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Passage of charge through an appropriate solution of reactants, solvent and electrolyte can achieve a selective chemical transformation, provided that the reference electrode potential is steady.  Changes in the applied potential during electrolysis are a major problem since different chemical reactions can be accessed at different potentials.  A bare silver wire (or a silver wire coated with AgCl) can't be used as a pseudo-reference electrode during a bulk electrolysis since its potential depends on the composition of the solution with which it is in contact.  The point of a bulk electrolysis IS to change the solution composition, so a shifting reference potential will result in drift in the applied potential.  The solution is to have the reference electrode in a separate compartment exposed to solution whose composition does not change.



A very good discussion of reference electrodes can be found at::


A simple bulk electrolysis application that might be found in a freshman lab is simply to pass a steady current through a solution of copper sulfate and to measure the change in mass at the copper anode and cathode.  After the 2:1 ratio of electrons to copper atoms has been taken into account, comparison of the charge passed (current x time) with the mass change should give an estimate of Faraday's number. Every copper atom that dissolves is balanced by one that precipitates, the solution composition does not change, and so the reference electrode potential does not drift.  This type of experiment does not require much in the way of preparation.



A little more planning is involved in the use of bulk electrolysis in organic synthesis.  An example is the dimerization of propionitrile to make adipic acid in the synthesis of nylon. The problem of reference electrode drift can be solved through the use of an aqueous SCE  or a Ag/AgCl electrode in a separate compartment, with a salt bridge (I particularly like the KCl/agar "Jello" type!) or some other means (Vycor frit, asbestos fiber etc) to allow ionic conduction.  If  the presence of water in the electrolyte is a problem, then a non-aqueous reference in a separate compartment is necessary.



My own electrolyses involve air-and moisture sensitive organometallic compounds, which only get more temperamental after electron-transfer.  I usually perform such experiments in an inert-atmosphere drybox with a dewpoint < -30°C and < 5 ppm oxygen.  I like the aqueous Ag/AgCl reference electrodes of the type available from Bioanalytical Systems, but the water is a problem.  I usually remove the Vycor frit, and use a tiny syringe to wash out the aqueous KCl solution with water and then ethanol.  After drying the electrode I take it into the drybox and then syringe in some blank electrolyte solution, being careful to avoid bubbles.  This reference electrode goes into a compartment separated from the rest of the cell by a frit. 



Without any bubbles to expand and contract in response to temperature and pressure, the solution in the reference electrode won't mix with the rest of the solution in the compartment.  With its own compartment, the reference electrode solution is less likely to become contaminated by electrode products diffusing in from the anode and cathode. 



The upshot is that the reference electrode's potential will be steady all the way through the experiment.  The downside is that there might be a lot of resistance between the other electrodes and the reference electrode.  One way of dealing with this is to add a "shunt" (as described on Gamry's website: to the cell.  Essentially, you put a Pt wire in the working electrode compartment, connect it to a capacitor, and connect the other end of the capacitor to the reference electrode.  The capacitor keeps charge from flowing through the shunt.

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Member since: Mar 13, 2009

I will probably use this blog to post random observations about my experiences with electrochemistry. Maybe attach some old code, too.

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