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Forum index » Science and Technology » Chem » Electrochem
Electrolysis - what's wrong here?
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Oscar Lanzi III
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Joined: 30 Apr 2005
Posts: 176

PostPosted: Sat Mar 04, 2006 2:13 pm    Post subject: Re: Electrolysis - what's wrong here? Reply with quote

The driving force for direcitonal movement does come from the electric
field. I misspoke when I said that without current you have only
diffusive movement. All you need is an electric field and you'll have
migration. To the extent that the ions tend to migrate in opposite
directions the migraiton is diminished, but as I have said there can be
a net migraiton when the ion mobilities are different.

--OL
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Chrisd5000rouge
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Joined: 08 Feb 2006
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PostPosted: Fri Mar 03, 2006 8:11 pm    Post subject: Re: Electrolysis - what's wrong here? Reply with quote

Thanks OL, I see your point. I see a flow of + and - ions going towards
the anode keeping the global neutrality. As consequence no flow of K+
towards the cathode, otherwise we would have no neutrality as MnO4-
does not follow K+, interesting. Sorry if I insist, but coming back to
my first question then: till this MnO4-/K+ flow reaches the anode, we
have no current, where does the energy come from? One could argue that
is due to the hydrophily of the paper but it is directional. Can we say
that the electrical field induces the direction?
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Oscar Lanzi III
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PostPosted: Wed Mar 01, 2006 11:45 pm    Post subject: Re: Electrolysis - what's wrong here? Reply with quote

It's the difference in ion mobility that decides this. The more mobile
ion wins the battle.

--OL
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Chrisd5000rouge
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Posts: 11

PostPosted: Wed Mar 01, 2006 7:01 pm    Post subject: Re: Electrolysis - what's wrong here? Reply with quote

....and the same should be true towards the cathode, the K(+) draggind
the MnO4(-), kicking and screaming. However there is no purple trace of
MnO4(-) ion towards the cathode! There must be some influence of the
electrical field. No, still something wrong I guess...
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Oscar Lanzi III
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Posts: 176

PostPosted: Wed Mar 01, 2006 3:26 am    Post subject: Re: Electrolysis - what's wrong here? Reply with quote

If we go with the idea that the K(+) and MnO4(-) ions have different
mobilities, then in this case the MnO4(-) ion is more mobile and drags
the K(+) ion, kicking and screaming, towards teh anode.

--OL
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Chrisd5000rouge
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Joined: 08 Feb 2006
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PostPosted: Tue Feb 28, 2006 9:11 pm    Post subject: Re: Electrolysis - what's wrong here? Reply with quote

well...what i see is that the purple moves to the + side. If it is only
a question of diffusion, mobility speed or hydrophily then there should
be also some trace towards the - and also happen when no voltage is
present. so?
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beav
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Joined: 18 Oct 2005
Posts: 63

PostPosted: Tue Feb 28, 2006 5:57 pm    Post subject: Re: Electrolysis - what's wrong here? Reply with quote

On Tue, 28 Feb 2006 05:46:24 -0600, ol3@webtv.net (Oscar Lanzi III)
wrote:

Quote:
But you can't separate the ions without introducing other ions
electrochemically, The K(+) and MnO4(-) have to go the same way!
(Electroneutrality principle.)

Rather than a separation of charges, we have a difference in ion
mobilities -- one ion is more mobile than the other and the latter ion
ends up being "dragged" the wrong way. When I look at the flux
equations it seems entirely possible.

--OL


agreed. thanks to the support material ( hydrophilic paper fibers).

hmmm... i wonder if the effect would be any different if you used
something more hydrophobic, like Tyvek and just enough water to
connect the circuit and solvate the KMnO4.....
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Oscar Lanzi III
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Joined: 30 Apr 2005
Posts: 176

PostPosted: Tue Feb 28, 2006 11:46 am    Post subject: Re: Electrolysis - what's wrong here? Reply with quote

But you can't separate the ions without introducing other ions
electrochemically, The K(+) and MnO4(-) have to go the same way!
(Electroneutrality principle.)

Rather than a separation of charges, we have a difference in ion
mobilities -- one ion is more mobile than the other and the latter ion
ends up being "dragged" the wrong way. When I look at the flux
equations it seems entirely possible.

--OL
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Chrisd5000rouge
science forum beginner


Joined: 08 Feb 2006
Posts: 11

PostPosted: Mon Feb 27, 2006 7:00 pm    Post subject: Re: Electrolysis - what's wrong here? Reply with quote

Thanks beav. About the electrophoretic gradient, could you tell me the
mechanism of it? does the paper play a specific role on it? I ask this
because Oscar says that the electrical field does not induce the
migration if only water is present. There is only diffusion.
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beav
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Joined: 18 Oct 2005
Posts: 63

PostPosted: Mon Feb 27, 2006 3:21 pm    Post subject: Re: Electrolysis - what's wrong here? Reply with quote

On Sun, 26 Feb 2006 19:58:10 -0600, ol3@webtv.net (Oscar Lanzi III)
wrote:

Quote:
Check how reproduciible your result is; you may get random variation in
the pemeability of the paper. Also put some litmus paper on the paper
as well. Your water could be picking up carbon doixide, which forms an
acid.

--OL


by applying the potential, you are creating an electrophoretic
gradient, using the paper as the medium.

turn off the power and all you'll get is a purple sheet all over.

like Oscar said, the only discharge you'll get is H2 and O2.
everything else will be well solvated.
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Oscar Lanzi III
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Joined: 30 Apr 2005
Posts: 176

PostPosted: Mon Feb 27, 2006 1:58 am    Post subject: Re: Electrolysis - what's wrong here? Reply with quote

Check how reproduciible your result is; you may get random variation in
the pemeability of the paper. Also put some litmus paper on the paper
as well. Your water could be picking up carbon doixide, which forms an
acid.

--OL
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Chrisd5000rouge
science forum beginner


Joined: 08 Feb 2006
Posts: 11

PostPosted: Sun Feb 26, 2006 10:38 pm    Post subject: Re: Electrolysis - what's wrong here? Reply with quote

Ok, but I made a simple experiment placing a permanganate cristal on
the center of a wet filter paper (wetted with deionized water) and
applying voltage at the ends. You can clearly see the red color moving
to the + pole, like in an electrophoresis. I agree that this does not
happen when I do this directly in water. What is the difference?
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Oscar Lanzi III
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Joined: 30 Apr 2005
Posts: 176

PostPosted: Sun Feb 26, 2006 7:49 pm    Post subject: Re: Electrolysis - what's wrong here? Reply with quote

At first, with no current, the crystal dissolves and the ions just
diffuse -- a process independent of any electric-field induced
migration. When they diffuse to the point where they contact the
electrodes and com,plete the circuit, then current flows and now you
have migration along with the diffusion.

And the ions do not necessarily "neutralize." That depends on the
electrolyte, the solvent, and even the electrode material. There are
other possible reactions involving the water solvent itself. With
sodium sulfate in water and chemically inert electrode materials, all
you get is water reacting to from H2+O2; the electrolyte, though
necssary to complete the circuit, does not do anything chemically. With
NaCl you do get the chloride ions to oxidize at the anode to chlorine,
but at the cathode you get hydrogen from water -- no sodium metal
(unless it's a mercury cathode and you're getting the Na amalgam, but
they don't do that anymore commercially).

I will not go into detail here. I assume that the OP has one of these
schoolbooks, or textbooks, which explains all this at some length.

--OL
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Chrisd5000rouge
science forum beginner


Joined: 08 Feb 2006
Posts: 11

PostPosted: Sat Feb 25, 2006 8:28 pm    Post subject: Electrolysis - what's wrong here? Reply with quote

I have always been puzzled about the ion migration theory in
electrolysis. Schools books tell that under influence of an electrical
field, ionic charges start to move towards the electrodes, then
neutralize there.
My questions is: suppose you drop a NaCl cristal in the middel of a
water-filled electrolytic cell. If we apply potential and supposing
pure water non-conductive, Na+ and Cl- would start migrating from the
middle of the solution towards the electrodes while no current would be
present till those reach the electrodes: how can we have movement
without spending energy? suppose you apply high frequency voltage. the
ions would move go and back continiously without never reaching the
electrodes, this would at least heat up the solution I guess? so what's
wrong here?
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