Introduction to titration of copper.

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INTRODUCTION

Copper was discovered by chance over 11,000 years ago and today remains one of the most versatile metals on earth1. It is for this reason that it remains invaluable to mankind in a variety of applications from power transmission to micro- electronics. Copper is generally found as the di-valent cation, copper (II) or mono-valent ion, copper (I).

At low concentrations, copper is an essential element but is toxic at high levels2 especially in water as it poses serious environmental and human health hazards. Industries involved in metal processing must therefore pre treat and/or detoxify the metal rich effluents before discharging them into the environment3 .

As well as removing water pollution, economic benefits ensue from this process, due to the high cost of copper on the World markets.

The following techniques can therefore be used to detect the presence of copper ions in water or solutions:

TITRATION METHOD

It involves a reduction-oxidation titration. Copper(II) ions react with excess iodide ions forming a copper (II) iodide precipitate and molecular iodine.

                 2Cu2+(aq) + 4I-(aq)           2CuI(s) + I2(aq)

The iodine is then titrated against a standard solution of sodium thiosulphate in the presence of a  starch indicator.

                I2 (aq) + 2S2O32-(aq)           2I-(aq) + S4O62-(aq)

An alternative to this would be the titration of copper (II) ions against a solution of EDTA using Fast Sulphon F as an indicator.

An acid base titration can also be carried out. An adequate amount of cation exchange resin in its hydrogen form is added to copper(II) ions, which stick onto the resin and displace twice as many hydrogen ions.

              2Resin(s) + Cu2+(aq)            Resin-Cu(s) + 2H+(aq)

The resin is then filtered off and the solution titrated with sodium hydroxide solution.

GRAVIMETRIC METHOD

It is also known as the weighing method. Zinc powder is added to a solution of copper (II) ions.

             Zn(s) + Cu2+(aq)              Zn2+(aq) + Cu(s)

Excess zinc is removed by the addition of sulfuric acid, before drying and weighing the copper. Zinc metal and copper (II) ions react exothermically and so make it possible to determine the concentration of copper (II) ions by measuring the heat evolved. That is, the more heat, the higher the concentration. This method however, is fairly complex and time consuming.

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COLORIMETRIC METHOD

When ionic concentrations are too low to be determined by the gravimetric method, chemists use a colorimeter or a spectrophotmeter. This is an instrument that measures quantity of light energy that dissolved ions absorb when light is passed through the solution.

Some ions do not produce an intensely coloured solution but instead form complex and brightly  coloured ions that absorb light in the visible range. For example, the formation the tetra amine complex ion Cu(NH3)42+, from copper (II) ions by the addition of aqueous ammonia NH3.

ELECTROCHEMICAL CELL METHOD

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