Background

A voltaic cell can be assembled from two half-cells and the potential difference measured using a high-resistence voltmeter, as in the diagram below.

Different investigations can be designed using varying concentrations of the solutions, or temperature.

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Requirements

Chemicals

• Copper(II) sulfate pentahydrate, CuSO₄.5H₂O
• Zinc sulfate heptahydrate, ZnSO₄.7H₂O
• Potassium nitrate, KNO₃
• Copper foil
• Zinc foil
• Distilled (deionized) water

Apparatus

• Beakers, 100ml
• Spatula
• Glass stirring rod
• Weighing boat
• Top pan balance
• Filter paper
• Cables and crocodile clips
• High-resistance voltmeter
• Pipette, 10ml, and pipette filler
• 2 x Volumetric flask, 100ml
• Digital or analogue thermometer

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Procedure - The effect of [Cu²⁺] on the EMF of a copper|zinc voltaic cell.

Preparing the solutions

• Prepare 100 cm³ of 1 mol dm^-3 copper(II) sulfate solution, by weighing an appropriate mass of copper(II) sulfate pentahydrate and stirring into 80 cm³ of distilled water in a 100 cm³ beaker.
• Transfer the solution to a 100 cm³ volumetric flask and make up to the mark with distilled (deionized) water.
• Repeat the procedure using zinc sulfate heptahydrate.
• Prepare a saturated solution of potassium nitrate using a 50 cm³ beaker.

Measuring the potential difference

• Using about 50cm³ of each solution, construct the voltaic cell as in the diagram above, and connect the two half-cells by a salt bridge made by soaking a strip of filter paper in the saturated potassium nitrate solution.
• Record the temperature of the two half-cells.
• Prepare 100 cm³ of a 0.1 mol dm^-3 solution of the copper(II) sulfate by dilution of the original solution.
• Measure the potential difference between the diluted copper(II) sulfate solution and the original zinc sulfate solution.
• Repeat the dilution process to obtain solutions of 0.01, 0.001 and 0.0001 mol dm^-3 concentration of copper ions and measure the new potential difference.
• Record all data in a suitable table.

Processing the data

• Plot a graph of potential difference (y-axis) against log₁₀[Cu²⁺] (x-axis).

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