hodges - 28-6-2004 at 16:25
When water is electrolysed using inert electrodes and a small amount of table salt to increase conductivity, hydrogen and oxygen are produced.
However, if a lot of table salt is added, the products are hydrogen and chlorine. How does one go about determining at what concentration this
transition occurs?
Here was my attempt at this. Cl to Cl2 requires a potential of 1.36V. OH to O2 requires a potential of 1.23V. Therefore, with equal concentrations
of OH- and Cl-, you get O2. Per the Nernst equation, a 10X increase in concentration affects potential by around 0.06 volts at room temperature.
Since 1.36 - 1.23 = 0.13V, somewhere just over a 100-fold increase in Cl- would be required to get Cl2 instead of O2. The OH- concentration of water
at room temperature is 10E-7 moles per liter. 120 times 10E-7 is around 10-5. Presuming adding NaCl does not change the OH- concentration (which
seems like a reasonable assumption since NaCl is neutral), a concentration of around 10E-5 moles per liter of salt is required. The molecular weight
of NaCl is 58, so 58 * 10E-5 = 0.0006 grams per liter. According to these calculations, less than 1/1000g of salt per liter would produce chlorine.
Obviously my calculation is not correct. Does anyone know the right way to do this?
Clonejeffie - 22-3-2017 at 14:46
no clue
Liamatpm - 22-3-2017 at 16:14
You usually will just get hydrogen gas and chlorine gas if you are using salt for your electrolyte. In this reaction at the anode the chloride
'becomes' CHlornine gas and on the cathode the Hydrogen (In H2O) is released as a gas. I don't know if this helps. I hope it did. I use around 5volts
DC at 2.24amps.
I have 2 not very good (or accurate) youtube videos on this type of thing.
https://www.youtube.com/watch?v=dr5UBI_Zfhs
https://www.youtube.com/watch?v=GopUQVRZPbU
[Edited on 23-3-2017 by Liamatpm]
WGTR - 22-3-2017 at 17:09
You also have electrode overvoltage to consider, that is different for each electrode material and product. On graphite anodes the overvoltage is
higher for oxygen than chlorine, enough that chlorine is favored over oxygen.
https://en.m.wikipedia.org/wiki/Overpotential
[Edited on 3-23-2017 by WGTR]