Advanced Projects in Chemistry - Analysis of Alloys

Chemistry Projects at the K-11 and K-12 grades require application of the theoretical concepts studied in class. Transition metals are studied under the d-block. One of the peculiar characteristics of transition metals is formation of alloys. Alloys are solid homogeneous solutions of two or more solids. In an alloy, two or more elements combine such that at least one is a metal, and the resultant material has metallic properties. Alloys are designed to have properties that are more desirable than those of pure metals. Steel is stronger than iron, while brass, an alloy of copper and zinc is more durable than copper and more attractive than zinc in appearance.

Unlike pure metals, alloys do not have a single melting point. They possess a melting range. Alloys improve properties of metals, by increasing hardness, tensile strength, chemical resistance and attractive appeal by modifying color.

Alloys of Zinc and Copper are widely used in everyday life and known to all of us. Detection of the presence of the metals used to prepare these alloys can be achieved through semi-micro qualitative analysis. These tests are useful both for the student of chemistry as well as for a forensic laboratory chemist.

Laboratory work in alloy analysis involves a three step protocol
1. Grinding the alloy sample to a fine particulate powder.
2. Digesting the sample in an appropriate acid.
3. Detection of the metal ions in the alloy.

Alloy %composition
Brass Zn-20-40% &Cu-60-80%
Bronze Zn-10% &Cu -90%

Tests for Brass & Bronze include tests for Zn and Cu metals
1. Zn was detected with potassium ferrocyanide test and the Sodium hydroxide test
2. Cu was detected with Potassium ferrocyanide, potassium iodide and liquid ammonia tests.

Uses of Brass - Resistance wires, cookware, condenser tubes, locks, gears, bearings, doorknobs, ammunition, and valves; for plumbing and electrical applications; in musical instruments such as horns and bells for its acoustic properties and manufacture of zippers.

Uses of Bronze - Statues, coins, windings of steel strings of various stringed instruments such as the double bass, piano, harpsichord, and the guitar, springs, turbines and blades, hammers, mallets, wrenches and other durable tools.

Chemical Reactions -
For Cu Analysis :
Liquid ammonia test -
Cu(NO3)2+4NH4OH--> [Cu(NH3)4][NO3]2+4H2O-Deep blue solution

Potassium ferrocyanide test -
[Cu(NH3)4]SO4+4CH3COOH--> CuSO4+ 4CH3COONH4
2CuSO4+ K4 [Fe(CN)6]-->Cu2[Fe(CN)6]+2K2SO4 - Chocolate brown precipitate

Potassium iodide test
2CuSO4+ 4KI-->Cu2I2 (white precipitate) +I2 (brown coloration)+2K2SO4

For Zn Analysis
1. Sodium Hydroxide test
ZnCl2+2NaOH-->Zn(OH)2 white precipitate + 2NaCl
Zn(OH)2+2NaOH--> Na2ZnO2 soluble in excess NaOH+2H2O

2. Potassium ferrocyanide test
2ZnCl2+K4 [Fe(CN)6]-->Zn2[Fe(CN)6] bluish white precipitate +4KCl

A transition metal forms an alloy with another transition metal ion with ease because of the similarity in atomic size. The lattice site in the crystal structure of one transition metal can be occupied by other transition metals giving a homogeneous solid solution termed as an alloy. Mutual substitution of one or more metals leads to the formation binary, ternary or tertiary alloys. Hardness, resistance to corrosion, tensile strength, load bearing capacity are some of the properties which can be bettered by alloy formation. Actual lab analysis impresses the composition of the alloy for the student.

(Acknowledgments - The experiments for the analysis of the metal ions as well as procuring samples of alloys, digesting alloys of bronze and brass were carried out by Anchit R.Giri and Shashwat Kishore of Class XII, R.N Podar CBSE Senior Secondary High School, Santacruz(W).)