OT: Making a zinc/copper/vinegar battery - internal resistance at about 1 megohm :-(

Both of those seem like they'll give quite a small metal surface area, is she allowed to use e.g. a bit of copper pipe and the flattened can of a ZnCl battery (does anyone buy them still?)

Noticing the dark tarnishing of the galvanised nails, I suddenly had a thought: are these "galvanised" nails (that's what it says on the pack) actually plated with zinc or some other metal?

I found some screws which actually said "zinc plated" and these worked a lot better - enough to give about 1.7 V under the load of an LED which was enough to make the LED glow very dimly.

Ideally you'd use a larger area of electrodes, as you say, and try to keep them as close together in each cell without touching.

So don't believe the pack - galvanised may not actually mean zinc plated :-(

Yes, ideally cannibalise some dead AA batteries for their zinc casing, and use larger pieces of copper - even multi-strand copper wire would have more area than single-core wire from twin-and -earth cable.

I would expect more than that, over 1V per cell.

I wouldn't rely on resistance measurements of a semiconductor.

Without current flowing the zinc will dissolve in the acid producing hydrogen and revealing the metal underneath, which is steel. Try using fresh nails. Don't leave them in the acid when you aren't using them. Take them out and wash the residual acid off.

A copper/iron couple will still produce a voltage but it will be lower. That may be what you are seeing.

No. LEDs will glow very dimly at less.

I think you have discovered why zinc copper batteries are not commercially made...

What you need is a bigger area of copper. Try pennies or something instead. Or get some copper foil off amazon

Albeit needing a higher voltage (maybe 3.5V), a high sensitivity Gallium Nitride blue LED can glow brightly with just a hundred micro-amps.

Silly question: The cells are isolated?

The voltage is lower than I would have expected using a DVM with many megohms resistance across probes. Does the voltage start higher and drop over time. And then recover?

The surface area is far too low to be useful. I'm also wondering about the connection between copper wire and zinc nail. If you place meter probes in the electrolyte you can check uniformity of connections.

A LED is a very non-linear device. You might see 300k at 1V but as the voltage increases the slope resistance will fall very rapidly.

You could use a meter that has a diode measurement to check knee voltage. They generally give you a voltage reading at 10mA and show it hasn't been damaged from reverse bias or other failure.

Does your meter provide a current measurement? That would take out the guesswork.

"The electrode potential for copper is +0.34 V and for zinc -0.76 V. In open circuit, the voltmeter measures the sum of these two voltages, 1.10 V, each electrode behaving anodically with respect to its local electrolyte."

I really think, as the science aspect of this experiment, you want to be observing a single cell producing 1.10V . That indicates that the full redox reaction between the electrodes is occurring. You don't want the half reaction between Zn and hydrogen, releasing bubbles of hydrogen gas and a potential related to just the Zinc component. A typical multimeter might have a 10 megohm input impedance and should be showing the full potential of the constructed cell.

*******

Somebody here optimizes the experiment and gets... 40 microamps. Which should be plenty to power the probe tips of the multimeter.

The potato battery, apparently there is phosphoric acid in the potato. I didn't know that.

"Potatoes have phosphoric acid and work well; they are the basis for commercial 'potato clock' kits."

"The current that is output by the battery through a meter will depend on the size of the electrodes, how far the electrodes are inserted into the fruit, and how close to each other the electrodes are placed; the voltage is fairly independent of these details of the electrodes."

Another example here, of an article on building electrochemical cells.

In this example, the experiment has been reduced to a commercial form. A ready-made four cell apparatus. Complete with a LED. Such optimism. The LED does not strike me as one of the high-efficiency type. A blue LED for example, requires 2.5V-3.0V (depends on current flow level) to fully light up. A red LED is, like, 1.6V. An infrared LED may have a lower forward voltage than that.

When you do a poor job of creating a plating bath, the metal can come out as "fines". The "fines" react with light in a particular way and the material may look dark as a result. Fines may wipe off on your fingers, whereas an electroplate may stay put when wiped. The colour of the result, may not necessarily indicate the species. When I attempted and failed at making a platinum plated fuel cell electrode, the material rather than being shiny platinum, was black in colour and wiped off on my fingers.

Summary: There is real science going on here. The student should be writing down the observations. Randomly adding reagents to the cell, you can rapidly lose control of the experiment by doing that. Each cell you construct, should be carefully tested to see what the reagent achieves in terms of cell potential.

Even when a curriculum has carefully constructed experiments that cannot fail, oh yesss, they can and do fail. I think one chemistry lab, it took me about six afternoons to get the miserable f***er to work. Angry? Yes.

Paul