blogfast25
International Hazard
Posts: 10562
Registered: 3-2-2008
Location: Neverland
Member Is Offline
Mood: No Mood
|
|
Electrolyte gel bridge?
I'm thinking of building a Cu/Cu2+ reference electrode from an old bottomless test tube, some 1 M (or 2M) Cu(NO3)2 solution, some sturdy copper
conductor wire coil and an electrolyte bridge in the form of some conductive gel, to hermetically seal off the bottom of the electrode but allow the
circuit to close (with the active electrode). A quick search here didn't unearth much...
Polyacrylamide crosslinkable gels for biochemical research seem to be the material of choice for these bridges but they seem inordinately expensive
and perhaps not the best for the intended purpose.
Anybody have any tangible experience with this? As replacement material I was thinking aloud:
• Agar Agar gel, made up with an electrolyte. Perhaps too soluble in water?
• PVA (wood and other glues), mixed with some Na2SO4/K2SO4...?
• Neoprene based contact glue, mixed with some Na2SO4/K2SO4?
• Araldite, mixed with some Na2SO4/K2SO4?
Any ideas most appreciated!
|
|
|
matei
Hazard to Others
Posts: 205
Registered: 16-9-2006
Member Is Offline
Mood: No Mood
|
|
IMO the simplest way of obtaining such a salt bridge would be with Agar mixed with KCl. The Agar (or agarose, which is purified Agar Agar) is mixed
with saturated KCl in water, heated until it dissolves and cooled. On cooling it transforms into a gel. In order to seal a glass tube, you insert one
end of it through a layer of gel (e.g. 5 mm thick), so the gel trapped inside the tube forms a plug.
Acrylamide and N,N-methylene-bis(acrylamide) (the crosslinking agent) are both known carcinogens.
[Edited on 30-10-2010 by matei]
|
|
|
blogfast25
International Hazard
Posts: 10562
Registered: 3-2-2008
Location: Neverland
Member Is Offline
Mood: No Mood
|
|
Thanks, Matei...
I was thinking of capping off the bottom end of the tube (with masking tape, a bung or whatever) and cast the hot Agar Agar/KCl mixture into the tube
where it could then set (gel) and form a plug?
|
|
|
matei
Hazard to Others
Posts: 205
Registered: 16-9-2006
Member Is Offline
Mood: No Mood
|
|
Well, if you prepare the Agar gel in a beaker, you can insert the tube vertically in the hot mixture and wait for it to cool unil it gels and than you
take out the tube and scrap the gel that's adhering to the outside wall of the tube...
|
|
|
blogfast25
International Hazard
Posts: 10562
Registered: 3-2-2008
Location: Neverland
Member Is Offline
Mood: No Mood
|
|
Yep, that's good too... Will try this shortly.
Actually I see that Agar Agar is mentioned in Wiki for making salt bridges, so finding recipes shouldn't be a problem.
[Edited on 30-10-2010 by blogfast25]
|
|
|
blogfast25
International Hazard
Posts: 10562
Registered: 3-2-2008
Location: Neverland
Member Is Offline
Mood: No Mood
|
|
Today I completed the reference cell of Cu/Cu(NO3)2 (0.2 M). A sturdy LDPE plastic tube, about 30 cm of coiled 1 mm conductor copper, 1 ml of bridging
gel (0.2 M KCl, 5 % w/v Agar Agar) caps off the bottom of the tube. The copper coil was activated with dilute HNO3 and rinsed with DIW, before
insertion into the electrolyte. The electrode will be kept in 0.2 M KCl when not in use.
Here it is - reference electrode and Pt wire connected:
The active electrode is a thin piece of platinum wire, about 3 cm long (15 quid!). Here I get a steady - 38 mV on the meter with DIW only:
Adding just one drop of 0.5 M Fe3+ and the potential changed to - 430 mV.
The readings seem fairly stable…
Then a real test: ‘classroom’ titration of 10 ml 0.1 N Mohr’s Salt with 0.1 N KMnO4 (both nominal, not standardised), recording potential every
1 ml. Titration set up:
Despite the rather cramped conditions and the fact that in my haste I’d forgotten to acidify the Fe2+ solution (thus Fe(OH)3.nH2O formed directly
during titration – far from ideal!) the results were in fact very good. At v = 0 ml, U was -190.3 mV, at 1.1 ml, U dropped to -283 mV, after that
and up to about 9 ml, the potential dropped slowly but very steadily by about 10 mV/ml permanganate added. From 9.5 ml (-441 mV) the potential changed
to -704 mV at 9.7 ml.
The early drop (0 to 1.1 ml) in cell potential is reminiscent of the titration of weak acids/bases.
The plotted results (Excel) showed a nice smooth S-shaped curve with a clear endpoint at 9.6 ml.
I hope to get as good results as this with the titration of Sn2+ with Fe3+…
I do wonder if some other material (cheaper!) than Pt for the active electrode could be used. NiChrome wire has been suggested on some educational
website. I also have some titanium fasteners (99 % Ti) but Ti does slowly react with HCl. Ideas most welcome...
[Edited on 5-11-2010 by blogfast25]
|
|
|
![[*]](images/xpblue/default_icon.gif){kind=icon}
