MadSci Network: Chemistry

Re: ? Acid or the Salt that carry the electrical charge for Potato Battery

Date: Fri Jan 23 15:24:45 2009
Posted By: Dan Berger, Faculty Chemistry/Science, Bluffton University
Area of science: Chemistry
ID: 1231788836.Ch

I'm getting confused about this part. Many of my references say the potato contains phoshoric acid which causes the chemical rections to occur at each of the electrodes. The copper electrode electrons and acid form hydrogen gas,then the phoshoric acid desolve the Zinc giving positive IONs from the electons that stay on the electrode. These Zn++ migrate in the electrolytic solution Acid Juices in the potato then if the Zinc and copper electrodes are connected a electric current flows. But, I read your achived message ID# 943757681.Ch and ID#916286185.PH, these say it's not the Acid in the potato, and state the Salts in the potato are the source of the ions, (Sodium +, and Chlorine -). In those references the Na+ and Cl- are what charge the electrodes and the elctrons are the carriers for the electricity. Can you please help me reconcile this information?

Let's take this one thing at a time.

  1. The charged particles that move through the potato are ions, of whatever sort happen to be present. Not being a biochemist, my guess is that they are largely sodium and potassium cations (positive ions), and chloride anions (negative ions); but there are certainly going to be other anions than chloride present in a potato or a lemon or whatever. The identity of the ions doesn't matter in terms of how electricity moves; it might matter in terms of the internal resistance of the potato, because some ions probably move more easily than others.
  2. Electricity is carried through the external circuit by electrons.
  3. The reason the food battery works is that electrons move spontaneously from zinc to copper as long as there's any way of balancing the charges that result when this happens.
  4. When electrons move away from the zinc, they leave behind zinc ions that move out into the potato to meet anions traveling from the copper electrode.
  5. When the electrons reach the copper electrode, they react with anything even mildly acidic that happens to be there, such as phosphoric or citric acid, to form hydrogen gas. The anion of the acid moves toward the zinc electrode to meet the zinc cations being formed there.
    • In this way, you could say that the acid is corroding the zinc electrode.
    • Any other anion could stand in for the acid's anion.
  6. The electrodes do not accumulate charge to any extent at all; if they did, the battery would not work. The reason positive ions move from the zince electrode to the copper one, and negative ions move in the other direction, is to balance the charges generated by the electrochemical imbalance between zinc and copper.

To sum up: any ions can carry the charges through the potato. Acid makes the battery work better because it provides a place for the copper electrode to dump electrons. You could get the same voltage with just salt water, but probably not as much current. The reason that hydrogen gas doesn't build up in commercial flashlight batteries (though you can get a hydrogen explosion if you work very hard at force-charging a car battery) is that they are designed to have other sorts of electron dumps than acid.

Incidentally, the reason you don't get much current from these beasties is that the transfer of electrons from zinc to copper is not very efficient. To get a higher-current battery you need something that's hungrier for electrons than a weak acid.

I hope this is helpful. Thanks for giving us the opportunity to clarify our answer!

Dan Berger

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