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Author: Subject: Electrolyte gel bridge?
blogfast25
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cool.gif posted on 30-10-2010 at 04:38
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!
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matei
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[*] posted on 30-10-2010 at 06:49


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]
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blogfast25
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[*] posted on 30-10-2010 at 09:15


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?
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[*] posted on 30-10-2010 at 09:51


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...
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[*] posted on 30-10-2010 at 10:53


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]
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[*] posted on 5-11-2010 at 10:05


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]
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