This experiment involves a cell that is, in principle, the same as the closed-circuit "gravity" cell invented in France and used on American land telegraph lines from the middle of the 19th century. It is a version of the Daniell Cell, which used the same components, but separated the two solutions with a porous cup rather than by their specific gravities. More information on the history of the gravity cell can be found in the pages on the Telegraph. The electrode reactions are Zn → Zn++ + 2e- at the anode and Cu++ + 2e- → Cu at the cathode. The zinc is supplied by a zinc metal anode, the copper ions by a solution of cupric sulphate. The role of the sulphate ions is to provide electrical neutrality.
It is easy to see how a gravity cell works for yourself, and to observe some of its characteristics. A simple cell that I set up in a 50 ml beaker is shown at the right. The black square is the zinc anode, and the loop of copper wire at the bottom of the beaker is the cathode, surrounded by crystals of cupric sulphate. A digital multimeter is connected to measure the open circuit voltage and the closed circuit current of the cell. All that is necessary to move between the two measurements is to change the range switch.
The zinc anode is 20 mm by 20 mm by 0.75 mm. A hole is drilled near the edge for a 6-32 x 3/8" machine screw and nut that holds a solder lug. A piece of #22 AWG wire is soldered to the lug, and bent to support the zinc anode inside the beaker. A piece of #18 AWG wire is stripped for about 2" and bent in a loop for the cathode. Connections are made with the two wires. A thin layer of cupric sulphate crystals are dropped around the cathode, and then the beaker is filled with water so that the zinc is submerged. Several drops of sulphuric acid are added to the top of the liquid, to make it more conducting.
The open circuit voltage was about 1.02 V, and remained constant until the end of the experiment, about 30 hours from the start. This constancy means that the cell does not "polarize" like the Volta pile, whose voltage dropped steadily as it was used. It is no accident that the cell voltage is close to 1.0 V, since the Daniell cell was the original standard of voltage. The cell was normally on short circuit throughout. At first, the current was about 10 mA, but increased slowly to 22 mA at the end, after the electrolyte had become more conducting. The blue solution at the bottom remains distinct from the clear solution at the top, but becomes a little thicker. No electrolysis occurs, except for a few bubbles of hydrogen clinging to the zinc that result from impurities and local action. The zinc soon turns black and scaly as zinc enters solution.
The standard reduction potential for Zn++ is -0.723V, and for Cu++ it is 0.344V. If the activity of the zinc and copper ions were unity, the cell potential would be 1.105V. The activities are certainly not unity, especially for the zinc, but still the observed potential of 1.02V is reasonably close.
Unfortunately, one cell does not provide sufficient voltage to turn on an LED, and insufficient current to light a flashlight lamp. Two or three cells would surely light an LED, and would provide enough current for a telegraph circuit. A larger cell with more zinc would be a practical current source. Actual gravity cells had an internal resistance of about 2Ω. The small one in this experiment showed about 50Ω when the electrolyte became reasonably concentrated. It is clear that the gravity cell is not a portable power source, since the electrolyte must be kept quite undisturbed. Also, the current must be kept flowing, in order to counteract diffusion of the two layers into each other.
The materials are easy to obtain, but you may have to purchase much more than you need. Science Lab sells 100 g of cupric sulphate for $7.89, and a 12" x 12" zinc plate for $11.24, plus shipping. This is enough to make about 100 of the cells described in this article, so would be fine for a class. It might be possible to salvage zinc from a carbon-zinc cell--these are the least expensive kind of batteries. An alkaline cell would be messy, because of the alkaline electrolyte. If the cell is in a metal case, simply cut the ends off with a hacksaw and push out the insides, discarding all but the zinc tube. I have not done this. This zinc may possibly be amalgamated, which would make it work even better. Cupric sulphate should not be taken internally, but is not a nasty poison by any means, and reasonable care should be sufficient. The few drops of sulphuric acid are not absolutely necessary; perhaps a little salt would improve the conductivity of the electrolyte if acid is not used, for safety reasons. My plate of zinc came with instructions not to eat it or breathe it, advice with which one can hardly argue. It is, of course, no hazard whatsoever.
Composed by J. B. Calvert
Created 16 January 2003