|
|
| Author |
Message |
Gordon
science forum Guru Wannabe
Joined: 12 May 2005
Posts: 115
|
 Posted: Thu Oct 27, 2005 12:54 am Post subject:
Stern layer
|
|
|
If one places two carbon or platinum electrodes into a saline
solution such as sea water, and applies a voltage below the
electrolysis potential of water, what ions make up the Stern
layers on the respective electrodes?
Do hydrogen and oxygen ions from the water form the Stern layers,
or do sodium and chlorine ions, and other salt ions form the
Stern layers? |
|
| Back to top |
|
 |
N:dlzc D:aol T:com (dlzc)
science forum Guru
Joined: 25 Mar 2005
Posts: 2835
|
| Posted: Thu Oct 27, 2005 3:09 am Post subject:
Re: Stern layer
|
|
|
Dear Gordon:
"Gordon" <gordonlr@DELETEswbell.net> wrote in message
news:5490m150u7c6mhqojoisnm2kncusfa29sd@4ax.com...
| Quote: | If one places two carbon or platinum electrodes into a saline
solution such as sea water, and applies a voltage below the
electrolysis potential of water, what ions make up the Stern
layers on the respective electrodes?
Do hydrogen and oxygen ions from the water form the
Stern layers, or do sodium and chlorine ions, and other salt
ions form the Stern layers?
|
If you don't get an answer here, you might try sci.chem.
I would (probably incorrectly) guess H+ and Na+ on one electrode,
and OH- and Cl- on the other.
David A. Smith |
|
| Back to top |
|
|
tadchem
science forum Guru
Joined: 03 May 2005
Posts: 1348
|
| Posted: Thu Oct 27, 2005 12:18 pm Post subject:
Re: Stern layer
|
|
|
Gordon wrote:
| Quote: | If one places two carbon or platinum electrodes into a saline
solution such as sea water, and applies a voltage below the
electrolysis potential of water, what ions make up the Stern
layers on the respective electrodes?
|
Ions in the Stern Layer are absorbed onto the surface of the electrode
and held in place by electrostatic attraction.
http://www.geocities.com/CapeCanaveral/Hangar/5555/zeta.htm
(scroll down to the middle, under 'Zeta Potential')
In aqueous *solutions* such as sea water, these will be dissolved ions
such as Na+ and Cl-. Pure water (conductivity grade) is a
non-conductor, and does not electrolyze easily, nor does it suppor true
ions of the form H+ or OH-.
In aqueous solutions the 'ion' H+ is more correctly a hydrated form
with several water molecules loosely bonded together, with a charge
from an extra H+. The general formula
H(2n+1)O(n)+
with n an integer from about 3 to 12 (it rapidly fluctuates) is
probably more accurate. Similarly 'hydroxide' is more accurately
represented as
H(2n-1)O(n)-
| Quote: | Do hydrogen and oxygen ions from the water form the Stern layers,
|
No, unless the electrode itself can form more-or-less stable
associations with either H+ or OH-. Usually electrodes are conductors
(with lots of 'loose' electrons available in conduction bands) and will
not form such associations.
| Quote: | or do sodium and chlorine ions, and other salt ions form the
Stern layers?
|
If these ions are available, electrostatics will drive the formation of
Stern layers with them.
Tom Davidson
Richmond, VA |
|
| Back to top |
|
|
N:dlzc D:aol T:com (dlzc)
science forum Guru
Joined: 25 Mar 2005
Posts: 2835
|
| Posted: Thu Oct 27, 2005 1:10 pm Post subject:
Re: Stern layer
|
|
|
Dear tadchem:
"tadchem" <thomas.davidson@dla.mil> wrote in message
news:1130415488.609291.322860@g47g2000cwa.googlegroups.com...
....
| Quote: | In aqueous *solutions* such as sea water, these will
be dissolved ions such as Na+ and Cl-. Pure water
(conductivity grade) is a non-conductor, and does not
electrolyze easily, nor does it suppor true ions of the
form H+ or OH-.
In aqueous solutions the 'ion' H+ is more correctly a
hydrated form with several water molecules loosely
bonded together, with a charge from an extra H+.
The general formula
H(2n+1)O(n)+
with n an integer from about 3 to 12 (it rapidly
fluctuates) is probably more accurate. Similarly
'hydroxide' is more accurately represented as
H(2n-1)O(n)-
|
That makes good sense. Glad you had your "ears on".
David A. Smith |
|
| Back to top |
|
|
Gordon
science forum Guru Wannabe
Joined: 12 May 2005
Posts: 115
|
| Posted: Thu Oct 27, 2005 1:19 pm Post subject:
Re: Stern layer
|
|
|
On 27 Oct 2005 05:18:08 -0700, "tadchem"
<thomas.davidson@dla.mil> wrote:
| Quote: |
Gordon wrote:
If one places two carbon or platinum electrodes into a saline
solution such as sea water, and applies a voltage below the
electrolysis potential of water, what ions make up the Stern
layers on the respective electrodes?
Ions in the Stern Layer are absorbed onto the surface of the electrode
and held in place by electrostatic attraction.
http://www.geocities.com/CapeCanaveral/Hangar/5555/zeta.htm
(scroll down to the middle, under 'Zeta Potential')
In aqueous *solutions* such as sea water, these will be dissolved ions
such as Na+ and Cl-. Pure water (conductivity grade) is a
non-conductor, and does not electrolyze easily, nor does it suppor true
ions of the form H+ or OH-.
In aqueous solutions the 'ion' H+ is more correctly a hydrated form
with several water molecules loosely bonded together, with a charge
from an extra H+. The general formula
H(2n+1)O(n)+
with n an integer from about 3 to 12 (it rapidly fluctuates) is
probably more accurate. Similarly 'hydroxide' is more accurately
represented as
H(2n-1)O(n)-
Do hydrogen and oxygen ions from the water form the Stern layers,
No, unless the electrode itself can form more-or-less stable
associations with either H+ or OH-. Usually electrodes are conductors
(with lots of 'loose' electrons available in conduction bands) and will
not form such associations.
or do sodium and chlorine ions, and other salt ions form the
Stern layers?
If these ions are available, electrostatics will drive the formation of
Stern layers with them.
Tom Davidson
Richmond, VA
Thanks, Tom, for your response. This is more or less the way I |
had understood the situation, but I kept getting conflicting
information from other documented sources.
The most prevalent conflicting information implies that the
saline ions are hydrated, and do not actually come into contact
with either electrode. The polar water molecules that form the
hydration capsule make contact with the electrode surface, and
the closest saline ions are located at the inner Helmholtz
boundary.
An example of this conflicting information may be viewed at:
http://www.dekker.com/sdek/abstract~db=enc~content=a713562271~words=
Tom, I'm not trying to present a case against what you've said.
In fact, I agree with your assessment, but am curious as to why
there seems to be a substantial conflicting opinion.
Gordon L. Richard
Oklahoma City, OK |
|
| Back to top |
|
|
tadchem
science forum Guru
Joined: 03 May 2005
Posts: 1348
|
| Posted: Thu Oct 27, 2005 5:54 pm Post subject:
Re: Stern layer
|
|
|
Gordon wrote:
<snip>
| Quote: | The most prevalent conflicting information implies that the
saline ions are hydrated, and do not actually come into contact
with either electrode. The polar water molecules that form the
hydration capsule make contact with the electrode surface, and
the closest saline ions are located at the inner Helmholtz
boundary.
An example of this conflicting information may be viewed at:
http://www.dekker.com/sdek/abstract~db=enc~content=a713562271~words=
|
This is in error. Solvated ions in aqueous solutions do not posess an
inviolate shell similar to that of complex ions, in which the attached
molecules are difficult to displace. If this were true, then
electroplating would be very difficult, as the ions could never make
physical contact with the electrode.
Rather the molecules providing the solvation shell around an ion are
comparatively labile. The solvent molecules themselves are
electrostatic dipoles, and it is energetically quite difficult and
expensive to arrange 6 or more such that they *all* have the same end
pointed inwards. The mutual electrostatic repulsion would become
significant.
In an actual solvation sphere the solvent molecules are arrayed around
the ion such that the majority are aligned with charged ends of the
dipoles to counter the charge on the ion, but some actually have the
end with the same charge as the ion pointed inwards. This not only
provides attraction to bind the whole of the solvation sphere together,
but is also the primary source of the entropy of solvation. (This is a
small truth I personally stumbled upon some years ago while solving the
Poisson Equation as a wave equation for an ion in a medium comprised of
dipoles.)
Most chemists, once they understand the concept of solvation, tend to
overlook the fact that solvation spheres can rearrange on a femtosecond
time scale just by shifting the position(s) of solvent molecule(s).
This makes it very easy for electrostatic forces between the electrode
and the ion to squeeze the water out from between them.
| Quote: | Tom, I'm not trying to present a case against what you've said.
In fact, I agree with your assessment, but am curious as to why
there seems to be a substantial conflicting opinion.
|
Some physical chemists are less willing than others to abandon the
models they learned as students, especially when the actual
measurements (in this case measurements of the distance between the ion
and the electrode - a few score picometers) are prohibitively
difficult, unreliable, or otherwise unavailable.
In solution chemistry there are still a lot of things going on for
which we *know* the models are wrong, but they are the best guesses we
have so far.
HTH
Tom Davidson
Richmond, VA |
|
| Back to top |
|
|
Gordon
science forum Guru Wannabe
Joined: 12 May 2005
Posts: 115
|
| Posted: Thu Oct 27, 2005 8:13 pm Post subject:
Re: Stern layer
|
|
|
On 27 Oct 2005 10:54:04 -0700, "tadchem"
<thomas.davidson@dla.mil> wrote:
| Quote: | Gordon wrote:
snip
The most prevalent conflicting information implies that the
saline ions are hydrated, and do not actually come into contact
with either electrode. The polar water molecules that form the
hydration capsule make contact with the electrode surface, and
the closest saline ions are located at the inner Helmholtz
boundary.
An example of this conflicting information may be viewed at:
http://www.dekker.com/sdek/abstract~db=enc~content=a713562271~words=
This is in error. Solvated ions in aqueous solutions do not posess an
inviolate shell similar to that of complex ions, in which the attached
molecules are difficult to displace. If this were true, then
electroplating would be very difficult, as the ions could never make
physical contact with the electrode.
Rather the molecules providing the solvation shell around an ion are
comparatively labile. The solvent molecules themselves are
electrostatic dipoles, and it is energetically quite difficult and
expensive to arrange 6 or more such that they *all* have the same end
pointed inwards. The mutual electrostatic repulsion would become
significant.
In an actual solvation sphere the solvent molecules are arrayed around
the ion such that the majority are aligned with charged ends of the
dipoles to counter the charge on the ion, but some actually have the
end with the same charge as the ion pointed inwards. This not only
provides attraction to bind the whole of the solvation sphere together,
but is also the primary source of the entropy of solvation. (This is a
small truth I personally stumbled upon some years ago while solving the
Poisson Equation as a wave equation for an ion in a medium comprised of
dipoles.)
Most chemists, once they understand the concept of solvation, tend to
overlook the fact that solvation spheres can rearrange on a femtosecond
time scale just by shifting the position(s) of solvent molecule(s).
This makes it very easy for electrostatic forces between the electrode
and the ion to squeeze the water out from between them.
Tom, I'm not trying to present a case against what you've said.
In fact, I agree with your assessment, but am curious as to why
there seems to be a substantial conflicting opinion.
Some physical chemists are less willing than others to abandon the
models they learned as students, especially when the actual
measurements (in this case measurements of the distance between the ion
and the electrode - a few score picometers) are prohibitively
difficult, unreliable, or otherwise unavailable.
In solution chemistry there are still a lot of things going on for
which we *know* the models are wrong, but they are the best guesses we
have so far.
HTH
Tom Davidson
Richmond, VA
Thanks, Tom, for your insights on this. I was reasonably |
confident that the saline ions would be the dominant entities in
the Stern layer on each electrode, but I was not sure enough to
let the argument rest. There does seem to be a great many who
would argue otherwise. I'm a physicist, and my chemistry
background is not as strong as I'd like it to be on matters such
as this.
Gordon Richard
Oklahoma City, OK |
|
| Back to top |
|
|
Google
|
|
| Back to top |
|
|
|
|
The time now is Fri Jan 09, 2009 10:10 pm | All times are GMT
|
|
Personal Loans | MPAA | Cash Back Credit Cards | Online Advertising | Credit Counseling
|
|
Copyright © 2004-2005 DeniX Solutions SRL
|
|
Other DeniX Solutions sites:
Electronics forum |
Medicine forum |
Unix/Linux blog |
Unix/Linux documentation |
Unix/Linux forums
|
Powered by phpBB © 2001, 2005 phpBB Group
|
|
FAQ
Search
Memberlist
Usergroups
Register
Profile
Log in to check your private messages
Log in
Forum index
»
Science and Technology
»
Physics
