Copper Titration

INTRODUCTION

Copper coins are made of coinage bronze, an alloy consisting mainly of copper, but which contains other metals as well. In this experiment we are going to determine the percentage of copper in 1p pieces using titrimetric analysis. Copper (atomic number =29) is the last member of the first series of d-block transition elements. Its electron configuration is 1s22s22p63s23p63d104s1 and this arrangement in which all five 3d orbitals are fully occupied is more stable than the alternative [Ar] 3d94s2. Copper is a relatively unreactive metal but is attacked by oxidising acids notably nitric acid and concentrated sulphuric acid.

Redox titration is a volumetric method that relies on the oxidation of the analyte and if the redox reaction doesn’t produce a well-defined colour change at the equivalence point, an indicator should be used which in this case the starch.

The coin which weighed 3.555g was placed in a 250cm3 beaker and dissolved in 20cm3 of concentrated nitric acid forming a solution of copper (II) ions. This was all done in the fume cupboard as the brown fumes ( exothermic reaction) that was coming out was the toxic nitrogen dioxide, Copper atoms are oxidised to Cu2+ at the same time NO-3 ions are reduced to molecules of NO which with O2 from the air become oxidised to molecules of NO-2. The solution of Cu2+ ions was then treated with aqueous potassium iodide. Copper (I) iodide is precipitated as a white solid and iodine is produced, according to the equation below: 2Cu2++ (aq) + 4I-(aq) - 2CuI(s) + I2 (aq). The amount of liberated iodine is found by titrating with standard sodium thiosulphate solution.

Charles' Law

As the temperature from the reaction warms the gas, it expands. Later, as it cools, the gas contracts. Although the nitrogen dioxide gas is noxious and toxic, it dissolves readily in water and make the solution acidic. This can be shown by adding a little indicator to the water and making the water slightly basic before the...