Science and Technology news... :: View topic - the porous plug in reference electrodes ?

Adam S · Guest

I>m trying to find a supplier of those porous plugs used in reference
electrodes ? I want to make a series of custom Ag/AgCl electrodes and
not pay too much money.

Searching google I came across names like Vycor frits, sintered glass,
and porous PTFE but were only in reference to the construction of
commercial electrodes. I>m thinking porous plugs too specialised for any
company to sell as a product.

Yevgen Barsukov · Guest

On Oct 12, 5:19 am, Adam S <not.valid@nosuchaddress> wrote:
[quote]I>m trying to find a supplier of those porous plugs used in reference
electrodes ? I want to make a series of custom Ag/AgCl electrodes and
not pay too much money.

Searching google I came across names like Vycor frits, sintered glass,
and porous PTFE but were only in reference to the construction of
commercial electrodes. I>m thinking porous plugs too specialised for any
company to sell as a product.
[/quote]
The cheapest but very good working solution for reference electrode
frits is
the graphite center of a pencil. It is actually made not of pure
graphite but
of graphite, clay and binder. Procedure is as follows:

- take a 1 cm long graphite rod from a pensil (thin or thick, does not
matter)
- hold it with a tweezers in a gas-burner fire so that it gets bright
red (just for a few seconds) to burn out the binder
- make a glass capillary by holding a glass tube in a fire of gas-
burner and pulling it apart so that thin part of the capillary is
slightly thicker than the graphite rod
- break the thin area of the pulled tube in the middle after
circularly scratching it with glass-cutter (we used a little synthetic
ruby)
- place graphite rod into the capillary so that it just slightly
sticks out
- hold the capillary with graphite in the flame of gas-burner and
rotate it until glass melts and builds homogeneous smooth layer around
the rod

Reference electrode container with porous separator is ready. It has
relatively low impedance
and very good insulating properties. You can keep even volatile
organic electrolytes such as acetonitryle in it for weeks without
drying out
(for long storage you can make a little plastic cap to close the
porouse end).

Regards,
Evgenij

Bill Penrose · Guest

On Oct 12, 3:19 am, Adam S <not.valid@nosuchaddress> wrote:
[quote]Searching google I came across names like Vycor frits, sintered glass,
and porous PTFE but were only in reference to the construction of
commercial electrodes. I>m thinking porous plugs too specialised for any
company to sell as a product.
[/quote]
We>ve used Teflon heat shrink with porous Teflon or glass. Even a
pinch of Pyrex glass wool has worked in a pinch, if constricted inside
Teflon heat shrink. Use a size of heat shrink that has 'zero'
recovered inside diameter.

Dangerous Bill

Bill Penrose · Guest

On Oct 12, 9:05 am, Yevgen Barsukov <evgen...@gmail.com> wrote:
[quote](for long storage you can make a little plastic cap to close the
porouse end).
[/quote]
What a great idea! Do you have problems with wetting the graphite?

DB

Bob M · Guest

A simple plug of plain pine timber works very well. I used one in the
reference electrode of my polarograph for many years. In fact I think
the use of wooden plugs preceeded the use porosints for this purpose.
Make sure that the plug is made of soft sap wood and only very
slightly larger that the hole it is going into.Push the plug into
place and trim off the excess wood to flush with the end of the glass
tube with a sharp scalpel.
In many respects the wooden plug is superior to the porosint. It
certainly leaks less and the addition of make up electrolyte is lower.
I am sure I saw a new commercial electrode recently that had a wood
plug or similar.

Bob M
www.molab.co.nz

Mark Thorson · Guest

Bob M wrote:
[quote]
A simple plug of plain pine timber works very well. I used one in the
reference electrode of my polarograph for many years. In fact I think
the use of wooden plugs preceeded the use porosints for this purpose.
Make sure that the plug is made of soft sap wood and only very
slightly larger that the hole it is going into.Push the plug into
place and trim off the excess wood to flush with the end of the glass
tube with a sharp scalpel.
In many respects the wooden plug is superior to the porosint. It
certainly leaks less and the addition of make up electrolyte is lower.
I am sure I saw a new commercial electrode recently that had a wood
plug or similar.
[/quote]
I just love the way you guys improvise stuff.
I>m tempted to do some of this stuff myself.
This could be turned into a good science kit
for high school students. Build your own pH
meter from scratch, except for a commercial
DVM from Radio Shack or Digi-Key.

Yevgen Barsukov · Guest

On Oct 12, 11:48 am, Bill Penrose <dangerousb...@gmail.com> wrote:
[quote]On Oct 12, 9:05 am, Yevgen Barsukov <evgen...@gmail.com> wrote:

(for long storage you can make a little plastic cap to close the
porouse end).

What a great idea! Do you have problems with wetting the graphite?

DB
[/quote]
No, probably because these "graphite" rods have large part of clay
which adds
porosity and some hydrophilic component. Also when binder is burned
out,
it leaves some additional pores.
So we were using such separators both in organic and aqueous
electrolytes and electrolyte was usually wetting the entire membrane
instantly after filling.

Regards,
Yevgen

Adam S · Guest

Yevgen Barsukov wrote:
[quote]On Oct 12, 5:19 am, Adam S <not.valid@nosuchaddress> wrote:
I>m trying to find a supplier of those porous plugs used in reference
electrodes ? I want to make a series of custom Ag/AgCl electrodes and
not pay too much money.

Searching google I came across names like Vycor frits, sintered glass,
and porous PTFE but were only in reference to the construction of
commercial electrodes. I>m thinking porous plugs too specialised for any
company to sell as a product.

The cheapest but very good working solution for reference electrode
frits is
the graphite center of a pencil. It is actually made not of pure
graphite but
of graphite, clay and binder. Procedure is as follows:

- take a 1 cm long graphite rod from a pensil (thin or thick, does not
matter)
- hold it with a tweezers in a gas-burner fire so that it gets bright
red (just for a few seconds) to burn out the binder
- make a glass capillary by holding a glass tube in a fire of gas-
burner and pulling it apart so that thin part of the capillary is
slightly thicker than the graphite rod
- break the thin area of the pulled tube in the middle after
circularly scratching it with glass-cutter (we used a little synthetic
ruby)
- place graphite rod into the capillary so that it just slightly
sticks out
- hold the capillary with graphite in the flame of gas-burner and
rotate it until glass melts and builds homogeneous smooth layer around
the rod

Reference electrode container with porous separator is ready. It has
relatively low impedance
and very good insulating properties. You can keep even volatile
organic electrolytes such as acetonitryle in it for weeks without
drying out
(for long storage you can make a little plastic cap to close the
porouse end).

[/quote]
That sounds like a great idea, but I>m wondering if the relatively high
electrical conductivity of graphite affects the electrode potential. I
imagine graphite in this situation would act as an intermediate
electrode with some charge transfer reaction. Electrochem is one of my
weak points so I>m not sure what the graphite would do.

Adam S · Guest

Bob M wrote:
[quote]A simple plug of plain pine timber works very well.
[/quote]
Hay, Bob I tried your approach and I think this is a winner. I cut off
a 6mm length of common wooden dowel, 12mm diameter, and epoxy into end
of ridged PVC tube. I was careful to only apply epoxy on the
circumference so as to keep the flat sides of the wooden plug exposed.
As you recommended, I trimmed the end down with a grinding disc and got
the end of the tube nice and flush. I then let it soak for 2 days in
electrolyte and wolla, I had a housing for a reference electrode.

Yevgen Barsukov · Guest

On Oct 14, 7:21 pm, Adam S <not.valid@nosuchaddress> wrote:
[quote]Yevgen Barsukov wrote:
On Oct 12, 5:19 am, Adam S <not.valid@nosuchaddress> wrote:
I>m trying to find a supplier of those porous plugs used in reference
electrodes ? I want to make a series of custom Ag/AgCl electrodes and
not pay too much money.

Searching google I came across names like Vycor frits, sintered glass,
and porous PTFE but were only in reference to the construction of
commercial electrodes. I>m thinking porous plugs too specialised for any
company to sell as a product.

The cheapest but very good working solution for reference electrode
frits is
the graphite center of a pencil. It is actually made not of pure
graphite but
of graphite, clay and binder. Procedure is as follows:

- take a 1 cm long graphite rod from a pensil (thin or thick, does not
matter)
- hold it with a tweezers in a gas-burner fire so that it gets bright
red (just for a few seconds) to burn out the binder
- make a glass capillary by holding a glass tube in a fire of gas-
burner and pulling it apart so that thin part of the capillary is
slightly thicker than the graphite rod
- break the thin area of the pulled tube in the middle after
circularly scratching it with glass-cutter (we used a little synthetic
ruby)
- place graphite rod into the capillary so that it just slightly
sticks out
- hold the capillary with graphite in the flame of gas-burner and
rotate it until glass melts and builds homogeneous smooth layer around
the rod

Reference electrode container with porous separator is ready. It has
relatively low impedance
and very good insulating properties. You can keep even volatile
organic electrolytes such as acetonitryle in it for weeks without
drying out
(for long storage you can make a little plastic cap to close the
porouse end).

That sounds like a great idea, but I>m wondering if the relatively high
electrical conductivity of graphite affects the electrode potential. I
imagine graphite in this situation would act as an intermediate
electrode with some charge transfer reaction. Electrochem is one of my
weak points so I>m not sure what the graphite would do.
[/quote]
Adam,
electric conductivity of graphite will play no role in this
application because
ion flow will never cross the "graphite/electrolyte" boundary (as it
is protected
by activation energy) so it will not matter if it was glass of
graphite.
In addition, there is only uA - pA range currents flowing through this
membrane
so there is no way that enough potential would accomulate to cross the
graphite/electrolyte
barrier.

Regards,
Yevgen

Yevgen Barsukov · Guest

On Oct 14, 7:32 pm, Adam S <not.valid@nosuchaddress> wrote:
[quote]Bob M wrote:
A simple plug of plain pine timber works very well.

Hay, Bob I tried your approach and I think this is a winner. I cut off
a 6mm length of common wooden dowel, 12mm diameter, and epoxy into end
of ridged PVC tube. I was careful to only apply epoxy on the
circumference so as to keep the flat sides of the wooden plug exposed.
As you recommended, I trimmed the end down with a grinding disc and got
the end of the tube nice and flush. I then let it soak for 2 days in
electrolyte and wolla, I had a housing for a reference electrode.
[/quote]
It looks to me that wood electrode is better used in water-based
electrolytes.
In organic electrolytes often used in electrochemistry, some parts of
wood
would become soluble and contaminate the electrolyte. Most of
electrochemical
experiments are extremely sensitive to any contaminants. For example
Li-ion batteries
that use organic electrolyte can not coexist with even minuscule
amounts of water,
that would be certainly extracted from the wood membrane.
Even in water-based electrolytes, for analytical chemistry experiments
wood could
leak too much Na, K and phosphate for comfort, although for simple
electrodeposition purposes
it should be fine.

Regards,
Yevgen


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