5%, Mg = 0.5%

The alloy dissolves in aqua regia. The solution is tested as in an elementary qualitative analysis.

Procedure :

1. Take about 1 g. of duralumin in a china dish. Add to it about 10 ml. of aqua regia (Con. HNO3 : Con. HCl, 1:3)

2. Heat the contents of china dish strongly till the whole of the alloy dissolves and then slowly to get a paste.

3. Dissolve the paste in dil. HCl in a test tube. Pass H2S gas in excess through it till the precipitation is complete. A black ppt. is formed.

4. Filter the solution. Test the black ppt. for copper and filtrate for aluminium.

5. Test of Black ppt. :

Transfer a little of the black ppt. into a test tube. Add to it 2-3 ml. of 50%. HNO3 and boil the contents of the tube. A light blue or green sol. indicates the presence of Cu+2. Divide this sol. into two parts.

a) To one part add excess of NH4 OH - a deep blued calouration confirms the presence of Cu+2.

b) Acidify the part with acetic acid and add K4[Fe(CN)6] i.e. potassium ferrocyanide solution. A reddish brown ppl. confirms the presence of Cu+2 ions.

6) Analysis of filtrate :-

Boil the filtrate till H2S is completely removed. Add a drop of conc. HNO3 and heat add 1g of solid NH4Cl, warm and cool. Add NH4OH in excess till the solution smells of ammonia - A gelatinous white ppt in minimum dil. HCl then add a drop of blue litmus solution. Add NH4OH solution till it smells of ammonia (till it is alkaline).

A blue ppt (lake) floating in the colourless solution confirms the presence of Al3+ ion.

Results

The given sample of duralumin contains - Cu and Al as the main constituents.

 

Uses of alloys

i) To modify chemical reactivity :-

When sodium is used as reducing agent it is too reactive to be used but its allay with mercury, called sodium amalgam can be safely used as reducing agent.

ii) To increase hardness :-

Hardness of gold is increased by adding copper to it. Also zinc is added to copper to make copper hard in form of brass.

iii) To increase tensile strength :-

Nickeloy, an alloy of Nickel (1%), Copper (4%) and aluminium (95%) has high tensile strength.

iv) To lower the melting point :-

Solder metal which is an alloy of Sn(30%) and Pb(70%) has very less meting point as compared to melting points of Sn and Pb.

v) To modify the colour :-

Aluminium bronze an alloy of Cu and Al has beautiful golden colour.

vi) To resist corrosion:-

Iron gets rusted and corroded. Its corrosion takes place with time but stainless steel, an alloy of iron and carbon get not rusted the composition of stainless steel is :

Iron - 98%

Carbon - 2%