Author Topic: Minimum voltage needed for electrolysis  (Read 3567 times)

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PeterXXL

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Minimum voltage needed for electrolysis
« on: November 19, 2015, 06:03:10 PM »
Kephra, at...

http://www.cgcsforum.com/index.php?topic=1147.0

you wrote:

Quote: "8 ) There is a minimum voltage necessary to create colloidal silver, but it is very low,  3.5 volts when using a sodium based electrolyte.  However, in practice, the applied voltage must be several times higher than this minimum to maintain reasonable current levels. " End quote.

Question 1: Where did you get the 3.5 V from?

Question 2: What is the equivalent minimum required voltage for gold?

kephra

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Re: Minimum voltage needed for electrolysis
« Reply #1 on: November 19, 2015, 07:43:37 PM »
Answer 1:  It comes from the electrochemical series. (Standard electrode potentials)

From:http://www.cgcsforum.com/index.php?topic=1147.0
Quote
10)  The voltage across the electrolysis cell is the sum of three separate voltages:
A)  The voltage drop across the anode boundary layer of 0.8 volts
B)  The voltage drop across the cathode boundary layer of 2.7 volts (when using electrolyte)
C)  The voltage drop across the bulk fluid between the electrodes.
This sets the minimum voltage of 3.5 volts plus the voltage loss of the bulk fluid.  To account for the bulk fluid the minimum voltage must be substantially higher and depends on the amount of electrolyte, and the electrode geometries.

And from: http://www.cgcsforum.com/index.php?topic=1143.0
Quote
When using sodium carbonate as an electrolyte, the MINIMUM theoretical voltage needed is 3.5 volts.  Below this there is not enough voltage to reduce the sodium at the cathode and oxidize the silver at the anode.  This comes from the electrochemical series which describes the voltage a metal creates when used with a different metal in an electrolytic cell (battery).  Experiments I have conducted seem to confirm this.  The sodium ion requires -2.71 volts to force an electron to it, and the silver atom requires 0.8 volts to remove an electron from it.  So the total is 3.51 volts.   In practice, the voltage should be several times this minimum because there is voltage lost in the bulk water itself.

Answer 2: It would be the voltage to reduce sodium (2.7 volts) plus the voltage to oxidize gold (look it up in the electrochemical series)  However, in practice a lot more voltage is required, so it is irrelevant.
Colloidal Silver is only a bargain if you make it yourself.

RickinWI

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Re: Minimum voltage needed for electrolysis
« Reply #2 on: November 20, 2015, 02:43:07 AM »
So when we hook a Volt Meter to the 2 Electrodes are we measuring 10 C from your first quote? ( " The voltage drop across the bulk fluid between the electrodes." )
So many VARIABLES & so little TIME.

kephra

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Re: Minimum voltage needed for electrolysis
« Reply #3 on: November 20, 2015, 07:21:18 AM »
You are measuring the sum of all three voltage drops.  Its not possible to measure the bulk fluid voltage drop directly unless you have 2 identical electrodes.  Even then, the voltage drop or conductivity of the bulk fluid depends on the electrode geometries.  A TDS meter reads conductivity by having two identical electrodes with known geometries.

Think of it like this...  The conductivity of the water depends on the number of ions existing in the volume of water defined by the surface area of the submerged electrodes.  Additionally, if you imagine a cross section of that area, a small cross section acts sort of like a choke point because a small cross sectional area will have fewer current carrier ions.  This means that having one of the electrodes be very large and the other very small, the conductivity will be mostly controlled by the smaller electrode.  This geometry along with the concentration of the electrolyte and current defines the voltage drop of the bulk fluid between the electrodes.

Now, the other voltages are caused by the difference in the two electrode materials, and placing different electrode materials in a cell with some electrolyte makes a galvanic cell (battery).  Battery voltage depends on the reduction/oxidation potentials of the individual electrode metals and is quantified by the electrochemical series.  So a sodium/silver galvanic cell would produce 3.5 volts if you could make one. (You cant because bulk sodium would explode in water due to its high reactivity with water).  So to force current in the opposite direction (electrolysis cell) you have to overcome this battery voltage of 3.5 plus whatever voltage drop in the bulk fluid develops because of the conductivity/resistivity of the water itself.

When the cell is operational, the cathode is actually a sodium cathode because the current is plating the cathode with sodium metal.
When the cell is not operational, the cathode is silver from the small of amount of silver plated onto it, so there will be no voltage across the cell when it is disconnected unless all the silver is removed and you have a dissimilar (non silver) cathode.

Colloidal Silver is only a bargain if you make it yourself.

PeterXXL

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Re: Minimum voltage needed for electrolysis
« Reply #4 on: November 20, 2015, 10:47:49 AM »
I found this...

http://chemwiki.ucdavis.edu/Analytical_Chemistry/Electrochemistry/Voltaic_Cells/The_Cell_Potential

...from where it can be read that...

Aq (Silver) = +0.800
Na (Sodium) = -2.713
Cl (Chloride) = +1.358

...and...

http://www.benjamin-mills.com/chemistry/ecells.htm

...says that...

Au (Gold) = +1.50

So will the total required voltage then be 1.50 (Au) + 1.358 (Cl) + 2.713 (Na) = 5.571 Volt then to create gold chloride in a water solution of sodium and chloride.?

RickinWI

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Re: Minimum voltage needed for electrolysis
« Reply #5 on: November 20, 2015, 11:57:29 AM »
Thanks for the explanation Kephra. I think I understand now why "We want at least 10 Volts" since it is the sum of ALL 3 in your first quote 10 A +B + C.

We Know that BARE minimum for A + B = 3.5V
And we cannot know the BARE Minimum for C because it is dependent on a bunch of different variables that will be different for each person's setup. You must be figuring that the worst case scenario for Minimum for C would be 3 or 4 Volts which would leave a little extra room to spare if we were at 10 V across the electrodes.

I remember from a different thread you saying that "further gains can be had by having a little more than 10V".  I have no problem getting to 10 V and still having my Anode : Cathode size ratio in the neighborhood of 3.4:1.  But to get to 12V, for example, I have to decrease the amount of submerged Cathode so my A:C ratio becomes 5:1 or 6:1 or whatever. Would you say that is a trade-off worth making? OR is it better to stay at 10V with an ideal A:C ratio?

So many VARIABLES & so little TIME.

kephra

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Re: Minimum voltage needed for electrolysis
« Reply #6 on: November 20, 2015, 12:59:47 PM »
It will make no discernible difference.  I just try to stay above 10 volts.  The magic ratio only becomes important when you are operating close to the 3.5 volt mark.  I do not obsess over the tiny difference it might make.
Colloidal Silver is only a bargain if you make it yourself.

kephra

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Re: Minimum voltage needed for electrolysis
« Reply #7 on: November 20, 2015, 01:01:58 PM »
So will the total required voltage then be 1.50 (Au) + 1.358 (Cl) + 2.713 (Na) = 5.571 Volt then to create gold chloride in a water solution of sodium and chloride.?
No, you do not include the chloride.  Its just the voltages needed to oxidize gold and reduce sodium.  The chlorine is already ionic.
Colloidal Silver is only a bargain if you make it yourself.

Gene

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Re: Minimum voltage needed for electrolysis
« Reply #8 on: November 20, 2015, 04:05:50 PM »
Rick,

There is another variable in the equation.  The actual current through the cell.  Given a fixed placement/geometry for the anode and cathode and how much electrolyte you've chosen to add (which changes the overall conductivity of the bulk fluid), thats going to create a resistance between the electrodes.  The more current, the higher the voltage will be, holding all else constant simply due to Ohms law.

This is why its an issue for processing cold with lower currents.  With 14 gauge silver wire anode submerged 3.5" into the bulk fluid at 2-3ma, the cathode needs to barely glance the surface of the fluid, not being submerged more than 1-2mm unless you want the cell voltage to really start dropping below 10V.  I even moved away from my 14 gauge copper wire cathode and now use a piece of bare 24 gauge wire to get the surface area to be less and thats worked out MUCH better for me as now I can submerge it a good 1/4" and I have some "wiggle room" too.

The only three ways I know to mitigate this given a fixed size/geometry silver anode is to 1) increase cell current which then potentially causes issues with particle size if you set it too high based on your electrode geometry and surface area or 2) use less electrolyte which now raises the possibility of more silver plate-out on the cathode or 3) use a finer gauge cathode to reduce overall surface area as I've done.

As Kephra has said many times, its all a big balancing act - you have to find the sweet spot and then keep doing it for each run.

Just like real life - no black or white but a million shades of gray in between where many of them are perfectly acceptable solutions, though here, the deviation between all of the acceptable solutions is small compared to that which is possible within the arena you're operating.  Basically "good enough" is a small, "fuzzy" circle of solutions and not one single black "dot".  So too is life!

RickinWI

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Re: Minimum voltage needed for electrolysis
« Reply #9 on: November 20, 2015, 08:36:45 PM »
Thanks for the explanations, Kephra & Gene.
So many VARIABLES & so little TIME.

kephra

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Re: Minimum voltage needed for electrolysis
« Reply #10 on: November 21, 2015, 11:20:59 AM »
If you want to go further down the rabbit hole, you can refine the 3.51 volt by applying the Nernst equations, which compensate for the concentration of reactants and temperatures.  However, it only makes a few tens of millivolts of difference, so I ignore it.
Colloidal Silver is only a bargain if you make it yourself.

RickinWI

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Re: Minimum voltage needed for electrolysis
« Reply #11 on: November 21, 2015, 12:10:56 PM »
I think I will skip on by that Rabbit Hole.
I am already WAY too far down a number of different Rabbit Holes and generally speaking I don't like the info I find down there......Depressing.
So many VARIABLES & so little TIME.

peri1224

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Re: Minimum voltage needed for electrolysis
« Reply #12 on: January 02, 2016, 01:42:02 PM »
Some of this chemical and electrical stuff is a bit over my head. Anyway, my setup is a flattened 1 oz. silver bar, measuring almost 7x6 cm, or about 40 sq. cm area under the water. The generator is 14.5 mA, operated with 3 9V batteries. Depending on how deep I put the copper wire anode into the water, the process usually starts with 20 or 15 V and stays quite stable when I process cold.
Does that put me in the middle of the sweet spot or somewhere near the edge?
The voltage fluctuates a bit with heated processing because of the evaporation loss (open beaker without cover), which I compensate for by dropping in new DW to maintain the water level.
The result is always very good, but I have no idea where I stand in relation to that sweet spot.

kephra

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