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A redox titration to determine the % of copper in a brass screw.

Objective
• Be able to find the percentage of copper in a sample of brass.
• Be able to perform an iodine-thiosulfate titration.

Procedure
1. Weigh and record the mass of the brass rivet.
2. Put the rivet into the small beaker and place the beaker in a fume cupboard. Add 10 cm3 of concentrated nitric acid to the rivet and leave until the rivet has completely dissolved.
3. Transfer the solution to the volumetric flask and make the solution up to 250 cm3 with the distilled/deionised water. Stopper and shake.
4. Pipette 25.0 cm3 of the solution into the conical flask. Add small quantities of solid sodium carbonate to the solution to neutralise the acid until a precipitate is present. Then add ethanoic acid solution drop-wise until the precipitate just dissolves.
5. Add 25.0 cm3 of potassium iodide solution to the conical flask.
6. Titrate the liberated iodine with the sodium thiosulfate solution. When the mixture is near the end point, use the starch solution as an indicator – a white precipitate will persist at the end point.
7. Record the readings for the titration in an appropriate format.
8. Repeat the titration until you have two sets of results that are within 0.10 cm3 of each other.

Analysis of results
(a) 2Cu2+ + 4I–  2CuI + I2
(b) I2 + 2S2O32–  2I– + S4O62–
• Using the equations and your titration data, calculate the mass of the copper in the rivet.
• Determine the percentage of copper in the rivet.

Questions
1. Make a list of any measurement errors for this experiment.
2. What is the white precipitate present at the end point?
3. Using oxidation numbers, show what has been oxidised and reduced in the given equations (a) and (b).
4. A similar solution of copper(II) ions was prepared from a rivet (2.1 g) and the concentration of copper was found using colorimetry. Explain how the concentration would have been found using this method.
5. The concentration of the copper in Question 4 was found to be 0.085 mol dm–3. Calculate the percentage of copper in the rivet.