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39. The determination of copper in brassTopic

Metals–chemical analysis.

LevelPost-16.

Timing25 min.

DescriptionIn this experiment students dissolve some brass in nitric acid and compare the colourof the solution against standard copper solutions in a well-plate. This experiment haspossibilities for use as an assessed practical. Two versions of the student worksheetare given (versions A and B).

In version A students are guided through the calculations at the end. This versioncould be used to assess skills in doing the experiment/following instructions. Inversion B no help is given with the calculations. This version could be used to assessskills in treatment of results.

Apparatus (per group)▼ One student worksheet and one sheet of white paper

▼ Access to a balance

▼ Access to a fume cupboard

▼ One 10 cm3 beaker

▼ One 10 cm3 volumetric flask

▼ One plastic well-plate (24 well) – eg Sigma ref: M9655

▼ One plastic pipette – eg Aldrich ref: Z13,503-8, fine-tip.

Chemicals (per group)Solutions contained in plastic pipettes, see p. 2

▼ Nitric acid 5 mol dm–3

▼ Deionised water

▼ Copper nitrate solution 0.50 mol dm–3

▼ Brass turnings

ObservationsThe brass dissolves quickly to form a blue solution. This colour is due to the copperpresent in the brass.

(This part of the experiment must be done in a fume cupboard since nitrogendioxide is formed.) The intensity of the colour of this solution should lie within therange of intensities of colour of the standard solutions. Students find the nearestcolour match and then calculate the copper content of the brass.

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164 Microscale Chemistry

DiscussionMost brass contains about 60% copper (the remainder being zinc). Brass forms aninteresting subject for a discussion on the structure of metals and alloys.

Copper metal has a face-centered cubic structure (fcc) while the structure of zincis hexagonal. As zinc is added to copper it substitutes in the lattice to form a distortedfcc structure (zinc atoms are ca 13% larger than copper). This distorted structure isdifficult to deform and accounts for the greater strength of brass compared to purecopper.

When the zinc content reaches about 36% a new body centered cubic phaseappears and the strength increases markedly although the ductility is reduced. Theoptimum properties of strength and ductility for most uses of brass occur at about40% zinc.

ReferenceA. Street and W. Alexander, Metals in the service of man, 10th edn. London:Penguin, 1994.

SafetyStudents must wear eye protection.

It is the responsibility of the teacher to carry out a risk assessment.

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39. The determination of copper in brassVersion A

In this experiment you will be finding out how much copper there is in brass (analloy of copper and zinc). You will dissolve the brass in nitric acid and compare thecolour of the solution with that of solutions of various concentrations of copper.

Instructions

Part A Preparing the brass solution

1. Weigh out, accurately, about 0.3 g of brass in a 10 cm3 beaker.

2. Put the beaker in a fume cupboard.

3. Add 10 drops of nitric acid.

4. When the reaction subsides add a further 10 drops of nitric acid.

5. Repeat until all the brass has dissolved.

6. Using the pipette, transfer the solution to a 10 cm3 volumetric flask. Add dropsof water to the beaker to rinse and then transfer the washings to the flask.Make the volume in the flask up to the line with more water. Stopper the flaskand then invert it a few times to mix.

Part B Preparing the standard copper solutions

1 2 3 4 5 6

A

B

C

D

1. Fill the well plate with solutions as indicated in the table below.

Drops of 0.50 mol dm–3

copper nitratesolution

A2 A3 A4 A5 A6

22 248

A1Well No

2616 28 3010 12 14 18 20

Drops of water 24 18 16 14 12 1032 30 28 26 22 20

C1 C2 C3 C4 C5 C6

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There should be a total of 40 drops in each well.Add 40 drops of the brass solution to well B3 (see diagram). Compare the intensity

of the colour of your brass solution with the wells around it. The well that matchesthe intensity of colour of your brass solution represents the copper concentration inyour brass solution – eg if well A6 matches the colour of your brass solution then thecopper concentration will be 0.50 x 18/40 mol dm–3.

Calculations1. Calculate the number of moles of copper in 10 cm3 (the volume of the brass

solution).

2. Multiply the value you obtained in (1) by the relative atomic mass of copper(63.5) to give the mass of copper in the brass solution.

3. Divide by the mass of brass used and express the result as a percentage.

Questions1. Does the zinc interfere in any way in this analysis? Give reasons for your

answer.

2. Can you suggest any way to improve the accuracy of this experiment?

Version BIn this experiment you will be finding out how much copper there is in brass (analloy of copper and zinc). To do this you will dissolve the brass in nitric acid andcompare the colour of the solution with that of solutions of various concentrations ofcopper.

Instructions

Part A Preparing the brass solution

1. Weigh out, accurately, about 0.3 g of brass in a 10 cm3 beaker.

2. Put the beaker in a fume cupboard.

3. Add 10 drops of nitric acid.

4. When the reaction subsides add a further 10 drops of nitric acid.

5. Repeat until all the brass has dissolved.

6. Using a pipette, transfer the solution to the 10 cm3 volumetric flask. Add dropsof water to the beaker to rinse and then transfer the washings to the flask.Make the volume in the flask up to the line with more water. Stopper the flaskand then invert it a few times to mix.

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Part B Preparing the standard copper solutions

1 2 3 4 5 6

A

B

C

D

1. Fill the well plate with the solutions as indicated in the table below.

Drops of 0.50 mol dm–3

copper nitratesolution

A2 A3 A4 A5 A6

22 24 32

A1Well No

2616 28 3010 12 14 18 20

Drops of water 24 18 16 14 12 10 830 28 26 22 20

C1 C2 C3 C4 C5 C6

There should be a total of 40 drops in each well.Add 40 drops of your brass solution to well B3 (see diagram). Compare the

intensity of the colour of your brass solution with the wells around it.From your results, calculate the copper content of your brass expressing your

answer as a percentage.

Questions1. Does the zinc interfere in any way in this analysis? Give reasons for your

answer.

2. Can you suggest any way to improve the accuracy of this experiment?