Introduction to titration of copper

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 .

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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.

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

I shall be looking at the colorimetric and electrochemical cell method in more detail.

RISK ASESSMENT

Care:

A lab coat and safety goggles must be worn when carrying out all experiments. The lab coat shields the skin and clothing from contact with the chemicals and the glasses protect the eyes from splashes.

1. Ammonia solution – toxic and irritant

Harmful if ingested in quantity. Fumes are toxic if breathed in.

First Aid

Eyes: Irrigate thoroughly with water. If discormfort persists seek medical attention.

Lungs: Exposure to adequate amounts of fresh air.

Skin: Wash off thoroughly with soap and water.

Mouth: Wash off thoroughly with water. Seek medical attention in severe cases.

Spillage disposal

Mop up with plenty of water and run to waste, diluting greatly with running water. Wash site of spillage thoroughly with detergent and water.

2. Zinc sulfate – irritant

Harmful if ingested or inhaled. Could result in corneal burn.

First Aid

Swallowing: Induce vomiting if person is conscious.

Skin: Wash thoroughly with soap and water. Contaminated clothing should be taken off.

Eyes: Flush with plenty of water for at least 15 minutes. Eyelids should be kept apart while doing this.

Lungs: Exposure to sufficient amounts of fresh air. Artificial respiration should be administered if breathing has stopped.

Spillage disposal

Should be carefully reacted with sodium carbonate to form insoluble zinc carbonate solid that can be disposed of easily.

3. Copper Sulfate – irritant

Harmful if ingested or inhaled. Copper Sulfate dust may ulcerate membranes. Prolonged exposure could cause dermatitis.

First Aid

Eyes: Irrigate thoroughly with water for 15 minutes. Seek medical attention if irritation persists.

Skin: Wash with water.

Swallowing: Vomiting should NOT be induced. Milk and water should be given to dilute the poison.

Spillage disposal

Sweep out without creating a dust, mark and store as a hazardous waste. If in solution form, collect on absorbent material and store as a hazardous waste.

MAKING UP OF SOLUTIONS

To make up 1 litre of a molar solution, the relative molecular mass(RMM) of the solute is to be dissolved in a litre of distilled water. A standard volumetric flask is used. The solute is dissolved in a beaker of water to ensure that it is easily soluble. Using a funnel (so as to avoid spillage), the solution formed is added to the volumetric flask. The empty beaker should be rinsed out with distilled water so as to ensure that all of the dissolved solution has gone into the flask. This is done as it ensures that the concentration is the one required rather than a weaker one.

When filling up the volumetric flask, the bottom of the meniscus should touch the line on the neck of the flask. The last cm3 must be added using a dropping pippette as this increases accuracy.

Once the exact amount of water is in the volumetric flask, shaking of the solution is necessary so as to ensure that the entire solid is dissolved.

Copper Sulfate

I am making up 1 litre of a 0.05 molar solution for the colorimetric method. Copper Sulfate has a relative molecular mass of 249.68g. Therefore, in order to make up 1litre of 0.05M, I will have to dissolve 249.68g x 0.05 = 12.48g in a litre of distilled water.

Zinc Sulfate

I will require 200cm3 of a 1M solution. Zinc sulfate has a RMM of 287.54. I will therefore dissolve 58g in 200cm3 of distilled water.

Sodium Sulfate

I require 100cm3 of a 1M solution and so will dissolve 32g in enough distilled water to make up 100cm3 of solution.

DETECTING CONCENTRATION OF COPPER (II) IONS USING COLORIMETRIC METHOD

Apparatus required:

25 beakers, 50cm3

2 burettes, 50cm3

2 burette stands

1 stirring rod (glass)

30 test tubes

26 cuvettes

1 colorimeter

distilled water

Copper (II) Sulfate, CuSO4, stock solution

Concentrated Ammonia, NH3, in dropping bottle

All apparatus should be cleaned with distilled water before use. This reduces the likelihood of contamination of the solutions by impurities, which could distort findings.

Method:

The copper (II) sulfate and distilled water each had to be placed in their own burette. This was done using a funnel thus ensuring that no solution was spilt. The volumes in the burettes were set to 50cm3 by letting out any excess solution through the bottom of the burette into a beaker. This was done until the bottom of the meniscus was lying on the 50cm3 line.

A standard run was carried out i.e. the reaction which all the results were to be compared to. This run used 10cm3 of copper (II) sulfate.

1. Prepare solutions of copper (II) sulfate that increase in dilution as shown in the table below. In each case, swirl the two liquids in the beaker so that they mix together.

Volume Copper (II) Sulfate (cm3)

Volume

Distilled water(cm3)

Concentration Copper (II) Sulfate (moldm3)

10

0

0.05

8

2

0.04

6

4

0.03

4

6

0.02

2

8

0.01

2. Obtain the test tubes and label them 1 to 5.

3. Pour 5cm3 of each prepared solution into its corresponding test tube and 5cm3 of distilled water into test tube 6.

4. Add two drops of concentrated ammonia in each of the test tubes 1 to 5 and stir. Ensure the bottle is stoppered when not in used as ammonia fumes are toxic.

5. Switch the sensitivity controls in the colorimeter to a minimum and check that the suitable filter is in position.

6. Place test tube 6 in the colorimeter and use it to check for 100% transmittance (%T).

7. Place test tube 1 to 5 into the colorimeter, one at a time and read off the %T for each solution.

8. Record observations.

9. Repeat steps 1 to 8 again at least five times so as to obtain average readings.

Note:

The standard run should be performed when commencing each day’s work so as to ensure that solutions have not gone off. When making a new batch of solutions, it must be checked that the concentrations are constant.

As this experiment was carried out over a long period of time, I had to re-calibrate the colorimeter periodically. This was done so as to avoid instrument drift, which would affect the results.

It must be ensured that the volume of the solution is kept standard, so that the concentration of the solutions are kept the same.

DETECTING THE CONCENTRATION OF COPPER(II) IONS USING ELECTROCHEMICELL METHOD

Apparatus required:

10 beakers, 150 cm3

3 burettes, 50cm3

5 zinc strips, 1.5cm x 1.5cm

5 copper strips, 1.5cm x 1.5cm

Filter paper strips, 2cm x 15cm

Copper wire with alligator clips at each end

Voltmeter

Emery paper

Canvas gloves

Distilled water

200 cm3 1M Sodium sulfate

200cm3 1M Copper (II) sulfate

100cm3 1M Sodium Sulfate

Method:

The metal strips were polished using emery paper, rinsed with distilled water and then dried. This was done to ensure that the metal surfaces were free from any oxides that may have formed thereby providing a clean surface for reactions.

The copper(II) sulfate, sodium sulfate and distilled water were placed in their own burettes (these are used as they allow the quantity of the solutions to be measured accurately). A funnel was used to ensure that there was no spillage. The volumes in the burettes were then set to 50cm3 by letting any excess solution out through the bottom and into a beaker. This was done until the bottom of the meniscus was laying on the 50cm3 line.

The standard run used 50cm3 of copper (II) sulfate and 50cm3 of zinc sulfate.

1. Measure out 50cm3 of copper (II) sulfate into one of the 150cm3 beakers.

2. Dip a copper strip into the solution and attach a crocodile clip to it. This forms the copper half cell.

3. Measure out 50cm3 of zinc sulfate into another beaker, dip a zinc strip into it and attach the other crocodile clip to it. This forms the zinc half cell.

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