Electrolysis of Copper (II) Sulfate

The experiment has been devised for microscale application, and requires access to a balance with milligram accuracy, and also a platinum wire electrode. Other equipment needed: one 10 mL conical flask, a few centimetres of clean copper wire approx 1 mm diameter, concentrated copper sulfate solution (at least 1M),  cotton wool, source of low-voltage DC current, with potentiometer to regulate cell voltage, electric leads, voltmeter (or multimeter) to monitor cell voltage.

It concerns the stoichiometry of copper(II) oxide. If a solution of copper(II) sulfate is electrolysed with a  platinum anode, oxygen will be produced at the anode, while copper is deposited at the cathode. The mass of the  whole electrolytic cell should decrease by the mass of the oxygen given off, while the mass of the cathode  should increase by the mass of copper ions reduced. The gain in mass of copper should be 63.5/16.0 (approx 4)  times the loss of mass from the electrolyte.

A piece of clean copper wire, about 1mm in diameter, is cut and bent so that it hooks over the rim of a 10 mL conical flask with the lower end of the wire a few millimetres above the bottom of the flask; this will be the  cathode. Its mass is recorded to the nearest milligram.

About 6 mL of fairly concentrated CuSO4 solution (at least 1M) is placed into the conical flask. A platinum wire anode and the copper-wire cathode are inserted and held in place by a plug of cotton wool. The electrodes are approximately 2cm apart. The mass of the whole assembly: flask, solution, two electrodes, and cotton wool is then recorded to the nearest millligram.

These are connected to a power source and the whole assembly is placed inside a plastic bag which has also a  little moisture in it, so that there should be no evaporative loss from the solution. After about two hours (this allows a good deposit of copper to form), the mass of the whole assembly is measured again (the difference from the first mass gives the mass of oxygen produced), then the mass of the washed and dried cathode gives the mass of copper produced.

The best voltage seems to be close to three and a half. Below three volts, the reaction rate is too slow, while  at four volts the deposit of copper is sometimes spongy and loose.

A graphite anode is unsuccessful: it tends to disintegrate. A useful platinum anode is made by attaching a  short piece of platinum wire to the end of a length of fine copper wire. The tip of a pasteur pipette is cut  into lenghs of about 1 cm; the wire is inserted through the narrowest of these until the junction of the copper  and platinum lies inside the glass. The glass is then melted using a small flame. The next bigger piece of  glass is then placed over the junction, and it too is melted; then the process is repeated a third time. The  effect is that the joint between the copper and platinum is sealed inside a bead of glass.

Some typical results:
at 4.0 volts for fifty minutes: 48 mg of hard copper and 11 mg of mass lost from the assembly.
at 4.0 volts for two hours, 144 mg of hard copper to 31 mg of mass lost from the assembly.
at 4.0 volts for two hours, 311 mg of spongy copper to 83 mg lost from the assembly.
at 3.0 volts for two hours, 58 mg of hard copper to 7 mg lost from the assembly,
at 3.5 volts for two hours, 136 mg for 26 mg lost from the assembly.

These inconsistencies have been typical of what I've observed in many trials, in which a variety of  experimental approaches have been tried.

Has anyone tried it to get consistent results? Contact Mike Clark