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B Seawater battery voltages

  1. Nov 10, 2016 #1
    We created a 5x5 seawater battery system to test voltages across multiple pHs and concentrations of NaCl. Concentration ranges go from 0g/L to 32g/L (near saturation) and the pHs range from 3 to 11. We used copper and magnesium electrodes to test the voltages in each cell and got some results we didnt' expect. With seawater being a better conductor than distilled water according to everything I've read, we expected the higher voltages to be seen on the more concentrated cells. Instead, the highest voltage potential we got was on the distilled water with a pH of 3.0.

    Why would we not see a definite increase in voltage from fresh to more concentrated saltwater even across the same pH?
     
  2. jcsd
  3. Nov 10, 2016 #2

    Bystander

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    You do understand the differences among "conductivity, ionic strength, voltage, saturation ... ?"
     
  4. Nov 10, 2016 #3
    Salt water being more conductive, I would have assumed it meant a higher voltage could be achieved. Is this not the case?
     
  5. Nov 10, 2016 #4

    Bystander

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    It is not.
     
  6. Nov 10, 2016 #5
    Then can you elaborate?
     
  7. Nov 10, 2016 #6

    Bystander

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  8. Nov 10, 2016 #7
    I'll gladly read that, as I came here for help.

    On another note, and I fully expect to be lectured on this as well, I come to this forum about once a year as an absolute last resort because of how quickly some of the members are to just say "no, you're wrong" and not even attempt to give guidance. It's condescending and rude. I can Google a Wikipedia page in half a second too, but it doesn't necessarily mean that it's making the connection to me (otherwise I wouldn't had to have come here). If the science community in general has any interest in passing the torch to a younger generation, perhaps it needs to not dismiss it so quickly.
     
  9. Nov 10, 2016 #8
    So the first thing that jumps out at me there is that it refers to fresh water as "almost an insulator", which is what I have seen before. So, again, why are we getting higher voltages across fresh water that has no to few dissolved ions? What am I missing here?
     
  10. Nov 10, 2016 #9

    Bystander

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    Voltage at constant current is inversely proportional to conductivity. Low conductivity means high voltage; high conductivity means low voltage.
     
  11. Nov 10, 2016 #10

    Nidum

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  12. Nov 10, 2016 #11

    davenn

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    you are missing the electrolyte that in dissolved within the fresh ( pure) water. in a say, car lead acid battery ... it is the sulphuric acid within the water that breaks the ionic bonds that allow the ions to freely move and produce a charge separation ... the positive cations and the negatively anions



    Dave
     
  13. Nov 10, 2016 #12
    Assuming it's related to resistance then in terms of how saltwater will have higher resistance because of the ions dissolved. Higher [ions] = Lower conductivity = Higher Voltage. Does that sound right?
     
  14. Nov 10, 2016 #13

    davenn

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    remember NO electrons flow through the electrolyte only from the battery terminals and the external circuit.
    Because you sea water is a good conductor .... lots of dissolved salts, it was basically internally short circuiting your battery
     
  15. Nov 10, 2016 #14

    Bystander

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    Exactly backwards.
     
  16. Nov 10, 2016 #15
    To simplify my case, just look across a single pH so that only the concentration of NaCl is changing. Why would I not see a distinct trend in voltage potential as concentration increases (either up or down)? It's all over the map....

    For example, across the pH 7.0 group I get the following (concentrations are 0g/L, 8g/L, 16g/L, 24g/L, and 32g/L): 0.530V, 0.528V, 0.528V, 0.535V, 0.532V
     
  17. Nov 10, 2016 #16
    Right - yeah I had that backwards in my head but it makes sense. Higher Ions = Higher Conductivity = Lower Voltage. That's fine on paper, but why would I be getting results consistently like what I posted in the last reply there?
     
  18. Nov 10, 2016 #17

    Bystander

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    Looks constant to me. You do not want to read too much into the last digit.
     
  19. Nov 10, 2016 #18
    Ok - with that in mind then, SHOULDN'T there be a change noticeable? I would think that between distilled water and nearly saturated water there would be something to see. There are noticeable trends up and down the pH scale with the rest of the data but I'm trying to focus on one variable at a time. The pH trends are odd too where the pH of 11 has everything jacked way up beyond everything else, like double the next highest values...but I'll deal with that later.

    Just within a single pH, is there a possibility that we would need to either steel wool or at least rinse the electrodes between beakers? We have taken data from low concentration to high so that salt residue didn't contaminate down the gradient hoping to avoid having to rinse between each but I'm wondering now if that's an issue. I may just scrap this round of data and start over to see what happens.
     
  20. Nov 10, 2016 #19

    Bystander

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    You are looking at the difference in solid copper and solid magnesium. What effect were you expecting to see between a short circuit and shorter circuits?
     
  21. Nov 10, 2016 #20
    I was expecting to see some kind of change in voltage potential based on the idea that saltwater is a better conductor than freshwater. If not those electrodes, what would you suggest? We tested several combinations and went with the ones that gave the highest values, which happened to be Mg paired with Cu.

    The whole idea of this came from the "seawater battery" demonstrations you see scattered around the internet and how each one says that you need to use saltwater instead of freshwater to get it to work. We were able to light small light bulbs and run small DC motors with this setup, but wanted some numbers to back up the changes between concentrations to see if that mattered and to then expand that result to test the differences in pH to see if that did anything.
     
  22. Nov 10, 2016 #21

    Bystander

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    These all depend upon dissimilar metals corroding differentially in electrolyte solutions; the more concentrated the solution, the higher the current and the faster the corrosion.
     
  23. Nov 10, 2016 #22
    So does that not follow the same set of principles as we were talking about before then? I get that this is a little different in that an it's not an external current passing through an electrolyte solution...
     
  24. Nov 12, 2016 #23

    dlgoff

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    No. You were asking about battery voltage. The voltage is dependent on the metals and the current is dependent on the electrolytes which @Bystander was explaining.
    Give Battery University a read. That should help explain the data you're seeing.
     
  25. Nov 12, 2016 #24

    sophiecentaur

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    But did you? I deprecate the habit of posts that contain nothing more than a reference but, if you really want the best outcome from PF (and any other good forum) then it is a good idea (polite, even) to include some idea of what "everything I've read" means, in your case. Everything I have read (about a half century ago and later) would actually suggest that the emf from an electrolytic cell would depend upon the Ionic Potentials, rather than the concentration. Needless to say, the concentration of ions would affect the available current (Internal Resistance).
    Your OP was actually just as vague and unhelpful as the first answer you got. The 'cost' of help on PF is that you present questions in a way that shows you have made some effort towards getting an answer on your own. Try that with your next question and you will probably find that many more people want to respond to an interesting post.
     
    Last edited: Nov 12, 2016
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