Science Buddies: "Ask an Expert"

My daughter completed an experiment using three types of potatoes and found that a purple sweet potato produced the most voltage. We thought that because the nutritional data we found indicated that the sweet potato contained more amounts of copper and zinc in them, it would therefore augment the chemical reaction, but are having a hard time finding anything to support this...or does this have nothing to do with it? Help!

Hi Manuula70,

The potato does not actively participate in the reaction. It contains phosphoric acid which acts as an electrolyte to faciliate the stripping of the electrons from the Zinc. This reaction produces Hydrogen gas which you may have seen bubbling out of the potato. For more information please see the following link http://www.physlink.com/education/askexperts/ae516.cfm

Best of Luck,

Marc

Thanks, Marc; I did see that page, but I just needed clarification. So the make-up of the potato has nothing to do with it? Would it matter that one potato has more phosphoric acid than another? Or more zinc and copper? We also read that the purple sweet potato is used for its antioxidant properties. Do antioxidants have anything to do with this? Sorry for all of the questions, but we are intrigued!!

According to this link http://hyperphysics.phy-astr.gsu.edu/HB ... al/ph.html, the ion concentration of the electrolyte (acid) directly affects the output potential of the galvanic cell. This means that the acid concentration / pH of the potato will make a difference in how much "power" you get from your potato cell. This ion concentration could relate to the type of potato, but probably is more influenced by the freshness of the potato? I'm not sure about that one

Keith

You can also try to measure the potato's PH level using litmus or PH paperhttp://sciencekit.com/wide-range-ph-tes ... IG0024657/. You may be able to get some from your child's science teacher. If the purple potato has the higher PH (more acidic) then this would support your conclusion of it producing the higher voltage since it has a higher ion concentration. Also check the nutritional information of the potatoes to see if there is a difference in the phosphorus content between the three potatoes.....since its the phosphoric acid that is running the show in this experiment.

In terms of antioxidants, ascorbic acid (Vitamin C) is found in potatoes. However, their job is to tackle free radicals once they're inside your body and it is the reaction of the copper and zinc with the phosphoric acid that is turing the potato into a battery.

Best of luck,

Marc

The water content (hydration) will significantly affect ion mobility which will affect the internal resistence of the "battery". The previous expert who mentioned "freshness" as a possible factor is correct if you consider a dried out potato as a "stale" potato. Differences in the amounts of zinc or copper in a potato would only affect the "battery" properties if they were free ions. If they are bound chemically to something that doesn't easily ionize at the pH in the potato, then they won't affect the result. The phosphoric acid content would definitely create free ions which would decrease internal resistance and increase voltage potential at the same time. The suggestion of measuring pH differences is a good one. Pre-soaking all of the potatoes in water over night might be a way to reduce the effects of different water contents if you want to try and isolate reasons for the differences you found.

Thank you all for your suggestions!!