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Old Nov 10th 2016, 12:11 PM   #1
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Voltages across saltwater

I am testing "seawater batteries" for a project and am looking for the best combination of pH and salt concentration. It's set up as a 5x5 grid in which the pH ranges from 3 to 11 on one side and the concentration of NaCl in the water ranges from 0g/L to 32g/L (near saturation).

We expected that, regardless of the pH, the voltages we got from this would increase as salinity increased because of the larger concentration of ions, BUT our highest voltage (1.53V) was actually found in the freshwater at a pH of 3.0.

Why would it not increase voltage as it went up in salinity?
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Old Nov 11th 2016, 11:20 AM   #2
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The (maximum) voltage available from a single chemical battery cell is determined almost exclusively by the chemical components involved in the reaction.
The current and power of the battery may be changed by its size, but the voltage is fixed by the mutual energy levels of the reacted and unreacted chemicals that are generating the voltage.
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Old Nov 13th 2016, 06:20 PM   #3
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Well, because this is changing both in pH and in salinity, each beaker is different from all the others. I would still think I should see some kind of pattern or trend though and I'm not. It seems to be all over the map.
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Old Nov 14th 2016, 11:10 AM   #4
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What are you using to alter the PH?
(I would guess that you are using HCl and NaOH).
What material are your electrodes?
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Old Nov 14th 2016, 11:12 AM   #5
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Originally Posted by Woody View Post
What are you using to alter the PH?
(I would guess that you are using HCl and NaOH).
What material are your electrodes?
Yes on both of those chemicals. Our electrodes are copper and magnesium, but only because they read the largest voltages. We also have aluminum, zinc, lead, iron, and carbon we could use if you think one would be better than the other.
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Old Nov 14th 2016, 11:17 AM   #6
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We redid the test this morning and got the following, which looks better to me. Found out that we got more reliable readings by rinsing the electrodes between each beaker rather than just between pHs.

But my question remains...for the acidic and neutral pHs, why does voltage decrease as salinity increases? According to Ohm's Law, shouldn't voltage increase as salinity increases?

I definitely see patterns here after this new data was taken. Almost constant decrease in voltage as we increase the pH, but you can clearly see the change in pattern as pH drops to 9 and then to 11 where all of a sudden the voltages increase as the concentration of salt increases (which is what I would expect, but it apparently isn't always the case because it consistently decreases when the pH is neutral to acidic)
Attached Thumbnails
Voltages across saltwater-data.jpg  

Last edited by trpnbils; Nov 14th 2016 at 11:22 AM.
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Old Nov 14th 2016, 01:49 PM   #7
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the cell giving the highest voltage is basically a magnesium copper and hydrochloric acid battery.
The addition of sodium ions seems to actually reduce the voltage.
I did chemistry years ago, and can't remember much now...
You might get a better response from the Chemistry Help Forum
One thought that might help is the question "what it being consumed"
To generate a voltage there must be a chemical reaction which is converting one chemical to another, with a consequent release of (electrical) energy

Last edited by Woody; Nov 14th 2016 at 02:17 PM.
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Old Jan 27th 2017, 01:24 AM   #8
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Voltage available from a single chemical battery cell is determined almost exclusively by the chemical components involved in the reaction.
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