The determination of copper in brass

In this experiment, students determine the copper content in brass (an alloy of copper and zinc) by dissolving brass turnings in nitric acid and comparing the colour of the solution with that of solutions of various concentrations of copper. It should take approximately 25 minutes. 

The experiment has possibilities for use as an assessed practical. Two versions of the student worksheet are available – versions A and B. In version A, students are guided through the calculations at the end. This version could be used to assess skills in doing the experiment/following instructions. In version B no help is given with the calculations. This version could be used to assess skills in the treatment of results.

Equipment

Apparatus

• Eye protection
• Student worksheet
• Sheet of white paper
• Access to a balance
• Access to a fume cupboard
• Beaker, 10 cm³
• Volumetric flask, 10 cm³
• Plastic well-plate, 24 wells (eg Sigma ref: CLS3526)
• Plastic pipette (eg Aldrich ref: Z13, 503-8, fine-tip)

Chemicals

• Nitric acid, 5 mol dm^–3
• Deionised water
• Copper nitrate solution, 0.50 mol dm^–3
• Brass turnings

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection throughout (splash-resistant goggles to BS EN166 3).
• Check our guidance on apparatus and techniques for microscale chemistry for instructions on preparing solutions.
• Nitric acid, 5 M HNO₃(aq) (CORROSIVE) – see CLEAPSS Hazcard HC067 and CLEAPSS Recipe Book RB061. Consider wearing protective gloves.
• Copper nitrate solution – see CLEAPSS Hazcard HC027B and CLEAPSS Recipe Book RB031.
• DISPOSAL: collect and retain copper/zinc solutions for appropriate disposal.

Procedure

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 ten drops of nitric acid.
4. When the reaction subsides add a further ten 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 drops of 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 flask and then invert it a few times to mix.

Preparing the standard copper solutions

For use with student sheet A (includes guide to calculations)

1. Fill the well-plate (see diagram) with solutions as indicated in the table below. There should be a total of 40 drops in each well.

2. Add 40 drops of the brass solution to well B3 (see diagram).
3. Compare the intensity of the colour of your brass solution with the wells around it. The well that matches the intensity of colour of your brass solution represents the copper concentration in your brass solution – eg if well A6 matches the colour of your brass solution then the copper concentration will be 0.50 × 18/40 mol dm⁻³.

For use with student sheet B (no guide to calculations)

1. Fill the well-plate (see diagram) with the solutions as indicated in the table below. There should be a total of 40 drops in each well.

2. 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.
3. From your results, calculate the copper content of your brass expressing your answer as a percentage.

Guide to calculations

1. 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.

Student questions

1. 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?

Teaching notes

Observations

The brass dissolves quickly to form a blue solution. This colour is due to the copper present in the brass. (This part of the experiment must be done in a fume cupboard since nitrogen dioxide is formed.)

The intensity of the colour of this solution should lie within the range of intensities of colour of the standard solutions. Students find the nearest colour match and then calculate the copper content of the brass.

Discussion

Most brass contains about 60% copper (the remainder being zinc). Brass forms an interesting subject for a discussion on the structure of metals and alloys. Copper metal has a face-centered cubic structure (fcc) while the structure of zinc is hexagonal. As zinc is added to copper it substitutes in the lattice to form a distorted fcc structure (zinc atoms are ca 13% larger than copper). This distorted structure is difficult to deform and accounts for the greater strength of brass compared to pure copper.

When the zinc content reaches about 36% a new body centered cubic phase appears and the strength increases markedly although the ductility is reduced. The optimum properties of strength and ductility for most uses of brass occur at about 40% zinc.